ABSTRACT
A review is presented of some of the ways in which electron spin
resonance (ESR) spectroscopy may be useful to investigate systems of
relevance to the environmental sciences. Specifically considered are:
quantititave ESR, photocatalysis for pollution control; sorption and
mobility of molecules in zeolites; free radicals produced by mechanical
action and by shock waves from explosives; measurement of peroxyl
radicals and nitrate radicals in air; determination of particulate
matter, polyaromatic hydrocarbons (PAH), soot and black carbon in air;
estimation of nitrate and nitrite in vegetables and fruit;
lipid-peroxidation by solid particles (silica, asbestos, coal dust); ESR
of soils and other biogenic substances: formation of soil organic
matter, carbon capture and sequestration (CCS) and no-till farming;
detection of reactive oxygen species in the photosynthetic apparatus of
higher plants under light stress; molecular mobility and intracellular
glasses in seeds and pollen; molecular mobility in dry cotton;
characterisation of the surface of carbon black used for chromatography;
ESR dating for archaeology and determining seawater levels; measurement
of the quality of tea-leaves by ESR; green-catalysts and catalytic
media; studies of petroleum (crude oil); fuels; methane hydrate; fuel
cells; photovoltaics; source rocks; kerogen; carbonaceous chondrites to
find an ESR-based marker for extraterrestrial origin; samples from the
Moon taken on the Apollo 11 and Apollo 12 missions to understand
space-weathering; ESR studies of organic matter in regard to oil and gas
formation in the North Sea; solvation by ionic liquids as green
solvents, ESR in food and nutraceutical research.
Keywords: quantititave ESR, photo-catalysis, zeolites, explosives,
peroxyl radicals, nitrate radicals, particulate matter, PAH, asbestos,
coal dust, photosynthesis, fuel cells, photovoltaics, kerogen,
carbonaceous chondrites, nutraceutical research
1. Introduction
The present review is a companion to a related survey published in
a previous issue of Science Progress entitled, "Electron spin
resonance: a diagnostic method in the biomedical sciences (1) ",
and extends its coverage into the application of ESR to the many and
various aspects of the environmental sciences. In reality, there is an
element of overlap and certain topics could legitimately have been
included in either review, since various environmental factors do indeed
influence human health and hence are of relevance to the biomedical
sciences. Therefore, while trying to avoid too much duplication, I
reiterate the following essentials of the method per se, of which a more
detailed coverage may be found in the first review (1). Electron spin
resonance (ESR)--also known as electron paramagnetic resonance
(EPR)--tends to receive far less coverage than its relative, nuclear
magnetic resonance (NMR). This is partly because NMR spectrometers and
their uses are more ubiquitous, and furthermore, unless there is a
specialist in the subject on the staff, ESR receives only scant mention
in university science courses. In principle, NMR spectra may be recorded
from dozens of different nuclei, whereas obtaining an ESR spectrum
requires some of the sample molecules to contain one or more unpaired
electrons, which might appear to be a curiosity feature. Nonetheless,
here lies the real crux of ESR, since the method is completely specific
for unpaired electrons, which are frequently formed in materials that
have encountered one or more of a range of important energetic
conditions, for which a signature is supplied in the form of the
consequent ESR spectrum. The unpaired electron-bearing sites are usually
termed "damage centres", "defects" or "trapped
electrons" by physicists, geologists, archeologists and
environmental scientists, but are normally referred to by chemists and
biologists as "free radicals", whose detailed molecular
structures may be revealed from the spectral parameter of hyperfine
splitting, where it is observed, and that of the g-factor.
Paramagnetic transition-metal cations, [Fe.sup.3+], [Mn.sup.2+],
[Cu.sup.2+], are quite commonly detected in environmental samples and in
biological tissues using ESR. Paramagnetic materials, including metal
cations and synthesised complexes or stable organic radicals (usually
nitroxides), may be deliberately added to samples as probes of local
molecular environments such as cell-membranes, and to determine their
dynamic properties. Along with other stable "organic" radicals
(carbon chars and lithium phthalocyanines), nitroxides may be used to
measure oxygen tensions in a variety of "soft" materials
including foodstuffs. When it is desired to investigate various reacting
systems for the intermediacy of free radicals, "spin-traps"
are often added. These are frequently of a structural type designed to
"trap" reactive radicals by addition to them, so forming
nitroxides in situ, which give rise to detectable concentrations in
consequence of their relative stability. Clearly, there are many and
varied important applications for ESR, and most importantly so in areas
of the biological and environmental sciences.
Unpaired electrons are created in a broad variety of samples which
have often encountered fairly extreme conditions; high-energy (ionising)
radiation (X-rays, [gamma]-rays), energetic particles (electrons,
protons, [alpha]-particles), UV light, high temperatures, combustion
processes, reactive chemical reagents, mechanical stress, explosions. As
we shall see, under both these and other sets of conditions, structural
dislocations may be introduced in the form of organic and inorganic free
radicals, which host unpaired electrons, and their ESR signal may
provide a marker of the kind of process which has created them. The
great power of ESR is its ability to identify the chemical nature of
free radical species, and from the intensity of the signal, the number
of radicals that have been formed in particular systems. From the
line-widths and line-shapes of the ESR spectra from radical species,
frequently as deliberately introduced to samples as spin-probes, various
features of the local molecular environment may further be deduced. For
example, details of the local molecular environment and
phase-transitions in such complex media as foodstuffs (ice-cream and
dough) may be deduced using spin-probes as additives.
Free radicals, moreover, play a central role in nature,
particularly in living systems, and consequently, current activity in
researching into these species is enormous. Mainly, this is because it
is widely held, in part through the agency of ESR measurements, that
radicals provide both the cause and mediation of many diseases, and
indeed of the ageing process itself. The generally reactive character of
radicals and unpaired-electron species overall (including metal ions,
which can participate in redox processes) further underpins much of
chemistry (it is, of course, "chemistry" which is implicit to
all the above), and it is largely on the findings from simpler chemical
systems that much of current biochemical thinking is based, and this
underlying free radical chemistry impacts further on a variety of
environmental aspects--especially in the role of pollutants in
atmospheric phenomena, such as ozone-loss, global warming and acid-rain
production. Furthermore, since the atmosphere is in direct contact and
exchange with the surface of the Earth--with the soil, the plants which
grow within and upon it (biosphere), the rocks and mountains
(lithosphere), and the rivers, seas and oceans (hydrosphere)--the
atmospheric chemistry takes-on a wider role. Remarkably, ESR can provide
insight into many of these phenomena too, both through its use in the
investigation of model systems, and more directly as an analytical
technique.
2. Quantitative ESR ("Q-ESR")
Since many of the applications to be described in which ESR is used
as an analytical technique involve the measurement of the unpaired
electron concentration (also known as the spin-concentration or
spin-count) in samples, it appears appropriate to discuss this at the
outset. Obtaining reliable measurements of ESR signal intensities is
extremely tricky (2), and it almost appears that all vital aspects of
the ESR experiment act in conspiracy against managing this successfully.
For a start, the sensitivity of the measurement depends acutely on the
position of the sample in the microwave cavity, and is greatest at a
single point at the cavity centre. On moving the sample away from this
point, in any direction, the ESR signal intensity is found to decrease.
The sample tube can be constrained by means of a "collet", so
that movement only up and down in the cavity is possible.
When the sample is a liquid (and therefore is homogeneous),
potential problems in its positioning may be circumvented by using a
completely filled sample-tube which extends through the full length of
the cavity. In this case, the exact position of the sample has far less
influence on the signal intensity, provided that the sample-tube is
homogeneous in its diameter and wall-thickness. This approach was used
in the study of relative antioxidant efficiencies (3). For many solid
materials, it is not possible to provide a homogeneous, cavity-length
sample. Therefore, the sample position needs to be determined precisely.
Various devices have been devised for this purpose; the simplest being
merely to draw a calibration mark on the sample tube, so that its
penetration-depth through the collet and into the cavity is known and is
consistent. For a series of samples, a set of matched tubes is required,
each calibrated to the same depth, which ideally brings the sample to
the cavity centre, in order to maximise sensitivity.
For quantitative comparisons between samples, they should be as
closely matched in composition as possible. This applies to the use of
standard samples against which may be measured unpaired electron
concentrations: these are more usually referred to as "spin
concentrations", and are quoted as the number of "spins"
(unpaired electrons) per gram of sample. [As a
"rule-of-thumb", it is often said that a typical X-band ESR
spectrometer can detect down to ca [10.sup.13] "spins" per
gram, but the actual achievable value depends on all kinds of things!].
So, at the very least, the sample tubes should all be filled to the same
depth, and at best, the standard should consist of the same material as
the sample. Since this is rarely possible, a more practical alternative
strategy is to match the materials as closely as possible in their
dielectric properties. For example, in quantifying the amount of
elemental carbon (EC) and organic carbon (OC) (mainly polyaromatic
hydrocarbons; PAH) in particles sampled from the air in Sofia
(Bulgaria), Yordanov and his coworkers have prepared a set of standards
by dispersing a synthetic carbon--Char in talc (4), since the dominant
sample component is of similar mineral (silicate) type. [In sampling air
from the Mersey Road Tunnel, in Liverpool, we found that silica powder
serves the same purpose.]
Finally, of course, the operating conditions of the ESR
spectrometer, gain, modulation and microwave power level, should be the
same for both sample and standard, although the signal intensity
response to the gain is probably sufficiently linear to permit
extrapolation from one sensitivity range to another, as is required when
the two specimens have widely different spin-concentrations. The
construction of the cavity has also been shown as critical in QESR
measurements (5).
2.1 Standards
Strictly, the spin concentration should be determined against a
standard and, in principle, any paramagnetic material of known spin
concentration might be used. In practice, DPPH
(1,1-diphenylpicrylhydrazyl) is widely used. The spin concentration of
the standard is determined from the quantity (number of molecules)
present, and a corresponding "area" under the absorption peak
is obtained by double integration of the first-derivative spectrum. Then
the sample is run, the spectrum is also double integrated, and from the
relative areas, the spin concentration of the sample may be deduced.
This is not always necessary, and good results may be achieved on the
basis of relative peak heights alone. Yordanov and his group have
constructed a special sample holder, into which a sample of [Mn.sup.2+]
is strategically sealed (6). By varying the relative height of the
sample holder in the cavity, and of the sample within the holder,
relative heights of sample/[Mn.sup.2+] signals may be obtained, which
give very good linear correlations, and which seem to be more reliable
than using a full "spin-count". It is also thought that this
approach offsets the loss of power of the klyston, which occurs
inevitably over time, since both sample and [Mn.sup.2+] signals will be
equally affected.
3. ESR studies of photo-catalysis for pollution-control
Considerable efforts have been made to optimise photo-catalytic
systems for use in cleaning water of organic pollutants (7-10). The
essential requirement is that the solution should be air-saturated, and
contain suspended particles of a semiconductor to serve as the
photo-catalyst, one such material being titanium dioxide. When
Ti[O.sub.2] (or indeed any other semiconductor) is irradiated with
light, electrons are promoted from the valence band (vb) to the
conduction band (cb) of the material. The "band-gap"
(excitation energy) for Ti[O.sub.2] is 3.2eV, which corresponds to a
wavelength of ca 400nm, and is at the border of the UV-Vis spectrum. The
process generates positive holes (electron vacancies) in the valence
band ([h.sub.vb.sup.+]) and electrons ([e.sub.cb.sup.-]) in the
conduction band. A number of events may then occur, which result in the
formation of reactive species capable of reacting with and decomposing
any organic pollutants that may be present. The initial species
([h.sub.vb.sup.+]) and ([e.sub.cb.sup.-]) can either react directly with
the organic agents as sorbed on the surface of the particle, or they may
be converted into reactive intermediates which react with the organic
pollutants either directly on the surface or close to it. In the case of
Ti[O.sub.2], it is normally thought that ([h.sub.vb.sup.+]) is
transformed into hydroxyl radicals (* OH) by the oxidation of
[H.sub.2]O, while, in the presence of molecular oxygen,
([e.sub.cb.sup.-]) forms the superoxide radical anion ([O.sub.2.sup.-*),
which serves to leave the holes free to react. The limiting factor in
determining the efficiency of the photo-catalytic process is the rate of
recombination of ([h.sub.vb.sup.+]) and ([e.sub.cb.sup.-]), in
competition with the formation of other reactive species, of which * OH
radicals are believed to be the major agents responsible for the
decomposition ("mineralisation") of organic pollutants (Figure
1) (11,12), since they are fiercely oxidising. Indeed, compelling
evidence has been gleaned from ESR spin-trapping studies that * OH
radicals are produced on light irradiation of aqueous solutions
containing Ti[O.sub.2].
The * OH radical trapping efficiency by DMPO
(5,5-dimethyll-pyrroline-N-oxide) has been estimated as ca 33% (13), and
so, in principle, might be used to estimate the yield of * OH radicals
in a photo-catalytic system; however, a much improved value of ca 80%
was demonstrated by trapping * OH with the stable free-radical
3-carboxyproxyl (3-carboxy-2,2,5,5-tetramethylpyrrolidine-N-oxyl), a
reaction that leads to the diamagnetic product proxyl-NH
(3-carboxy-2,2,5,5-tetramethylpyrrolidine) (14). Therefore, the latter
is the better method. The main difference between the two methods is
that, in the spin-trapping procedure, the appearance of the spin-adduct
of DMPO/ * OH is monitored, while the latter measurement relies upon the
disappearance of the initial signal from 3-carboxyproxyl, as it
scavenges * OH radicals. It is thought that this occurs by an initial
coupling of the two radical species, followed by elimination of
[O.sub.2] from the primary product, proxyl-NOOH, to give proxyl-NH which
is the species mainly isolated (along with minor amounts of proxylNOH).
From a measurement of the line heights and widths, a rotational
correlation time of 38 picoseconds was deduced for 3-carboxyproxyl in
water, which increased to 45 picoseconds when Ti[O.sub.2] was added:
although this change is not large, it does indicate some interaction
between the probe and the catalyst surface. The rate of formation of *
OH radicals was found to be a linear function of the light intensity at
low intensities, and serves as a mechanistic parameter with which to
investigate the mechanism of formation of this reactive species by the
photo-excitation of Ti[O.sub.2] (14)
[FIGURE 1 OMITTED]
4. Studies of zeolites
Zeolites have a profound ability to absorb molecules of water and
of many other substances. Zeolites contain small holes (micropores),
which are generally of less than 13 [Angstrom] in diameter, and hence of
molecular dimensions, and it is into these that molecules are actually
absorbed. The essential framework of a zeolite is an aluminosilicate
which carries an overall negative charge. This is counterbalanced by the
presence of metal cations (15-17). In consequence, zeolites can be used
as ion-exchangers, and may be used to "soften" water, since
cations such as [Ca.sup.2+] and [Mg.sup.2+] which make water
"hard" are absorbed into the zeolite, while [H.sup.+] or
[Na.sup.+] originally present are displaced. Of greater importance is
that radioactive cations, particularly [Sr.sup.2+] and [Cs.sup.+] (17),
can be removed from the cooling water output produced during the
operation of nuclear power plants (NPPs), in order to minimise their
contaminating the environment in the first place, and in remedial
clean-up operations when radioactive material has inadvertently been
released. Indeed, around 500,000 tonnes of zeolites were used in the
aftermath of the disaster at Chernobyl in 1986, supplied from mines
across the former USSR (18). Zeolites have many and various important
environmental applications, as has been summarised. (17).
* Buildings: four million tonnes of natural zeolites are mined
annually mainly to be used in the construction industry, of which 2.5
million tonnes are shipped to China to make a light-weight concrete.
Volcanic tuff may be cut with handsaws and used directly to fabricate
houses and indeed all kinds of buildings, in regions where it is
plentiful, Republic Square in Yerevan, Armenia, with its architectural
splendour.
* Around 1.4 million tonnes of zeolite A are synthesised each year
for use as a "builder" in detergents, to remove and
encapsulate [Ca.sup.2+] and [Mg.sup.2+] cations which make water
"hard", rather than polyphosphates which cause algal bloom in
lakes and rivers.
* Toxic heavy metal cations, [Pb.sup.2+], [Cd.sup.2+], [Zn.sup.2+],
may also be removed from the environment by cation-exchange into
zeolites.
* Toxic anions may also be removed by reaction with heavy metal
cations previously exchanged into the zeolite. For example, a
silver-exchanged zeolite can be used for removing radioactive iodine
(iodide ions) in the form of insoluble AgI which stays in the zeolite.
Other toxic anions: cyanide, arsenite, arsenate, chromate and molybdate
may similarly be removed so long as the precipitated salt has a
sufficiently small solubility product.
* [H.sup.+]-exchanged zeolites, ultrastable zeolite Y (USY) are
used as solid acid catalysts in the petrochemical industry. Around
300,000 tonnes of synthetic zeolites are manufactured annually for his
purpose.
* [Ni.sup.2+] exchanged zeolites have been demonstrated to absorb
sulfur compounds (thiophene derivatives and thiols) from model
"diesel" in accord with a desire to to reduce
transportation-based S[O.sub.2] emissions, and hence the damaging
effects of "acid rain" on rivers and the facades of buildings.
* Reduction in N[O.sub.x] emissions from vehicles, using
zeolite-loaded "catalytic converters".
* Surfactant-modified zeolites have a potential to remove toxic
anions, chromate and organic pollutants, trichloroethylene,
simultaneously from the environment. It is common that a region is
co-contaminated with a mixture of pollutants and such a multifunctional
decontamination agent might be very useful. Contaminated groundwaters
are a good example.
* Molecular sieves: small-pore zeolites (such as zeolite-A)
selectively absorb small polar molecules, water, and so zeolite
"molecular sieves" are highly efficient drying agents for
removing traces of water from other solvents.
* Hydrocarbon sieving: linear n-alkanes (needed to make detergents)
can be separated from branched alkanes, since the former pass more
slowly through a colunm packed with zeolite 5A in consequence of their
preferential penetration of the zeolite pores, which results in a more
tortuous passage through the material. Millions of tonnes of n-alkanes
are produced annually by this method.
* [H.sup.+]-exchanged zeolites (H-ZSM-5) are used as solid acid
catalysts, for "cracking" in the petrochemical industry.
* Medical applications: Hemosorb and QuikClot are commercial
products based on zeolites which when applied to wounds (occasioned in
accidents, battle-field situations or by surgery) are said to cause an
"instant" cessation of bleeding. Zeolites are also used in
kidney dialysis machines, to absorb ammonia from blood and prevent it
from building up in the body (a job that healthy kidneys normally do).
* Separation of gases: there are commercial units that can provide
oxygen of 95% purity for use in hospitals or for patients, those
suffering from emphysema and other forms of obstructive pulmonary
disease (OPD), by separating it from air. Nitrogen (80% of air) is
preferentially absorbed over oxygen by a zeolite because of its much
larger molecular electric quadrupole moment, and so enables oxygen to
separate from air almost in a state of purity.
* Agriculture: for supplying [K.sup.+] and N[H.sub.4.sup.+] to
plants from soils that have been enriched with zeolites exchanged
specifically with these cations. It is suggested that such
"zeoponics", as the strategy is called, might be used to grow
food on long space missions, if we ever send "a man to Mars".
