05/143671

10/30/1973

05/14/1971

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Barringer Research Limited (Ontario, CA)

73/28.010

55/459.100, 436/92, 55/417, 95/73, 96/417, 436/25, 96/413, 96/43, 436/178, 96/73

G01N33/00; G01V9/00; G01N31/06

73/19,28,17R,421.5R,421.5A,53 55/2,131,133,136,137,138,270 23/23EP

3205700

Apparatus for recovering minute quantities of volatile compounds from inert solids

September 1965

Lively et al.

Goldstein, Herbert

What I claim is

1. A method of identifying the presence of a substance in an area, comprising:

2. A method as claimed in claim 1 wherein said area is an area of the earth, wherein said substance is indicative of underlying geological conditions, wherein said method is practised from an aircraft and wherein said method includes the additional step of correlating said analyzed matter with said known locations.

3. A method as claimed in claim 2 wherein said matter is removed from said particulates by heating said particulates to a temperature sufficient to drive off said matter and wherein said particulates are transferred to an inert carrier gas prior to the analysis thereof.

4. Apparatus for performing a rapid geochemical survey of an area of the earth from a moving vehicle comprising:

5. Apparatus as claimed in claim 4 wherein said concentrating means includes at least two chambers, each for collecting said particulates, and means for diverting at least a portion of said air stream cyclically between said chambers, so that particulates are collected cyclically in one of said chambers and then in the other, wherein said removing means includes means for heating at least some of the collected particulates to remove matter therefrom in gaseous form, wherein there is further provided means for operating the heating means in said chambers cyclically so that while particulates are collected in one chamber, the concentrated particulates in the other chamber are being heated, and connecting means for connecting said chambers to said analyzing means.

This invention relates to a method and apparatus for detecting volatile components in the atmosphere for the purpose of identifying mineral deposits. It has been found that a number of volatile components are present at an unusual concentration in the atmosphere overlying certain classes of mineral deposit. Thus, for example, mercury vapour increases from typical backgrounds of one to ten nanograms per cubic meter to concentrations varying between 10 and 50 nanograms per cubic meter over gold and silver deposits as well as over many types of base metal orebody. This is attributed to the fact that mercury tends to concentrate in most metallic ores to an amount which is greater than the surrounding rocks and since mercury and some of its compounds have an appreciable vapour pressure at normal atmospheric temperatures, some of this mercury is dispersed into the overlying atmosphere above orebodies. Similarly, there is a geochemical association of chlorine, fluorine, bromine and iodine with many classes of mineral deposit, and since a number of these elements and their compounds have a significant vapour pressure, these elements and their compounds can also occur in the atmosphere over mineral deposits. In the case of the halogen elements, there is also a strong association with phosphate deposits and oil fields so that these elements can also be used to prospect for oil and phosphates. In addition to these elements there are a number of others that have much lower vapour pressures, but nevertheless can occur in minute but measureable traces in the atmosphere. These include the halide compounds of elements such as tellurium and selenium which have close affinity with certain types of copper deposit, and the halide compounds of elements such as arsenic, antimony and bismuth, all of which have sufficient vapour pressure to be present in minute traces. Given sufficient sensitivity in an analytical system, even more elements can be detected in the form of volatile inorganic compounds. In general, the halides of most metals are substantially more volatile than the metals themselves and can occur in minute quantities in the atmosphere.

In addition to the occurrence in the atmosphere of volatile inorganic compounds there are also present significant amounts of organic vapours. Organic vapours are generated by living forms of all types including plants, soil bacteria, insects and animals. Large quantities of volatile organic vapours are generated by trees in the form of natural oils known as turpines. Some of these organic vapours oxidize in the atmosphere to form minute liquid and solid particulate matter, a fact which can cause the development of atmospheric haze over forests under some weather conditions. A fractional component of organic vapours in the atmosphere is comprised of metallo-organic compounds. It has been shown that certain metals that are present in soils are converted into volatile organic compounds by bacteria. A well established example is that of mercury which is converted into di-methyl mercury by a variety of bacteria and other micro-organisms in soils. Di-methyl mercury has a lower boiling point than water and readily evaporates into the atmosphere as it is formed. It has been demonstrated that soils containing living micro-organisms liberate substantially more mercury into the atmosphere than the same soils that have been sterilized.

