US3593018A

US3593018A - Time of flight ion analysis with a pulsed ion source employing ion-molecule reactions

Info

Publication number: US3593018A
Application number: US812285A
Authority: US
Inventors: Martin J Cohen
Original assignee: FRANKLIN GMO CORP
Current assignee: PCP Inc A CORP OF FLORIDA
Priority date: 1969-04-01
Filing date: 1969-04-01
Publication date: 1971-07-13
Anticipated expiration: 1988-07-13

Abstract

Apparatus and methods for sorting and detecting ions in a drift cell, the electric fields applied to the cell being controlled at appropriate times to minimize dispersion of bunched ions produced by a pulsed source. Bunched product ions produced by ion-molecule reactions are analyzed in accordance with their velocity in a drift field.

Description

United States Patent Inventor Appl. No.

Filed Patented Assignee Martin J. Cohen West Palm Beach, Fla. 812.285

Apr. 1, 1969 July I3, 1971 Franklin GMO Corporation West Palm Beach, Fla.

TIME OF FLIGHT ION ANALYSIS WITH A PULSED ION SOURCE EMPLOYING ION-MOLECULE REACTIONS I9 Claims, 2 Drawing Figs.

U.S. Cl 250/413 TF, 250/419 G, 250/42.9 SB

Int. Cl ..H0lj 39/34, BOId 59/44, HOlj 37/08 Primary Examiner lames W. Lawrence Assistant Examiner-C. E. Church Attorney- Raphael Semmes ABSTRACT: Apparatus and methods for sorting and detect ing ions in a drift cell, the electric fields applied to the cell being controlled at appropriate times to minimize dispersion of bunched ions produced by a pulsed source. Bunched product ions produced by ion-molecule reactions are analyzed in accordance with their velocity in a drift field.

PULSED SOURCE CURRENT TIME 1 ION PULLOUT (b) V REACTION DRIFT 50m sec 50m sec TIME 5m see A ION PULLOUT REACTION DRIFT K n r1 r1 in LJ LJ Li Li L! TIME? PATENT-ED JUL I 3 IQTI 2 IO I --A K PULSED ION SOURCE STATIC AND DYNAMIC POTENTIAL SUPP L Y INVENTOR MARTIN J. COHEN TIMEOF FLIGHT ION ANALYSIS WITH A PULSED ION SOURCE EMPLOYING ION-MOLECULE REACTIONS BACKGROUND OF THE INVENTION This invention relates to apparatus and methods of ion classification and more particularly is concerned with enhancing the resolution of ion measurements performed in a drift cell.

The copending application of Martin J. Cohen, David I. Carroll, Roger F. Wernlund, and Wallace D. Kilpatrick Ser. No. 777,964, filed Oct. 23, i968, and entitled Apparatus and Methods for Separating, Concentrating, Detecting, and Measuring Trace Gases," discloses "Plasma Chromatography" systems involving the formation reactant ions and the reaction of these ions with molecules of trace substances to form product ions, which may be concentrated, separated, de-

tected, .and measured by virtue of the velocity or mobility of the ions in an electric field. The production and analysis of ions takes place in a chamber, the length ofthe meanfree path of the ions being very much less than the dimensions of the chamber under operating pressure conditions, such as atmospheric. The reactant ions may be produced by subjecting the molecules of a suitable host gas, such as air, to ionizing radiation, for example. The reactant ions are 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 the reactant ions and the trace gas molecules produce product 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 product ions are also subjected tothe electric drift field and may be sorted in accordance with their velocity or mobility. A specific system of the copending application employs a pair of successively arranged ion shutter grids or gates for segregating the ion species in accordance with their drift time. The opening, of the first gate. is timed to pass a group of ions, which may comprise unreacted primary or reactant ions'as well as secondary or product ions, and the opening of the second gate is timed to pass a portion of the group to an ion detection means. Only those ions passed during the short period when the first shutter grid is open form the mixed ion bunch which is analyzed by ion drift time in the ion drift analyzer space. Using a radioactive or other continuous ionizing source, 99 percent or more of the available ions are not utilized.

The copending application of Martin J. Cohen entitled Apparatus and Methods for Improving the Sensitivity of Ion Detection and Measurement, filed concurrently herewith, discloses apparatus and methods for ion measurements,.employing continuous ion sources, in which a much larger percentage of the available ions is passed to the ion analysis region.. The invention of that application is capable of producing signal improvements of the order of to 100 times and involves the. rapid withdrawal of the continuously formed ions from the ion-molecule reaction region and the bunching of such ions before drift-time analysis.