* Contaminated, brown field land may be rendered fit for building
and even for agriculture by treating the soil with sufficient quantities
of zeolites, which remove heavy-metal cations.
* Use in more efficient heating systems. Essentially, the adsorbed
water can be driven out of a zeolite by heat, but when the water is
readsorbed, heat is given out. The principle can be incorporated into a
heat-pump system which uses more of the available energy for actual
heating, most of which would otherwise be wasted.
ESR has proved its utility in elucidating many essential properties
of molecular adsorption, mobility of contained molecules and catalysis
by zeolites, as is illustrated in the following examples.
4.1 Studies of liquids sorbed in zeolites using ESR
Different kinds of materials may be used, including nitroxides, to
probe the motional behaviour of a liquid: this is often different for
the material in bulk form or as confined in micro-pores (19). In
general, it is found that the differences vary markedly according to
pore size, but depend less on the nature of the porous material (whether
it is, say, a silica-gel or a zeolite). Since zeolites contain cations
that occupy particular sites in an overall negatively charged porous
framework, further differences are expected according to the
charge-state of the nitroxide probes, which are available (Scheme 1) in
neutral, cationic or anionic forms, the spin probes, 4-Hydroxy-TEMPO
(TEMPOL), CAT-1 ([Temp-TMA.sup.+]) and and [TEMPYO.sup.-]
([Tempyo.sup.-]).
[FORMULA NOT REPRODUCIBLE IN ASCII]
In one example, each of the three probes was sorbed (20), as a 4 x
[10.sup.-4] M solution in ethanol, into samples of the synthetic zeolite
NaX (13X) which had been deprived of water by heating at 200[degrees]C
under vacuum for 3-4h. ESR spectra were recorded in the temperature
range 138-298 K both for solutions in bulk form and as sorbed into the
zeolite. For TEMPOL, the sharp "three-line" spectrum of the
probe undergoing "fast-motion" persisted down to 160 K in the
bulk solution, below which spectra characteristic of
"slow-motion" were recorded; this transition occurred at a
rather higher temperature of 180 K in the micro-pore contained solution.
[Temp-TMA.sup.+] displayed the transition below 180 K in bulk ethanol,
but as sorbed in 13X, although a sharp "three-line" pattern
appeared above 210K, it remained superimposed on the signal from an
immobilised fraction which persisted up to room temperature. In the
sorbed situation, the mobility of the probe decreases in the order:
[Tempyo.sup.-] > TEMPOL > [Temp-TMA.sup.+], in line with
increasingly strong interactions with the zeolite surface. The
interesting observation of two distinct motional regimes for
[Temp-TMA.sup.+] is interpreted in terms of probe molecules which are
located at different sorption sites in separate cavities. [The absence
of line broadening immediately excludes that two such molecules are
present together in the same cavity]. Distinct cation sites are well
established in fuajasites (15-17), such as zeolite X and zeolite Y, and
are called simply site II and site III (Figure 2), and so it is not
surprising that the cationic probe molecules would seek these out. A
species sorbed thereon might be expected to give slow-motion spectra,
whereas a probe free to move within the cavity fluid should give
fast-motion spectra. Clearly any exchange of [Temp-TMA.sup.+] molecules
between sites II and III and the cavity fluid occurred slowly enough to
give separate ESR absorptions. For linewidths of the order of 1-2 G, the
exchange rate must be less than (2-3) x [10.sup.7] [s.sup.- 1]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
MCM-41 (mobil crystalline material) is an ordered mesoporous
material (Figure 3), with a microporous structure of uniform mesopores
(3 nm in diameter) running through a matrix of amorphous silica. As a
working definition, micropores are those of less than 2 nm, while
mesopores range in size from 2 to 50nm. Pores larger than this are
termed macropores. MCM-41 is highly porous (pore volume > 1.0mL
[g.sup.-1]) and has a high associated surface area (1000[m.sup.2]
[g.sup.-l]). It is therefore of interest as a potential support material
in fabricating heterogeneous catalysts, since it offers the possibility
to provide highly dispersed catalytic phases. Unfortunately MCM-41 has a
limited robustness in the presence of various reagents and there is an
ongoing effort to introduce catalytically active phases, especially
nickel/nickel oxide and molybdenum oxide, inside the mesopores of the
material (21). The effect of confinement on molecules in MCM-41 was
investigated using a spin-probe as detected by ESR spectroscopy. When
the solid was added to an aqueous solution of a nitroxide spin- probe,
the initially sharp ESR spectrum was found to broaden, while a
superimposed sharper signal remained as a minor component. It appears
that MCM-41 traps the solute molecules within the nanochannels, which
cause the solvent water molecules to form a relatively stable molecular
cage. A detailed analysis of the ESR spectrum indicates that the
nitroxide molecules undergo anisotropic reorientation without being
physically adsorbed by the channel wall (22). The mobility of 2-propanol
and water molecules within the MCM-41 nanochannel as a function of
temperature was further determined. The results demonstrate that two
phases are formed by the 2-propanol molecules: in one the molecules are
partially immobilised on the ESR lime scale (even at temperatures of 40
K above the bulk melting point of the pure material) along with a far
more mobile second phase. Water, in contrast, showed only an
"immobilised" phase even when the temperature was increased to
313K. At higher temperatures still, the spin-probe molecules undergo
anisotropic rotational diffusion which reduces motional resistance from
the solvent molecules in the nanochannel (23). At high concentrations
(30 mM), spin probes exhibit very broad ESR spectra from Heisenberg spin
exchange. When the solution is encapsulated in the nanochannel of
MCM-41, however, a much sharper spectrum is recorded, which suggests
that collisions between the solute molecules within the nanochannel are
quenched. Superimposed ESR patterns are detected: a sharp signal from
the probe in the nanochannel and a broad spectrum from the bulk
solution. When the system is cooled to temperatures in the range of
10-20 K above the melting point of the alcohol, the ESR spectrum from
molecules within the nanochannel changes to that characteristic of a
glassy state. A steady sharpening of the spectrum occurs as the
temperature increases, and the glassy signature finally disappears at
around 293 K. It may be that the solvent molecules form a
liquid-crystalline-like structure involving hydrogen bonding which
prevents the solute from undergoing translational diffusion within the
nanochannel (24).
Aqueous solutions of di-tert-butyl nitroxide (DTBN) (Scheme 2) and
2,2,6,6-tetramethylpiperidine-l-oxyl-4-ol (4-Hydroxy-TEMPO) flowing in a
quartz column of 0.81 mm internal diameter packed with well-dried MCM-41
were studied by ESR. In both cases, a very broad signal which accounts
for > 98% of the total is ascribed to spin-probe molecules that are
located in the MCM-41 nanochannel, along with a minor sharp signal from
probe molecules that occupy the bulk space between the MCM-41 particles.
[FORMULA NOT REPRODUCIBLE IN ASCII]
Although the spin probes are located deep in the cylindrical (3 nm
diameter channel) nanospace of MCM-41, they are transported downstream
fairly rapidly. It is concluded that the aqueous solution is transported
through the nanochannel of MCM-41 relatively slowly but, nonetheless,
more quickly than is predicted from conventional flow-considerations.
With solutions of the same spin probes at high concentration in ethanol
solution, the ESR spectra also show different spectral forms in the two
spaces but the profiles are less distinct. Since the translational
diffusion of the individual molecules is quenched in the nanochannel,
both the solute and solvent molecules must move collectively through it.
It is proposed that this method with which to study the fluid flow in
nanospaces may be called "spin probe nano flowmetry" (25). In
another study by this group, highly concentrated solutions of DTBN were
condensed on several silica materials, such as MCM-41, two types of
SBA-15, and fumed silica. At a very low level of doping only the ESR
spectrum of an immobilised nitroxide radical is observed, but as more of
the solution is adsorbed, the spectrum sharpens until a well-resolved
three-line spectrum is observed, when sufficient of the solution has
been added as is estimated to cover the surface with a monomolecular
layer. Hence, the DTBN molecule can tumble rapidly on/in this solvent
layer. As yet more solution is added, the ESR spectrum changes
differently and characteristically from system to system: thus, the
line-width increases in a practically linear fashion for the SBA-15 and
fumed silica systems. However, in the case of MCM-41 it remains nearly
constant until the quantity of solution added is greater than the total
capacity of a nanochannel. The increase in line-width as more solution
is added is small for the SBA-15 system but large for the fumed silica
system. An interpretation is offered in terms of the structural geometry
of their silica materials and with a condensation model for the alcohols
on these surfaces. Furthermore, a model of the collective probe/solvent
molecular flow of the alcohol solutions through the nanochannel of
MCM-41 is derived (26).
4.2 Non-nitroxide spin-probes in zeolites
Lund and his co-workers have published a series of papers
(27-29,31) on the structure and dynamics of nitrogen-containing radical
cations, [R.sub.3][N.sup.+*] and [R.sub.3][N.sup.+] - C[H.sub.2], formed
by radiolysis of zeolites containing adsorbed trialkylamine or (as
synthesised) containing [R.sub.4][N.sup.+] as the organic template (a
material that assists the crystallisation of the particular zeolite
form). Even when the [R.sub.4][N.sup.+] cation is too large to enter the
pores of a zeolite by standard cationexchange procedures, it may be
introduced in situ during the zeolite synthesis. The internal dynamics
of the [Me.sub.3][N.sup.+] - C[H.sub.2] radical cation as produced in
[gamma]-irradiated Al-offretite was investigated (27). The spectra were
strongly temperature dependent, within the interval of 4-300K. From a
spectrum measured at 110K, the hyperfine splitting could be simulated
using a dominant anisotropic coupling to two equivalent protons of ca 22
G (the C[H.sub.2] group), which is axially symmetric according to
exchange of the protons by rotation about the N--C[H.sub.2] bond. Over
the temperature range employed, this changes little, nonetheless the
initial substructure on each line of the 1:2:1 triplet of a 1:3:3:1
quartet (from an equivalent coupling to one proton from each of the
methyl groups) changes from 4.6G to a "decet" of splitting
1.5G, from nine equivalent protons at 300 K. This effect is due to
restricted rotation of the methyl groups, and can be analysed to yield
an activation energy of 8.1kJ [mol.sup.-1] The coupling to the [sup.14]N
nucleus is unchanged by the motional processes and remains at a constant
3.5 G (27). The research was extended to a study of the influence of
cage size/type using Al-offfetite, SAPO-37 and SAPO-42 in their ability
to stabilise the radical cations [Me.sub.3][N.sup.+*] and
[Me.sub.3][N.sup.+] - C[H.sub.2.sup.*] (28), in which the radicals were
produced by [gamma]-irradiation of these zeolites which all contained
[Me.sub.4][N.sup.+] cations as the organic template.
[Me.sub.3][N.sup.+*] was stable at room temperature in the relatively
small, sodalite, cages of SAPO-37 and in the [beta]-cages of SAPO-42.
[Me.sub.3][N.sup.+] - C[H.sub.2.sup.*] was also stable in the sodalite
cages of SAPO-37, and also in the relatively large, gmelinite, cages or
in the main channels of Al-offretite and in the [alpha]-cages of
SAPO-42. The exchange rates of the methyl group hydrogen atoms in
[Me.sub.3][N.sup.+] - C[H.sub.2.sup.*] are in the order: SAPO-37
(sodalite cages) < Aloffretite (gmelinite or the main channels) <
SAPO-42 ([alpha]-cages), and follows the order of the relative cage
sizes: sodalite (ca 6 [Angstrom]) < the gmelinite cage (ca 6 x 7.4
[Angstrom]) or main channels (ca 6.5 [Angstrom]) < the [alpha]-cage
(11 [Angstrom]).
In a second paper (29), is reported an investigation of the
structure and dynamic properties of the radical cations of triethylamine
([Et.sub.3][N.sup.+*]) and tripropylamine (n - [Et.sub.3][N.sup.+*] ),
formed by exposure to [gamma]-radiation of AlP[O.sub.4]-5 in which the
neutral precursor amines were incorporated as an organic template during
synthesis. Both [Et.sub.3][N.sup.+*] and n - [Pr.sub.3][N.sup.+*] were
stable in AlP[O.sub.4]-5 up to room temperature, and gave well resolved
spectra throughout the entire temperature range 4-300K. The temperature
dependent spectra were analysed using a two-site exchange model in which
the two inequivalent [beta]-C[H.sub.2] hydrogen atoms of the
C[H.sub.3]C[H.sub.2]- or (C[H.sub.3]C[H.sub.2])C[H.sub.2]-groups
mutually interchange their positions, becoming equivalent on the ESR
timescale. The exchange rates were evaluated by computer simulations of
the ESR lineshapes, and were found to increase by over two orders of
magnitude, in the range [10.sup.8] x [10.sup.7] [s.sup.-1] to 6.6 x
[10.sup.9] [s.sup.-1], as the temperature increased from 110K to 270K.
As expected, the fully averaged coupling constant (20G) is close to half
that measured for the frozen conformation (36 G) on a simple
[cos.sup.2][THETA] basis for [beta]-protons. From appropriate Arrhenius
plots of the exchange rates as a function of temperature, energy
barriers to internal rotation were determined at 9.1 and 11.4 kJ
[mol.sup.-1] for [Et.sub.3][N.sup.+*] and n - [Pr.sub.3][N.sup.+*]
respectively. Density functional theory (DFT) calculations predicted
energy barriers of 8.7 and 7.7kJ [mol.sup.-1], respectively and it is
thought that the discrepancy from the value deduced experimentally for n
- [Pr.sub.3][N.sup.+*] arises from interactions between the radical
cations and the zeolite wall (29). We note that a similar increase in
the activation energy for the interconversion of the radical cation of
9-octalin (1,2,3,4,5,6,7,8-octahydronaphthalene) to 28 kJ [mol.sup.-1]
as adsorbed in the zeolite H-mordenite, from the value of 14 kJ
[mol.sup.-1] measured in liquid solution, according to dynamic
simulations of its ESR spectra (30). The last paper (31) in this
interesting set of reports provides an overview of the above work, and
furthermore reports experiments in which nitric oxide (NO) is used as a
probe, which forms a dimeric [(NO).sub.2] triplet state (S = 1) species
in Na zeolite-A. At 5 K an anisotropic interaction with a single
[Na.sup.+] cation was observed for this species.
NO has been employed more generally as a probe molecule for cations
present in zeolites (32,33). For example (32) it is shown that ESR
spectra of the NO mono-radical adsorbed on a zeolite surface is
characterised by three g-tensor components. This is as expected since NO
is electronically orbitally degenerate in an unperturbed condition, but
such degeneracy may be removed by electrostatic interactions with its
local environment, such as being adsorbed on a surface. A 1 : 1 : 1
triplet hyperfine splitting is resolved on the [g.sub.yy] feature
stemming from coupling to the [sup.14]N nucleus. It is the [g.sub.zz] g-
tensor component which is especially sensitive to the local environment
in the zeolite and provides a measure of the surface electrostatic
field. Since the magnetic orbital-coupling along the molecular axis,
which is responsible for the [g.sub.zz] shift, occurs between the
unpaired electron orbital and a vacant orbital, this is negative in
respect to the free-spin value (g = 2) and typically [g.sub.zz] lies in
the range 1.8-1.9. This is the opposite to the effect for the superoxide
radical anion ([O.sub.2.sup.-*])-which has two more valence electrons
than NO--in which the dominant coupling is with a filled orbital, hence
with [g.sub.zz] values that are greater than g = 2. We shall see more in
Section 4 about the adsorption of homonuclear diatomic gases in zeolites
and the induction of infra red absorption bands from them, but we note
that electrostatic fields in zeolites have been estimated from the IR
spectra of adsorbed molecular oxygen ([O.sub.2]), in which the field
induces a dipole moment, and the intensity of the IR band at ca 1555
[cm.sup.-1] from the O-O stretching vibration increases in proportion to
the square of the magnitude of the molecular dipole. ESR measurements of
NO as a probe molecule may well provide an effective but complementary
alternative. Automated simulation of ESR spectra are possible through a
new procedure which facilitates the analysis of a series of closely
related spectra, that from NO adsorbed in different zeolites (32). Lund
and his coworkers have made a review on ESR and ENDOR studies of NO,
N[O.sub.2] and [Cu.sup.2+] cations adsorbed in zeolites, based mainly on
their own extensive work in this field (33). The underlying purpose of
this work is to understand the diffusion and bonding of NO, N[O.sub.2]
and [Cu.sup.2+] involved in the catalytic removal of NOx with
Cu-zeolites. Translational motion of N[O.sub.2] in mordenite, ZSM-5 and
K- and L-type zeolites could be investigated from an analysis of the
temperature-dependent ESR spectra using the slow-motional ESR theory
developed by Freed and his coworkers. The spectral broadening that
became apparent at increasing temperatures can be explained by
Heisenberg exchange between the N[O.sub.2] molecules as they diffuse
along the zeolite channels. In Na-ZSM-5, the spinexchange rate increased
markedly as the Si/Al ratio increased, and is understandable if the
major barrier to its diffusion is caused by the interaction between
N[O.sub.2] and [Na.sup.+] cations. Another paper reports on the
influence of different zeolite structures on the motional dynamics of
N[O.sub.2] (34), as determined from ESR measurements, from which the
following deductions could be made: (1) in zeolites with similar channel
structures, the diffusion rate of N[O.sub.2] is proportional to the
channel size, leading to an order of diffusion rates: Beta-type >
ZSM-5 > ferrierite and L-type > mordenite; (2) the diffusion is
faster in those zeolites that have multi-dimensional channels
(Beta-type, ZSM-5 and ferrierite) than in those with unidimensional
channels (L-type and mordenite).
Turro and his coworkers have generated benzyl and other related
kinds of delocalised radicals in zeolites as probe molecules for these
solids in relation to their role in reactivity. They have coined the
term supramolecular steric effects, to describe phenomena which
stabilise diphenylmethyl radicals (DPM) formed by photolysis of
1,1,3,3-tetraphenylacetone as adsorbed onto the surface of the MFI
zeolite LZ-105 from a solution in isooctane. The solvent was evaporated
in a stream of argon, leaving a loading of 0.3-0.5%, which is enough to
fill all the holes on the LZ-105 surface with the ketone and yielding a
supramolecular system termed I@LZ-105 (35). On photolysis, DPM was
formed. While the supramolecular effect could prevent the dimerisation
of DPM to form 1,1,2,2-tetraphenylethane--a reaction which proceeds in
solution at near diffusion-controlled rates-reaction of DPM with
molecular oxygen was possible, since it could diffuse into the zeolite.