In addition to mercury, there are a number of other metals that can be converted by micro-organisms into volatile metallo-organic compounds which provides an important mechanism for introducing metallic vapours into the atmosphere. The problem in measuring all these components both organic and inorganic is to achieve sufficient sensitivity to be able to make rapid measurements during mobile traversing. U.S. Pat. No. 3,309,518 of O. Weiss, dated Mar. 14, 1967 discloses a method of aerial prospecting in which solid particulates are gathered on a piece of filter paper, following which the filter paper is dissolved, the particulates are concentrated, and then the particulates are analyzed by an electron probe analyzer which excites X-ray emissions from the particulates. By analyzing the X-rays, information is said to be obtained concerning the identity of the elements present in the particulates. Instead of analyzing the particulates themselves for the presence of metals, the present invention treats the particulates as adsorbers of volatile components which are indicators of the presence of mineral deposits. Particulates are normally present in the atmosphere to a mass concentration of at least 10 micrograms per cubic meter, and are frequently up in the range of several hundred micrograms per cubic meter. It has been shown that volatile components in the atmosphere are adsorbed by particulates and reach an equilibrium. A good example is that of iodine vapour. It has been demonstrated that free iodine vapour can be 75 percent adsorbed onto natural particulates in the atmosphere in a short period of time. The high adsorption is a function of the very large surface area of particulates in the atmosphere in relation to their mass, since particle sizes extend below one-tenth of a micron and particle concentrations run in the vicinity of thousands of particles per cubic centimeter. Nearly all surfaces are natural adsorbers for gases and vapours and the ability of particulates to scavenge traces of condensable gases and vapours from the atmosphere is a function of this phenomena and their large ratio of surface area to mass.

Metallo-organic vapours are condensable and susceptible to adsorption by particulate matter and particularly by organic liquid particulates. Thus over heavily forested areas high concentrations of organic liquid particulates may occur which can adsorb the metallo-organic vapours generated by micro-organisms in the highly active and humus rich forest soils. Thus the volatile components adsorbed on particulates in the atmosphere may be indicators of anomalous metal concentrations in the underlying terrain in vegetated areas as well as in the more obvious arid regions where vegetation is scarce and conditions are dusty.

In the present invention, atmospheric particulates are collected and subsequently heated to drive off adsorbed gases and vapours, which are then analyzed. The particulates may be collected continuously from a moving aircraft, for example, and concentrated if necessary. A number of analytical techniques may be employed for the purpose of identifying the adsorbed gases and vapours.

As used herein, the word "particulates" refers to minute solid or liquid particles in suspension in a gas, particularly the atmosphere. For convenience, the word "gas" will usually be used to refer to either gases or vapours or both gases and vapours. "Adsorbed matter" refers to atoms, molecules, or ions of a solid, liquid or gas held in contact with surfaces or interfaces of finely divided particulates.

FIG. 1 is a diagrammatic sectional view of apparatus according to the invention;

FIG. 2 is a diagrammatic perspective view of the apparatus according to FIG. 1, partly broken away; and

FIG. 3 is a diagrammatic perspective view of a modified form of a cyclone for concentrating solid particulates.

The drawings show an embodiment of the invention in which large volumes of air are sampled through an electrostatic precipitator. The air enters through a sampling duct 1, goes through an electrostatic charging grid 2 and then through a grounded collecting grid 3. The charging grid 2 may consist of a set of grounded wires alternating with a set of wires connected to a source of negative high voltage. The grounded collecting grid 3 is made of nichrome resistance wire which can be heated to a temperature of up to 1,000°C by the application of a current through the wires. The aIr stream is split by a perforated baffle 4 which can be rotated into an alternative position 5 about a pivot point 6. The perforations are arranged in the baffle such that a small percentage of the air stream passes through the perforations and the remainder is diverted into the other half of the duct. The collecting grid 3 is split into two portions which can be heated independently and are both kept at ground potential so that at all times they will collect and retain particles. A second unperforated baffle 7 is arranged to swing about pivot point 8 and can be rotated to point 9. Two butterfly valves 10 and 11 can be operated to open and close exit tubes 13a, 13b.

