US3626180A

US3626180A - Apparatus and methods for separating, detecting, and measuring trace gases with enhanced resolution

Info

Publication number: US3626180A
Application number: US780851A
Authority: US
Inventors: David I Carroll; Martin J Cohen; Roger F Wernlund
Original assignee: Franklin Gno Corp
Current assignee: PCP Inc A CORP OF FLORIDA
Priority date: 1968-12-03
Filing date: 1968-12-03
Publication date: 1971-12-07
Anticipated expiration: 1988-12-07

Abstract

Apparatus and methods for sorting and detecting trace gases which undergo ion-molecule reactions, trace ions being formed in a reactive gaseous medium and being analyzed in a nonreactive gaseous medium. The ions are classified in accordance with their velocity in an electric drift field.

Description

United States Patent David 1. Carroll Lantana;

Martin J. Cohen, West Palm Beach; Roger F. Wernlund, Lake Worth, allot Fla. 780,851

Dec. 3, 1968 Dec. 7, 1971 Franklin GNO Corporation West Palm Beach, Fla.

Inventors Appl. No. Filed Patented Assignee APPARATUS AND METHODS FOR SEPARATING, DETECTING, AND MEASURING TRACE GASES WITH ENHANCED RESOLUTION 22 Claims, 3 Drawing Figs.

U.S. Cl 250/419 TF, 250/41.9 G

Int. Cl l-I0lj 39/34i Field of Search 250/41.9 G,

SYNC PULSER References Cited UNITED STATES PATENTS 11/1956 Washbum 250/41.9 (1) 10/1965 Fox eta1...... 250/43.5 X 5/1966 Fite et al... 250/41.9 ISB 1/1968 Gregory 324/33 Primary Examiner-James W. Lawrence Assistant Examiner-A. L. Birch Attorney-Raphael Semmes ABSTRACT: Apparatus and methods for sorting and detecting trace gases which undergo ion-molecule reactions, trace ions being formed in a reactive gaseous medium and being analyzed in a nonreactive gaseous medium. The ions are classified in accordance with their velocity in an electric drift field.

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Milliseconds MARTIN J. COHEN ROGER F. WERNLUND ATTORNEY APPARATUS AND METHODS FOR SEPARATING, DETECTING, AND MEASURING TRACE GASES WITH ENHANCED RESOLUTION BACKGROUND OF THE INVENTION This invention relates to apparatus and methods of ion-classification and more particularly is concerned with improving the resolution of measurements performed upon trace gases which undergo ion-molecule reactions.

The copending application Ser. No. 777,964, of Martin J. Cohen, David I. Carroll, Roger F. Wernlund, and Wallace D. Kilpatrick, filed Oct. 23, 1968 and entitled Apparatus and Methods for Separating, Concentrating, Detecting and Measuring Trace Gases, discloses Plasma Chromatography systems involving the formation of primary or reactant ions and the reaction of the primary ions with molecules of trace substances to form secondary or product ions, which may be concentrated, separated, detected, and measured by virtue of the difference of velocity or mobility of the ions in an electric field. The primary ions may be produced by subjecting the molecules of a suitable host gas, such as air, to ionizing radiation, such as beta rays from a tritium source, corona from a multipoint or wire array, electrons produced by photoemission from a cathode, etc. The primary ions are then subjected to an electric drift field, causing them to migrate in a predetermined direction through a reaction space into which the sample or trace gas is introduced. The resultant collisions between primary ions and the trace gas molecules produce secondary ions of the trace gas in much greater numbers than can be produced by mere electron attachment, for example, to the trace gas molecules. The secondary ions are also subjected to the electric drift field and may be sorted in accordance with their velocity or mobility. The specific systems of the said copending application employ ion shutter grids or gates for segregating the ion species in accordance with their drift time. The pressure of the gas in the Plasma Chromatograph cell is maintained high enough (preferably atmospheric) to ensure that the mean free path length of the ions is very much smaller than the dimensions of the cell. 7

A possible limitation upon the Plasma Chromatography technique disclosed in the said copending application results from the fact that the ion-molecule interactions may not stabilize" or reach completion. The apparent mobilities measured may be the result of ion species changing identity one or more times during their transit through the velocity analysis region of the drift cell. The apparent mobilities measured under these conditions are a rapidly varying function of trace concentration and cannot always be uniquely identified with the trace material. Moreover, it appears that the additional interactions which occur in the analysis region are due to the trace material itself, rather than to the carrier gas.