The signals from DPM persisted over several weeks if air was excluded,
but the spectrum was immediately replaced by another one from peroxyl
radicals, R[O.sub.2.sup.*], when air was admitted. Since the spectrum
did not change appreciably over the temperature range - 23 to -
150[degrees]C, it was concluded that the species DPM[O.sub.2] was
immobilised by adsorption on the zeolite internal surface. By pumping
the sample down to a vacuum of 5 x [10.sup.-5] mmHg, the spectrum of DPM
was recovered by dissociation of the corresponding peroxyl radical; the
latter could be recovered by further admission of air, and restored to
DPM by pumping, as could be repeated several times, showing that the
reaction is fully reversible. It is proposed that supramolecularly
stabilised reactive intermediates might be used to explore extensive
intramolecular reactivity, and in this regard, a contrast may be drawn
between a supramolecularly isolated and a matrix isolated system. Thus,
while a "matrix" impedes all reactions of a given
intermediate, a supramolecular medium is more selective and allows
considerable rotational and diffusional freedom. As an example of this,
we see a complete preclusion of dimerisation for DPM radicals, however,
a complete and reversible equivalent bimolecular reaction with [O.sub.2]
is possible. Additionally, the usual subsequent bimolecular reaction
between DPM[O.sub.2.sup.*] radicals, as is observed in solution for
organic peroxyls, is restricted, since the ESR signal from the primary
peroxyl species is persistent (35). The term supra-molecular may be
taken to mean "one level up from [the] molecular", hence
addressing the immediate environment within which the reactive molecules
are occluded. The strategy has been used in a study of
"recombination stereoselectivity" with which to probe magnetic
isotope and magnetic field effects for the coupling of 1-phenylethyl
radicals, as generated within the micropores of NaY type zeolites by the
photolysis of meso- or dl-2,4-diphenylpentan-3-one, present as
co-adsorbed with a chiral inductor (diethyl tartrate or ephedrine) (36).
The reactions were found to be little affected by an external magnetic
field of 2,000 G, but the geminate radical recombinations were sensitive
to intramolecular magnetic isotope effects, such that an enhanced
enantiomeric excess (ee) was obtained for the [sup.13]C isotopomeric
radicals in all cases (36). A range of persistent radicals was similarly
studied as adsorbed "on" ZSM-5 zeolites, formed by photolysis
of a range of substituted dibenzyl ketones (37). The word
"on", used in the title of the paper, is significant in that
radicals formed on the external zeolite surface are not persistent if
they are unable to diffuse into the interior, a matter that depends on
the nature ("supramolecular structure") of the initial
radical@zeolite complex and the diffusion and reaction dynamics of the
radicals produced by photolysis, which determines whether they are
stable for periods of seconds to many hours. In order for persistence to
be obtained at all, the radicals must initially separate and diffuse
apart, whether they are formed on the external surface or on the
internal pore surface. If they are formed on the external surface, the
radicals must diffuse to the internal surface into the supramolecular
state, or if they are produced by photolysis of a ketone adsorbed at the
internal surface, they must separate and become located by diffusion at
regions some distance apart to avoid rapid radical coupling. Size/shape
features are important factors in determining molecular adsorption
capacity and diffusional properties of molecules hosted in zeolites, and
accordingly the structures of the parent ketones and daughter radicals
were selected to impose variable steric constraints on the properties of
these molecules. Radicals which are confined to the external surface by
steric factors are transient since they undergo rapid dimerisation
reactions there. The persistence of radicals located at the inner
surface is a result of inhibition of bimolecular radical combination or
disproportionation reactions, and in order to benefit from such
stabilisation this is where they must be able to diffuse to. As an
example of the sensitivity of the approach, the presence of an ortho
methyl group is sufficient to prevent diffusion of a benzyl radical to
the internal surface. This is clear from the ESR spectra, which shows
that only the "unmethylated" benzyl radical remains persistent
following photolysis of a ketone that must give this, along with a more
hindered radical which is lost by reactions on the external surface.
Analogous radicals with additional methyl substitution at the exocyclic
positions show similar behaviour, which demonstrates that only those
radicals that do not have an ortho ring-methyl group are persistent.
Further extension of the alkyl chain apparently does not impede internal
diffusion, since spectra from benzyl radicals bearing even exocyclic
n-pentyl substituents are observed. Reactions of these persistent
radicals with [O.sub.2] and NO were further investigated, and in the
case of reaction with [O.sub.2], a high yield of persistent peroxyl
radicals was observed. However, the addition of NO scavenges persistent
benzyl-type radicals, which leads initially to the diamagnetic
nitroso-adduct, which on further photolysis is transformed into a
persistent nitroxide radical by subsequent radical trapping. A model is
presented to account for the influence of structural variation in both
the radicals and their parent ketones on their reactivities under the
prevailing "supramolecular" conditions (37).
In one study, ortho-methyldibenzyl ketone, which is ESR silent, was
co-adsorbed with an ESR detectable nitroxide spin-probe, for the purpose
of characterising the external surface of silicalites, which possess MFI
structures, but are made entirely of Si[O.sub.4] tetrahedra, in contrast
to the ZSM-5 zeolites which contain aluminium centres
(Al[O.sub.4.sup.-]), and exchange counter-cations. The external surface
area of the silicalite (a practically all-silica ZSM-5 structure) can be
related to the amount of ESR-silent co-adsorbate molecules required to
attain a certain surface coverage by monitoring the change in the
nitroxide ESR parameters, an increase in rotational freedom when some of
the nitroxide molecules are displaced from the strong binding sites by
the co-adsorbate molecules (which are characterised by
"powder" type spectra). Isotopically labelled ([sup.15]N,
[sup.14]N) nitroxides were used and from the results obtained from them
it was concluded that the molecules adsorbed on the strong binding sites
are able to exchange efficiently with those in an isooctane solution or
on the weak surface-binding sites on a time-scale of a few hours up to a
day (38). The supramolecular model derived from the photolysis of
dibenzyl ketones adsorbed on zeolites is emphasised to take into account
the effect of the physical and chemical nature of the structure of the
zeolites and their effect on the radical species formed. Within this
model are a number of phenomena including surface coverage, adsorption
at internal and external surfaces, diffusion over the surfaces,
molecular sieving of radicals, and the eventual product distributions. A
novel method is introduced for "titrating" the binding sites
via EPR spectroscopy. The influence of co-adsorbed spectator molecules
of varying polarities, namely water, pyridine, and benzene, on the
photolysis of o-methyldibenzyl ketone and dibenzyl ketone adsorbed on
MFI zeolites is investigated. Insights are provided into a displacement
mechanism prompted by the spectator molecules and further demonstrates
how the product distribution of photolysis of sorbed ketones can be
controlled (39).
Using an adaptation of a simple electrostatic model (40), combined
with spin-populations derived from ESR measurements, the degree of
N([[delta].sup.+]) - O([[delta].sup.-]) charge separation in a nitroxide
spin-probe ([R.sub.2]N--[O.sup.*]) is equated with the surface electric
fields in a series of cation-exchanged samples of zeolite-X: LiX, NaX,
KX, MgX, CaX, SrX, BaX. Electric fields in the range 2.0-8.0V/A (0.2-0.8
V/nm) are deduced (40), in good accord with values obtained from
theoretical and other experimental procedures. The surface fields are of
similar magnitude to those measured using the interaction between the
molecular electric quadrupole of NO and zeolite surfaces, according to
the perturbation of the g-tensor (41). We propose that the
ESR/spin-probe method might be used very effectively in the estimation
of a fundamental feature which under-pins many of the properties and
utilities of zeolites. The presence of metal-cations in zeolites results
in strong local electric fields which can promote chemical
transformations, for example photochemical oxidations of hydrocarbons
(hc) with molecular oxygen, using light of longer wavelengths than would
be the case in solution, since the surface electric-field is able to
stabilise the necessary, initial collisional ([hc.sup.+*]
[O.sub.2.sup.-*]) charge-transfer complexes, rendering their formation
thus accessible at lower energies. For example, the C-T complex formed
by photoexcitation of tetramethylethylene (2,3-dimethylbut-2-ene) in
zeolites X and L is stabilised to the tune of 3.5 eV, causing the
absorption wavelength to tail into the visible region of the
electromagnetic spectrum at 350nm (42).
5. Free radicals produced by mechanical action
It is well known that the influence of mechanical stress on
materials can cause the fracture of chemical bonds, or the transfer of
electrons between donor-acceptor molecules, such that free radicals are
formed. A good example of this is the detection of free-radical signals
from samples of bone following milling or grinding (43).
The effect of mechanical action on synthetic polymers, and even on
quite low molecular weight compounds, is also known to produce radicals.
Such mechanical stress may come from milling, the action of pressure,
compression combined with shearing deformation or shock waves, from
explosives. As a potential practical application, the mechanical
degradation of 1,2,3-trichlorobenzene, monochlorobiphenyl and other
compounds was undertaken using a ball-mill with inorganic materials such
as CaO added. On prolonged grinding, a degree of dechlorination close to
100% was achieved during a process that involved the formation of free
radicals according to ESR measurements and yielded inorganic chloride
anions (44). Thus mineralisation of such materials might be achieved in
this way. The types of compound which have been investigated include
compounds with a weak covalent bond, donor-acceptor mixtures, energetic
organic compounds (explosives), etc. Collectively, this field of
chemistry has been termed "mechanochemistry", meaning the
investigation of the influence of mechanical energy on the chemical
(molecular) properties of materials. Some of these topics are now
outlined.
5.1 Mechanical decomposition of compounds with a weak covalent bond
There are compounds (mainly dimers of very stable free radicals)
which contain exceptionally weak bonds. The dimer of the archetypal free
radical, triphenylmethyl ("trityl") has a bond dissociation
energy (45) of ca 45 kJ/mol, sufficiently low that it exists in
equilibrium, in solution, with the free radical (46). Other compounds
have stronger, but still weak bonds, for instance alkylindanedione
dimer, in which the interconnecting bond has a dissociation energy of 72
kJ [mol.sup.- 1] (47). This is far smaller than the typical bond
energies for organic compounds; for comparison, the dissociation energy
for the central C--C bond in n-butane is ca 340[mol.sup.-1] (45). Other
compounds which demonstrate this feature are
bis(2,4,5-triphenylimidazolyl) (48),
3,3'-bis(3-aryl-2-benzofuranosyl) (48),
2,2'-bis(2,3,4-triarylchromenyl) (48) and
2,2'-bis(2-aryl-3-benzothiophenolyl) (49); all of which are
susceptible to pressure, combined with a sheafing deformation of the
solid, which results in the formation of the respective (monomer) free
radicals. Pressure alone does not have the same effect, since the
sheafing force is required to literally tear the two halves of the dimer
apart.
Suitable apparatus permits the sample to be subjected to pressure
within the cavity of the ESR spectrometer. During my time in the
Chemistry Departments at Queen Mary and Westfield College, London
University, I was given a Varian E9 ESR spectrometer by the Materials
Department, which had been purchased with the intention of subjecting
strands of polymers to mechanical stress in an attempt to correlate the
free radical concentration resulting from "snapped"
main--Chain chemical bonds with the mechanical load imposed. Apparently
it worked well, but the project was abandoned when complex hyperfine
structure emerged in some samples, rather than a single line with which
to measure the unpaired electron concentration. The spectrometer is long
gone, but I will be ever grateful both for this quirk of fate and to Dr
Peter Reid for the machine itself.
ESR signals were detected in samples of Baltic amber (a resin),
whose intensity was increased by milling with anthracene, acridine or
calophony (50). Immediately following the milling procedure, the
intensity of the ESR signal increases about six times, but then decays
once the mechanical process is terminated, and may be restored by
further milling (50). Indicators like phenolphthalein are also
susceptible to this kind of mechanical stress, which induces both ESR
signals and colour changes. The explosives, 1,3,5-trinitro-1,3,5-
triazacyclohexane (RDX) and pentaerythritol tetranitrate, provide an ESR
spectrum of * N[O.sub.2] on milling at 77 K (51).
5.2 Formation of radicals by the action of shock-waves from
explosives
Another route, by which radicals are generated in solids by
mechanical force, is through shock-waves, generated by explosions (52).
The mechanical energy is sufficiently great that even strong covalent
bonds may be cleaved. It is believed that an initial decay of individual
molecules takes place, releasing energy which sets-off secondary decays
of other molecules. When the rate of localised heat generation
("hot spots") exceeds the rate of heat loss, the explosion
itself occurs, and radicals are produced by the rapid-heating of
molecules. The ESR spectrum of * N[O.sub.2] is detected following
explosions of various materials, including RDX and ammonium nitrate
(52). Radicals are also detected in non-explosive materials, which have
been subjected to shock-waves from explosives present in intimate
mixtures with them.
5.3 Radical-pair generation by mechanical stress on a
donor-acceptor pair
So far, we have considered radicals that are generally formed by
cleavage of a weak covalent bond. One further possibility is that
radicals may arise from the interactions between an electron donor and
an electron acceptor. There are, in fact, numerous compounds known to
behave in this way, under conditions of mechanical stress, giving rise
to radical pairs (53). If the two radicals are close enough that the
unpaired electron from each may interact with the other, then the ESR
spectrum takes on the particular form of a "triplet-state".
The spectrum is characterised by the parameters [D.sub.[parallel]] and
[D.sub.[perpendicular to]]: in practical terms, [D.sub.[parallel]]
provides an indication of the distance between the two unpaired
electrons, while [D.sub.[perpendicular to]]: measures the symmetry of
their interaction. In some cases, the "radical pair" may
withstand extraction into a solvent, and so is a "biradical"
product of chemical bond formation between donor and acceptor,
otherwise, the "pair" will be put asunder by the action of the
solvent (54). The simple action of grinding the materials together in a
mortar for some minutes is all that is required to produce a detectable
ESR spectrum.
6. Measurement of peroxyl radicals and nitrate radicals in air
Organic peroxyl radicals (R[O.sub.2] * ) are the main chain-carders
in the atmospheric oxidation of hydrocarbons. They are formed during the
daytime by reactions between * OH radicals and hydrocarbons and CO, and
at night by reactions between hydrocarbons and "nitrate"
radicals (* N[O.sub.3]), and from reactions of the Criegee intermediate
produced by the interaction between alkenes and ozone. The night-time
chemistry of * N[O.sub.3] is as follows: * N[O.sub.3] radicals are
produced by the reaction between ozone and * N[O.sub.2], then a rapid
equilibrium is established between * N[O.sub.3], * N[O.sub.2] and
[N.sub.2][O.sub.5]. Both * N[O.sub.3] and [N.sub.2][O.sub.5] can be
removed by reaction with hydrometeors (literally, falling objects
composed of water; precipitation consists of a stream of hydrometeors,
in the form of droplets or ice crystals). This leads to the formation of
dissolved * N[O.sub.3], a process which provides a night-time sink for
atmospheric N[O.sub.x]. In addition, * N[O.sub.3] radicals undergo
addition reactions with alkenes present in the atmosphere, followed by
addition of oxygen, to form [beta]-nitratoalkylperoxyl radicals
([O.sub.2]NO - C[R.sub.2] - C[R.sub.2] - OO * ). * N[O.sub.3] also
reacts with aldehydes to yield R[O.sub.2] * radicals and nitric acid,
while reactions of R[O.sub.2] * with * N[O.sub.3] lead to the formation
of alkoxy (RO *) radicals [equation (1)]. Hence, the * N[O.sub.3]
radical can play a similar role as does NO during daytime (55), namely
by initiating chain reactions that lead to the formation of H[O.sub.2].
and * OH radicals at night [equations (2) and (3)]. In equation (2), the
radical RO * is in fact one of type RC[H.sub.2] - O. which transfers a
hydrogen atom to [O.sub.2], forming H[O.sub.2] * :
R[O.sub.2] * + * N[O.sub.3] [right arrow] RO * + * N[O.sub.2] +
[O.sub.2] (1)
RO * + [O.sub.2] [right arrow] RCHO + H[O.sub.2]* (2)
H[O.sub.2] * + * N[O.sub.3] [right arrow] * OH + * N[O.sub.2] + 02
(3)
Measurements of these radicals have been made in the troposphere by
sampling air at the top of a mountain (Schauinsland a mountain in
Southern Germany with an elevation of 1,284 m) using a matrixisolation
apparatus (56). The radicals were trapped from 8 L of air, with an
efficiency of > 95%, in a [D.sub.2]O matrix at 77 K over a period of
30 minutes. The samples were then transported, frozen in liquid
nitrogen, to a laboratory at ground level for ESR measurements. The ESR
spectra were analysed using a numerical procedure which fits the
individual spectra of each radical component, and provides a measure of
their concentrations: the detection limit is 5 parts-per-trillion-by-
volume (pptv) for H[O.sub.2] * , R[O.sub.2] * and * N[O.sub.2], and 3
pptv for * N[O.sub.3] due to its narrower ESR linewidth. From a typical
ESR spectrum, recorded from one experiment, the uppermost trace (A) is
the original spectrum, and is dominated by * N[O.sub.2], at a
concentration of 0.65 parts-per-billion-by-volume (ppbv); subtraction of
the * N[O.sub.2] signal (B) yields the residual signal (C), which
matches closely the structure of the * N[O.sub.3] reference spectrum
(D): a simultaneous fit of the reference spectra of * N[O.sub.3],
H[O.sub.2] * and various peroxyl radicals yielded a concentration of *
N[O.sub.3] at 9.5pptv, whereas the total amount of peroxyl radicals was
less than 1 pptv.
The * N[O.sub.3] levels were always found to be quite low, < 10
pptv. The concentrations of peroxyl radicals varied between almost
40pptv and values below the detection limit of 5pptv. The H[O.sub.2] *
concentration was mainly also at or below the 5pptv detection limit
(56). The ESR method is, of course, specific for free radicals, and can
form part of an overall sampling strategy in which other analytical
methods are employed to analyse hydrocarbon and other trace gas
components from the atmosphere, to provide an overall view and a test of
atmospheric chemical kinetic models (55)
7. Determination of polycyclic aromatic hydrocarbons (PAHs) using
ESR
Polycyclic aromatic hydrocarbons (PAHs) are probably the most
widespread of all potentially carcinogenic and mutagenic chemical
pollutants in the environment, and so the determination of PAHs is of
vital importance in environmental monitoring (55). They are produced in
virtually all combustion processes and are generally present in
environmental samples as very complex mixtures. Since the molecules of
PAHs are diamagnetic, ESR does not immediately come to mind as the most
obvious means for their identification and quantification; however, PAHs
are quite readily oxidised to their corresponding radical cations, using
a silica-alumina catalyst (57), of the kind used in the petrochemical
industry for hydrocarbon "cracking". The catalyst is in an
amorphous form, rather than being of the zeolite type, which can also
produce radical cations from molecules with a sufficiently low
ionisation potential. This has the advantage that larger PAH molecules
can access the catalytic sites, whereas they would be excluded from the
pores of a zeolite, zeolite Y does not admit molecules of anthracene or
larger (58). The resulting radical cation species are stable in the
sorbed state on the catalyst surface, and the conversion is
quantitative. The catalyst is first heated in air at 800[degrees]C for
at least two hours in a muffle furnace, and then is allowed to cool,
under vacuum, to room temperature. The PAH solution, in toluene, is
introduced to a sample tube, to which the catalyst is then added.
When known concentrations of perylene, anthracene, chrysene, pyrene
and 3,4-benzopyrene were added sequentially, firstly in the range up to
0.5 nmol, then from 1 to 5 nmol, a straight line plot is observed of ESR
signal intensity vs concentration. This indicates that the method is
valid for the estimation of PAHs generally, and as present in mixtures,
the experiment is measuring the total number of PAH molecules present,
irrespective of their precise nature. In order to measure an overall
signal intensity in mixtures of PAHs, it is necessary to over-modulate
the spectrum: this has the effect of broadening the spectral lines so
that all PAH radical cations are revealed in one single first derivative
signal, whose peak-to-peak intensity may be taken as a measure of
radical concentration (57).