The particulates in the air entering at 1 are charged on the grid 2 and collected on the grid 3, the air substantially devoid of its particulates then being expelled out of the duct at 12. With the baffles in the position shown in FIG. 1, a small portion of the air passes over the right hand side of the grid 3 and a heating current is applied to this right hand side. Particulates adhering to the right hand side of the grid 3 are raised to a sufficiently high temperature to drive off adsorbed gases and volatile components which then pass out through the open valve 11 along the right hand exit tube 13b. The baffle 7 is closed in order to seal off this small flow of air and channel it out through the tube 13b. At the same time the main body of air passes over the cold left hand portion of the collection grid 3 and particulates are collected. After a short period (e.g., 1 to 5 seconds more or less), the perforated baffle 4 is swung to the position 5, the unperforated baffle 7 is swung to position 9, the valve 10 is opened and the valve 11 is closed, and heat is applied to the left hand side of the collection grid 3 and removed from the right hand side. The particulates collected on the left hand side of the collection grid 3 are now heated to a temperature sufficient to drive off adsorbed gases and volatile components and a small air stream through perforated baffle 4 carries them through the exit valve 10 and out to the tube 13a. Also, the heating current in the right hand side of the collection grid 3 is switched off and particulates are collected on this portion of the grid 3 from the main body of the air stream. The cycle is then repeated.

Appropriate control means is provided to control the functions described. The details of these control means are well within the skill of those skilled in the art, and the control means is therefore shown in block form. Typically the control means includes a control box 14 having shafts 15, 15 connected to the baffles 4, 7 at pivot points 6, 8 to swing the baffles in unison from one side of the device to the other, to close off first one grid containing chamber and then the other. Electric valve actuators 17, 18 are connected to the butterfly valves 10, 11 and are connected by electrical leads (not shown) to the control box 14 and are controlled thereby so that the appropriate exit tube 13a or 13b will be connected to the device. Control leads 19, 20 also extend from the respective halves of the grid 3 to the control box so that each half grid will be heated when the chamber in which it is located is closed.

Although only two chambers, each containing half of the collection grid 3, have been shown, additional collection chambers can be used if desired. Alternative means may also be used to collect and heat the particulates, but desirably a continuous record should be provided as the air over the terrain under investigation is traversed.

In this fashion a continuous air stream flows through the exit tube 13 containing a concentration of the volatile components that were originally present in adsorbed form in the incoming air stream. The rate of flow through the perforated baffle 4 can be arranged to be very small such as 1 percent of the total incoming air or less. This provides a high degree of concentration. Furthermore, in a simple modification of the system, the perforations in the baffle 4 can be entirely removed and the closed chamber so formed when the baffles are in position can be flushed out with argon from a gas cylinder if so desired. This can provide an inert carrier gas through the heated particulates instead of using oxygen. This can be particularly convenient for some types of emission spectrographic analysis of the vapours. Since the baked particulates accumulate the collection grid 3 must occasionally be cleaned or replaced to maintain efficient operation.

A variety of analytical techniques can be used for analyzing the gases and vapours emerging from the exit tube 13. For example the gases and vapours can be scrubbed with a water spray and the liquid solutions so obtained can be passed over a specific ion electrode. Electrodes are available which can have high sensitivity for the halogen elements such as fluorine and can provide a continuous electrical reading of fluorine concentration. In the case of measuring for mercury vapour, the vapours can be passed through an optical cell and the absorption of a 2537A beam of light can be measured. This absorption is related to the concentration of mercury vapour present. In a more sophisticated analytical arrangement, an argon carrier gas can be used as described above and the gas stream can be passed through a microwave cavity in order to generate a microwave plasma. The emission light from this plasma can be passed into a spectrometer and measurements made of specific emission lines corresponding to elements such as mercury, fluorine, iodine, bromine, chlorine, tellurium, arsenic, antimony, bismuth, etc. Since various kinds of analytical apparatus may be used, the analytical apparatus connected to exit tube 13 is indicated in block form at 21.

It will be appreciated that a number of well known methods can be used for collecting particulates which are adaptable to this invention. Similarly, a number of well known analytical techniques can be employed for analysing the vapours liberated from the particulates. The principal feature of the invention is the concept of utilizing the particulates as the carrier for the trace gases and vapours, since the particulates can be readily concentrated and purged of their gas and vapour contents.

In an aircraft installation, it is desirable for the apparatus to ingest in the order of 10 cubic meters per minute of air. This represents a cubic meter every 6 seconds which will carry 10 micrograms or more of particulates through the system every 6 seconds. Plasma analytical techniques are capable of achieving sensitivities typically of the order of 10 ^-11 grams for a wide range of elements so that such a system is able to see as little as 10 ^-11 grams of an element adsorbed in gas or vapour form on 10 ^-5 grams of solid material. This represents an absorption of 1 ppm by weight. In some cases, sensitivities for elements can be achieved as high as 10 ^-14 grams (e.g., cadmium). In these cases 10 ^-14 grams of, for example, cadmium compounded as a halide or organic vapour and dispersed in adsorbed form through a cubic meter of air can be detected. The utility of such a device for detecting minute dispersions of vapours above mineral deposits is readily apparent. Normalization against the atmospheric particulate concentration and size can be carried out by using conventional equipment such as an optical monitor or dust concentrator which is connected continuously on stream during survey.

