US3009063A - Gas analysis - Google Patents

Description

Nov. 14, 1961 .1. R. ROEHRIG GAS ANALYSIS Filed Oct. 2, 1957 Compensufing Curren+ INVENTOR. JB'MAFLGI x p d United States Patent "ice iiflfflli 3,009,063 GAS ANALYSIS Jonathan R. Roehrig, Sndbury, Mass., assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed Oct. 2, 1957, Ser. No. 687,800 4 Claims. (Cl. 25083.6)

This invention relates to radioactive ionization detectors for use in quantitative and qualitative determination of gases when used with gas chromatographic separation equipment.

It is an object of invention to provide a beta particle ionization chamber detector for use in chromatographic analysis of gas which can detect smaller concentrations of separated sample gas than detectors heretofore known.

Another object is to provide a means to determine the quantities of gas given ofl from a chromatographic separation unit and to determine the time of evolution of increments of the flow more accurately than possible heretofore.

Another object is to produce a radioactive ionization chamber detector for use with chromatographic separation equipment which is more rugged and cheaper to manufacture than other known detectors but having at least comparable sensitivity.

A further object of the invention is to produce a gas chromatography detector which operates with an amount of radioactive material which produces a iarger number of disintegrations per unit time than radioactive detectors heretofore known but which does not create radiation which will endanger operating personnel even though no shielding is used other than that inherent in the formation of a chamber isolated from the atmosphere.

A still further object is to produce a gas detector for use with gas chromatographic separation apparatus which, for a given number of beta particles traversing the gas, has the highest possible ion current output, when the ionizing path is limited to a suitably low value.

Other objects will in part be obvious and in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts, and the method involving the several steps and the relation and the order of one or more such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 is a diagrammatic, sectional, schematic drawing illustrating one preferred embodiment of the invention;

FIG. 2 is a diagrammatic, sectional, schematic drawing illustrating another preferred embodiment;

FIG. 3 is a cross-section of the apparatus of FIG. 2 taken on plane 33 indicated in FIG. 2.

The objects of this invention are realized by utilizing tritium (H as a source of the ionizing particles in an ionization chamber particularly constructed so as to serve as a detector for gases being evolved from chromatographic separation.

Though radioactive ionization chambers are old to the art, and tritium has been known for some years, it has never been appreciated prior to this invention that tritium will yield distinct advantages when used in radioactive chamber detectors in gas chromatographic analysis.

In gas chromatographic analysis one accepted method of gas determination has been to ionize the gas evolved from chromatographic separation equipment in an ion chamber by subjecting the gas to a stream of ionizing particles such as beta particles, to collect the ions, and to measure the current thus produced. When the stream of ionizing particles is fairly constant both in flow and in particle energy level, it has been found that the current produced is a function of the cross-section area (area for ionization) of the gas molecules being subjected to the flow of ionizing particles. This relation is useful only if the energy of the particles is not entirely absorbed by the gas so that at least some of the particles completely traverse the portion of gas being analyzed.

Where a first gas (sample gas) appears in an unknown concentration with a second gas (the carrier gas), the current produced by the ionization of the binary combination is difierent from that obtained by the ionization of the carrier gas alone (at the same prwsure and temperature) if the cross-sections for ionization of the two gases are different. Where the carrier gas is one which has an extremely small cross-section for ionization, the presence of even a small amount of sample gas will appreciably increase the ionization current. The current thus resulting from the ionization of the binary combination is directly related to the concentration of the sample gas in the carrier gas.

The lower limit of detectable concentrations using ionization detector methods is defined by the detector signalnoise ratio, I/I where I is the electric current in the detector attributable to the presence of a sample gas (in mixture with the carrier gas) and I is the R.M.S. current attributable to unpredictable, random disturbances in the detector (i.e. noise). In general if NI is less than about unity it is assumed by those skilled in the art that the signal, I, is undetectable and that the detector cannot be used to measure the gas concentration which produces that current. The noise in ionization chambers utilizing the products of radioactive disintegration as ionizing particles is primarily attributable to the variation of the rate at which disintegrations occur in radioactive material from instant to instant with the consequent variation of flow of ionizing particles from the source. This necessarily causes the ion current to vary because the number of ionizations produced varies with the flow of the ionizing particles.

