WO1988005535A2

WO1988005535A2 - Ion mobility detector

Info

Publication number: WO1988005535A2
Application number: PCT/US1987/003178
Authority: WO
Inventors: Joseph E. Roehl
Original assignee: Allied Corporation
Priority date: 1987-01-27
Filing date: 1987-12-03
Publication date: 1988-07-28
Also published as: WO1988005535A3

Abstract

An ion mobility detector (10) for measuring the mobility of ions incorporating a cylindrical shutter grid (18) concentric with a cylindrical collector (20) having an electric field radially therebetween. The invention overcomes the problem of recombination at exposed sidewalls of the drift region. The invention further overcomes the problem of sensitivity by maximizing the area of the shutter grid (18) and collector (20).

Description

Ion Mobility Detector Background of the Invention

Field of the Invention:

This invention relates to ion mobility detectors and more particularly to an ion mobility cell.

Description of the Prior Art:

An ion mobility detector (IMD) or ion mobility spectrometer (IMS) is an apparatus for identifying chemical compounds by ionizing an unknown gas sample and allowing it to drift in an electric field thus separating out various ionic species by their physical properties such as scattering cross section. Ion mobility detectors, as they are designed today, consist of a reaction region where the gas sample is ionized, a shutter grid which turns the ion flow on and off, a drift tube which is a cylindrical tube where a uniform field is maintained and a collector plate. Often an aperture grid is placed in front of the plate to shield it from the charge applied to the shutter grid.

One example of a cylindrical drift tube is shown in U. S. Patent 4,378,499, which issued on March 29, 1983 to G. E. Spangler et al. In U. S. Patent 4,378,499 the drift region is formed in a cylindrical shape by a plurality of concentric rings which are at a plurality of potentials and spaced apart from one another along a longitudinal axis to form a uniform electric field in the direction of the longitudinal axis formed by the concentric rings. The shutter grid is at one end of the cylinder and the collector is positioned at the other end.

A second example of a drift tube is shown in U. S. Patent 4,390,784, which issued on June 28, 1983 to D. R. Browning et al. In U. S. Patent 4,390,784 a film resistor was disposed on the inside of a non-conductive tube such as a ceramic tube. A potential was placed across the film resistor to provide an electric field in the direction of the longitudinal axis of the ceramic tube. The shutter grid was positioned at one end of the ceramic tube and the collector was positioned at the other end. A drift tube having a cylindrical geometry is convenient when theoretical calculations are to be made and compared with laboratory data, however, the cylindrical geometry has several limitations: ions collide with the cylindrical wall in the drift tube and are lost; and for a given volume of a cylindrical tube, the area of the collector plate and shutter grid is quite limited. These limitations reduce the sensitivity of the ion mobility detector although they allow precise control over the electric field in the drift tube.

Summary of the Invention

An apparatus is described for measuring the mobility of ions comprising a housing enclosing a drift region, the drift region having a shutter grid and collector spaced apart, the shutter and collector being concentric about a longitudinal axis, a passageway for introducing the ions to a first side of the shutter away from the collector, a voltage supply for applying a potential whereby an electric field between the shutter grid and the collector is generated therebetween, and a circuit for applying a first potential at first times to the shutter grid to hold the ions and for applying a second potential at second times to allow the ions to pass through the shutter grid, a current or voltage amplifier having an input coupled to the collector for detecting the time of arrival of the ions at the collector.

It is an object of this invention to provide an ion mobility detector wherein the ions drift radially between two concentric cylinders.

It is a further object of this invention to provide an ion mobility detector having a drift region geometric configuration which maximizes the area of the shutter grid and collector plate with respect to the side walls therebetween.

Brief Description of the Drawing

Fig. 1 is a perspective view of one embodiment of the invention.

Fig. 2 is a cross section and perspective view of a portion of the embodiment of Fig. 1. Fig. 3 is a perspective view of a collector.