An alternative application of the invention is in the detection of minute traces of hydrocarbons and other organic volatiles in connection with oil exploration. In this case, a similar technique is used except that the electrostatic collecting grid is heated to a lower temperature which does not pyrolise and destroy the organic compounds. A typical temperature is 200°C. The output pipe of the instrument 13 is now taken to a gas chromatograph for analysis or to a silicone rubber membrane. The membrane is used to seal off a low pressure zone in front of the entrance port of a mass spectrometer. Organic vapours have the property of passing rapidly in one or two seconds through such a membrane leaving behind the accompanying molecules of air or other carrier gas. Rapid real-time analysis can then be carried out in the mass spectrometer which may conveniently be of a lightweight type. A suitable instrument is an RF Quadrapole mass spectrometer. Such a system can achieve extreme sensitivity and specificity for organic compounds. It has great utility in oil exploration where the presence of sub-surface oil fields can be indicated by hydrocarbon seeps at the surface. Such seeps can be gaseous and contain methane, ethane, pentane, etc. or they may involve liquid hydrocarbons such as benzene which can have sufficient vapour pressure to escape into the atmosphere.

The embodiment of the invention described and shown uses heat for removing volatile components from the particulates. It will be appreciated, however, that the equipment can be modified in order to remove absorbed gases by reduction in pressure. Thus the chamber that is sealed during the heating cycle can be pumped down to low pressure instead, in order to cause degassing of the particulates. Alternatively, a combination of heat and low pressure can be used in order to minimize heating and obtain a liberation of organic compounds without altering their chemical structure.

In another variant of the invention, adsorbed gases and vapours can be removed by flushing the collected particulates with a gas or liquid that has complexing capabilities for the vapour in question. Thus, certain adsorbed metallic vapours can be removed by passing chlorine over the particulates. The bonding forces of adsorption is this case are overcome by the stronger bonding force between the chlorine and the metal. The chlorine or other carrier agent can then be analyzed for adsorbed components leached from the particulates.

Although the invention has been described with reference to an electrostatic method of particle collection, other techniques may be employed, such as the use of a cyclone separator. A modified type of cyclone suitable for concentrating dust from a large volume of air is shown in FIG. 3. In FIG. 3, incoming air containing particulates enters a cone shaped cyclone 22 through a duct 23 which directs the flow of air tangentially into the cyclone 22. The air swirls around a perforated cone shaped separator 24, and much of the air escapes through the openings in the separator 24 to a discharge duct 25 having a flow balance valve 26. The particulates tend to settle towards the bottom of the cyclone 22, and thus a stream of air that is enriched in particulates is caused to flow through a discharge duct 27. A still further method is the use of filters of the paper or fibreglass type. The particles from large volumes of air can be collected on filters and the adsorbed gases or volatile components subsequently removed by heating, gas or liquid leaching or vacuum degassing. This can be operated on a semi-continuous real-time basis in the fashion of the electrostatic embodiment already described in detail, or the filter can be stored for later analysis. Thus, for exploration purposes, a continuous strip filter can be used adapted from a standard and commercially available continuous pollution sampler and provide a flight record of particulate matter. Special equipment can be constructed to provide subsequent analysis of the adsorbed gases and volatiles held on the particulates. Correlation can then be made between the analytical data derived from the particulates and the locations which the said particulates were collected.

As in the case of any geophysical prospecting apparatus, it is necessary to correlate the data obtained from the particulates with the locations at which the particulates were obtained. This may be accomplished in the case of an airborne instrument by providing a strip camera to record photographically the flight lines of the aircraft.

Whereas applications of the invention that have been described relate particularly to mineral and oil exploration, in general the invention is applicable to the solution of problems involving the detection of minute quantities of organic or inorganic vapours. The detection of the presence of concealed narcotics is one example. Many narcotics have sufficient vapour pressures for them to be detectable at some distance by dogs. The present invention can be used to sample large volumes of air from aircraft cabins, baggage holds, custom areas, holds of ships and the like. The particles in the atmosphere act as scavengers for traces of narcotic vapours which are concentrated and liberated for analysis by the method of the invention.

In the foregoing description, reference has been made to the detection of mineral deposits. It is to be understood that the term "mineral deposits" is intended to generally refer to any and all deposits in the earth of economic importance, which are capable of being sensed by the method and apparatus of the present invention, including for example metalliferous orebodies and petroleum deposits.