Apparatus And Method For Improving The Sensitivity Of Time Of Flight Ion Analysis By Ion Bunching

Abstract

Apparatus and methods for sorting and detecting ions in a drift cell, the electric fields applied to different regions of the cell being controlled at appropriate times to ensure the rapid withdrawal of the ions from a reaction region to an analysis region, the bunching of the ions in the analysis region, and thereafter the separation of the bunched ions in accordance with ion drift velocity, and detection of separated ion species.

Description

BACKGROUND OF THE INVENTION

This invention relates to apparatus and methods of ion classification and more particularly is concerned with enhancing the sensitivity 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, 1968 and entitled "Apparatus and Methods for Separating, Concentrating, Detecting and Measuring Trace Gases," disclosed "Plasma Chromatography" systems involving the formation reactant ions and reaction of these ions with molecules of trace substances to form 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 production and analysis of ions take place in a chamber, the length of the mean free 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 to the 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 reactant ions as well as product ions, and the opening of the second gate is timed to pass a portion of the group to an ion detection means. In accordance with the technique described in the said copending application, the first shutter grid may sample the ions in the reaction region by opening for 0.1 to 0.5 millisecond every 50 milliseconds. 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.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is concerned with apparatus and methods for ion measurements in which a much larger percentage of the available ions is passed to the ion analysis region. It is accordingly a principal object of the invention to provide apparatus and methods of this type which are capable of signal improvements of the order of 10 to 100 times when compared with the operation set forth in the said copending application.

Briefly stated, the concept underlying the present invention involves the rapid withdrawal of the ions from a reaction region to an ion analysis region, the holding up of the ions in the analysis region until substantially all of the ions are drawn out of the reaction region, the concentration or bunching of the ions withdrawn from the reaction region, and thereafter the analysis of the ion bunch in accordance with the velocity of the various species comprising the bunch, and the detection and measurement of the ion species of interest.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described in conjunction with the accompanying drawings, which illustrate a preferred and exemplary embodiment of the invention, and wherein:

FIG. 1 is a diagrammatic longitudinal sectional view illustrating a drift cell and potential supply employed in the invention;

FIG. 2 is a graphical diagram illustrating qualitatively the electric field at different regions of the drift cell during successive intervals of time;

FIG. 3 is a graphical diagram illustrating qualitatively the ion density at different regions of the cell during successive intervals of time;

FIG. 4 is a graphical diagram illustrating quantitatively the voltages applied to different elements of the drift cell during the successive intervals of time; and

FIG. 5 is a graphical diagram illustrating quantitatively the electric field at different regions of the drift cell during the successive intervals of time and the condition of the ion gates during such intervals .

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 application 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 an ionizing source, such as a tritium foil forming part of the electrode. 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.sup.-.sup.15 amps. at a time constant of 300 milliseconds. The drift cell employs a pair of shutter grids or ion gates G1 and G2. Each gate comprises two sets of interdigitated parallel wires, alternate wires of each grid being connected together to form the two sets. Grids G1 and G2 are arranged in spaced sequence between the electrodes K and A and define therewith sequential regions within the envelope 12. In the illustrative form of the invention the distance between grid G1 and electrode K is 2 centimeters, between grids G1 and G2 is 8 centimeters, and between G2 and electrode A is 1 centimeter. Inlet tube 14 permits the introduction of gas into the region between K and G1, while outlet tube 16 permits gas to be exhausted from the envelope.

As will be set forth more fully hereinafter, a static and dynamic potential supply 18 provides static and dynamic potentials appropriate to the various electrodes of the drift cell 10, which may also include a series of guard rings 20 spaced along the envelope for maintaining the uniformity of the drift field in the different regions of the envelope. The guard rings may be connected to taps of the static supply providing successively greater potentials along the series of rings in each region.

Adjacent elements of each shutter grid are normally maintained at equal and opposite potentials relative to a grid average potential established by the static supply. Under these conditions the shutter grid is closed to the passage of electrically charged particles. At predetermined times all of the elements of the grid are driven to the same potential, the grid average potential, by the use of suitable grid drive circuits. The grid drive circuits open the ion gates in sequence, the G1 drive circuit producing sync pulses and the G2 drive circuit producing pulses delayed relative to the G1 pulses. The dual grids per se and their drive circuits are known in the prior art and do not constitute the present invention.