Accurate analysis of samples by chromatographic methods depends upon precise determination of the transit time through the column and magnitude of each increment of gas evolved. It is therefore necessary to avoid mixing successively evolved increments of the gas. Moreover, it is necessary that the instantaneous indication of the detector be related to only a small increment of flow (i.e. gas evolved during a short period) in order to produce accurate resolution of the signals produced from successively evolved constituents. For these reasons the dimensions of the ionization chamber must be limited in order to define a small increment of the flow for ionization at one time. The cross-section of the chamber must be such that unimpeded flow can be maintained, but the total volume should not be so large that mixing results. Chromatographic columns now generally used have cross-sections on the order of one square inch. It has been found that ionization chambers through which all of the evolved gases flow from such columns cannot exceed 10 cc. in volume and provide good resolution. For columns of smaller area correspondingly smaller detector volumes are required. It should be noted that as the size of columns increases it is possible to ionize larger quantities of gas at any instant for comparable resolution.

In order to be commercially practical an ionization detector must not create a radioactive hazard to operating personnel nor should it require heavy shielding.

A further desideratum in an ionization detector is that the signal current produced be large in order to permit the use of rugged instruments for measuring the ion current which inherently respond only to readily measurable electric Cl rrents, i.e. currents on the order of amperes. Minimum measurable current may be a limitation on sensitivity should the effect of noise be eliminated.

By utilizing the methods and apparatus of this invention, tritium can be used to produce a very sensitive chromatographic sensor with high signal-noise ratio, high ion current and no radiation hazard, even though no shielding is utilized.

Tritium can be combined with many materials (e.g. titanium and zirconium) with a sufficiently stable chemical bond so that little tritium is lost by chemical decomposition over an extended period of time. All materials with which hydrogen stably combines to form solids can be utilized in the invention. However, it is preferred to use those metals which have the characteristic of exothermically occluding hydrogen. Such metals can be impregnated with tritium so that very large quantities of tritium are located near enough to the surface of the resultant metal-tritium compound so as to supply beta particles that penetrate the space adjoining that surface. The half-life of tritium is sufliciently long to permit prolonged use without adjustments.

Referring now to the drawings the apparatus of this invention will be described in more detail.

In FIG. 1 an ionization chamber 4 fabricated of brass with a wall thickness of inch and an internal volume of 3 cc. is provided with a central electrode 6 which is electrically isolated from the housing by suitable insulation indicated at 7. Inside the ionization chamber is an ionizing particle source 8 comprising a foil of titanium impregnated with tritium. The source foil 8 is in a cylindrical configuration concentric with and contacting the inner wall of the ionization chamber. About 1 curie of H is contained in the foil.

An inlet 10 is provided to introduce the gas to be analyzed to the chamber 4 from a chromatographic gas separation column 9 which may operate on the displacement, elution, or other separation principle. An outlet 12 is provided in the chamber to allow the gas to leave, thus maintaining a flow of the gas to be analyzed.