BRIEF DESCRIPTION OF THE INVENTION It is accordingly a principal object of the invention to provide improved apparatus and methods for separating and detecting molecular quantities of trace substances with greater resolution and reliability than has heretofore been possible, and to quench undesirable ion-molecule reactions.

Briefly stated, preferred embodiments of the apparatus and methods of the invention are concerned with Plasma Chromatography systems which involve the formation of positive or negative ions by reactions between the molecules of trace substances and primary ions. The secondary ions are separated and detected in a drift cell by utilizing the difference in velocity or drift time of ions of different mass in an electric field. In order to obtain the improved results of the invention, ion formation occurs in a reactive gaseous medium, and ion drift velocity (mobility) measurements are performed in a nonreactive gaseous medium.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and wherein:

FIG. I is a longitudinal sectional view, somewhat diagrammatic, illustrating a trace gas detector system of the invention;

FIG. 2 is a similar view illustrating a modified Plasma Chromatography cell which may be employed in the invention; and

FIG. 3 is a graph illustrating a typical output curve obtained by virtue of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. I, Plasma Chromatography cell 10 in accordance with the invention may comprise a gastight envelope l2 enclosing a series of electrodes, which may be of plane parallel geometry, for example. Principal electrodes I4 and 16 may be arranged adjacent to opposite ends of the envelope, which may be a circular cylinder. When the apparatus is used to detect negative ions, as will be assumed for example, electrode 14 will be a cathode and electrode 16 an anode. When the apparatus is used to detect positive ions, the polarities will be reversed. As described in the said copending application, the Plasma Chromatography cell preferably includes a pair of shutter grids or

ion gates

18 and 20, each of which comprises two sets of interdigitated parallel wires, alternate wires of each grid being connected together to form the two sets. Cathode 14 or the region of the envelope near this electrode is pro vided with an ionizing source, which may be of the type mentioned previously. Anode 16 may be a collector plate connected to an output device, such as electrometer 22, which may be Cary Instruments Model 401 (vibrating reed) type with current sensitivity of 10" ampe'res at a time constant of 300 milliseconds.

An electric drift field is provided between the principal electrodes I4 and 16. In the form shown the source of the drift field is a chain of batteries 24, the negative end of the chain being connected to the cathode I4 and the positive end to ground. Anode I6 is connected to ground through the input circuit of the electrometer 22. Alternatively, a resistor chain voltage divider may be employedin conjunction with a battery connected across the chain. Taps on the chain are connected to a series of guard rings 26 spaced along the length of the envelope 12, which maintain the uniformity of the drift field.

Adjacent elements of each

shutter grid

18 and 20 are normally maintained at equal and opposite potentials relative to a grid average potential established by the battery chain 24. Under these conditions the shutter or gate is closed to the passage of electrically charged particles. Potential sources which provide the equal and opposite potentials just referred to may be considered to be part of grid drive circuits within

blocks

28 and 30 entitled Sync Pulser and Delayed Pulse." The components of these blocks are effective to drive the adjacent elements of each shutter grid to the same potential, the grid average potential, at predetermined instants, alternate grid wires being shown connected to the battery chain by

re sisters

32 and 34 to establish the grid average potentials.

A sample comprising a suitable host or carrier gas, such as air, carrying an appropriate gaseous trace substance, such as triethyl phosphite, for example, flows into the envelope by means of a gas inlet pipe 36 at one end of the envelope. Inlet pipe 36 may terminate in an opening through the cathode I4, which may include an ionizing source, such as tritium foil, indicated at 38. An outlet pipe for all gas within the envelope is indicated at 40 and may be partially concentric with inlet 36. Portions of the cathode may be porous to permit gas to flow therethrough. A nonreactive or inert buffer gas, such as nitrogen, enters the envelope through an inlet pipe 42 at the opposite end of the envelope. This gas, which may be termed the ion velocity sorter gas, fills up most of the cell, and in particular, the region from the first shutter grid 18 to collector electrode or anode 16. Any suitable sources of flow pressure, such as a fan or pump, may be employed to move the gases through

inlets

36 and 42 into the envelope.