7.1 Measurement of soot and PAH in urban air
The procedure has been applied to the analysis of the air in Sofia,
Bulgaria (59). Aerosols containing both mineral dust and soot with
adsorbed PAH were collected by drawing 5 [m.sup.3] of air through a
Whatman No. 1 filter paper. It is important to introduce some
terminology here: PAH is often referred to by environmental workers as
"organic carbon" (OC), while soot is called "elemental
carbon" (EC); EC refers to the hard carbon-rich material on which
OC is adsorbed, and both are produced together by combustion processes,
as in incinerators, vehicle engines, and indoors by smoking and burning
fuels. Three to five samples were collected at each of four locations:
(1) by the side of a motorway, (2) 100m away from the motorway (urban
air), (3) in an office (where smoking is forbidden), (4) in a cafeteria
(where smoking is allowed). The soot collected on the filters was
measured directly by ESR, since it contains unpaired electrons (at this
point, the PAH, OC is invisible). For the evaluation of the PAH, the
filter was extracted with 10mL of toluene, and 0.3mL of the resulting
solution was transferred to an ESR sample tube with a cracking catalyst
introduced, in the form of a pellet. After one hour, the ESR signal from
the total PAH radical cations was recorded, thus providing a measure of
the OC content of the soot: both EC and OC concentrations were
calibrated against a standard sample containing known quantities of a
carbon-Char, prepared by pyrolysing sucrose at 550[degrees]C. To ensure
reproducibility in the relative peak-to-peak signal intensities and
line-widths, the spectra were recorded simultaneously with that from a
reference standard containing [Mn.sup.2+] ions. It was found that the EC
content decreased in the order: motorway > urban air > cafeteria
> office, with values of 91.1 [micro]g [m.sup.-3], 49.5 [micro]g
[m.sup.-3], 18.7 [micro]g [m.sup.-3] and 7.8 [micro]g [m.sup.-3].
However, the OC content (from PAHs) had values of cafeteria (76.6
[micro]g [m.sup.-3]), motorway (78.1 [micro]g [m.sup.-3]), urban air
(32.5 [micro]g [m.sup.-3]), and office (2.7 [micro]g [m.sup.-3]). It is
reasonable that the EC levels indoors are less than those outside, and
are greatest on the motorway; the high level of OC in the cafeteria is
probably caused by smoking, while that on the motorway must arise from
exhausts.
The method has the advantage of simplicity over other, rather
complex, procedures that are employed in the analysis of EC and OC. For
example, these components may be determined separately using a thermal
treatment to drive-off the volatile OC, followed by catalytic oxidation
to form C[O.sub.2], which, after reduction to C[H.sub.4], may be
quantified by various methods of gas-analysis. The temperature is
increased up to 600[degrees]C at which the OC (principally PAH) becomes
volatile and is removed in a stream of helium prior to their oxidation.
The next step involves the removal of carbon particulate (EC) remaining
on the filter, which is then also converted to C[O.sub.2] in an
[O.sub.2]-He stream (59). ESR has been similarly employed in the
analysis of diesel exhaust particles (DEP), taken at various points in
the exhaust-pipe of a diesel engine, at the dust sampler of a highway
tunnel (standard DEP), on the soundproofing wall alongside a heavy
traffic road and on the filters. A very broad signal was detected from
iron oxides and a second much sharper signal from carbon radicals. On
annealing the DEP sample at 250[degrees]C, a dramatic increase was
observed in the intensity of the latter signal, which suggests that a
thermally activated carbonization of residual organics had occurred
(60). An investigation was made of summertime carbonanceous aerosols
collected from the marine boundary layer of the Arctic Ocean, in which
the relative concentration of what is described as black carbon (BC)
which is the same as EC in the Sofia study was determined using ESR. It
was found that the spatial distribution of BC from ship emissions was
concentrated around the periphery of the Arctic Ocean, which suggests
relatively intensive contamination by ships in the Russian and Canadian
Arctic. As determined by gas chromatography-mass spectrometry
(GC-MS)--rather than by ESR, as in the Sofia study-the abundance of PAHs
on the BC particles was determined in the range 142-2672pg [m.sup.-3]
(mean=702pg [m.sup.-3]), which is much greater than previously
determined using land-based observation, and hence ships appear to be a
major contributor to PAH concentrations, certainly at some regions of
the Arctic Ocean during the summer months (61). It is thought that such
carbon-contamination from shipping may contribute to the melting of the
Arctic ice, potentially urging environmental consequences. If the Arctic
continues to warm at its current rate a further decline in the amount of
Arctic summer sea ice is expected. The consequences are that the global
sea level will rise, with less habitats for polar bears but additionally
an increased level of ship traffic which may further accelerate climate
change.
8. ESR studies of soils and environmental biogenic substances
Soil is a complex material. It is heterogeneous, consisting of both
organic and mineral components. The organic fraction is usually termed
humus, and is composed predominantly of two types of material, humic
substances and polysaccharides, which may constitute up to 80% of the
total extractable matter in soil. Humic materials contain free radicals,
which are of the semiquinone type, and are a mixture of
"transient" species (with lifetimes of several hours), and
others which are stabilised within the complex chemical structure of the
material matrix. It is thought that the latter participate in the soil
chemistry by acting as electron donors and acceptors (62). Natural
[Fe.sup.3+]-fulvic acid complexes have also been characterised in some
extracts. It has been shown that all 3d-transition metals form
"inner-sphere" complexes with humic acids: [Mn.sup.2+] ions
are coordinated to raw peat or to peat humic acids octahedrally, whereas
[Cu.sup.2+] ions occur in square-planar arrangements with two
carboxylate and two aliphatic nitrogen-functional ligands. It is
reported that [VO.sub.2.sup.+] ions occur in a square-pyramidal
structure with four oxygen-containing ligands. In acidic solutions,
diamagnetic Mn(VII), Cr(VI), Mo(VI) and V(V) oxoanions are reduced by
humic acid to paramagnetic Mn(II), Cr(III), Mo(V), and V(IV) ions, but
Cu(I) is instead oxidised to paramagnetic Cu(II) (63). According to
Cheshire et al. (64) copper is present in soil humic acid partly as a
copper-porphyrin type complex, but in fulvic acid it occurs in some
other complexed form. [VO.sub.2.sup.+] ions form more covalently bonded
complexes in fulvic acid than is the case in humic acid. In contrast,
[Mn.sup.2+] complexes of both humic and fulvic acids are highly ionic in
their bonding. In another study, Boyd et al. (65) investigated the
mechanism of Cu(I) binding by sewage sludge humic acid, and concluded
that Cu(II) forms two equatorial bonds with oxygen donor atoms
originating from functional groups of the humic acid. The ESR data also
indicate that the two Cu(II)-humic acid oxygen bonds occupy
cis-positions in the square-plane of Cu(II), a result that is consistent
with the formation of a Cu-Chelate. DFT calculations have been used to
calculate [pK.sub.a] values and g-tensor components for semiquinone
radicals as a mimick for the kind of radicals present in humic acids.
The results accord with the notion that naturally occurring stable
semiquinone radicals (native form) are trapped in the humic acid
macromolecule matrices which precludes their direct involvement in the
acid-base equilibrium with short-lived radicals (transient form). The
differences calculated for g-values for protonated and deprotonated
model radicals are similar to those observed for humic acids, which
permits an identification of the stable radicals to protonated
semiquinones, and the short-lived radicals to deprotonated radicals
(66).
[FIGURE 4 OMITTED]
Jezierski et al. (67) have made a quantitative ESR study of
radicals stabilised in the polyphenolic matrices of various biogenic
materials: lichens, mosses, composts, soils, peats, brown coals and
sewage sludge sediments. Both the raw materials and extracted fractions
of humic acids were investigated. It was found that the g-value may be
used as an indicator of the extent of transformation of organic matter.
Composting of municipal solid wastes results in an increase of the
g-value as the process proceeds. When the stage of compost maturity has
been reached, the g-value changes become insignificant. For lichens,
mosses, sewage sludge sediments, soil, peat and brown coals, the g-value
is an important parameter, which correlates with the redox properties of
the environment and the chemical composition of the material. For brown
coal, the g-value decreases during coalification and correlates with
humic acid content; carbonization of the coal also results in the same
trend in the g-value. Treatment of these materials with ammonia gas also
causes an elevation of the g-value, which is greatest for lignites.
Changes in the spin-concentrations are found to occur as a result of
quite different phenomena: in lichens, there is an increase in the
spin-concentration in response to air-pollution; in degraded soil, the
spin-concentration increases with its organic carbon content; in
lignites the spin-concentration increases with the humic acid content.
It was also found that the interaction of humic substances with metal
ions also alters the spin-concentration: in the interaction of peat
humic acid and lignite humic acids (both raw and carbonized), both with
[VO.sub.2.sup.+], complex variations in the spin- concentrations are
found, which depend on the ion concentration and on other conditions of
sample pre-treatment such as temperature (67). Changes in soil organic
matter (SOM) fractions under no-till cropping systems have been
investigated using ESR (68). Though the topic is controversial, it is
claimed that up to 40% of the C[O.sub.2] emitted by human activities (26
billion tonnes/year) could be sequestered in the form of soil SOM (69)
if no-till/regenerative farming practices (Figure 4) were employed
rather than intensively fertilised tillage as in modern industrialised
agriculture which breaks-down soil and causes a substantial loss of its
carbon content in the form of greenhouse gases. The energy-inputs might
also be reduced to one-quarter for regenerative no-till farming, of
those required for fertilised tilled crop production (69). On the basis
of a negative relationship between decay rates of SOM and the
concentration of iron oxides, it was concluded that the SOM was
physically stabilised by interactions with variable charge materials.
This was confirmed by power-saturation curves measured by ESR for 20-53,
2-20 and <2 [micro]m grain size fractions, in which saturation
occurred less readily as the grain size decreased, indicating stronger
interactions between semiquinone radicals and mineral components, as
confirmed by higher concentrations of ironoxides and kaolinite in the
smaller particles (68). The bioavailability of xenobiotics in soils and
sediments depends on a number of interlocking factors. In order to
obtain direct information on the molecular level as to the environment
of xenobiotics in natural porous media, an ESR study was conducted using
Tempol and [Tempamine.sup.+] as spin-probes. In batch experiments with
Cahectorite suspensions and pastes, it proved possible to distinguish
between probe molecules in different locations: adsorbed, in bulk
solution or in large interstitial pores. The spin-probe underwent
degradation in the bulk solution and the kinetics for the release of the
probe molecules from the clay aggregates and/or paste could be
monitored, but in any case the process was complete in under a day,
indicating that the probes are only weakly contained by the hectorite.
Further potential applications of this technique in soil science are
considered (70). It is, of course, the mobilisation of pollutants in
soils, pesticides such as carbofuran (71) that permits their degradation
(mineralisation) by microbial action and hence the ESR-based method
might cast some light on the mechanisms by which this occurs.
[FIGURE 5 OMITTED]
ESR has also been applied to the determination of the effects of
air-pollution on over 800 samples of lichens (72). In a study of lichens
from Lower Silesia (southwest Poland) a statistical correlation was
found between the annual average atmospheric concentration of S[O.sub.2]
and the concentration of semiquinone radicals in Hypogymnia physodes
thalli. Similar results were obtained for Umbilicaria species from the
Karkonosze Mountains. A plot of the distribution of the semiquinone
radicals within the lichen thalli was also derived. Interestingly, the
action of N[O.sub.2] on Umbilicaria species resulted in the production
of iminoxyl radicals in the thalli. It was concluded that the increased
production of semiquinone radicals in lichen thalli by atmospherically
polluted environments and the degradation of lichen acids to
[beta]-diketones are probably parallel processes. The iminoxyl radicals
derived from the [beta]-diketones in the lichen matrix showed
anisotropic ESR spectra at room temperature; and when extracted into a
range of organic solvents, a linear correlation was found between the
isotropic [sup.14]N couplings and the Dimroth-Reichardt solvent polarity
parameters ([E.sub.T]).
9. Consecutive estimation of nitrite and nitrate ions in vegetables
and fruits
Nitrite ions can be measured using ESR spectroscopy, based on their
quantitative liberation of NO in an acidic solution of Fe[SO.sub.4]
[equation (4)]. NO is swept from the reaction vessel using a stream of
air, and into a trap containing a 0.01 M solution of [Fe[(DETC).sub.3]],
to form [Fe(NO)[(DETC).sub.2]], whose concentration may be measured
using ESR. Both the production and trapping of NO are quantitative. The
nitrite concentration having been so determined, it is possible to make
a sequential and quantitative determination of nitrate, since, in the
same acidic Fe[SO.sub.4] solution, [NO3.sup.-] is converted to
[Fe(NO)[SO.sub.4]], according to equation (5), which remains in
solution, during the nitrite measurement.
N[O.sub.2.sup.-] + nFe[SO.sub.4] + [mH.sub.2][SO.sub.4] [right
arrow]NO + ... (4)
N[O.sub.3.sup.-] + nFe[SO.sub.4] + [mH.sub.2][SO.sub.4] [right
arrow][Fe(NO)[SO.sub.4]] + ... (5)
On addition of sodium diethyldithiocarbamate to this solution,
[Fe(NO)[(DETC).sub.2]] is formed, and may be extracted with a known
volume of toluene, and its concentration determined by ESR. Full
experimental details for this consecutive estimation of nitrate and
nitrite in vegetables and fruits by ESR are described, based on the
selective reactions of nitrate and nitrite ions to yield a
stoichiometric amount of the ESR-active
mononitrosyldiethyldithiocarbamate complex of iron in aerobic
conditions. Since, NO and N[O.sub.2] (which may be present in the air)
give the same reaction, the detection limit depends on their background
concentration in the air. The ESR response is found to be linear up to
2500 [micro]g of N[O.sub.2.sup.-] and 16 000 [micro]g of
N[O.sub.3.sup.-]. The applicability of ESR spectrometry for estimation
of both nitrate and nitrite content is demonstrated by the analysis of
18 vegetables and fruits, available in the local markets in Sofia
(Bulgaria), grown with the use of mineral fertilisers. It is concluded
that the amounts of both nitrite and of nitrate differ considerably from
one species to another, and that both are far lower in home-grown
produce than in fruit and vegetables bought in a local market (73).
10. Lipid Peroxidation by solid particles: silica, asbestos,
coal-dust
There is current concern over the toxicity of various minerals,
especially asbestos caused by their inhalation, leading to lung
diseases, including specific types of cancer, although their precise
mode of action remains uncertain. However, as shown in the examples
below, there is evidence from spin-trapping studies that these materials
can initiate free radical formation, and so might precipitate lipid
peroxidation (1) of the pulmonary cell membranes (74). In crocidolite
asbestos (75), it is known that iron is present and that it can reduce
[O.sub.2] and participate in Fenton-type reactions. Because of the
importance of these reactions in crocidolite-induced toxicity, studies
have been made, on three different types of crocidolite fibres, to
determine the factors which control the activity of iron in catalysing
the two reactions. Results show that the total concentration of iron in
crocidolite fibres is not an appropriate parameter for characterising
the activity of this mineral, which seems rather to be controlled by the
valency and the location of the iron in the lattice, and also by its
availability for mobilisation from these minerals.
Following this is a study of crocidolite which has been
"detoxified" (76). The fibres were treated with ferric oxide
salts to form a metal-micelle polymer surface coating which prevented
physiological reactions with the mineral. This detoxified crocidolite
was tested for its ability to produce OH radicals from
[H.sub.2][O.sub.2]. It was found that the intensity of the DMPO-OH
radical adduct signal was indeed reduced from that obtained from the
native crocidolite fibres. Similar experiments showed that the ability
of the detoxified crocidolite to reduce oxygen was also decreased
compared with the native mineral. The availability of ferrous iron
present in the two crocidolite fibres to catalyse the above reactions
was investigated with the chelating agent ferrozine. The results
indicate that ferrozine was able to remove fewer ferrous ions from
detoxified crocidolite than the native form; moreover, Mossbauer
spectroscopy shows that the detoxification process results in both bulk
and surface changes in the co-ordination chemistry of the detoxified
sample. The detoxification process also introduces a surface coating
comprising ferric ions which shield near-surface ferrous ions and
consequently reduces the Fenton-type reactivity of the fibres. On the
subject of "detoxification" of crocidolite, the same group
(77) report the treatment of crocidolite fibres with microwave radiation
at different temperatures: this reduced the [Fe.sup.2+]/[Fe.sup.3+]
ratio, according to Mossbauer measurements, and produced a concomitant
decrease in the ability of the fibres to peroxidise linoleic acid.
At least one in vivo study has been made of the toxicity of
asbestos, using a spin-trapping technique (78). 180 day-old rats were
instilled intratracheally with either 500 micrograms of crocidolite or
saline; 24 hours later, histologic examination of the lungs revealed a
neutrophilic inflammatory response. ESR examination of the chloroform
extract from lungs exposed to asbestos showed a spectrum consistent with
a carbon-centred radical adduct while those spectra from lungs instilled
with saline revealed a far weaker spectrum. The adducts are nearly
identical with ethyl and pentyl radical adducts, providing evidence of
in vivo lipid peroxidation resulting from asbestos exposure (78). The
adsorption of neutral and charged nitroxides from their solutions onto
asbestos fibres was investigated using ESR, some of which contained an
hydrophobic chain attached to the nitroxide group while in others it was
absent. The four different asbestos fibres studied were chrysotile
(which belongs to the serpentine group), and anthophyllite, amosite, and
crocidolite (all of which belong to the amphibole group). It was found
that the "chain-free" nitroxides (Scheme 1) 4-hydroxy-TEMPO
and [TEMPYO.sup.-], being neutral and negatively charged respectively,
were barely adsorbed by the positively charged chrysotile surface (<
10%), while the positively charged nitroxide CAT1 (Scheme 1) was better
adsorbed to the extent of 25% by the negatively charged anthophyllite
fibres. As expected, a reduction in the rotational correlation time was
observed by interection of the spin-probe with the asbestos surface. The
presence of the hydrophobic chain attached to the nitroxide group
encouraged the formation of surface aggregates and led to a string
enhancement of surface adsorption. The doxylstearic acids were
preferentially adsorbed by chrysotile to the extent of 80% an effect
that was enhanced as the solvent polarity increased and as the chain
length between the carboxylic and the doxyl groups increased. The
positively charged surfactants (Scheme 1) CAT10 and CAT16 were adsorbed
preferentially by anthophyllite fibres. Amosite fibres showed poor
adsorption, whereas the ESR spectra from crocidolite samples were
scarcely detectable, because of spin-spin interactions between the
adsorbed radicals and paramagnetic surface metal ions. The close
proximity of the surface adsorption sites favoured a high local
concentration of radicals adsorbed on the chrysotile fibres, while in
contrast low-packed surface aggregates were formed at the anthophyllite
surface, since the interacting sites were rather more widely dispersed
(79).