A potential difference is maintained between the ionization chamber walls 4 and the central electrode 6 by an electric circuit 14 including a voltage source 15 so that the collector 6 is maintained at about 100' volts negative with respect to the foil 8. In the circuit 14 is provided an electrometer 16. The operation of the apparatus is as follows: Gas is introduced into the chamber 4 through the inlet 10. Beta particles emanating from the foil 8 ionize the gas and then impinge upon the Walls of the chamber after traversing the gas. The positive ions created by the ionization of the gas are collected on the central electrode 6 and there is thus created a flow of current which is measured by the electrometer 16. A compensating current is provided for bucking out substantially all of the background current Which is attributable to the ionization of the carrier gas so that the residual signal current measured by the electrometer 16 is related to the concentration of sample gases. This compensating current can be obtained, in one preferred embodiment, from a second ionization chamber (not shown) identical to chamber 4 but containing only the carrier gas. The amount of sample gas in the chamber is thus indicated. Subsequent to ionization, the gas flows out of the chamber 4 through outlet 12, thus maintaining a smooth flow. With this apparatus no detectable radiation occurs outside the walls of the chamber. Since shielding power of material is dependent largely upon mass, it is seen that any material can be used for the walls provided it defines a chamber from the atmosphere and provided that the mass per unit area occurring in the chamber Walls is on the order of that occurring using inch brass in the Wall.

It is preferred that the gas entering the ionization chamber at any instant of time be only a binary combination of the carrier gas (i.e. hydrogen or helium) and a sample gas. Helium and hydrogen have small crosssections for ionization as compared to the cross-sections of gases commonly detected. Since the ion current is dependent upon the cross-section of the total gas being ionized, the ion currents produced from large quantities of helium or hydrogen are small as compared with the current that would be obtained using other carrier gases at the same pressure. With helium or hydrogen as the carrier gas the ion current produced from the ionization of the sample gas evolved from the chromatographic column can most easily be distinguished from the current attributable to the carrier gas.

In the apparatus of FIG. 1 all of the gas flowing from the chromatographic separating means is subjected to the flow of ionizing particles. FIGS. 2 and 3 present another embodiment of this invention which is particularly adaptable to subjecting only a portion of the gas flowing from the chromatographic separating means to beta particle radiation. With this arrangement the nature of a flowing gas is determined by analyzing only a portion of the total flow.

Chromatographic separation means 18 is provided with an outlet tube 20 connected thereto. In a portion of the tube a tritium foil 22 is provided. Beta particles emanating from the tritium foil 22 ionize gas flowing from the separating means 18 through the tube 20. An ion barrier 24 is placed at a distance from the tritium foil equivalent to about one-half the mean range of the beta particles as measured normal to the surface of the foil. (The mean range of beta particles is the average distance the particles will travel through gas, at given pressure conditions, after emanating from the surface and before the energy level of the particles drops to that of thermal electrons.) This mean range is about 2 cm. when the carrier gas is helium at atmospheric pressure. The ion barrier 24 limits the significant ionizing effect of the beta particles to the gas flowing between the foil 22 and the barrier 24, by creating an obstruction to ion movement. Accordingly, that portion of the gas between the foil and the barrier is the only portion from which ions are collected and from which the nature of the flowing gas determined. In a preferred embodiment the barrier is a continuous surface which absorbs all of the beta particles which completely traverse the portion of gas being ionized for analysis. The means for collecting the ions is an electrode 26 inserted between the tritium foil 22 and the ion barrier 24, which together constitute an ionization chamber through which gas flows. An external electrical circuit which is similar to FIG. 1 and includes an electrometer 16 and a voltage source 15 is connected between the electrode 26 and the tube 20 by means of circuit 14.

In preferred embodiments of the invention the distance between the tritium source and the ion barrier should be at least 0.1 of the mean range of the particles emanating from the source but not greater than about 1.5 times this mean range. The dimension selected for any particular chromatographic application will be the result of a compromise between resolution and linearity on one hand and minimum detectable ion current on the other hand. When linearity and resolution are of utmost importance the dimension must be as small as is consistent with generation of a minimum useable ion current signal. When trace quantities are to be detected, a maximum signal must be generated and a larger portion of the ionizing potentiality of the beta particles must be utilized. Accordingly, thelarger chamber dimensions must be used for trace detection.