Fig. 4 is a perspective view of an aperture grid. Fig. 5 is a perspective view of a shutter grid. Fig. 6 is a perspective view of a portion of the inside of the embodiment of Fig. 1. Fig. 7 is a block diagram of a ion mobility detector. Fig. 8 is an alternate embodiment of the invention.

Description of the Preferred Embodiment

Referring to Fig. 1, an ion mobility detector cell 10 is shown. Ion mobility detector cell 10 has a housing 12 enclosing a drift region 14 having a longitudinal axis 16. Drift region 14 has shutter grid 18, which is positioned axially along longitudinal axis 16 and may be, for example, concentric with longitudinal axis 16. Drift region 14 also has a collector 20 positioned axially along longitudinal axis 16.

Collector 20 may be concentric with longitudinal axis 16 to provide a uniform spacing between the surface of shutter grid 18 and the surface of collector 20 in the radial direction from longitudinal axis 16. Between shutter grid 18 and collector 20 may be positioned an aperture grid 22 which functions to provide an electrostatic shield to collector 20 to isolate collector 20 from the charge at shutter grid 18 and in the drift region 14 between shutter grid 18 and aperture 22. The shutter grid may be positioned at a radius Rl shown by arrow 23. Collector 20 may be positioned at a radius R2 shown by arrow 24 and aperture grid 22 may be positioned at a radius R3 shown by arrow 25.

A sample gas 28 having gas constituents to be detected and carrier gas 26 may enter inlet 29 having a passageway 30 in housing 12 leading to reaction region 32. Reaction region 32 may contain an ionizing source for ionizing atoms in carrier gas 26 and sample gas 28 to form ion products which leave outlet 33 and pass into drift region 14 along longitudinal axis 16 and inside of shutter grid

18. An ionization source in reaction region 32 may be,

63 for example, nickle foil. An alternate ionization source in reaction region 32 may be, for example, an alkali salt which may be heated to a predetermined temperature and exposed to gas 28.

A drift gas 36 enters inlet 38 and flows through passageway 39 in housing 12. Passageway 39 is coupled to an annular distribution channel 40 shown in Fig. 2 for releasing drift gas 36 into drift region 14. Gas 36 flows axially along longtudinal axis 16 and through housing 12 via passageway 44 and out outlet 45. In drift region 14 the gas flow of carrier gas 26, sample gas 28 and drift gas 36 should be laminer and travel in a path in the direction of longitudinal axis 16 as shown by arrows 46 and 47.

A voltage supply having a predetermined potential may be coupled to shutter grid 18 with respect to collector 20 to cause ions of either positive or negative polarity to move at times shutter grid 18 is pulsed open to collector 20. Aperture grid 22 may be placed at a potential of a few volts such as 5 volts above the potential of collector 20. Collector 20 may be at ground potential. A voltage amplifier or current amplifier may be coupled to collector 20 which is isolated from ground to provide an indication of the arrival of ions at collector 20.

In operation, a carrier gas 26 and sample gas 28 enter inlet 29 to reaction region 32 wherein product ions are formed which are carried by the gas stream shown by arrow 46 into the region inside shutter grid 18. Shutter grid 18 may be pulsed to allow ions to pass through shutter grid 18 and travel radially toward collector 20. The time of arrival of various ions at collector 20 is an indication of the ion mobility. While ions are drifting toward collector 20, drift gas 36 enters inlet 38 through passageway 39 and channel 40 into drift region 14 which provides a gas flow transverse to the direction of ion movement in drift region 14 as shown by arrow 47. The ions when neutralized by collector 20 are carried out of drift region 14 with drift gas 36 through passageway 44 and outlet 45. Drift gas 36 and carrier gas 26 may be, for example, purified air, dry nitrogen or a gas of predetermined composition to enhance positive or negative ion response at collector 20.