In the region between the cathode I4 and first grid 18, closely adjacent to the radioactive cathode 14, primary ions of the reactive carrier gas, or one or more of the main constituents thereof, such as oxygen, are formed under the influence of the ionizing source at this region. For example, negative oxygen ions may be formed at cathode 14, as by direct attachment of electrons to the oxygen molecules of the air host gas, the sample being subjected to beta rays produced by the tritium foil cathode. The primary ions drift toward the anode 16, and in the reaction space between the cathode l4 and the first shutter grid 18 the primary ions encounter other molecules. A majority of these collisions will be with oxygen, nitrogen, or other nonreactive molecules. A small fraction of the collisions will be with the trace molecules of interest. In these cases the primary ions will interact with the trace molecules to form secondary ions. The secondary ions, will have, in general, an appreciable difference in mobility from the primary ions. These ion-molecule reactions take place in the region near the radioactive foil cathode as the gaseous sample passes out of the inlet tube 36. The volume of inert gas flow from inlet 42 is kept large relative to the sample gas flow from inlet 36 (for example, at least twice as large) to ensure that the cell is primarily filled with inert gas. Thus, the ionmolecule reaction region is quite limited in volume, but nevertheless there is sustained ion-molecule reaction in this region.

The ion flux at the first shutter grid 18 will consist of both the primary ions and possibly several species of secondary ions. A sample of this mixed ion population is periodically admitted to the drift region between the first and second shutter grids when the first shutter grid 18 is opened by momentarily driving all of the grid wires to the grid average potential. The second shutter grid 20 is opened momentarily at a predetermined time after the opening of the first grid. The ions that pass through the second grid drift toward and are collected by the anode l6, and the resultant output current is integrated over several cycles to give a measurable current. By scanning the time of opening of the second grid relative to the first, a drift time spectrum of the ion population can be obtained in the output and recorded to produce an output curve (current vs. drift time) as shown in FIG. 3. This permits the various ion species to be separated and identified.

Because of the presence of the inert buffer gas within the ion velocity analysis region, high resolution is obtained, as can be readily observed in the output curve illustrated in FIG. 3. A total of separate ion peaks is resolved. Interference from moisture in the sample, which heretofore has tended to blur or obscure desired trace gaspeaks, is eliminated. The apparent mobilities are no longer a function of trace material concentration. Moreover, since the reactive volume is greatly reduced, the effective time constant is also greatly decreased. Effective time constants of the order of 20 to 100 milliseconds can be realistically achieved.

FIG. 2 illustrates a modified form of Plasma Chromatography cell which may be employed in the invention. Parts corresponding to those of FIG. 1 have been designated by the same reference numerals with the addition of a prime. While the sampling grid 18' and the timing grid 20 have been designated diagrammatically as single grids, the same dual grid structures as those previously described are employed. A small, slow stream of sample gas is directed through a small inlet tube 36' which enters the envelope from the side and exits near the cathode 14 in the region between the cathode and sampling grid 18. An ionizer 38 (such as tritium) is located in the tube 36 near its exit. lens are present in the sample gas stream from the inlet tube 36' because of primary ion formation and ion-molecule reactions which occur in the sample gas input tube. The nonreactive buffer gas enters the envelope at 42, and all gases exit from the envelope at 40'.

the gas flow, which may be of the order of 50 centimeters per second, for example. Because of the small active or sample volume of the invention, the invention is ideally suited for gas chromatograph detector use. For such use with the apparatus of FIG. 1, for example, the effiuent from the gas chromatograph (inert carrier plus trace gas) is inserted into the inlet 36. A separate reactive gas for formation of primary ions may also be inserted into the inlet tube.