Another report shows that the "OH-generating potential of coal
dust correlates positively with the surface iron content of the coal
dust (80). Two other papers describe results for the inhibition of
quartz-induced lipid peroxidation. In one (81), an alkaloid used in
China to treat the lesions of silicosis, is tested for its antioxidant
activity: it is found that tetrandrine reacts efficiently with "OH
radicals generated by the reaction of freshly fractured quartz particles
with an aqueous medium, and also scavenged [O.sub.2.sup.-]. radicals
produced from xanthine/xanthine oxidase. A significant inhibition of
linoleic acid peroxidation by freshly fractured quartz particles was
also found. Taurine-based compounds were similarly investigated (82): it
was discovered that hypotaurine, but not taurine, caused a significant
reduction in silica-induced peroxidation, again using linoleic acid as a
model lipid.
11. Detection of reactive oxygen species in the photosynthetic
apparatus of higher plants under light stress (83)
Plants which perform photosynthesis using oxygen have developed a
balanced system of defence against reactive oxygen species (ROS) which
involves both enzyme and non-enzyme mediated pathways. This is
necessary, since such reactive species are produced in chloroplasts,
under the influence of sunlight, to which the lipid components of their
thylakoid membranes would be sensitive to attack. Under normal
conditions, the system is efficient, and prevents significant damage,
but under conditions of stress, as caused by both natural and artificial
agents of pollution, the system may become overloaded, to the detriment
of the plant. The principal activating factor is sunlight itself, since
it can, under appropriate circumstances, activate oxygen into reactive
forms.
When there is an excessive flux of solar radiation, such species
can initiate the breakdown of membranes, proteins and pigments.
Alternatively, damage may occur in consequence of a reduced efficiency
in the capacity of the plant to utilise protons. The overall and complex
interplay of these processes is called photoinhibition (PI) (which
results in a decrease in photosynthetic activity), and photosystem (PS)
II is thought to be the primary target of damage.
PS II is a pigment protein complex with a reaction centre that
contains a heterodimer of two membrane spanning proteins, D1 and D2,
which either bind or contain the redox cofactors involved in the central
electron transport. The bound components are: the primary electron
donating chlorophyll dimer (P680); the primary electron acceptor,
pheophytin (Pheo); the secondary electron accepting quinones [Q.sub.A]
and [Q.sub.B]. The catalytic cleaving of water occurs at a
manganese-containing cluster bound to the lumenal side of D1/D2,
resulting in electrons which are transferred to P680 via [Tyr.sub.z], a
redox-active residue on D1. Electrons are transported from P680 to a
mobile pool of plastoquinone molecules by subsequent redox reactions via
Pheo, [Q.sub.A] and [Q.sub.B] (Figure 5).
Good model systems for studying light-induced stress in plants are:
oxygen-evolving thylakoid membrane, PS II and other subthylakoid
membrane preparations. Such in vitro studies have disclosed two main
routes, which are dubbed "acceptor site induced" and
"donor site induced" photoinhibition (API and DPI,
respectively). Both API and DPI cause the impairment of PS II electron
transport, followed by the specific degradation mainly of the D I
reaction centre protein and, to a lesser extent, of the D2 protein;
while the result of more protracted PI is more general membrane
damage-as signified by the detection of lipid peroxidation products. The
two forms of PI are distinguished on the basis of differences in the
primary site of electron transport malfunctioning, fragmentation pattern
of the subsequent D1 protein degradation, as well as in the light
intensity and oxygen requirement of the two processes. A third,
alternative pathway of PI has been preposed to occur under conditions of
low light intensities. ROS are also likely involved in this process, but
their predicted yield lies below the limits of detection for the methods
normally used to investigate them. Both models of PI assume the
formation of active oxygen (Figure 6). In API, singlet oxygen production
is believed to be caused by increased formation of a triplet state of
chlorophyll in the reaction centre, and is a consequence of
over-reduction of the first quinone electron acceptor in PS II. DPI
occurs when electron flow from water to P680 is insufficient. There is
consensus that the damage is triggered by the strong oxidants (P680+ and
Ty[r.sub.z.sup.+]) created by the primary charge separation and whose
lifetime is prolonged in consequence of inoperative water splitting. In
such case, both electron transport and protein damage proceed in the
ansence of oxygen, even upon illumination with relatively lower
intensities of PAR.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Similarly to PI caused by excessive PAR, UV-B (280-320nm)
irradiation causes a multitude of physiological and biochemical changes
in plants, although these two types of light stress differ in a number
of respects. Increased doses of UV-B radiation reaching the Earth's
surface because of stratospheric ozone depletion have encouraged
investigations into this form of light stress. It is well known that
UV-B radiation causes the rapid inactivation of photosynthetic electron
transport, altered pigment composition, loss of membrane integrity, and
that it may result in the dimerisation of thymine bases and lesions in
DNA. The increased synthesis of flavinoids, which are very efficient
quenchers of ROS, as well as the increased expression of genes for
flavinoid biosynthesis imply the involvement of ROS in the process. In
the thylakoid membrane, the primary target for UV-B is PS (II); damage
by UV-B involves functional impairment of PS II electron transport and
degradation of PS II reaction centre proteins, primarily D1 and D2.
11.1 In vitro studies.
In vitro, it is found that the production of singlet oxygen is a
unique characteristic of API, among the three forms of light-stress, and
it does indeed arise in PS II. Singlet oxygen may trigger the subsequent
D1 protein damage either directly or by causing a specific change in
conformation which renders the protein susceptible to proteolytic
damage. In addition to the role of this reactive but non-free radical
oxygen species in API, the involvement of oxygen free radicals is quite
possible, considering the proposed molecular mechanisms of both API and
DPI. Indeed, the formation of radicals is evidenced by the reduced
photoinhibition caused when radical scavengers or antioxidant enzymes
such as superoxide dismutase (SOD) are introduced. PI induced free
radical formation has been supported by spectrophotometric measurements
in nonoxygen-evolving PS II preparations, and also directly, using ESR,
in thylakoid membranes. In DPI, it is mainly * OH radicals and
superoxide radical anions that are produced. It was found that free
radical formation required the presence of oxygen during API but not in
DPI, demonstrating that the production of ROS and the propagation of PI
are closely related. UV-B radiation also results in the production of
free radicals but not of singlet oxygen, which demonstrates that the
primary site of UV-B induced electron transport impairment is different
from that of PI by excessive levels of PAR. In thylakoid membranes, UV-B
radiation results in a parallel production of several different, mainly
* OH and carboncentred, radicals. Comparing a series of samples (Figure
7) with increasing PS II purity (thylakoid membranes, PS II, PS II core
complex preparations) suggests that the primary event is * OH radical
production, which leads-on to other radicals as products of its
reactions with various components of the systems being investigated.
Superoxide radical anions are formed as by-products during the
operation of photosynthetic electron transport. Their site of origin is
PS I, where molecular oxygen provides an alternative sink for electrons
in illuminated thylakoid membranes, when NADP is absent. Accordingly,
superoxide radical anions can be trapped in isolated thylakoid membranes
with Tiron (the disodium salt of 4,5-dihydroxy-1,3- benzenedisulfonic
acid). The signal was inhibited by DCMU (which blocks the electron
transport between PS II and PS I), increased by the addition of
methylviologen, and no superoxide trapping was observed from the PS II
core complex, even under conditions of API. These results demonstrate
that the observed ESR signal does indeed arise from the trapping of
superoxide from PS I by Tiron. In addition, API caused only a marginal
increase in the ESR signal intensity from thylakoid membranes,
indicating that superoxide is not the main promoter of API. In contrast,
DPI resulted in a dramatic production of superoxide in PS II core
complexes. Superoxide radical anions are also produced in thylakoid
membranes and PS II preparations exposed to UV-B radiation; however,
Tiron is itself slightly sensitive to UV-B which limits the precision of
the conclusions which may be drawn.
ROS in vivo
While these in vivo experiments indicate that ROS are plausible
candidates in the in vivo processes of light stress, such an actual
investigation is tricky. Introduction of 2,2,5,5-tetramethylpyrrolidine
into intact leaves indicates that singlet oxygen is being produced;
however, it is found that the resulting nitroxide is rapidly metabolised
in the leaf, which may make quantitative studies difficult. This
difficulty in studying actual plant tissue is really the same as is
encountered in studying animal tissue, and it is plausible that the
procedure of extraction of the hydroxylamine product of the nitroxide
formed from 2,2,5,5-tetramethylpyrrolidine by oxidation by singlet
oxygen, followed by re-oxidation to the nitroxide would prove fruitful.
Spin-trapping studies of radicals in plant tissue is similarly
difficult, since the almost ubiquitous presence of ascorbate and other
reducing agents also reduces the resulting spin-adduct nitroxides,
rendering them undetectable. Rapid post-stress isolation of thylakoid
membranes with the spin-trap present can diminish this effect, as may be
illustrated in the example of detecting flee radicals in leaves that
have been exposed to UV-B radiation. It is, however, important to bear
in mind that the radicals detected may not be the primary species formed
in vivo, but are products of such initial damage. Alternatively, the
increased quantities of ascorbyl radical, formed from ascorbate, may
provided a natural marker of stress in plants. It is hoped that the
detection of singlet oxygen using 2,2,5,5- tetramethylpyrrolidine or
Tiron, and of radicals with spin-trapping and the "natural"
ascorbyl radical marker, may be used in the development of more rugged
crop strains, able to resist an increasingly harsh environment.
12. Molecular mobility and intracellular glasses in seeds and
pollen
Deterioration of seeds and pollen during storage involves many
physical and chemical changes, including the disruption of the integrity
of cells, decreased enzyme activities, lipid peroxidation,
de-esterification and Maillard reactions (84,85). Since it is known that
glasses are formed in biological tissues during their dehydration, it
has been proposed that their formation is a major factor in determining
the rates of deterioration during storage (86,87). It is thought that
the presence of intracellular glasses decreases molecular mobility and
impedes diffusion within the cytoplasm, thus slowing-down harmful
reactions and changes in structure and chemical composition during
ageing (88,89). Using a spin-probe method, motional correlation times
can be estimated (90) and correlated with bulk properties such as glass
formation and storage stability.
In one study (91), mature male inflorescences of cattail (typha
latifolia L.) were collected from fields near Wageningen, in The
Netherlands, and allowed to shed their pollen in the lab. Pollen (94%
germination) was exposed to a liquid germination medium containing 2.5
mM of the polar spin probe CP (3-carboxylproxyl). There are a number of
interesting details forthcoming from this investigation. In the first
place, as measured at -150[degrees]C, when molecular motion occurring at
a rate > [10.sup.-8]s is quenched, the separation between the
extremum features of the CP spectrum ([2A.sub.zz]) provides information
about the polarity of the local environment of the spin-probe. In pea
axes, the maximum [2A.sub.zz] was found to decrease with decreasing
water content from 74G to 70G, whereas in cattail pollen, it changed
accordingly from 72.5 G to 71.5 G. On increasing the temperature, the
ESR spectra reflect the onset of molecular motion by an initially steady
decrease in [2A.sub.zz], which at a particular point falls abruptly. It
was found that with an increasing water content, the sharp decrease in
[2A.sub.zz] began at lower temperatures, and at a loading of 0.16 g
water/g it coincided with the melting point of ice, which indicates that
bulk water is present in the pea axes cells. Similar results were found
for pollen. Saturation-Transfer measurements (ST-ESR) were employed to
gain a more detailed insight into the kind of molecular motion involved,
by estimating values of [[tau].sub.R] for the CP probe molecules. In
summary, it was found that during the drying of both cattail pollen and
pea-axes, the [[tau].sub.R] for the spin-probe decreased from
[10.sup.-11]s in the fully hydrated state to [10.sup.-2s] in the dry
state (91). This essential strategy was subsequently employed to
demonstrate a likely correlation between molecular motion in the
cytoplasm of lettuce seeds and storage lifetime, for which a
"viability equation" was derived (92).
13. TEMPOL radicals in dry cotton
I have followed this project with some interest, since it was first
initiated in my laboratory in Liverpool on a consultancy basis with
Unilever Research (93), but via a brief consultation in Cardiff, later
moved to the University of Stuttgart, along with its funding. The basis
of the work is the adsorption of the spin probe, TEMPOL onto cotton,
which is the world's most important clothing material. The
absorptive properties of cotton are well known, when clothes are washed.
This is due to the presence of amorphous regions where the pores of the
material are located, and that the material is polysaccharide in nature,
so favouring hydrogen-bond formation with water molecules. Cotton (94)
contains both regularly arranged crystallites, and irregular
arrangements which provide the amorphous regions. The crystallites are
roughly 10 [nm.sup.3] cubes, which are divided by amorphous areas,
containing pores with a distribution in their average cross-sections
ranging up to 3 nm. The amorphous regions are mainly responsible for the
overall absorptive character of cotton, since they provide 42% of the
volume of the material, and are freely accessible to external molecules.
In a spin-probe study (95), 10mm x 20mm pieces of cotton were
soaked in solutions of TEMPO dissolved in ethanol, in the concentration
range, 6 x [10.sup.-5] to 5 x [10.sup.-2]M, then the residual solvent
was evaporated. The probe was adsorbed in three main forms, in relative
proportions that depended on concentration: (1) a mobile fraction,
probably occupying the amorphous pores, in association with water; (2) a
truly surface adsorbed immobilised fraction, adsorbed on the surface of
the crystallites, (3) a bulk fraction (aggregate) of TEMPOL radicals
which presents itself by a relatively broad single line. As the TEMPOL
loading increases, the apparent viscosity in the pores also increases,
and it is concluded that the TEMPOL molecules are deposited in pores of
differing viscosity: those of lower viscosity are filled first, and then
steadily pores of increasing viscosities, so that there is an apparent
increase at higher loadings. When the TEMPOL loading is less than 3 x
[10.sup.-3] tool [kg.sup.-1], the aggregated fraction is present in
negligible quantities, such that an equilibrium can be considered
between the mobile and adsorbed fractions only: mobile[??]adsorbed. The
equilibrium, mobile[??]adsorbed, may be expressed by an equilibrium
constant, K, from the temperature dependence of which, values for the
molar enthalpy ([DELTA][H.sup.o]) and entropy ([DELTA][S.sup.o]) of the
adsorption process were obtained by fitting the data to equation (6)
[where R is the molar gas-constant (8.314J [mol.sup.-1] [K.sup.-1]].
-ln K(T) = -[DELTA][S.sup.o]/R + {DELTA}[H.sup.o]/RT (6)
It is a general feature of adsorption-literally on surfaces, which
are in general, inhomogeneous, with sites that provide differential
sorption energetics--that the derived values of enthalpy
([DELTA][H.sup.o]) and entropy ([DELTA][S.sup.o]) differ according to
the degree of loading of a specified probe molecule. Thus, as TEMPOL is
adsorbed onto cotton at the higher loading (>3 x [10.sup.-4] mol
[kg.sup.-1]) the adsorption enthalpy is ca 50 kJ [mol.sup.-1], but
increases up to ca 80 kJ [mol.sup.-1] at the lowest loadings, which
correspond to the viscosity changes already mentioned. Thus, the regions
with lower viscosity and greater reorientational freedom also contain
more strongly binding adsorption sites. The entropies of adsorption are
surprisingly high, with values between ca 0.2 and 0.25 kJ [mol.sup.-1]
[K.sup.-1], and it is suggested that they reflect the loss of motional
freedom of the polysaccharide chains as TEMPOL is adsorbed on the
surface of the crystallites (95).
14. ESR characterisation of the surface of carbon black, used for
chromatographic applications
Porous carbon occurs in many forms, and indeed can be fabricated
for particular applications. As a bulk material of substantial porosity
and high surface area, porous carbon is used in environmental
remediation strategies, either in clean-up operations when contamination
has already occurred, or preventatively, to minimise the release of
pollutants into the environment. Of environmental significance too, is
that carbon materials are used as "packings" for
chromatographic columns, and these are employed ubiquitously to analyse
the nature and extent of general environmental contamination by organic
materials. We have seen numerous applications of nitroxyl radicals, in
their role as stable "organic"--free radical--probes.
"Inorganic chemistry" also provides probes, in particular
those based on M[n.sup.2+], [Cu.sup.2+], [V.sup.n+] and [Mo.sup.n+]. The
presence of F[e.sup.3+] is readily apparent in many samples both of
inorganic and biological origin-but the varied and subtle range of
coordination geometries inherent to the nature of iron renders
considerable complexity in many samples; nonetheless, a rough range of
"g-values" serves to provide "fingerprints" of
particular kinds of local environment, tetrahedral, octahedral, and
their variants of lower symmetry.
ESR has been used to investigate the adsorption capability and the
surface interacting ability towards Cu(II) solutions (Cu[Cl.sub.2],
Cu[(N[O.sub.3]).sub.2], CDSO4 in water or ethanol) of various carbon
blacks such as are used in gas/liquid/solid chromatographic
applications, both graphitized and ungraphitized, selected on the basis
of the surface area, namely, Carbographl (area = 100 [m.sup.2]
[g.sup.-1]), Carbograph4 (area = 210 [m.sup.2] [g.sup.-1]), and
Carbograph5 (area=560 m2 [g.sup.-1]), which were indicated as Clg, C4g,
C5g (g=graphitized), and C1ng, C4ng, C5ng (ng=ungraphitized). The EPR
analysis was supported by surface analysis, by which means the surface
area, the pore volume and the porosity were determined, and by atomic
absorption to obtain the amounts of adsorbed Cu(II). Graphitization was
found to cause a decrease in surface area, but C lg, at low surface
area, showed a unexpected increase of the adsorption ability which was
explained by the generation of a new surface porosity closed by graphite
layers. The carbon samples showed a broad unresolved EPR signal due to
mobile unpaired electrons in the carbon matrix. Graphitized samples
presented a narrower signal than ungraphitized samples, the width of
which increased as the surface area increased (with the exception of
C5ng in consequence of the large surface area being subject to the
action of oxidizing agents) and upon prolonged thermal treatment. The
signal intensity of the paramagnetic carbon centres was found to
decrease upon the adsorption of Cu(II). Computer simulation of the EPR
spectra of the solids following Cu(II) adsorption provided structural
information on the nature of the Cu-surface site complexes, in which the
[Cu.sup.2+] ions were coordinated with surface, mainly oxygenated, polar
sites. The process of adsorption was found to depend on the nature of
the different Cu(II) salts, in regard to the solubility of the salt and
the interacting ability of the counter-anion. Ethanol solutions are more
strongly adsorbed at the carbon surface than water solutions. Adsorption
is favoured by ungraphitized carbons with respect to their graphitized
counterparts due to both the higher surface area, and the higher
hydrophilicity of the surface. In summary, these carbon powders, widely
used for chromatographic applications, show an adsorption capability
towards Cu(II) solutions that is higher than expected as a result both
of a defined porosity, and the presence of polar groups which are not
eliminated by chemical surface treatments (96).
15. ESR dating
The Earth's crust is sufficiently abundant in radioactive
elements that, over time, radiation-damage is caused to the materials of
which it is composed and to artefacts contained within it. Therefore, if
the background radiation dose-rate is known and the total dose received
by a sample can be measured, it is possible to estimate the age of the
sample. Ionising radiation (y-rays from radioactive isotopes of elements
such as uranium, thorium and potassium) causes "valence band"
electrons in an insulating material to be ejected from atoms, leaving
positively charged holes, while some of the electrons are captured at
trapping-sites which they may be thought to enter via the higher energy
"conduction band"; the majority of the ejected electrons
simply re-combine with the positive holes (in radiation chemistry, it is
frequently the energy released during such charge- neutralisation events
which causes the formation of free radicals by breaking chemical bonds).