Fig. 2 is a cross section view of end 50 of housing 12. End 50 may have a surface 51 having channels 52—54 for receiving one end of shutter grid 18, aperture 22 and collector 20, respectively. Channels 52-54 may be circular on surface 51. End 50 may be non-conducting or end 50 may be conducting with channels 52-54 having an insulating layer or coating therein. End 50 may have a cylindrical portion 56 with threads 57 formed thereon for permitting the mating portion of housing 12 to be threaded thereon to form a rigid housing 12. Reaction region 32

63 may include a strip of nickle foil 58. Annular channel 40 has a plurality of openings 60 to surface 51 to permit even and adequate air flow of drift gas into drift region 14. Fig. 3 is a perspective view of a collector 20. Collector 20 may be, for example, cylindrical in shape, and made of conductive material such as stainless steel having a radius R_. One end of collector 20 will mate with channel 54 in end 50. Fig. 4 is a perspective view of aperture grid 22. Aperture grid 22 is made of conducting material such as stainless steel which may have a cylindrical shape having one end suitable for inserting into channel 53. Aperture grid 22 may be formed from a sheet of stainless steel by removing panels by stamping or etching to form a grid structure having parallel members which when positioned in ion mobility detector 10 are parallel to longitudinal axis 16. The cylindrical shape of aperture 22 may be formed by rolling the flat grid sheet into a cylinder and welding two opposite edges together. The thickness of the material in aperture grid 22 should be sufficient to provide a rigid aperture in ion mobility detector 10. Fig. 5 is a perspective view of shutter grid 18 having a cylindrical shape and having one end suitable for positioning in channel 52 shown in Fig. 2. Shutter grid 18 may be formed from conductive material such as stainless steel having alternating members 61 coupled electrically together and insulated from the other alternating members 62 which are electrically connected together. Shutter grid 18 is made of material of appropriate thickness to form a rigid aperture grid when positioned in ion mobility detector 10. Fig. 6 is a top view of end 50 from the inside of the embodiment in Fig. 1. Fig. 6 shows entire surface 51 of end 50.

Fig. 7 is a block diagram of ion mobility detector 64. A purified carrier gas 65 leaves scrubber 66 over duct 67 and duct 68 to inlet 38 of ion mobility detector cell 10. Purified gas 65 from scrubber 66 also travels over duct 67 and 69 to one side of membrane 70, the other side exposed to an ambient gas 71. Constituents of a gas 71 pass through membrane 70 into purified gas 65 to become gas sample 28 which passes over duct 74 to inlet 29 of ion mobility detector cell 10. One example of a membrane interface for an ion mobility detector cell is described In U. S. Patent 4,311,699, which issued on January 19, 1982 to G. E. Spangler which is assigned to The Bendix Corporation now merged into Allied Corporation. U. S. Patent 4_Λ 311,669 is incorporated herein by reference.

Gas 76 at outlet 45 of ion mobility detector cell 10 passes through duct 77 to an inlet of pump 78. The outlet of pump 78 is coupled over duct 79 to an inlet of scrubber 66. Pump 78 functions to draw gas through ion mobility detector cell 10 and to recirculate gas 76 through scrubber 66. 5 Power supply 82 which is referenced to ground potential over lead 83 provides voltages V over lead 84 to shutter grid 18 of ion mobility detector cell 10. Power supply 82 also provides volage V over lead 85 to aperture grid 22 of ion mobility detector cell 10.

10 Collector 20 of ion mobility detector cell 10 is coupled to detector circuit 92 over lead 87 to an input of amplifier 88 and through resistor 89 to lead 90 which is coupled to the output of amplifier 88. A second input of amplifier 88 is coupled over lead 91 to ground potential.

15 Amplifier 88 which may be, for example, a differential amplifier, functions to amplify the current or voltage on lead 87 to provide an output on lead 90. Housing 12 is coupled to ground potential over lead 93. Collector 22 is usually at ground potential but isolated from ground via

20 amplifier 88. The electric field in drift region 14 of ion mobility detector cell 10 is generated between shutter grid 18 and aperture grid 22 by the difference in voltages V _G and V . The ion mobility of positive ions may be detected by having voltage V_ςr positive with respect to^'

25 voltage V . The ion mobility of negative ions may be detected by having the V_ςr negative with respect to voltage V_A. In both cases, collector 20 is at or near ground potential such as 0 volts plus or minus one volt.