The invention is not restricted to velocity analysis by means of ion shutter grids, but is also applicable, for example, to analysis involving gas flow as a parameter, as set forth in the copending application of Martin J. Cohen, David I. Carroll, and Roger F. Wernlund, filed Nov. 26, 1968, and entitled "Apparatus and Methods for Separating, Detecting, and Measuring Trace Gases." For such application the ions are injected into an inert gas stream in a duct after formation in a separate chamber.

While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.

The invention claimed is:

l. A method of detecting a trace material in a gaseous sample, which comprises reacting said trace material with primary ions to form secondary ions of said material at a first region, analyzing said secondary ions in an inert gaseous medium at a second region, and maintaining the length of the mean free path of the ions at said regions very much smaller than the dimensions of the regions.

2. A method in accordance with claim 1, in which the analyzing of said ions comprises subjecting said ions to a drift field at said second region.

3. A method in accordance with claim 1, and in which the analyzing of said ions comprises electrically sampling a portion of the ions at said second region.

4. A method in accordance with claim 3, in which said sampling comprises admitting a group of said ions to a first location in said drift field and thereafter admitting a portion of said group to a second location in said drift field for detection.

5. A method in accordance with claim 1, in which the primary ions are formed by ionizing a gaseous carrier of said trace material.

6. A method in accordance with claim 5, in which said regions are in gas communication and said carrier is supplied to said first region at a rate substantially less than that at which said inert gaseous medium is supplied to said second region.

7. A method in accordance with claim 2, in which said inert gaseous medium is caused to flow through said second region in a direction opposite to the drift of said ions under the influence of said drift field.

8. Apparatus for detecting the presence of a substance in a gaseous sample, which comprises an envelope, means for forming ions from said sample at a first region in said envelope by ion-molecule reactions, means for applying a drift field to the ions in said envelope to cause them to drift in a second region, means for producing an electrical output from said envelope in accordance with the velocity of predetermined ions in said drift field, and means for supplying an inert gas to said second region, said regions being in substantially unrestricted gas communication with each other, and the length of the mean free path of said ions in said envelope being very much smaller than the dimensions of said envelope.

9. Apparatus in accordance with claim 8, in which said means for forming ions from said sample comprises means for forming primary ions from the molecules of a first gas and for reacting said primary ions with the molecules of said substance to form secondary ions.

10. Apparatus in accordance with claim 8, in which said means for applying said drift field comprises a pair of spaced electrodes in said envelope, one of which is adjacent to said first region and the other of which is adjacent to said second region, said means for forming said ions being adjacent to said one electrode and said means for producing an electrical output comprising said other electrode.

11. Apparatus for detecting the presence of a substance in a gaseous sample, which comprises an envelope having a pair of spaced principal electrodes therein, one of said electrodes having associated therewith ionizing means and the other of said electrodes constituting an output electrode, means for establishing an electric field between said electrodes, means for introducing a gaseous sample into said envelope at the region of said one electrode, and means for introducing an inert gas into the space between said electrodes, said space being substantially unrestricted for gas flow between said electrodes and the length of the mean free path of ions of said sample in said envelope being very much smaller than the dimensions of said envelope.

12. Apparatus in accordance with claim Ill, further comprising outlet means for withdrawing gas from said envelope.

13. Apparatus in accordance with claim 12, said outlet means being closer to said one electrode than to said other electrode.

14. Apparatus in accordance with claim 111, said means for introducing said sample into said envelope comprising a pipe terminating in the vicinity of said one electrode.

15. Apparatus in accordance with claim 14, said ionizing means being located within the end portion of said pipe.

l6. Apparatus in accordance with claim 14, said ionizing means being located exteriorly of and adjacent to the end portion of said pipe.

17. Apparatus in accordance with claim lll, further comprising a pair of ion gates arranged in succession between said electrodes.

18. Apparatus in accordance with claim 17, further c0mprising means for opening said ion gates in succession.

19. Apparatus in accordance with claim lll, wherein said means for introducing said sample comprises means for supplying a trace gas in an inert carrier gas and for introducing a separate reactive gas.

20. Apparatus in accordance with claim 11, said electrodes being located adjacent to opposite ends of said envelope, said means for introducing said sample and said inert gas into said envelope comprising pipes terminating in said envelope adjacent to said opposite ends.