In accordance with the techniques set forth in the said copending application, molecules of a host gas, such as air introduced by inlet 14 into the space K-G1 are ionized by the ionizing source, such as the tritium foil on the electrode K, and these reactant ions react with the molecules of the trace gas of the sample in the space K-G1 to produce product ions, the pressure in the envelope 12 being such that the length of the mean free path of the ions is very much less than the dimensions of the envelope. At a predetermined instant the first shutter grid G1 is opened momentarily for passing a group of ions from the mixed ion population presented thereto from the space K-G1, and the ions of this group migrate into the analysis region G1-G2 under the influence of an electric drift field applied between principal electrodes K and A. At a time delayed relative to the opening of grid G1, grid G2 is opened momentarily for passing a selected ion species to the detection space G2--A, the various ion species having separated in the analysis region G1--G2 in accordance with their velocity in the drift field. The ions which reach the electrode A produce a current or signal in the output circuit which may be measured, as by the electrometer referred to previously. By scanning the opening of grid G2 relative to grid G1 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.

In accordance with the present invention, the techniques set forth in the said copending application are modified to enhance the sensitivity of the measurements, as will now be described in conjunction with a specific example.

Electric fields which will be referred to hereinafter are defined in terms of the O.sub.2.sup.- ion velocity at atmospheric pressure. For example,

where

2.7 cm..sup.2 /volt sec. is taken as the mobility of O.sub.2.sup.-.

The cell dimensions are as assumed previously.

At the beginning of a cycle, time t = 0.000, assume that the grid G1 has just opened, and ions produced by the tritium source and subsequent reactions have filled the space K-G1. At this instant, an electric field which produces an O.sub.2.sup.- velocity of 800 cm./sec. is applied to the space K-G1. (Arbitrarily it may be assumed that the heaviest ion of interest has a velocity of one-half that of O.sub.2.sup.-.) For a 2 cm. space, the voltage across this space is approximately 600 volts. At the same time in the region G1-G2 the electric field may be made equivalent to 10 cm./sec. O.sub.2.sup.- velocity, which is 30 volts across the 8 cm. space. Curve (a) of FIG. 2 shows the electric field conditions in the reaction region K-G1 and the analysis region G1-G2 at time t = 0.000. The letter "O" under G1 indicates that this grid is open, while "end scan" under G2 indicates that this grid has ended its scan period, during which it is opened at the appropriate moment. Curve (a) of FIG. 3 shows, by the shaded block, the formation of ions throughout region K-G1.

Now suppose that these conditions remain for 5 milliseconds. During this time, the heaviest ions produced in region K-G1 migrate through G1 at a velocity of 1/2 .times. 800 cm./sec. As these ions enter the G1-G2 region, they slow down to the lower velocity of 1/2 .times. 10 cm./sec. In a time of 0.005 second the O.sub.2.sup.- ions move 10 cm./sec. .times. 0.005 sec. = 0.05 cm. Thus ideally a bunch of ions approximately 0.05 cm. long is presented adjacent to grid G1 in the region G1-G2 at the end of the 0.005 sec. interval, as indicated by the shaded block, curve (b), FIG. 3. There is, of course, a certain amount of sorting, with the O.sub.2.sup.- ions ahead of the slower ions in the bunch.

At time t= 0.005 sec. the potentials are changed to favor the analysis of the bunch of ions for 50 milliseconds in the region G1-G2 and the reformation of the ion-molecule reaction cloud in the region K-G1. The electric field conditions are shown by curve (b), FIG. 2. In each region the field is selected for its objective. In the region K-G1 the field is reduced at t= 0.005 sec. to produce a velocity of 80 cm./sec. for O.sub.2.sup.-, so that the heavier ions fill the 2 cm. space in 50 milliseconds. At time t= 0.005 sec. shutter grid G1 is closed (indicated by "C" in FIG. 2, curve (b) ), and this grid remains closed for the next 50 milliseconds. During these 50 milliseconds the bunch of ions admitted to the analysis region G1-G2 drifts rapidly through the analysis region under the influence of the greatly increased field in this region, and the ion species separate in accordance with their drift velocity in the field between G1 and G2. Grid G2 is opened briefly at an appropriate instant during the 50 millisecond scan period, and passes selected species of ions to the detection region G2-A. By opening grid G2 at different times delayed relative to the opening of grid G1, different species of ions may be selected, as set forth previously.