The trapped electrons (and positive holes) may be detected directly by
ESR (97-112); alternatively, by subsequently heating the sample or by
exposing it to light, they may be released back to the positive holes,
leading to light emission (luminescence). The release of trapped
electrons by heating is called thermoluminescence (TL) and by light,
optically stimulated luminescence (OST). A major advantage of ESR is
that it provides a "fingerprint" of the type of damage centres
produced, and which may be identified as molecular ("Free
Radical") species, in addition to determining their concentration.
In the archaeology literature, the defects are referred to
indiscriminately as "trapped electrons" (97-112). ESR has the
further advantage that it conveniently spans the time interval between
the older limit of [sup.14]C dating and the younger limit of K-Ar
dating, a period of roughly [10.sup.3] - [10.sup.6] years.
Estimates of the age of a sample are made from ESR measurements
using equation (7), in which DE is the "accumulated dose", the
total radiation dose that the sample has received over the time it has
existed for, and is determined by the "additive dose method"
(next section). The dose-rate, D', is considerably more tricky to
determine, since it requires a knowledge of the concentration of
radioactive elements (mainly, U, Th and K) both within the sample and in
its surroundings, and the effect of cosmic rays must also be considered.
The rather demanding and exacting procedures necessary to ensure a
reliable evaluation of the dose-rate have been described fully by Grun
(98,99).
Age = ([D.sub.E])/(D') (7)
15.1 The additive dose method
The procedure is based upon the manner in which the ESR signal from
a sample at its "natural intensity" is enhanced by artificial
irradiation with [gamma]-rays from a [sup.60]Co source. The fit of ESR
intensity to radiation dose is called the "dose response
curve". In the simplest case, this is actually a straight-line, but
more complex functions are often required to get a good fit to the data.
In any event, the curve is extrapolated to the point of "zero"
ESR intensity, and the accumulated dose (DE) (the dose which had created
the initial "natural" signal) is determined as the point of
intersection with the x-axis. Background radiation creates unpaired
electrons which are responsible for the ESR signal, whose intensity
grows with time (97-100).
15.2 Methods of ESR dating
15.2.1 Tooth enamel
In part, ESR has a niche in dating since it can access samples
which are too old for radiocarbon dating, and do not contain the vital
elements for such dating from the uranium series or from K/Ar ratios.
Bones and teeth are found in a majority of archaeological sites, and
while some workers are of the opinion that bones cannot be dated
reliably using ESR, it is agreed that tooth-enamel can be, and so might
provide chronological data as a general device in archaeology
(108,111,112). Tooth enamel is formed from hydroxyapatite to the extent
of > 96%, in which this mineral is in contact with more organically
rich materials (dentine and cementum). A comparison was made (99)
between the spectrum of natural tooth-enamel and one from the same
material following [gamma]-irradiation. There are essentially two
signals: one at g=2.0018 and another at g=1.9976, both from
[CO.sub.2.sup.-] radicals in hydroxyapatite. The spectrum was recorded
using a modulation amplitude of 5 G, which enhances the relatively broad
signals from [CO.sub.2.sup.-]. radicals, and means that a smaller
receiver gain is required to detect them. This has the effect of
reducing the intensity of signals from organic radicals that are
produced simultaneously by the radiation. This method is often useful in
suppressing overlapping signals from organic radicals which might
otherwise introduce errors into the dating procedure by distortion of
signals, from [CO.sub.2.sup.-]. radicals, which are being used to gauge
the "additive dose".
15.2.2 Dating of shells
Shells are found in relative abundance at many archaeological
sites, and ESR dating has been applied particularly to coastal deposits.
Attempts to date ostrich egg shells resulted in dramatic underestimates
of their ages, in comparison with other methods (amino acid
racemisation), which is thought to be the consequence of a low thermal
stability of the ESR signals (99). The spectra recorded from shells are,
in fact, often fairly complex patterns arising from mixtures of
radicals, and it is sometimes difficult to deduce reliable dose-response
curves from them. The main problem appears to be that of "where to
measure", since the radiation response which leads to different
spectral features and from different radicals is not uniform, and it
appears necessary to identify appropriate deconvolution procedures (to
separate signals from different radicals) prior to the measurement of a
signal intensity from a particular region of the experimental spectrum.
However, since shells are the only datable material available in some
deposits, and ESR determinations can be made on relatively small
samples, systematic efforts to optimise the reproducibility and accuracy
of such measurements should prove worthwhile.
15.3 Some examples
There are many examples, which are described along with the whole
basis of ESR dating, in excellent reviews written by Grun (98) and by
Ikeya (97), and some recent studies are now mentioned. As alluded to in
the last section, there are efforts ongoing to improve the precision and
accuracy of spectral measurements, involving "fitting"
procedures (110), either using simulations with a theoretical (Gaussian
or Lorentzian) line-shape, or an experimental spectrum; the latter, not
unexpectedly, so far gives the better result, as demonstrated by fewer
"residuals" (which are the components effectively
"left-over" by the fitting procedure, and hence reflect the
errors implicit to the fit).
Most ESR dating measurements in archaeology have been made on
tooth-enamel, and procedures are described by Grun for the extraction of
this component of teeth in order to minimise contamination (sorption),
principally by uranium, along with necessary corrections for the
influence of irradiation by more weakly penetrating [beta]-rays
(electrons formed by the decay of certain radioactive nuclei) (97), both
of which would otherwise complicate, or introduce errors into, the
calculation of the dose-rate.
Matters of reproducibilty and errors have received attention (109).
Some of the oldest tooth-enamel dated using ESR comes from a hominin
tooth (Australopithecus robustus), for which a best estimate of 1630 [+
or -] 160ka was made (Figure 8) (101), and a maximum age of around
2100ka (over two million years old!), using a different model for the
dose-rate. Interestingly, two teeth from a bovid (hollow-homed, ruminant
animal) were dated as being in the range of 100-200ka, despite previous
estimates of between 1000ka and 2000ka (101). A revision of the estimate
of the age of tooth enamel samples taken from the sedimentary sequence
at Border Cave (Australia) has been made on the basis of a detailed
gamma-ray survey and newly calculated beta-ray attenuation. It is
concluded that the total dose rate is between 0 and 30% smaller than
previously estimated, and hence the samples are between 0 and 30% older
(102); the results are now in better agreement with those obtained from
[sup.14]C dating and amino-acid recaemisation measurements. It is
further concluded that the sedimentation sequence began around 200ka
ago. A revision of the age of dental material from Neanderthal remains
at 122 [+ or -] 16ka was also reported (107). Other notable results are
that the oldest human remains in Australia have been established at Lake
Mungo (103), with an antiquity of 62,000 [+ or -] 6,000 years, and it
has been shown that Humans occupied Devil's Lair in Southwestern
Australia as long as 50,000 years ago (104). ESR can also be used to
estimate the date of sea-level changes, from barnacles and corals.
Barnacles live mainly in the intertidal zone and die when decreases in
sea level changes cause them to be permanently exposed. Hence, if the
barnacles can be dated, so therefore can past sea level changes,
including those that occur because of ocean volume changes, crustal
isostasy, and tectonics. Aragonitic mollusc shells can be dated by ESR
from as young as 5 ka to ages of at least 500 ka. By chemically
dissolving 20 [micro]m from the shells, six barnacle samples from
Norridgewock, Maine, and Khyex River, British Columbia, were tested for
their suitability for ESR dating. It was found that the four Maine
barnacle samples were not datable by ESR in consequence of interfering
signals from [Mn.sup.2+]. From two barnacles from BC, from which
[Mn.sup.2+] signals were absent, a mean ESR age of 15.1 [+ or -] 1.0 ka
was determined, in good accord with dates obtained from
[sup.14]C-measurements of the barnacles themselves and of wood present
in the overlying glaciomarine sediment. While stressing the need to
ensure the stability of the ESR signals and to make a comparison with
other barnacles of known age to validate the accuracy and reliability of
the method, it would appear that ESR can indeed date Balanus, and
accordingly associated sea level changes (105). While coupled
[sup.230]Th-[sup.234]U-ESR measurements have become practically routine
for dating teeth, this is not the case for dating corals. The age
according to ESR depends on the time-averaged cosmic dose rate,
[Doos.sub.(t)] (about which it is necessary to draw various
assumptions), but the [sup.230]Th/[sup.234] determined date does not.
Since the [D.sub.cos(t)] received by corals depends on attenuation of
the radiation by all attenuating material, the [D.sub.cos(t)] response
varies according to the depths of overlying water and of sediment cover.
By combining the two methods, both the age and a unique [D.sub.cos(t)]
can be simultaneously determined. [D.sub.cos(t)] can be predicted from
the depth under water of a coral and its sedimentary history according
to a given sea-level curve. Hence one can predict [D.sub.cos(t)]--if
[D.sub.(cos,coupled)(t)] is in good agreement with
[D.sub.(cos,sealevel)(t)]--which provides an independent validation for
the curve used to construct [D(cos,sealevel)(t)]. For six coral samples
dated at 7-128ka taken from Florida Platform reef crests, results were
obtained that agreed well with previously determined sealevel curves. It
is concluded that, where an entire reef can be sampled over a transect,
a precise test for sealevel curves might be developed (106).
[FIGURE 8 OMITTED]
16. Measurement of the quality of tea-leaves using ESR
Tea is the most widely consumed beverage in the world, and it has
been used as a daily drink and crude medicine in China and Japan for
thousands of years. Green-tea, particularly, is considered to be an
effective source of antioxidants, which are believed to be responsible
for some of the positive health effects noted above. The main
antioxidant components of green tea extracts include (+)-catechin,
(+)-gallocatechin, (-)-epicatechin, (-)-epicatechin gallate and
(-)-epigallocatechin gallate (EGCG).
The ESR spectra of tea leaves generally consist of two components:
a single-line from semiquinone type radicals of 10G width and a g-value
close to free-spin (2.0023), resulting from oxidation of the plant
material, and a broad component of > 1000 G width and a g-factor of
2.025, from protein-bound [Mn.sup.2+] ions (113). The latter signal is
interesting, since it provides a measure of protein degradation in the
leaves: as the protein is degraded through oxidation, [Mn.sup.2+] ions
are released from the protein and are able to tumble freely, hence the
resolution of six broad lines. It is notable that in cases where the
protein degradation is greatest, the semiquinone signal is also at its
most intense. In green tea leaves there is no indication of the six-line
pattern, which is fully resolved in the spectrum of black tea leaves.
The oxidation processes may be stimulated by irradiation with UV light,
a procedure to which the different kinds of tea leaves respond
differentially. For green tea leaves, the semiquinone radical signal
increases slightly but steadily up to 8 hours of irradiation, while a
more rapid increase is noted for black tea leaves. The results imply
that the antioxidative potential of green leaves is about four times
that of black leaves, in good accord with other studies which indicate a
factor of six in the relative antioxidant effects of extracts of the two
kinds of tea.
17. Green catalysts and catalytic media
The chemical industry faces unparalleled challenges, in terms of
providing for growing markets and meeting the needs of a rising world
population which aspires to a more advanced material quality of life,
while minimising the environmental impacts of pollution from
process-Chemistry. Nevertheless, the resources of this world are
limited, particularly those of crude oil, and of the other fossil fuels,
and hence an improvement in the efficiency of chemical manufacture is
demanded, as met by higher product yields and selectivities, along with
reduced energy costs. More effective catalysts and better reactor
designs are key factors in realising these aims (114). As noted
previously, efficient photo-catalysts for the mineralisation of
pollutants, in groundwaters, using visible-light are also highly
desirable and are being sought. A review has been published entitled,
"Electron paramagnetic resonance: a powerful tool for monitoring
working catalysts," the substance of which may be summarized thus:
The method may be applied to the online monitoring of catalytic
reactions on the basis of paramagnetic centres formed within the
catalyst or free radicals formed as intermediates. EPR finds application
for monitoring the synthesis and equilibration of solid catalysts and
also their role in gas- and liquid-phase reactions, vanadia-containing
oxides and FeMFI zeolites. The use of spin probes and spin traps for
detecting hydrocarbon radicals in photocatalysis is illustrated. The
potential advantage of making simultaneous EPR measurements with Raman
and UV/vis is illustrated and discussion is given of limitations and
future potential of the method in the field of catalysis (115).
Reactions of [Al.sub.2][O.sub.3]-supported TEMPO with NO and [H.sub.2]
and of Si[O.sub.2]/A[l.sub.2][0.sub.3]-supported
[H.sub.4][PVMo.sub.11][O.sub.40] with methanol and formaldehyde were
studied up to 400[degrees]C using a homemade heatable probe-head
equipped with a flow reactor. The TEMPO radicals were immobilised on the
support in positions which impose a different reactivity to NO and
[H.sub.2], as may be due to different accessibility, which changes
during thermal treatment. [Mo.sup.5+] is formed above 180[degrees]C, In
addition to [VO.sup.2+], depending on the [O.sub.2] content of the feed,
which is easily resolved Q-band but not at X-band (116). Highly
photoactive, tetrahedral [Ti.sup.4+] sites can be created in mixed-phase
Ti[O.sub.2] nanocomposites, and which are shown to be an intermediate
formed during the thermally driven phase-transformation from anatase to
rutile (117). Nitrogen-doped Ti[O.sub.2] is a novel photocatalyst which
can promote the decomposition of organic pollutants using visible light,
and contains a number of different paramagnetic centres, such as NO and
N[O.sub.2] radicals and other species, that interact strongly with the
Ti[O.sub.2] structure and are related to specific properties of the
solid. In one paper, attention is focused on molecular species generated
during the sol-gel synthesis process and segregated in cavities of the
Ti[O.sub.2] structure (118). A review has been published of highly
efficient titanium oxide-based photocatalysts with potential
applications for the photo-degradation of organic pollutants,
specifically methyl orange. Focus is given to the preparation and
characterisation of Ti[O.sub.2] photocatalysts prepared by transitional
metal doping and noble metal deposition, and particularly a combination
of the two methods. [Fe.sup.3+] doped together with Au deposition on
Ti[O.sub.2] shows an excellent photocatalytic activity to degrade methyl
orange (MO) under both UV and visible light illumination at wavelengths
> 420 nm. EPR demonstrates that [Fe.sup.3+] substitutes for
[Ti.sup.4+] in the Ti[O.sub.2] lattice, and XRD shows that Au exists as
Au(0) on the catalyst surface. A mechanism is proposed to account for
the synergistic influence of [Fe.sup.3+] and Au in enhancing the
photocatalytic activity of the catalyst (119). Photoirradiation of
Ti[O.sub.2] nanoparticles by visible light in the presence of
water-soluble, natural polysaccharide, arabinogalactan-complexes of
[beta]-carotene leads to a greater yield of the hydroxyl (*OH) radicals
than in the absence of the complex. Methyl and methoxyl radicals are
formed when *OH radicals react with DMSO and are trapped using PBN.
Carotenoid-arabinogalactan complexes exhibit an enhanced quantum yield
of free radicals and stability toward photodegradation over pure
carotenoids, and the greater photocatalytic efficiency for carotenoid
complexes is a consequence directly of the decrease in the rate constant
for the back electron transfer to the carotenoid radical cation.
Applications are stressed for Ti[O.sub.2], in which these results might
prove important, particularly in photodynamic therapy and in the design
of artificial light-harvesting, photoredox, and catalytic devices (120).
A multifrequency (9-95 GHz) EPR study has been reported for Ti[O.sub.2]
nanocrystals (NCs) capped by organic moieties prepared by both a
nonhydrolytic and a hydrolytic procedure, respectively with a spherical
or rodlike shape. The behaviour of the electrons promoted in the
conduction band by UV irradiation and of the holes in the valence band
has been monitored by means of EPR-detectable paramagnetic species so
generated. The presence of paramagnetic species on the surfaces of
Ti[O.sub.2] NCs was used to account for the catalytic performance of
these nanostructured materials which are highly catalytically efficient.
An observed carbon-centred radical is proposed to be responsible for the
higher catalytic activity of organic-capped nanosized catalysts, and
indeed, by irradiation, the intensity of this signal with respect to the
bulk [Ti.sup.3+] signal is larger in the NCs prepared with a hydrolytic
procedure, which are the more catalytically-active (121). In the
photooxidation of cyclohexane in mixtures with [CH.sub.2][Cl.sub.2] over
Ti[O.sub.2], singlet oxygen, [sup.1][O.sub.2], formation was determined
by the specific 2,2,6,6-tetramethyl-4-piperidone EPR assay. The yields
of [sup.1][O.sub.2] correlate with the amount of cyclohexanol detected
in the mixtures, which provides convincing evidence that cyclohexanol is
formed through the recombination of cyclohexylperoxyl radicals. Chloride
yields were also determined and provide direct evidence for the active
participation of dichloromethane in the photocatalytic system. A
mechanism is proposed (122). The role of dissolved oxygen in the
photocatalytic degradation of phenol was investigated using polymer
[poly(fluorene-co- thiophene) with a thiophene content of 30% (dubbed
PFT30)] sensitized Ti[O.sub.2] (PFT30/Ti[O.sub.2]) with visible light
irradiation. The photoluminescent (PL) quantum yield of
PFT30/Ti[O.sub.2] was about 30% that of PFT30/[Al.sub.2][O.sub.3], which
shows that electron transfer occurs between the polymer and Ti[O.sub.2].
The virtually complete quenching of the reaction when the solution was
saturated with [N.sub.2] shows the key role of [O.sub.2]. When
Na[N.sub.3] was present, which is an efficient quencher of singlet
oxygen ([sup.1][O.sub.2],), a 40% reduction in the degree of degradation
of phenol occurred. A roughly 60% decrease in the phenol
photodegradation ratio was observed when alcohols were added to the
medium, accounted for in terms of the scavenging of hydroxyl radicals
(*OH), the presence of which was confirmed by ESR/spin trapping, and
that these are the major reactive species present in aqueous solution.
In the absence of water, singlet oxygen ([sup.1][O.sub.2],) was the
predominant species. It is clear that oxygen and reactive intermediates
derived from it are critical in the photocatalytic degradation of phenol
(123). The encapsulation of microperoxidases (NAPs) into molecular
sieves of defined pore size, the mesoporous silica MCM-41 with 3 nm
diameter channels, is a nanotechnological attempt to mimic the enzymatic
activity of haemoproteins. Ferric microperoxidase-11 (MP-11) was
entrapped in MCM-41, to provide a catalyst (Fe(III)MP11MCM41) with both
the properties of catalase and monooxygenase. In a similar manner to
catalase, Fe(III)MP11MCM41 was specific for hydrogen peroxide, leading
to a high-valence oxidized intermediate, Compound II. In the presence of
phenol (a reducing agent) a complete peroxidase cycle was confirmed by
UV-visible spectrometry and EPR. Analysis of the reaction mixture by
(HPLC/MS) showed that the product of phenol oxidation was
2,4-dihydroxyphenol, and hence in addition to catalase activity, the
catalyst MP11 MCM41 also displayed monooxygenase properties. The latter
feature is due to the fact that the MP-11 haem iron is able to promote
homolytic cleavage of [H.sub.2][O.sub.2] with the generation of hydroxyl
radicals (124).