Shutter grid 18 functions to prevent ions from - 30 entering drift region 14 until a pulse is received from grid pulse generator 80 over lead 86. Shutter grid 18 may consist of a cylindrical array of parallel members 61 and 62 with every other member 61 in electrical contact with each other and to lead 86. Alternately, shutter grid 18 35 may be a cylindrical shaped shutter grid consisting of two grids displaced from each other in the radial direction from longitudinal axis 16. The other members 62 are coupled together and to lead 84. When grid pulse generator 80 provides a first voltage to lead 86, ions generated in reaction region 32 are collected by the grid wires and are not allowed to enter drift region 14. When grid pulse generator 80 momentarily provides a second voltage to lead 86, ions generated in reaction region 32 are allowed to enter drift region 14 without being collected by the grid wires. Grid pulse generator 80 is referenced to voltage V_SG by means of lead 84. Grid pulse generator 80 may be freerunning or may receive a timing control signal over lead 81.

The output of grid pulse generator 80 is coupled over lead 95 to an input of clock 96 which measures the time elapsed since the last trigger pulse and provides a signal on lead 97. When the lapsed time on lead 97 is correlated with the appearance of ion current as sensed by collector 20 and detector circuit 92.,-the -mobility for various ions to drift through drift region 14 may be measured. Alternatively, the ion mobility spectrum as a function of time can be taken with an oscilloscope or similar equipment. The inverse of the elapsed time is a measure of the mobility of the ions. Due to the geometry of drift region 14, the electric field strength may be a function of distance from shutter grid 18 such as 1/R where R is the distance from longitudinal axis 16.

Fig. 8 is an alternate embodiment of the invention. In Fig. 8, like references are used for functions corresponding to the apparatus of Fig. 1. In Fig. 8, a wire or rod 98 is positioned on one side of shutter grid 18 such as along longitudinal axis 16 to provide an electric field to cause ions leaving reaction region 32 to flow toward shutter grid 18. Wire or rod 98 may be coupled to a voltage source to place either a positive or negative potential on wire or rod 98 depending upon whether positive or negative ions are desired. Wire or rod 98 should have a potential which is either more negative or more positive than shutter grid 18. Wire or rod 98 may be positioned along longitudinal axis 16 by support 99.

An ion mobility detector has been described for measuring the mobility of ions comprising a housing enclosing a reaction region and a drift region, the drift region having a shutter grid and collector spaced apart, the shutter grid and collector being concentric about a longitudinal axis, the reaction region includes a means for generating ions such as a radioactive source, a passageway to couple ions to a first side of the shutter grid which is on the side away from the collector, a volage potential for generating an electric field between

^■ the shutter grid and the collector, a grid pulse generator for applying a first potential at first times to the shutter grid to hold the ions and for applying a second potential at second times to the shutter grid to allow the ions to pass through the shutter grid and a detector circuit for detecting the time of arrival of the ions at the collector.

Claims

The Invention Claimed is:

1. Apparatus (10) for measuring the mobility of ions comprising: a housing (12) enclosing a drift region (14) , said drift region (14) having a shutter grid (18) and collector (20) spaced apart, said shutter grid (18) and collector (20) being concentric about a longitudinal axis

(16) , means (32) for introducing said ions to a first side of said shutter grid (18) away from said collector (20) , means (82) for applying a potential between said shutter grid (18) and said collector (20) whereby an electric field is generated therebetween, means (80) for applying a first potential at first times to said shutter grid (18) to hold said ions and for applying a second potential at second times to allow said ions to pass through, said shutter grid, and means (92, 96) for detecting the time of arrival of said ions at said collector (20) .

2. The apparatus of Claim 1 wherein said housing (12) has a first end (50) and a second end spaced apart along said longitudinal axis (16) , said first end (50) having a first inlet (29) for receiving a gas sample (28) and a passageway (30) connecting said first inlet (29) to a reaction region (32) , said reaction region (32) having means (58) for generating ions from said gas sample (28) .