One significant advantage obtained by the use of tritium as a source of ionizing particles is that when tritium is combined with metals which exothermically occlude hydrogen to form a solid state compound, a source material is produced which can effect a useful increase in the ratio of signal to the noise attributable to random nuclear disintegration of the source which emits the ionizing beta particles. The signal current, I, which is related to the cross-section of the molecules, increases directly with the number, N, of ionizing particles traversing the gas being ionized per unit time. However, current I related to random nuclear disintegrations of the source, increases only by the square root of N. Accordingly, the signalnoise ratio, I /I increases directly with the square root of the number of disintegrations per unit time which supply ionizing particles which create ions in the chamber.

Using tritium, extremely large amounts of disintegrations yielding beta particles can be provided with a resultant increase in the signal to noise ratio. Since tritium, in disintegrating, produces beta particles with maximum energy of 18,000 e.v. and an average energy of about 6,000 e.v., no radiation hazard exists with tritium no matter how large the amounts are that are used inside an ionization chamber. Particles with energy of 18,800 e.-v. do not penetrate the atmosphere more than about 0.2 cm. or more than 2 cm. in helium at atmospheric pressure. Any material which has sufiicient strength to define an ionization chamber will be sufiicient to stop all tritium radiation occurring inside the chamber. X-rays excited by tritium beta particles striking the inner walls of a chamber are extremely soft and will likewise be absorbed by the walls defining the chamber. As an example, no detectable X-ray radiation or beta particles are observed at the surface of a wall of .01 inch of brass with a flat 1 square inch plaque containing 1 curie of tritium bound to zirconium attached to the opposite side of the wall.

Increase in amount of the other radioactive materials commonly used in detectors causes a radiation hazard which can be controlled only by utilizing shields which create a bulky, expensive instrument. Moreover, in attempting to accommodate such other radioactive materials in increased quantities in order to increase signalnoise ratio, the size of chambers is increased which effects detrimental mixing of successive increments of the gas evolved and causes poor resolution of the separated gases flowing from gas separating equipment.

A further advantage achieved in passing extremely low energy beta particles through a given distance is that more ionization occurs per low energy beta particle that passes through the gas being ionized than occurs per high energy particle.

When using tritium a high significant ion current and the absence of radiation hazard without shielding is obtained by placing the tritium within the ionization chamber, there being no Walls or membranes which the emanations must pierce prior to passing through the gas to be analyzed.

Two atoms of tritium can be combined with titanium or zirconium to form TiH or ZrH The theoretical maximum amount of tritium which can be exothermically occluded is dependent upon the valence of the metal. The metals which exhibit the property of exothermically occluding tritium are: lanthanum, cerium, praseodymium, neodymium, saman'am, titanium, vanadium, manganese, zirconium, hafnium, niobium (columbium), palladium, tantalum, thorium, and uranium.

Simple geometric configurations of the emanating source are needed for ease of fabrication and to ensure unimpeded flow of the gas being detected. For this reason, simple cylindrical shapes are preferred. A foil source provides an extremely large surface area with little mass of carrier material which minimizes beta particle absorption by the source itself. With a volume of 3 cc. and a height of 3 cm., a cylindrical ionization chamber provides about cm? available for use as emanating sur- 6 face area. Beta emissions equivalent to over 2 curies can be inserted into a chamber which is useful with some of the smallest chromatographic columns known. In a 3 cc. ionization chamber actually constructed, using a titanium tritide source containing 1 curie of tritium with a source area of about one square inch, and without extensive experimentation to determine optimum adjustments, the signal to noise ratio was sufficiently high to detect 0.1 microgram pentane in helium. Much greater sensitivity is attainable within the spirit of this invention when the maximum area in the ionization chamber is utlized.

One method of utilizing the maximum surface area available is by depositing by vacuum deposition, or other coating means, a coating of one of the metals which exothermically occludes tritium upon the inner walls of an ionization chamber and thereafter impregnating the coating with tritium.

As has been pointed out, the use of tritium as a source of ionizing particles in a detector for use with gas chromatographic separation equipment gives distinct advantages even if the signal-noise ratio of the detector is the same as that of other detectors utilizing radioactive material. These advantages are the lessening of the health hazard and the increased significant ion current. A detector which does not detect micrograms of a material such as pentane being eluted from a column would not be very useful. To provide a low enough noise level to permit detection of that amount, at least 10 tritium emanated beta particles per second traversing the ionization chamber are required. This invention is therefore limited to chambers having at least 10 beta particles traversing the chamber per second.