These more fundamental principles have been applied to more
pragmatic situations. For example, a traditional and popular lime tile
using for decorating houses in Japan, "Limix", was coated with
a titania sol and used to photochemically decompose formaldehyde, to
address the problem of "sick-building syndrome". The
photocatalytic activities of five types of lime tile material both with
titania sol coating and without it were examined in a flow-type
photoreactor based on the MS (Japanese Industrial Standard). ESR was
further employed to explore the potential photocatalytic mechanism in
the coated tile. It was discovered that a higher efficiency was achieved
when the lime tile was combined with a commercially available zeolite in
comparison with the lime tile coated with an immobilised titania
photocatalyst. The EPR spectrum indicates that oxygen-centred radicals
and surface-trapped holes are formed in the titania sol, but this is
entirely different from the spectrum observed in pure anatase and
Degussa P25 titania, implying that the photocatalytic mechanisms are
different (125). A Ti[O.sub.2]-Ag doped slurry has been synthesized for
concrete impregnation, to furnish a material with photocatalytic
properties, for example of self-cleaning to prevent the build-up of
organic debris on buildings. Ti and Ag centres were characterised by
EPR, and also the activity of the material in generating *OH radicals in
model situations (126). The surface chemistry of activated carbons used
as sorbents in a variety of applications has been studied using a
combination of methods, including ESR (127). An ESR-based
characterisation of kaolin, used in large quantities in the paper
industry as a filler and coating material has been reported. For this
application a high brightness grade of kaolin is required which depends
greatly on its iron-mineral content, which can be assessed using ESR
(128).
18. Fuel-related applications
The main waste material from biodiesel production is glycerol, raw
fractions of which obtained from different kinds of transesterified oil,
were investigated for their antioxidant and anticorrosive properties
using respectively EPR and the Herbert method. It was found that those
fractions rich in glycerol exhibited strong radical scavenging
properties, in comparison with biodiesel and the pure oil.
Interestingly, it was those fractions which displayed the greatest
radical scavenging abilities that also showed the highest anticorrosive
behaviour. It is proposed that the glycerol fractions may have
applications as lubricants, hence the raw glycerol might be used in the
absence of costly and lengthly purification procedures (129). EPR
spectra of crude petroleum contain an intense single line which is
thought to arise from the superposition of signals from different
radical species with very similar g-values, and moreover, the mobility
of free radicals in crude oil is restricted by its high viscosity. Hence
a low-viscosity oil was studied [marine diesel (bunker)] by X-band EPR
spectroscopy in the temperature interval 170 to 400K. Despite the
viscosity at room temperature (2.5 x [10.sup.-3] kg [m.sup.-1]
[s.sup.-1]) and the tumbling correlation time for free radicals of about
[10.sup.-7] s suggesting a high mobility of free radicals in marine
diesel, the EPR spectra still showed poor resolution up to 373K.
However, above 373K, resolved lines were recorded: a superposition of a
septet-quartet, a sextet-quartet and a quintet-quartet group of lines
which were attributed to phenalenyl radicals and their derivatives. It
is concluded that below 373K, these radicals were present as diamagnetic
dimers and hence undetectable to EPR (130). In contrast with the
isotropic X-band spectra, W-band spectra of diesel were found to show
poorer signal-to-noise ratios and anisotropy in the A- and g- parameters
(131). EPR has also been used to characterise oil shale residue and
rejects, calschist, shale file and retorted shale from the Irati
formation in Brazil (132). A study was reported of various fuel-samples
made using ESR. As recorded at 298 K, spectra from heavy fuel samples
proved to be characteristic of vanadyl porphyrins, along with a sharp
intense signal close to g=2 from organic radicals as are always present
in asphaltenes. Two distinct spectral patterns were discerned among the
gas-oil samples: the straight run gas-oil showed a single peak at g=2,
while the light cycle oil spectra were characteristic of the phenalenyl
radical. The pyrolysis fuels showed a single resonance from an organic
radical and the mixed samples gave spectra similar to those of heavy
fuels (133). A study was made of the photochemical
"weathering" of Brazilian petroleum from the Compos Basin, Rio
de Janiro, as a film on seawater in relation to pollution by oil spills.
The material displayed ESR signals from organic free radicals, vanadyl
and iron(III) cations. After 100 hours of solar radiation exposure, the
linewidth of the organic signal decreased by 10.6% and the g-value of
the vanadyl signal was also decreased as a consequence of the
degradation of porphyrin systems. It is concluded that solar radiation
promotes the partial destruction of the asphaltene portion of the crude
oil (134). ESR has also been used to characterise radicals present in
source rocks, by which means it has been shown that samples of kerogen
collected at the shallow part of a petroleum deposit gave a broader peak
than samples taken from deeper levels. Narrowing of the linewidth was
caused by treatment of the sample with pridine but not by heating. A
septet-quartet pattern is described which curiously is attributed to the
tertbutyl radical, though it is quite obviously from phenalenyl radicals
(135). Samples of whole rock and isolated kerogen from three Alaskan
North Slope wells were measured by ESR. Power-saturation studies of the
single-line spectra showed a distinct correlation with the downhole
maturation, the degree of aromaticity according to [sup.13]C NMR, and
with the beginning of the oil-window for the Kingak formation. Abrupt
changes in ESR parameters indicate a change in the nature of the organic
matter present and may reflect a facies (a body of rock with special
characateristics) change (13).
18.1 ESR in fuel-cell research
While it is extremely debatable whether the putative "hydrogen
economy" will ever become a reality, both on grounds of production
(137) and storage (137,138) of hydrogen, and the shortage of platinum
(137,138) to fabricate fuel cells (Figure 9) based on proton exchange
membranes on a scale comparable with the 700 million vehicles on the
roads worldwide, research continues into understanding the fundamentals
of such devices, and an in situ study has been reported which used
spin-traps to detect free radical intermediates in the
[H.sub.2]/[O.sub.2] catalytic cycle. Thus detected are hydroxyl radicals
(138) at the cathode side of an intriguingly designed fuel cell
(139,140) of appropriate dimension to fit within the resonant cavity of
an ESR spectrometer. The issue of fuel cells is vexed, since there is
only sufficient platinum recovered annually that even if it were all
used to make the so named proton exchange membrane (PEM) cells, which
are those most suitable for use with hydrogen as a "fuel", if
they are manufactured continually for the next 30 years, still only 7%
of the world's present transportation fleet of 700 million vehicles
can be so matched (141). The major obstacles to such an entire
transformation from an oil-based to a hydrogen-based transportation
infrastructure are that firstly the hydrogen has to be "made"
since it is not a fuel but an energy carrier. Most of the world's
hydrogen is manufactured by catalytically steam-reforming natural gas
(137,138), which results in an overall emission of C[O.sub.2], and thus
a "green" source must be found, e.g. electrolysis of water
using renewable electricity, e.g. from wind-power. The engineering feat
required to inaugurate sufficient generating capacity by this means,
however, is stupendous, and would take decades even were the
manufacturing capacity for it already in existence. There is a further
issue, namely of the availability of manufacturing raw materials, namely
metals: as noted above, there is insufficient platinum available (141)
to fabricate the PEM fuel-cells that would be required to
"bum" enough hydrogen to propel more than a small fraction of
the equivalent of 700 million vehicles as are currently on the
world's highways, and indeed the currently vexed issue of rare
earth (RE) metals, particularly neodymium needed for high-torque
electric motors/dynamos, such as are used in wind-turbines, most of
which are exported from China (142). However, Chinese home-demand for
REs is enormous given the thirst for all kinds of energy (including
wind-power and other kinds of renewable energy) to underpin the
relentless industrial growth of a nation that is predicted to overtake
the United States as the world economic superpower by the end of this
decade. In consequence, China has announced it will restrict its exports
of REs onto the world markets, in order to meet its own agendas, which
casts doubt on the growth of renewable energy in the world overall
(142). Hence, in the absence of alternative materials to replace
platinum in this particular application, it is unlikely that
transportation can be maintained at anywhere near current levels by
means of PEM fuel cells, in addition to making enough hydrogen to run
them on in the first place (141). Nonetheless, fundamental studies
continue in an effort to determine the durability of the PE-membrane
itself under the harsh and aggressive working conditions of a working
fuel cell. By introducing a spin-trap to a working fuel cell, specially
designed to fit inside the cavity of an ESR spectrometer, one group has
investigated a Nation 115 membrane and another membrane free from
fluorine. Immobilised organic radicals were measured at the cathode
side, which demarked the initiation of oxidative damage to the membrane.
The yield of radicals was significantly greater for the fluorine-free
membrane than for Nation. Free radical intermediates were trapped at the
anode which are involved in the fuel-oxidation process but evidence of
membrane degradation was absent (143). Another group has undertaken an
investigation of the stability of polymer membranes, principally for use
in PEM fuel cells. To whit, the comparative robustness of Nation,
stabilised Nation (StNafion), and 3 M and Aquivion membranes toward
attack by the hydroxyl radical, x OH was assessed using aqueous
dispersions of the polymers at 300K. Two types of adducts were detected
in all dispersions: DMPO/OH and DMPO/CCR, a spin-adduct of
polymer-derived carbon-centred radicals (CCRs). The results indicated
that StNafion was more robust than Nation, while the 3 M and Aquivion
polymers were significantly the most stable. It would appear that the
absence of the ether group and of the tertiary carbon in the side chain
is the cause of the greater stability of these ionomers. A competition
kinetics approach was used to determine the reaction rate constant of
Ce(III) with HO x radicals to yield k = 2.8 x [10.sup.8][M.sup.-1]
[s.sup.-1], which agrees excellently with that 3 x [10.sup.8] [M.sup.-1]
[s.sup.-1] given in the literature (144). A fuel-cell (FC) running in
the cavity of an ESR spectrometer was used to monitor the formation of
HO. and HOO. radicals, H x and D x. atoms, and radical fragments derived
from the Nation membrane, using DMPO as a spin-trap. Measurements were
made with [H.sub.2] or [D.sub.2] at the anode and [O.sub.2] at the
cathode. The FC was operated at 300K with a membrane-electrode assembly
(MEA) based on Nation 117 with a Pt catalyst, causing closed and open
circuit voltage conditions, CCV and OCV, respectively. Both anode and
cathode chemistry was investigated. The DMPO/OH adduct was detected only
at the cathode for CCV operation, implying that x OH radicals were
produced from [H.sub.2][O.sub.2] formed electrochemically via the
two-electron reduction of oxygen. The DMPO/OOH adduct, which was
detected in this study for the first time in a FC, appeared at both the
cathode and the anode for OCV operation, and at the cathode after an
operating time of > 2 h. A mechanism is proposed in which HOO x
radicals are generated electrochemically at the cathode (HO x +
[H.sub.2][O.sub.2] [right arrow] [H.sub.2]O + HOO x) and at the anode
from hydrogen atoms and crossover oxygen (H x + [O.sub.2] [right arrow]
HOO x). DMPO adducts of H x atoms and D x atoms were detected at the
anode and cathode sides, for CCV and OCV operation. The detection of H x
and D x atoms is significant in that these species are sufficiently
reactive to abstract F-atoms from the tertiary carbon in the polymer
membrane chain, leading to chain fragmentation, rather in the manner
that ionizing radiation "unzips" the C--C backbone of
polytetrafluoroethylene (PTFE) by forming initial carbon-centred
radicals. CCR adducts were detected at the cathode after CCV FC
operation; while weak signals from CCR adducts were also detected at the
anode. Since the only component in the FC that could provide CCRs is the
Nation membrane, their presence indicates membrane fragmentation. In
conclusion, the operation of a FC has been shown to include such actions
as gas-crossover, surface-reactions at the catalyst, and it is
conceivable that reactive H x or D x atoms attack the membrane. Since
the latter do not occur in ex situ laboratory experiments it appears
that different mechanisms may pertain in the real in situ case (145). A
study has been made by another group using EPR to monitor
radical-induced degradation of partially fluorinated aromatic model
compounds for fuel cell membranes (146). A detailed investigation has
been reported of the microscopic viscosity, ordering and polarity in
Nation membranes containing methanol-water mixtures, using Tempone as a
spin-probe (147). A related issue to the use of fuel cells, particularly
of the PEM type intended for hydrogen as the "fuel", is the
storage of hydrogen itself for which various materials have been
explored including zeolites, but without success in meeting the
stringent requirements of either volumetric or weight density in respect
to the volume or mass of the fuel-tank. For commercial applications, an
acceptable energy density for a hydrogen storage tank is deemed to be
that it can efficiently hold an amount of hydrogen equal to 6.5 wt% of
the weight of the tank and 62 kg [H.sub.2]/[m.sup.3] in terms of volume.
However, although investigations of hydrogen storage methods have been
carried out for over 30 years, there has been no single one developed
which fulfils these demanding criteria. Some approaches meet the weight
target, but occupy unsatisfactorily large volumes (tanks of compressed
hydrogen gas) yet others achieve the volume target but not the weight
ratio (metal hydride absorbents) (138). A solid consisting of
single-walled carbon nanotubes (SWCNTs) encapsulated by thin Pd layers
onto a Pd substrate has been investigated as a novel material for
hydrogen storage which shows a synergetic effect from a combination of
the adsorptive affinities for hydrogen of Pd and the SWCNTs. A high
effective [H.sub.2] pressure is provided from the highly H-loaded
massive Pd substrate into a small fraction of deposited SWCNT which
allowed the attainment of a net capacity of 8-12 wt%. [H.sub.2]. ESR
measurements established that the Pd-[C.sub.x]-- complexes which formed
at the openings of the SWCNTs can be considered as adsorption sites for
[H.sub.2], providing both a high gravimetric capacity (H/C > 1) and
low hydrogen binding energy in the Pd encapsulated SWCNT, thus the gas
can be released for application, to a fuel cell (148).
[FIGURE 9 OMITTED]
18.2 Methane hydrate
Methane hydrate (137) (Figure 10) is formed when methane gas and
water are brought together under suitable conditions of low temperature
and elevated pressure, such that an "ice" type structure is
formed containing methane molecules in considerable quantity. The solid
material can hold 164 times its own volume of methane. The temperature
at which methane-hydrate is stable depends on the prevailing pressure.
For example, at 0[degrees]C, it is stable under a pressure of about 30
atmospheres, whereas at 25[degrees]C, nearer 500 atmospheres is needed
to maintain its integrity. The occlusion of additional gases within the
ice structure tends to add stability, whereas the presence of salts
(NaCl, as from sea water) requires higher stabilising pressures.
Appropriate conditions of temperature/ pressure exist on Earth in the
upper 2000 metres of sediments in two regions: (i) in permafrost at high
latitudes in polar regions where the surface temperatures are very low
(below freezing), and (ii) submarine continental slopes and rises, where
not only is the water cold (around freezing), but the pressures are high
(greater than 30 atmospheres), since the pressure increases by one
atmosphere for every 10 metres of depth. Thus, in polar regions,
methane-hydrate is found where temperatures are cold enough for onshore
and offshore permafrost to be present. In offshore sediments,
methane-hydrate is found at water depths of 300 500m, according to the
prevailing bottom-water temperature. There are reported cases where
"chunks" of methane-hydrate break-loose from the sea bottom
and rise to the surface, depressurizing and wanning, where they
"fizz" from the release of methane as they decompose to the
gas/water state. There are manifold and widely disputed estimates of
exactly how much methane-hydrate there is. However, a figure of
[10.sup.16] cubic metres ([m.sup.3]) of methane gas occluded within the
entire global deposits of this material is probably a reasonable
estimate. One estimate (149) puts the total at nearly
[10.sup.19][m.sup.3], but this is the only one of such magnitude.
Notwithstanding, the quantities of methane-hydrate are vast, and in view
of this, it is thought that it might provide a potentially significant
energy source, probably at least four times the entire reserve of fossil
fuels (gas, oil, coal) known (estimated). As "peak oil" (150)
bares its teeth, the possibility appears increasingly attractive.
However, the actual extraction of methane from this source is beset by a
number of difficulties: low permeability of sediments, which restrict
the actual flow of methane; lack of sustained interest from the oil/gas
industry (though this may well change, vide supra, according to rising
pressures of demand upon the existing limited resource); current limited
gas-industry infrastructure at methane-hydrate locations; and the fact
that no good field example has yet been demonstrated of the successful
production of methane from its gas-hydrate. All these considerations
score on the negative side as far as methane-hydrate becoming a serious
fuel source is concerned. It seems clear that in a warming world (for
whatever reason), methane will be released in increasing quantities,
from warming permafrost, thus augmenting global warming. Disturbances on
the sea bed may also cause the decomposition of methane-hydrate. It is
known that drilling into methane hydrate poses a hazard to oil
prospecting operations, and it is also thought that decomposition of
methane hydrate with an eruption of methane could trigger a tsunami.
More catastrophically, it is believed by some that world-scale eruptions
of methane from these "ice" deposits can have triggered
climate--CHange (global warming) on a cataclysmic level, most notably
the Permian--Triassic (P-T or PT) extinction event, sometimes informally
called the Great Dying, which was an extinction event that occurred
approximately 252 million years ago, forming the boundary between the
Permian and Triassic geologic periods. It was the Earth's most
severe extinction event, with about 90% of all marine species, and 70%
of terrestrial vertebrate species going extinct. For some time after the
event, fungal species were the dominant form of terrestrial life, and
perhaps this is once again the destiny for life on Earth.
[FIGURE 10 OMITTED]
In the context of the present review, we note that methane hydrates
may be investigated using ESR, on the basis that methyl radicals are
induced by exposure to ionizing radiation and which remain trapped
within the ice matrix, so long as the overall structural integrity of
the material is preserved. However, on raising the temperature,
structural changes occur for which energetics can be determined
according to diffusion and combination of the radical species resulting
in reduction and ultimately loss of the initial ESR signal. In a series
of measurements made in the temperature range 210-260K, the methyl
radical signal was found to decay according to second-order kinetics by
combination to form ethane. However, two quite separate
temperature-dependent processes were observed with respective activation
energies of 20.0 [+ or -] 1.6 kJ [mol.sup.-1] for the lower temperature
range 210 230K and 54.8 [+ or -] 5.7kJ [mol.sup.-1] for the higher
temperature range 235-260K. The former value accords with the enthalpy
change of methane hydrate dissociation into ice and gaseous methane,
while the latter agrees well with the enthalpy change into liquid water
and gaseous methane. It is clear therefore that methane hydrates
dissociate into liquid (supercooled) water and gaseous methane in the
temperature range 235-260K (151). The approach was extended to a sample
of natural methane hydrate collected from the deep ocean floor in Ocean
Drilling Program (ODP) Leg 164. Following [gamma]-irradiation, ESR
signals from the methyl radical were observed along with those from an
unidentified radical species. As expected, the decay energetics and
thermal stability were very similar to those from the synthetic sample,
but the signal intensity was about one eightieth of that from the
synthetic sample, because the methane hydrate had partially lost methane
gas and been converted to ice. It is proposed that the intensity of the
ESR signal from the methyl radical detected following exposure to
[gamma]-rays might be used to estimate the amount of methane hydrate in
a sample mixed with ice (152). ESR measurements were also made on a
sample of ethane hydrate, which had been exposed to [gamma]-radiation.