At time t= 0.055 sec. the cycle is complete, and during the next 0.055 sec. the cycle repeats the procedure commenced at time t= 0.00 sec., as shown by curves (c), (d), (e), FIGS. 2 and 3. There is thus a 0.055 second period in which, in 5 milliseconds, those ions which were formed during the previous 50 milliseconds are bunched. By virtue of the invention the loss of ions is reduced to less than 5 percent.

The basic period is unsymmetrical, 5 milliseconds devoted to ion pulse forming and 50 milliseconds to ion analysis. The field is high in the reaction region and low in the analysis region during pulse forming. The reverse conditions prevail during ion analysis. The values of the field are selected to collect the ions rapidly from the space K-G1 and cause them to drift very little in the space G1-G2 during the 5 millisecond bunching period. During the analysis period the field in the reaction space K-G1 is reduced to permit all the slow ions to form with a minimum number collected, and the field in the analysis region is increased to provide the relatively rapid ion separation required.

During the 5 millisecond bunching time, the ions in the analysis region are in a very low field. Diffusion effect is relatively small. For trace material at low concentrations the space charge effect due to the trace ion is also small. If the reactant or primary ion is not fully converted (by ion-molecule reactions), it will contribute to the space charge. This condition may be alleviated by utilizing the reaction region as a coarse time-of-drift analyzer. For a fast reactant ion compared to a slow product ion, the opening of grid G1 can be delayed past the time used in the foregoing example in order to collect the unreacted ion upon the grid G1.

FIG. 4 illustrates typical potentials of the various electrodes during the intervals of time described above. For example, the voltage V.sub.A of the electrode A may be 0 volts relative to ground. The voltage V.sub.2 of grid G2 may be -100 volts. In the intervals 0-5 milliseconds, 55-60 milliseconds, and 110-115 milliseconds, the voltage V.sub.1 of grid G1 may be -130 volts, while the voltage V.sub.K of electrode K may be -730 volts. In the intervals 5-55 milliseconds and 60-110 milliseconds the voltage V.sub.1 may be -1300 volts and the voltage V.sub.K may be -1360 volts.

Curve (a) of FIG. 5 illustrates the electric field strength in the reaction, analysis and signal regions during the successive intervals. Curve (b) of FIG. 5 illustrates the opening and closing of grid G1. Grid G2 may be closed during the opening of grid G1 and opened at selected times during the G2 scan intervals illustrated.

The static and dynamic potential supply 18 may be of conventional design, a nominal potential for each element being established by a bleeder resistor string from a negative power supply, for example. Thus, V.sub.1 may have a static potential of -130 volts and V.sub.K may have a static potential of -730 volts. V.sub.A may always be at 0 volts and V.sub.2 may always be at -100 volts. A 50 millisecond pulse is generated with an amplitude of about -1170 volts for V.sub.1 and -630 volts for V.sub.K. This pulse may be generated with solid state circuitry using a transformer to carry an RF frequency which is rectified at the electrode connection. The pulse voltage can be clipped with Zener or corona tube regulation. Additionally, of course, the grid opening and closing pulses for the shutter grids G1 and G2 are superimposed upon the corresponding grid elements. Thus, as pointed out above, when it is desired to open a shutter grid, both sets of wires of that grid are driven to the same potential, the grid average potential existing at that time. When the grid is closed, the sets of grid wires are maintained at equal and opposite potentials relative to the grid average potential at the time.

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".

While a preferred embodiment of the invention has been shown and described, it will be apparent to those skilled in the art that changes can be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims. In the claims reference is made to first, second and third intervals of time, for ion formation, ion bunching, and ion analysis, respectively, as described above. For ease in correlating these intervals with the curves of FIGS. 2-5, it should be noted that the first or ion formation interval occurs between t= 0.005 and t= 0.055, the second or ion bunching interval occurs between t= 0.055 and t= 0.060, and the third or ion analysis interval occurs between t= 0.060 and t= 0.110. In repetitive cycles ions for the next cycle are formed during the interval between t= 0.060 and t= 0.110, and at t= 0.00 such ions are ready for bunching in the interval between t= 0.00 and t= 0.005. Of course with a continuous ion source ions are actually formed continuously, but the ions constituting a particular pulse or bunch are formed during a finite interval.

Claims

The invention claimed is:

1. A method of ion analysis which comprises forming product ions by reacting reactant ions with neutral trace particles, moving said ions, concentrating said product ions into a bunch by decelerating, and separating the bunched ions in accordance with their velocity in a drift field, the recited steps being performed in a space maintained at a pressure such that the length of the mean free path of said ions is very much smaller than the dimensions of the space.