BRIEF DESCRIPTION OF THE INVENTION The present invention is concerned with apparatus and methods for ion measurements in which a pulsed source,

rather than a continuous source, is employed to produce a bunch or pulse of product ions by ion-molecule reactions with a bunch or pulse of reactant ions, and in which dispersion of the product ions is minimized. It is accordingly a principal object of the invention to provide apparatus and methods of this type which permit measurements with greatly enhanced resolution.

Briefly stated, the concept underlying the present invention involves rapid pullout of primary or reactant ions from the region of a pulsed ion source, the holding up of the pulled-out primary ions at a region adjacent to the region of the source, in order to permit ion-molecule reactions to proceed for a predetermined period andin order toforrn a bunch-of product ions, and thereafter the analysis of the product ion bunch in accordance with the velocity of the various species comprising the bunch, and the detection and measurement of the ion species of inter.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION OF THE INVENTION The drift cell 10 which may be employed in the present invention is of the type set forth in the :said copending applica-' tion Ser. No. 777,964, and comprises an envelope 12 enclosing a series of electrodes, which may be of parallel plane geometry, for example. Principal electrodes K and A may be arranged adjacent to opposite ends of the envelope. When the apparatus is used to detect negative ions, electrode K will be a cathode and electrode A an anode. When the apparatus is used to detect positive ions, the polarities will be reversed. Electrode K or the region of the envelope near this electrode is provided with a pulsed ionizing source, such as a pulsed corona source, apulsed spark gap, a pulsed RF source, or a pulsed ultraviolet source, all of which are well known in the art. Electrode A may be a collector plate constituting an output electrode and may be connected to an electrometer (not shown), such as a Cary Instruments Model 401 (vibrating reed) type with sensitivity of 10' amps at a time constant of 300 milliseconds. The drift cell employs a grid 14 adjacent to electrode A, which may be a dual grid ion gate comprising two sets of interdigitated parallel wires, the sets normally being maintained at equal and opposite potentials relative to a grid average potential established by the static and dynamic potential supply 16. In this condition, the grid is closed to the passage of electrically charged particles, but when, at predetermined times, all of the elements of the grid are driven to the same potential (the grid average potential) by the use of appropriate grid drive circuits in the supply 16, the shutter grid opens. Such a dual grid and its drive circuits are well known in the prior art and do not per se constitute the present invention. Alternatively, and for simplicity, a passive grid of simple parallel wire form may be employed, in which event the output electrode A may be connected to the input of a fast amplifier and a boxcar integrator, for example. Both types of detection and readout circuits are described in the copending application of Roger F. Wernlund, entitled Apparatus and Methods for Detecting, Indicating and Recording Signals Produced by Ionized Trace Substances in a Gaseous Sample," Ser. No. 798,399, filed Feb. 11, 1969.

The static and dynamic potential supply 16 provides static and dynamic potentials appropriate to the various electrodes of the drift cell 10, which may also include a series of guard rings 18 spaced along the envelope for maintaining the uniformity of the drift field in the different regions of the envelope. Inlet 20 isprovide'd for introducing gaseous samples to the envelope, and outlet 22 permits the withdrawal of gas from the envelope. The pressure in the envelope 12 between the electrodes K and A is substantially uniform, since the. space between the electrodes is unrestricted, and is such that the length of the mean free path of the ions is very much less than the dimensions of the envelope, atmospheric pressure being.

molecules of a host gas, such as air introduced through inlet 20, to electrons generated by the pulsed source. The primary ions, which may be positive oxygen ions, for example, are indicated in curve (a) of FIG. 2 at time t=0.00. The problem is now to provide sufficient time for ion-molecule reactions between the primary ions and the molecules of the trace substances to be detected and yet to preserve the bunch while avoiding recombination losses at the source. In accordance with the invention, this is achieved by the use of a properly times sequence of voltage pulses applied across the cell.

First, a voltage pulse (eg. 3,000 volts) is applied across the cell to separate the desired sign of charged particles from the electrode K. For positive ions, the ion pullout pulse is positive, as shown by the potential V,,-, curve (b) of FIG. 2, that is, the electrode K is driven positive relative to electrode A, which may be at ground potential. The magnitude and duration is such as to move the positive ions in 5 milliseconds, for exam ple, to a position 1 cm., for example, from electrode K, as indicated by the shaded block in FIG. 1. Then the ion pullout pulse voltage (and, preferably, the grid average voltage) is reduced to zero, as shown by curve (b). At this time, the bunch of reactant ions, which may extend over a distance of 0.1 cm., for example, will remain in position in the absence of a field at the region of the ions. The pulse cloud tends to widen somewhat by diffusion and space charge repulsion.