Though in the preferred embodiments 'shown the cen tral collecting electrode has been described as being maintained at a potential negative in relation to the walls of the chamber, like results can be obtained by maintaining the chamber walls at a negative potential in relation to an electrode whereby the positive ions are collected at the walls.

Since certain changes may be made in the above apparatus and method without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Means for analyzing gas, to be used with a chromatographic gas separation column having a cross-section of flow less than one square inch from which is obtained individual components of the gas being analyzed each in a binary combination with a gas with a small cross-section for ionization as compared to the individual component, comprising a means to successively conduct said binary combinations to a detector, and a detector through which said binary combinations successively flow, said detector comprising an ionization chamber with a volume of less than 10 cc. so shaped that no appreciable mixing of successive increments of the binary combination occurs, said chamber being provided with ion collecting electrodes and electrical circuitry to collect any ions produced, means to measure ion current in said circuitry, and a source of beta particles within said chamber to successively ionize each of said binary gas combinations and produce ions of said combinations, said source comprising at least 0.1 curie of tritium in combination with at least one metal with which tritium is exothermically occluded, said chamber and source being so spacially related that the path length of most of the beta particles moving in the gas in the chamber is less than the mean range of the beta particles emanating from the source.

2. An apparatus for analyzing a mixture of gases which comprises a chromatographic gas separation means and means for flowing the separated gases successively through a beta particle ionization sensor, said beta particle ionization sensor comprising a gas tight chamber, means for creating a flow of gas through said chamber, means to ionize, said gas, means to collect all the ions produced from said gas and means to measure the current resulting from-the collections of said ions, said gas tight chamber having a volume of no more than 10 cc. and' said ionizing means being located Within said chamber and comprising tritium in combination with zirconium.

3. An apparatus for analyzing a mixture of gases which comprises a chromatographic gas separation means and means for flowing the separated gases successively through a beta particle ionization sensor, said beta particle ionization sensor comprising a gas tight chamber, means for creating a flow of gas through said chamber, means to ionize said gas, means to collect all the ions produced from said gas and means to measure the current resulting from the collection of said ions, said gas tight chamber having a volume of no more than 10 cc. and said ionizing means being located within said chamber and comprising tritium in combination with titanium.

4. An apparatus for analyzing a mixture of gases which comprises a chromatographic gas separation means and means for flowing the separated gas successively through a beta particle ionization sensor, said beta particle ionization sensor comprising a gas tight chamber, means for creating a flow of gas through said chamber, means to ionize said gas, means to collect all the ions produced from said gas and means to measure the current resulting from the collection of said ions, said gas tight chamber having a volume of no more than 10 cc. and said ionizing means being located Within said chamber and comprising tritium in combination with hafnium.

References Cited in the file of this patent UNITED STATES PATENTS 2,387,550 Winkler Oct. 23, 1945 2,641,710 Pompeo June 9, 1953 2,740,894 Deisler et a1 Apr. 3, 1956 2,761,976 Obermaier Sept. 4, 1956 OTHER REFERENCES Thompson: Biological Applications of Tritium, Nucleonics, vol. 12, No. 9, pages 31-35, September 1954.

Deal et al.: A Radiological Detector for Gas Chromatography, Analytical Chemistry, vol. 28, No. 12, December 1956, pages 1958-1964.

Brown et al.: Tritium As a Tool for Industrial and Chemical Research, Peaceful Uses of Atomic Energy, United Nations, vol. 15, pages 16-23, August 1955.

Wallhausen: Use of Radioisotopes in the Production of Self-Luminous Compounds, Peacefiul Uses of Atomic Energy, United Nations, vol. 15, pages 307-309, August 1955.

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