2. A method in accordance with claim 1, wherein the ions are formed during a first interval of time and are bunched during a second interval of time shorter than the first.

3. A method in accordance with claim 2, wherein the method steps are repeated cyclically, ions formed during one cycle being separated while ions of the next cycle are formed.

4. A method in accordance with claim 1, wherein the ions are subjected to a first drift field during formaction and to drift fields stronger than the first field during concentration and separating.

5. A method in accordance with claim 1, wherein the ions are formed in a first region and are rapidly withdrawn to and held at a second region during concentration.

6. A method of improving the sensitivity of ion analysis in a drift cell having sequential ion formation and ion drift regions, which comprises forming ions in the ion formation region during a first interval of time, causing said ions to move into the drift region during a second interval of time but to decelerate upon entering the drift region and become bunched near the entrance to said drift region, and thereafter moving the bunched ions through the drift region and causing them to separate in accordance with their drift velocity, the recited steps being performed in a space maintained at a pressure such that the length of the mean free path of said ions is very much smaller than the dimensions of the space.

7. A method in accordance with claim 6, further comprising passing at least part of said separated ions to a signal detection region.

8. A method in accordance with claim 6, wherein a first drift field is applied across the ion formation region and a second drift field across the ion drift region during said second interval of time, the first field being stronger than the second field, and the relative field strengths across said regions being reversed during said first interval of time.

9. A method of operating a drift cell having an envelope with first and second principal electrodes spaced therein, first and second ion gates spaced apart between and spaced from the principal electrodes, and an ionizing source associated with said first principal electrode, which comprises applying, during a first interval of time, a first drift field between the first ion gate and the first principal electrode, opening the first ion gate at the end of said first interval of time, applying, during a second interval of time, a second drift field between said first principal electrode and said first ion gate and a third field between said first ion gate and said second ion gate, closing said first ion gate at the end of said second interval of time, applying a fourth drift field between said first ion gate and said second ion gate during a third interval of time, opening said second ion gate during said third interval, and applying a fifth drift field between said second ion gate and said second principal electrode, said first field being weaker than said second field, said third field being weaker than said second field, said fourth field being stronger than the third field, the recited steps being performed in a space maintained at a pressure such that the length of the mean free path of said ions is very much smaller than the dimensions of the space.

10. Apparatus for ion measurements, comprising an envelope, a pair of principal electrodes spaced apart in said envelope, a pair of ion gates spaced between and from said electrodes, means for introducing a gaseous sample into said envelope, means for forming ions from said sample between one of said electrodes and one of said gates, means for applying a first drift field between said one electrode and said one gate during a first interval of time, means for applying a second drift field between said one electrode and said one gate and a third drift field between said one gate and said other gate during a second interval of time, means for closing said one gate during said first interval of time and for opening said one gate during said second interval of time, means for applying a fourth drift field between said gates during a third interval of time, means for opening said other gate during said third interval of time, and means for applying a fifth drift field between said other gate and said other electrode, said first field being weaker than said second field, said third field being weaker than said second field, said fourth field being stronger than the third field, and means for maintaining the pressure in said envelope at a level such that the length of the mean free path of said ions is very much smaller than the dimensions of the envelope.

11. Apparatus in accordance with claim 10, wherein said means for opening said one gate maintains said one gate open throughout said second interval of time and wherein said means for opening said other gate opens said other gate during a portion only of said third interval of time.

12. Apparatus in accordance with claim 11, further comprising means for varying the delay between the opening of said other gate and said one gate.

13. Apparatus for analyzing ions, comprising an envelope, means for forming ions at a first region of said envelope during a first interval of time, means for rapidly withdrawing the ions formed from said first region to a second region of the envelope during a second interval of time, means for slowing the movement of the withdrawn ions and forming them into a bunch near the entrance to said second region during said second interval, means for more rapidly moving the ions of said bunch through said second region during a third interval of time and for causing them to separate in accordance with their ion velocity, and means for maintaining the pressure in said envelope at a level such that the length of the mean free path of said ions is very much smaller than the dimensions of the envelope.

14. Apparatus in accordance with claim 13, further comprising means for passing a portion of the separated ions to a third region of the envelope for detection.

15. A method in accordance with claim 6, wherein said ions are formed by reacting other ions with neutral trace particles.