Ion-molecule reactions between the primary or reactant ions and the molecules of trace substances in the sample proceed for 50 milliseconds, for example, the objective being to form a bunch of product ions atthe position of the reactant ions. The 50-millisecond reaction interval is indicated in curve (b). Since the number of reactant ions is many orders of magnitude less than the number of trace molecules, depletion of the trace material is not a problem.

After the 50-m'illisecond reaction interval, the potential across the cell is raised to the normal ion-drift potential (e.g. 3,000 volts), and with a drift distance of, say, l0c., 50-milliseconds drift time is sufficient to accomplish the desired ion separation. The drift interval is indicated in curve (b) also. It is desirable to maintain the ion pulse width as small as possible, say 0.2 cm. Then the resolution is roughly 50in space dimensions. During the drift interval, the average potential of grid 14 is also raised (say to 2,700 volts).

If grid 14 is a dual grid of the type set forth previously, this grid will be opened at some time during the drift interval to pass selected ion species to the output electrode A. By scanning the time of opening of grid 14 relative to the commencement of the drift interval, a complete spectrum of the ion population within the analysis region may be produced for recording as a curve of output current versus time. Peaks in this curve represent the different primary and secondary ion species. If grid 14 is a simple, nonshutter grid, it will serve to shield the output electrode A and associated circuits from iondisplacement currents and will always be open.

Instead of reducing the drift voltage to zero during the reaction interval, an oscillating potential may be applied across the cell to jiggle the ions back and forth for 0.1 cm. distance, for example, as indicated by the pulsating drift potential in the 50- millisecond reaction interval shown in curve (c) of FIG. 2. For example, a 200 Hz oscillating field of 15 volts per centimeter may be used.

The static and dynamic potential supply 16 may be of conventional design. For example, nominal electrode potentials may be established, where required, by a bleeder resistor string from a DC supply of appropriate polarity. The required pulses may be generated with solid-state circuitry utilizing a transformer to carry an RF frequency which may be rectified at the electrode connections. Pulse voltages can be clipped with Zener or corona tube regulation. Grid opening and closing pulses for the shutter grid 14, if such a grid is employed, are of course superimposed upon the corresponding grid elements. Alternatively, the electrode potentials may be supplied by a bleeder string having taps for explicitly providing all of the voltages needed. A relay or motor-driven switch may then be employed to connect the electrodes to the appropriate points of the bleeder in sequence.

A special nonreactant or inert gas, such as nitrogen, may be introduced to the analysis region, as set forth in the copending application of David I. Carroll, Martin J. Cohen and Roger F. Wernlund, Ser. No. 780,851, filed Dec. 3, 1968, and entitled Apparatus and Methods for Separating, Detecting, and Measuring Trace Gases with Enhanced Resolution. Since the sample volume in the drift cell is defined by the region where the reactant ions rest, the volume for nonreactant gas is defined accordingly. Similarly, when the apparatus of the invention is used as a gas chromatography detector (the gas chromatograph effluent being introduced to the envelope 12 in addition to a suitable reactant gas), an appropriate small gas chromatography sample volume is defined by the reactant ion rest region.

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 I claim is:

l. A method of ion analysis in a drift field which comprises forming a bunch of reactant ions, adjusting the drift field to hold said bunch of ions in a predetermined region while reacting said ions with molecules of a sample to form a bunch of product ions, increasing the drift field to cause ions present after reaction to separate in accordance with the velocity of the ions in the drift field, the foregoing steps being performed under substantially the same gas pressure, detecting at least a portion of the se arated ions.

2. A method in accordance with claim 1, wherein the bunch of reactant ions is formed during a first interval of time and is reacted during a second interval of time which is longer than said first interval.

3. A method in accordance with claim 1, wherein the reactant ions are moved to said predetermined region by the application ofa drift field thereto.

4. A method in accordance with claim 1, wherein the reac tant ions are maintained substantially stationary during the reacting.

5. A method in accordance with claim 1, wherein the reactant ions are jiggled during the reacting.

6. A method in accordance with claim 1, wherein the pressure is substantially atmospheric.

7. A method of improving the resolution of ion analysis in a drift cell, which comprises forming a bunch of reactant ions at a first region of the cell, moving said ions from said first region to a second region of the cell as a bunch then maintaining said ions substantially at said second region while reacting said ions with sample gas molecules to form a bunch of product ions, thereafter moving the bunch of product ions, and any reactant ions remaining after reaction, through an analysis region and causing them to separate in accordance with their drift velocity, detecting at least a portion of the separated ions.