ESR signals were recorded from the ethyl radical (g 2.0031 [+ or -]
0.0005, [A.sub.[alpha] 1] = 2.2 [+ or -] 0.1 mT, [A.sub.[alpha]
[parallel]] = 2.5 [+ or -] 0.1 mT, [A.sub.[beta]] = 2.7 [+ or -] 0.1mT)
and atomic hydrogen (g = 2.0026 [+ or -] 0.0005, A = 50.5 [+ or -] 0.1
mT). From the results of ESR analysis and gas mass spectroscopy, it is
concluded that the ethyl radical decays into butane by dimerisation in
the first-order reaction in the temperature region of 250-265 K. The
activation energy of the decay reaction is 73.1 [+ or -] 6.3kJ
[mol.sup.-1], which is close to the dissociation enthalpy for the
conversion of ethane hydrate to liquid water and gaseous ethane (152).
It is concluded therefore that ethane hydrate does not dissociate into
ice but supercooled water, at least in this temperature range, which is
similar to the dissociation behaviour of methane hydrate as determined
previously (151).
18.3 ESR of coal
ESR has also been applied to study the quality of coal in which
complex paramagnetic centres have been found. Samples of flame coal
(71.4 wt% C), medium-rank coal (85.6 wt% C) and anthracite (94.9 wt% C)
were analysed along with macerals (exinite, vitrinite, inertinite) of
coals containing 73.8wt% C and 85.6wt% C. It was found that the
spin-concentration increased both in coal and macerals as the carbon
content increases. Four groups of paramagnetic centres were identified
in medium-rank coal, but only two groups in flame coal and anthracite.
The ESR spectrum of medium-rank coal is a superposition of two broad and
two narrow lines. Both broad and narrow lines were recorded from
flame-coal, and two narrow lines in anthracite. Two kinds of
paramagnetic centres with broad lines and one group with narrow lines
exist in exinite and vitrinite from medium-rank coal. Two different
broad ESR signals were detected for macerals from low-coalificated coal.
The most complex paramagnetic centres were found in medium-rank coal
samples. Broad ESR lines were not observed from the higher coalificated
samples, anthracite and inertinite from medium-rank coal while narrow
lines are not recorded in the ESR spectra of low-coalificated macerals.
Strong dipolar interactions and rapid spin-lattice relaxation processes
are characteristic for paramagnetic centres with broad lines. Strong
exchange interactions and short spin-lattice relaxation times were
measured for paramagnetic centres with the narrowest ESR signals (154).
18. ESR of photovoltaic systems
ESR and spectroelectrochemical studies of
poly(3,4-ethylenedioxythiophene) (PEDOT) have been carried out. It was
found that concentrations of paramagnetic centres in PEDOT vary from
0.02 spins per mer in the dedoped state to a maximum of 0.12 spins per
mer at a doping level of 0.15 [e.sup.-]/mer, which corresponds to 1 spin
per ca 8.5 metric units. Such high spin-concentrations indicate that
polarons are a major charge carrier group in PEDOT, which contrasts with
observations made for other members of polythiophene family. Moreover,
polarons do not disappear at high doping levels of PEDOT but rather
their numbers decline gradually down to 0.08 spins per mer at a maximum
doping level of 0.55 [e.sup.-]/mer as met in this study. Based on
information about concentrations of spins and polymer doping charges,
concentrations of bipolarons have been evaluated as a function of doping
level from which it appears that bipolaron formation begins at a doping
level of ca 0.06 [e.sup.-]/mer (155). The magnetic susceptibility of
solutions and powders of polyaniline was measured by ESR in the
temperature range 123-423K, and can be described by the integral of the
susceptibility of the polymer fragments in the triplet state over the
singlet-triplet energy level splitting. Mainly, the dependences can be
described in terms of the sum of the temperature-independent
susceptibility and the Curie-law susceptibility. In some cases, by
comparing the calculated and experimental dependences, the length of the
fragments L can be determined. A similar analysis can be applied to
other conducting polymers (156). So-called charge transfer complexes
(CTCs) in plastic solar cell materials influence the properties of
organic solar cells. In one study it is shown that CTCs can be captured
by light-induced electron spin resonance (I-ESR) with a much higher
sensitivity than is possible with conventional methods, such as
photoinduced absorption spectroscopy. Thus, the I-ESR technique rendered
possible the demonstration that CTC states exist by their direct
excitation with low-energy photons in an organic polymer/fullerene blend
and, also in hybrid blends of poly(3-hexylthiophene) and CdSe
nanoparticles. Furthermore, the recombination kinetics are analysed and
some discussion given of them (157).
19. ESR studies of ionic liquids
Being composed entirely of ions, ionic liquids were once mainly a
province of electrochemists. Recently, however, it has become apparent
that, inter alia, their lack of measurable vapour pressure characterises
them as green solvents, and that a wide range of chemical reactions can
be performed in them (158). In one study, the spin-probes TEMPO, TEMPOL
and CAT-1 (4-trimethylammonium-2,2,6,6- tetramethylpiperidine-l-oxyl)
were used to investigate both the microviscosity and micropolarity of
imidazolium-based ionic liquids. It was found that the mean rotational
correlation times (r) obtained by detailed simulation of the X-band ESR
spectra of TEMPO, TEMPOL and CAT-I increase with increasing viscosity of
the ionic liquid as expected, but the normal Stokes-Einstein behaviour
is not found. It was noted that ther probe molecule jumped into the free
volume of the ionic liquids by a non-thermally activated process, which
is most surprising. The [sup.14]N coupling constants show a
micropolarity for the ionic liquids comparable with that for
dichloromethane for TEMPO and with dimethyl sulfoxide for TEMPOL. The
micropolarity monitored by CAT-1 is found to depend strongly on
variations in the structure of the ionic liquid (159). TEMPO, TEMPOL and
CAT-I were employed along with their [sup.15]N variants ([sup.15]N-TEMPO
and [sup.15]N-TEMPOL- [D.sub.17]) similarly as probes of microviscosity
and micropolarity in ionic liquids. It was concluded that microviscosity
effects according to the Gierer-Wirtz theory may explain the spin-probe
behaviour. On the basis of spin exchange effects measured using TEMPO,
TEMPOL and CAT-1 dissolved in ionic liquids, it is concluded that there
is a greater degree of aggregation in the case of the non-polar
spin-probe TEMPO. Both isolated and aggregated species were observed for
the more greatly polar probes TEMPOL and [CAT-1.sup.160].
20. ESR studies of organic matter and kerogen in regards to oil and
gas formation
ESR has also been used as a guide to determine the maturation of
organic matter and for kerogen typing in the North Sea. In early ESR
analysis of North Sea wells, maturation of organic matter (OM) was
expressed in terms of maximum palaeotemperature (MPT) based on North
American calibrations that did not consider the influences of kerogen
composition or overpressure. In the North Sea, the MPTs were anomalous
in overpressured sequences and relative to other indices of OM
maturation such as vitrinite reflectance, so the ESR method was
abandoned there in geochemical studies. However, early empirical study
of North Sea ESR data indicated that, in relation to functions that
linked temperature and pore pressure, some ESR parameters were
predictable without reference to MPTs. In order to re-evaluate ESR
parameters as indices of OM maturation, the physical factors
(temperature and pressure) which affect OM maturation were related in to
the g-value and the spin concentration (Ng) (spin density) at six well
locations in the northern North Sea. A third ESR parameter, W (line
width), is not an effective guide to maturation levels due to its
complex relationship to the physical factors and kerogen types. However,
cross-plots of W versus "g" and Ng appear to be as effective
as pyrolysis for kerogen typing. Levels of maturation investigated in
the North Sea wells range through the equivalent vitrinite reflectance
values of about 0.50-1.50%. The values of "g" and Ng have been
differentiated for kerogen type, but undifferentiated values of
"g" have also been studied. Regression analysis has shown that
there are linear relationships between the ESR parameters "g"
and Ng, and the physical factors present-day temperature (To),
"effective" temperature (Te), and differential pressure (Pd).
Correlation coefficients for both "g" (undifferentiated and
differentiated) and Ng (differentiated) relative to the physical factors
are high; the highest values are for "g" and Ng relative to Te
and Pd (r = -0.950 for "g" differentiated or undifferentiated,
r = 0.944 - -0.976 0.976 for Ng differentiated, respectively). However,
correlation coefficients were lower for "g" and Ng relative to
To. More frequent high correlation coefficients and larger sample
populations suggest that "g" (undifferentiated) is a more
reliable index of OM maturation than Ng (differentiated). However, the
estimation of levels of OM maturation is improved if both indices are
used together. The ESR method appears to be effective both for
estimating levels of OM maturation and for kerogen typing. It has a
number of potential advantages over other geochemical methods: firstly,
it is more sensitive for estimating OM maturation than most other
methods; secondly, it can be used to analyse organic matter which is as
old as Proterozoic; thirdly, it does not destroy the samples analysed
(161).
21. ESR applied to extraterrestrial samples
ESR measurements were made on samples collected from the Moon
during the Apollo 11 and Apollo 12 missions. It was Apollo 11 that first
landed humans on the lunar surface on July 20 1969. Apollo 12's
mission was to prove that a "pin-point" landing was possible,
since Apollo 11 had actually landed some 4 kilometers away from its
intended target, and landing accuracy was important for taking samples
to compile a geological map of the Moon. A broad signal was recorded
centred at g = 2.09 [+ or -] 0.03 and assigned to ferromagnetic centres,
specifically from small metallic iron particles. In some samples, weak
signals were also observed from Mn[([H.sub.2]O).sub.6.sup.2+] in which
the metallic iron content is either low or partially removed. No signals
were detected from either free electrons or holes, which might be
expected as indicative of radiation damage, nor from [Ti.sup.3+] (162)
S-type asteroids are thought to be parent bodies of ordinary chondrites
and yet theft reflectance spectra are different from ordinary
chondrites. It is thought that the source of the difference is space
weathering, where impacts from high-velocity dust particles change the
optical properties of the uppermost regiolith surface of asteroids. By
means of a laser whose pulse-duration and energy were chosen to match
real conditions of heating by dust impacts, similar optical changes can
be induced. Nanophase iron particles were detected by ESR, which are
considered as the essential result of space weathering (163). A detailed
paper on space weathering essentially validates the idea that it is
caused by metallic iron nano-particles smaller than the wavelength in
ubiquitous vapour-deposited coatings on soil particle surfaces and
inside agglutinates. The vapour is produced by both solar wind
sputtering and micrometeoric impact vapourisation and injected
preferentially downward into the porous regiolith. The iron is reduced
by a physical process, the selective loss of oxygen that occurs during
deposition of the vapour, and does not require heating, melting or a
reducing environment (hydrogen). The single-domain iron particles are
detected by ESR (164). The 1976 Viking landers on Mars performed a
series of experiments in which soil samples were analysed for evidence
of life. Although no biological responses were elicited, samples of soil
from 10 cm beneath the surface on contact with water released oxygen in
larger amounts than would be expected from a simple physical desorption.
A sample of plagioclase feldspar was irradiated with UV light for 20
hours under a simulated Martian atmosphere, which caused the formation
of superoxide radical anions according to ESR. It is proposed that it is
the reaction between superoxide and water that is responsible for the
liberation of oxygen in the Martian samples (165). Another group has
investigated this interesting topic. Their results show that when finely
ground basaltic minerals are added to water, [H.sub.2][O.sub.2] is
produced and in sufficient quantities to explain the Viking Lander
results. It is proposed that reaction occurs between water and defects
at the surface of mechanically pulverised minerals, in analogy with
carcinogenic quartz dusts. It appears too that the yield of
[H.sub.2][O.sub.2] is increased by dehydroxylation of the mineral
surface, as is especially significant for dusts on the airless surface
of the Moon. It is suggested that a significant health hazard might be
posed by fine-grained basaltic dusts to astronauts visiting planetary
bodies whose surfaces have been impacted to generate such materials
(166). The stability of radiation-induced radicals in solid [H.sub.2]O
and C[O.sub.2] was investigated from the point of view of using these
materials for future ESR dating of outer planets, their satellites and
comets in the solar system. The samples were irradiated at 77 K using a
[sup.60]Co [gamma]-ray source and measured at several temperatures with
ESR to derive an isothermal decay time for the signal intensity. By
extrapolation of an Arrhenius plot, the lifetime of the defects was
deduced at the ambient temperature of the outer planets, and their
satellites: Uranus, Neptune, Pluto and Triton (the largest moon of
Neptune). The signals are stable over periods of years and it appears
that by using a portable ESR with a lightweight Nd-Fe-B magnet it might
prove possible to date remote samples in the solar system. In view of
the nature of the outer planets (gas giants) it is thought that similar
measurements on solid methane might prove worthwhile (167). A search was
made for ESR-based markers of the history and origin of the insoluble
organic matter (IOM) in extraterrestrial and terrestrial rocks. Three
samples were taken of the IOM from carbonaceous chondrites, (Orgueil,
Murchison and Tagish Lake meteorites) and three samples of cherts
(microcrystalline silicate rock) containing microfossils ranging in age
from 45 million years to 3.5 billion years. It was found that the
organic matter in the meteorites (whose age is the age of the solar
system) contains a high concentration of diradicaloid molecules with a
diamagnetic ground state and a thermally accessible triplet state (S =
1). The latter is significant because such species are unknown in IOM of
terrestrial origin. It is further shown that the ESR linewidth recorded
from the IOM in cherts and coals decreases with the logarithm of the age
of the material. The conclusion is drawn that the organic matter in the
oldest cherts (3.5 billion years) has the same age as the silica matrix,
and does not originate from contamination by bacteria (168). Elsewhere,
the nature of the diradicaloid species has been addressed. The
macromolecular IOM from the Murchison and Orgueil meteorites is highly
aromatic and contains organic radicals concentrated in microregions
rather than the homogeneous distribution found in terrestrial samples.
On the basis of the signal intensity increase found above 150K (due to
population of the S = 1 state), and electronic structure calculations
made using extended Huckel and Density Functional methods, it was
concluded that the diradicaloid species could be assigned to aromatic
fusions of between 10 and 15 benzene rings, with two unpaired electron
functions attached (-C[H.sub.2] x) in a quinoidal structure (169). An
important feature of meteoric IOM compared with terrestrial IOM is a
systematic enrichment in deuterium, which is highly heterogeneous. It
has been shown that organic radicals in the IOM constitute the D-rich
carriers in the D-rich hotspots. The electronic structure of the
radicals has been deduced from the measurement of the spin states S by
transient nutation in pulsed-EPR. It is shown that these deuterium-rich
radicals are dominated by biradicaloids (species with an S = 0 ground
state and a thermally accessible S = 1 state) and biradicals (species
with an S = 1 ground state) representing ca 61% and ca 31% of the
radicals in the IOM of the Orgueil meteorite, respectively, while single
radicals (S = 1/2) are present as only ca 8% of the total. In contrast,
mature terrestrial IOMs contain almost exclusively S = 1/2 radicals. A
structural model is proposed, in which the occurrence of dominant
biradicaloids and biradicals is a direct consequence of the structure of
the IOM, which consists of a network of small aromatic rings linked by
branched and short aliphatic units. This implies that the formation of
stable biradicaloids and biradicals by C-H breaking and their deuterium
enrichment occurs after the formation of the IOM in the primitive solar
system. The idea therefore is reinforced that the formation of the IOM
and the deuterium-rich hotspots are the product of ion chemistry in the
solar disk (170).
22. ESR in food and nutraceutical research
An ESR-based method for the quantitative/qualitative determination
of the purity of cane sugar has been proposed. The sucrose radical is
produced when sugar is pulverised, the concentration of which increases
as the particle size decreases. It is found that the ESR peak area for
the radical increases linearly as the sucrose content of the material
(refined sugar, plantation white sugar, soft brown sugar and raw sugar
were used as samples) (171). A review has been published on applications
of ESR in nutraceutical and food research, in particular to determine
the free radical scavenging capacity, oxidative stability evaluation and
[Cu.sup.2] + chelating capacity of foodstuffs. The potential of the ESR
spin-label oximetry technique is exemplified (1) in the determination of
lipid peroxidation and oxygen diffusion--concentration products in
liposomes, oxygen transport and depletion, along with membrane structure
and dynamic properties. The use of ESR in determining whether foods
including meat, fruits, vegetables, spices, cereal grains, and oil seeds
had been irradiated was surveyed. In a final section an account was
given of the investigation of microstructural changes, phase transitions
and viscosity changes during food formulation, processing, and storage,
along with the potential of the method to investigate the
radio-stability of food components (172). The spinprobe technique has
been applied to foodstuffs by Sutcliffe and his coworkers (173-175). The
spin-probe 1,1,3,3-tetramethylisoindolin-2-yloxyl (TMIO) and its sodium
sulfonate salt derivative, sodium
1,1,3,3-tetramethylisoindolin-2-yloxyl-5-sulfonate (NaTMIOS), were used
to monitor the microviscosity changes of water during starch
gelatinization. In cereal starch, which contains mostly A-type
polymorphs, evidence was found for the amylopectin and amylase regions:
the latter exhibited a phase transition at about 55[degrees]C and a
substantial increase in the microvoscosity was noted on cooling (172).
The same approach was also used to study changes in microviscosity in
the aqueous and lipid phases of flour dough during heating and
subsequent cooling. Hence, starch gelatinisation could be studied in
detail and it was shown that the process results in a fall in the
dielectric constant of the material (174). Another foodstuff so
investigated is ice-cream. Usin the TMIO probe, it was found that when
ice-cream is cooled, the fat phase is composed of a mixture of solid and
liquid fat down to a temperature of ca - 60[degrees]C. By means of the
water-soluble probe NaTMIOS, it was found that the aqueous phase changes
entirely from the solid to the liquid within I[degrees]C of
-18[degrees]C. On cooling further to -24.7[degrees]C, and then allowing
the sample to warm-up to + 25[degrees]C, the rotational correlation
times of the latter probe were slow to recover their original values.
For the lipid phase, a value of 65.7 [+ or -] 2.0 ps was obtained for
the rotational correlation time determined from the B parameter along
with an activation enthalpy of 32.5 [+ or -] 0.9kJ [mol.sup.-1]. These
values are typical of those expected to be found in the type of fat used
to make ice-cream. The water phase gave corresponding values of 32.2 [+
or -] 0.5 ps and 24.5 [+ or -] 0.4 kJ [mol.sup.-1], which are as
expected for a sucrose concentration of 24% (171).
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Professor Chris Rhodes has a visiting position at the University of
Reading and is Director of Fresh-lands Environmental Actions. He has
catholic scientific interests (www.fresh-lands.com) which cover
radiation chemistry, catalysis, zeolites, radioisotopes, free radicals
and electron spin resonance spectroscopy, which more recently have
developed into aspects of environmental decontamination and the
production of sustainable fuels. Chris has given numerous radio and
televised interviews concerning environmental issues, both in Europe and
in the United States-including on BBC Radio 4's Material World.
Latest invitations include a series of international Cafe Scientifique
lectures regarding the impending depletion of world oil and the need to
develop oil-independent, sustainable societies. He has published more
than 200 peer-reviewed scientific articles and five books. He is also a
published novelist, journalist and poet. His novel "University
Shambles" has just been released as an eBook and has been nominated
for Brit Writers' Awards 2011: Published Writer of the Year.
E-mail: cjrhodes@fresh-lands.com
doi: 10.3184/003685011X13130481780217