21. Apparatus in accordance with claim 20, further comprising a gas outlet pipe leading from said envelope adjacent to said pipe for introducing said sample.

22. A method in accordance with claim l, in which said secondary ions, together with unreacted primary ions, are analyzed in an inert gaseous medium at a second region.

Claims

2. A method in accordance with claim 1, in which the analyzing of said ions comprises subjecting said ions to a drift field at said second region.
3. A method in accordance with claim 1, and in which the analyzing of said ions comprises electrically sampling a portion of the ions at said second region.
4. A method in accordance with claim 3, in which said sampling comprises admitting a group of said ions to a first location in said drift field and thereaftEr admitting a portion of said group to a second location in said drift field for detection.
5. A method in accordance with claim 1, in which the primary ions are formed by ionizing a gaseous carrier of said trace material.
6. A method in accordance with claim 5, in which said regions are in gas communication and said carrier is supplied to said first region at a rate substantially less than that at which said inert gaseous medium is supplied to said second region.
7. A method in accordance with claim 2, in which said inert gaseous medium is caused to flow through said second region in a direction opposite to the drift of said ions under the influence of said drift field.
8. Apparatus for detecting the presence of a substance in a gaseous sample, which comprises an envelope, means for forming ions from said sample at a first region in said envelope by ion-molecule reactions, means for applying a drift field to the ions in said envelope to cause them to drift in a second region, means for producing an electrical output from said envelope in accordance with the velocity of predetermined ions in said drift field, and means for supplying an inert gas to said second region, said regions being in substantially unrestricted gas communication with each other, and the length of the mean free path of said ions in said envelope being very much smaller than the dimensions of said envelope.
9. Apparatus in accordance with claim 8, in which said means for forming ions from said sample comprises means for forming primary ions from the molecules of a first gas and for reacting said primary ions with the molecules of said substance to form secondary ions.
10. Apparatus in accordance with claim 8, in which said means for applying said drift field comprises a pair of spaced electrodes in said envelope, one of which is adjacent to said first region and the other of which is adjacent to said second region, said means for forming said ions being adjacent to said one electrode and said means for producing an electrical output comprising said other electrode.
11. Apparatus for detecting the presence of a substance in a gaseous sample, which comprises an envelope having a pair of spaced principal electrodes therein, one of said electrodes having associated therewith ionizing means and the other of said electrodes constituting an output electrode, means for establishing an electric field between said electrodes, means for introducing a gaseous sample into said envelope at the region of said one electrode, and means for introducing an inert gas into the space between said electrodes, said space being substantially unrestricted for gas flow between said electrodes and the length of the mean free path of ions of said sample in said envelope being very much smaller than the dimensions of said envelope.
12. Apparatus in accordance with claim 11, further comprising outlet means for withdrawing gas from said envelope.
13. Apparatus in accordance with claim 12, said outlet means being closer to said one electrode than to said other electrode.
14. Apparatus in accordance with claim 11, said means for introducing said sample into said envelope comprising a pipe terminating in the vicinity of said one electrode.
15. Apparatus in accordance with claim 14, said ionizing means being located within the end portion of said pipe.
16. Apparatus in accordance with claim 14, said ionizing means being located exteriorly of and adjacent to the end portion of said pipe.
17. Apparatus in accordance with claim 11, further comprising a pair of ion gates arranged in succession between said electrodes.
18. Apparatus in accordance with claim 17, further comprising means for opening said ion gates in succession.
19. Apparatus in accordance with claim 11, wherein said means for introducing said sample comprises means for supplying a trace gas in an inert carrier gas and for introducing a separate reactive gas.
20. Apparatus in accordance with claim 11, said electrodes being located Adjacent to opposite ends of said envelope, said means for introducing said sample and said inert gas into said envelope comprising pipes terminating in said envelope adjacent to said opposite ends.
21. Apparatus in accordance with claim 20, further comprising a gas outlet pipe leading from said envelope adjacent to said pipe for introducing said sample.
22. A method in accordance with claim 1, in which said secondary ions, together with unreacted primary ions, are analyzed in an inert gaseous medium at a second region.