8. A method in accordance with claim 7, wherein a drift field is applied across said drift cell during reactant ion formation and movement to said second region, the field is then substantially reduced across said cell during the reacting, and the field is then increased during the separating.

9. A method in accordance with claim 7, wherein said bunch of reactant ions is formed by actuating a pulsed ionizing source.

10. Apparatus for ion measurements, comprising an envelope, a pair of electrodes spaced apart in said envelope, a pulsed ionizing source adjacent to one of said electrodes, means for introducing a gaseous sample into said envelope whereby a bunch of reactant ions is formed adjacent said source from reactant molecules of said sample when said source is actuated, means for applying a drift field between said electrodes during a first interval of time to draw said ions away from said source, means for substantially reducing said drift field during a second interval of time for maintaining said bunch of ions at a predetermined region to permit said ions to react with'trace moleculesfrorn said sample to form a bunch of productions, means for applying a drift field between said electrodes during a third interval of time for causing the product ions and any unreacted reactant ions to be separated in accordance with their drift velocity, and means for detectin g at least a portion of the separated ions.

11. Apparatus in accordance with claim 10, wherein the space between said electrodes is substantially unrestricted and has a substantially uniform gas'pressure distribution.

12. Apparatus in accordance with claim 10, wherein said detection means comprises the other of said electrodes, there being a grid between said electrodes adjacent to said other electrode.

13. Apparatus in accordance with claim 12. wherein said grid has means for applying a potential thereto for shielding said other electrode from ion displacement currents in said cell.

14. Apparatus in accordance with claim 12, wherein said grid is an ion gate and has means for opening the gate at a a predetermined time.

15. Apparatus in accordance with claim 10, further comprising an ion gate between said electrodes adjacent to the other electrode, and means for opening said gate at a predetermined time for passing at least a portion of the separated ions to said other electrode.

16. Apparatus in accordance with claim 10, wherein said means for reducing said drift field comprises means for producing an oscillating drift field between said electrodes.

17. Apparatus in accordance with claim 10, wherein said second and third intervals of time are substantially longer than said first interval of time.

l8. A method in accordance with claim 7. wherein the recited steps are performed under substantially the same gas pressure.

[9. A method in accordance with claim 18, wherein the pressure is such that the length of the means free path of said ions is very much less than the dimensions of the cell.

Claims

1. A method of ion analysis in a drift field which comprises forming a bunch of reactant ions, adjusting the drift field to hold said bunch of ions in a predetermined region while reacting said ions with molecules of a sample to form a bunch of product ions, increasing the drift field to cause ions present after reaction to separate in accordance with the velocity of the ions in the drift field, the foregoing steps being performed under substantially the same gas pressure, detecting at least a portion of the separated ions.

3. A method in accordance with claim 1, wherein the reactant ions are moved to said predetermined region by the application of a drift field thereto.

4. A method in accordance with claim 1, wherein the reactant ions are maintained substantially stationary during the reacting.

10. Apparatus for ion measurements, comprising an envelope, a pair of electrodes spaced apart in said envelope, a pulsed ionizing source adjacent to one of said electrodes, means for introducing a gaseous sample into said envelope whereby a bunch of reactant ions is formed adjacent said source from reactant molecules of said sample when said source is actuated, means for applying a drift field between said electrodes during a first interval of time to draw said ions away from said source, means for substantially reducing said drift field during a second interval of time for maintaining said bunch of ions at a predetermined region to permit said ions to react with trace molecules from said sample to form a bunch of product ions, means for applying a drift field between said electrodes during a third interval of time for causing the product ions and any unreacted reactant ions to be separated in accordance with their drift velocity, and means for detecting at least a portion of the separated ions.

11. Apparatus in accordance with claim 10, wherein the space between said electrodes is substantially unrestricted and has a substantially uniform gas pressure distribution.

13. Apparatus in accordance with claim 12, wherein said grid has means for applying a potential thereto for shielding said other electrode from ion displacement currents in said cell.

14. Apparatus in accordance with claim 12, wherein said grid is an ion gate and has means for opening the gate at a predetermined time.

18. A method in accordance with claim 7, wherein the recited steps are performed under substantially the same gas pressure.

19. A method in accordance with claim 18, wherein the pressure is such that the length of the means free path of said ions is very much less than the dimensions of the cell.