3. The apparatus of Claim 2 wherein said reaction region (32) has a passageway (33) connected to said first side of said shutter grid (18) .

4. The apparatus of Claim 3 wherein said shutter grid (18) extends from said first end (50) of said housing (12) to said second end of said housing (12) , said second end of said housing (12) having an outlet (45) for expelling said gas sample (28) and neutralized ions therefrom.

5. The apparatus of Claim 1 wherein said shutter grid (18) has a cylindrical shape comprised of a plurality of conductive members (61) each member substantially parallel to one another and extending from a first end (50) of said housing (12) to a second end of said housing (12) .

6. The apparatus of Claim 5 wherein said alternating members (61) of said shutter grid (18) are electrically coupled together and wherein said other alternating members (62) of said shutter grid (18) are electrically coupled together.

7. The apparatus of Claim 1 wherein said collector (20) has a cylindrical shape extending from a first end (50) of said housing (12) to a second end of said housing (12) .

8. The apparatus of Claim 7 wherein said radial spacing between said shutter grid (18) and said collector (20) is equal to. or less—than the distance between said first and second ends (50) of said housing (12) .

9. The apparatus of Claim 7 wherein said first end (50) has an inlet (38) for drift gas (36) and said second end has an outlet (45) for said drift gas (36) .

10. The apparatus of Claim 9 wherein said inlet (38) at said first end (50) is connected by a passageway (39) to an annular channel (40) having a plurality of openings (60) for releasing drift gas (36) into the region (14) between said collector (20) and said shutter grid (18) .

11. The apparatus of Claim 10 wherein said openings (60) are evenly spaced apart along said annular channel (40) and wherein the number and size of each opening (60) permits a predetermined flow of carrier gas (36) into said drift region (14) .

12. The apparatus of Claim 11 wherein said second end of said housing (12) has a corresponding annular channel and a plurality of openings spaced apart to remove gas from said drift region (14) to provide a flow of drift gas (36) through said drift region (14) in the axial direction with respect to said longitudinal axis (16) .

13. The apparatus of Claim 5 wherein said first end (50) includes a circular channel (52) for receiving said shutter grid (18) .

14. The apparatus of Claim 7 wherein said first end (50) of said housing (12) includes a circular channel (54) for receiving said collector (20) .

15. The apparatus of Claim 1 further including an aperture grid (22) concentric with said shutter grid (18) and positioned between said shutter grid (18) and said collector (20) .

16. The apparatus of Claim 15 wherein a first end (50) includes a channel (53) for receiving said aperture grid (22) .

17. The apparatus of Claim 2 further including a pump (78) connected to an outlet (45) of said housing (12) , a scrubber (66) coupled to said outlet of said pump (78) for removing contarainents from said gas (76) exiting said housing (12) , and means (68) for flowing said gas (65) from said scrubber (66) to a second inlet (38) of said housing (12) .

18. The apparatus of Claim 17 further including means (69, 74) for flowing said gas (65) from said scrubber (66) past one side of a membrane (70) and into said first inlet (29) of said housing (12) .

19. The apparatus of Claim 18 wherein a second side of said membrane (70) is exposed to a gas ambient (71) to be monitored.

20. The apparatus of Claim 2 wherein said first and second ends (50) of said housing (12) are joined by a cylindrical member having threads at one end for engagement with matching threads (57) on said first end

(50) .

21. The apparatus of Claim 1 wherein a first end (50) of said housing (12) is non-conductive.

22. The apparatus of Claim 1 wherein a first end (50) of said housing (12) is conductive including means for non-conductively supporting said shutter grid (18) and said collector (20) .

23. The apparatus of Claim 1 wherein said means for introducing said ions (32) includes means (98) for generating an electric field to move said ions of a selected polarity toward said shutter grid (18) .

24. The apparatus of Claim 23 wherein said means (98) for generating an electric field includes an electrode (98) positioned from said shutter grid (18) and means for placing a potential on said electrode.