US 3639757 A
Discrete (nanogram) samples of organic materials are volatilized, and trace molecules of the samples are converted to ions by ion-molecule reactions, the ions being subject to a drift field and analyzed by a mass analyzer. The ions passed to the mass analyzer may be subjected to preliminary separating, filtering and focusing. The mass analyzer output device may include an electron multiplier and a multichannel analyzer. Mass analyzer is performed under high-vacuum conditions, while ion formation and drift take place under relatively high-pressure conditions.
Description (OCR text may contain errors)
United States Patent Caroll et al.
[ 1 Feb. 1, 1972  APPARATUS AND METHODS EMPLOYING ION-MOLECULE REACTIONS IN BATCH ANALYSIS OF VOLATILE MATERIALS  inventors: David 1. Caroll, Lantana; Roger F. Wernlund, Lake Worth; Martin J. Cohen, West Palm Beach, all of Fla.
 Assignee: Franklin GNO Corporation, West Palm Beach, Fla.
 Filed: Aug.4, 1969  Appl. No.: 847,115
 US. Cl. ..250/41.9 TF, 250/419 DS, 250/41.9 S, 250/419 D  int. Cl ..1101j 39/34, 801d 59/44  Field of Search ..250/41.91, 41.92, 41.9 R, 41.9 SB, 250/419 S  References Cited UNlTED STATES PATENTS 2,686,880 8/1954 Glenn ..250/41.9 2,772,362 11/1956 Dietz ..250/41.9
Fete et al. ..250/41.9
FOREIGN PATENTS OR APPLICATIONS 1,230,714 6/1959 France ..250/4 1 .9
OTHER PUBLICATIONS semiautomatic Data Collection System For Mass Spectrometers," Moreland, The Review of Scientific Instruments, Vol. 38 No. 6, June 1967 pp. 760- 764 Mass Spectrometry and Its Applications To Organic Chemistry," Beynon, Elsevier Publishing Company, New York, 1960 pp. 147- 172 Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church Attorney-Raphael Semmes  ABSTRACT Discrete (nanogram) samples of organic materials are volatilized, and trace molecules of the samples are converted to ions by ion-molecule reactions, the ions being subject to a drift field and analyzed by a mass analyzer. The ions passed to the mass analyzer may be subjected to preliminary separating, filtering and focusing. The mass analyzer output device may include an electron multiplier and a multichannel analyzer. Mass analyzer is performed under high-vacuum conditions, while ion formation and drift take place under relatively highpressure conditions.
43 Claims, 3 Drawing Figures REIADOUT PAIEN IEU m H972 SHEEI 1 [IF 2 INVENTORS DAVID I. CARROLL ROGER F. WERNLUND MARTIN J. COHEN .rDOnzwm ATTORNEY INVENTOBS DAVID I. CARROLL ROGER F. WERNLUND MARTIN J. COHEN mm mm SHEET 2 OF 2 Q ATTORN BY PAIENIED FEB H972 APPARATUS AND METHODS EMPLOYING ION- MOLECULE REACTIONS IN BATCH ANALYSIS OF VOLATILE MATERIALS BACKGROUND OF THE INVENTION This invention relates to apparatus and methods for detecting and identifying either positive or negative ions of trace gases, especially trace gases produced by volatilizing discrete samples. The invention is particularly concerned with improvement in the speed and sensitivity of analysis of volatile organic materials, generally containing three or more carbon atoms and additional elements such as oxygen, nitrogen, sulphur, phosphorus, or other metals. Such materials are characterized by the rapid rate at which they form positive or negative ions through ion-molecule reactions with selected reactant ions.
Prior technology used in the analysis of the above types of materials is exemplified by the gas chromatograph, the mass spectrometer, and the combination gas chromatograph-mass spectrometer. Sensitivity of the gas chromatograph is limited by: l background arising from effluents from the chromatograph column, (2) loss and tailing of the column material at the level of parts of gas chromatograph trace peak in the carrier gas, and 3) finite width of the gas chromatograph peak, and therefore sample measurement time.
Limitations upon the conventional mass spectrometer are imposed by the use of an energetic electron beam to ionize all gas molecules in its path with very little regard to their chemical nature. Thus the background gas as well as the desired chemical form ions. This universal ionizing effect places a limit of about 10' wanted ions to unwanted background ions. Conventional ionizing'techniques produce positive ions more efficiently than negative ions, the latter being produced inefficiently, if at all. A further sensitivity limitation is the destructive ionization effect of the energetic electron beam. Beams up to 100 volts may be used, the desired being to have a high beam current to obtain efi'rciency and a low beam energy to reduce the fragmentation. Generally, for the complex organic molecule under analysis, the amount of positively charged parent ion remaining after this beam ionizing mechanism takes place may be less than 10 The bulk of the ions are smaller fragments which are difficult to distinguish from fragments arising from other organic chemicals present. This leads to the concept of fragmentography which attempts to identify a complex molecule from a signature involving many ion fragment masses and their relative abundance. This procedure becomes quite complicated when more than one organic chemical is present, and it has even been proposed to employ computers to unravel the complexity.
The combination of the gas chromatograph and mass spectrometer is an attempt to separate the organic chemicals in the gas chromatograph in order to reduce the complexity of the mass spectra in the mass spectrometer.
The copending application of Martin J. Cohen, David I. Carroll, Roger F. Wernlund, and Wallace D. Kilpatrick, Ser. No. 828,402, filed May 27, I969, and entitled Apparatus and Methods for Detecting and Identifying Trace Gases, discloses a combined Plasma Chromatograph and 'mass analyzer having greater sensitivity and resolution than instrumentation available heretofore. The term Plasma Chromatography" refers to a technique described more fully in an earlier 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," by which measurements upon trace gases can be performed at atmospheric pressure without modification of the parent materials. Moreover, the measurements can be performed very rapidly (in seconds) and at high sensitivity (of the order of 1 part in 10", for example). Succinctly stated, the system of the earlier copending application involves the formation of primary or reactant ions from a reactant gas 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 velocity or mobility of the ions in an electric field. A significant advantage of this system is that measurements are preferably performed at or about atmospheric pressure, so as to maintain the mean free path of the ions much smaller than the dimensions of the cell and so that the ions reach statistical terminal velocity in the drift field dependent upon their mass and may be sorted in accordance with their mobility. The resolution of the basic Plasma Chromatograph is. however, not as great as that of the gas chromatograph, for example, but the Plasma Chromatograph-Mass Analyzer described in the later copending application has far better resolution, while maintaining the sensitivity of the basic Plasma Chromatograph.
BRIEF DESCRIPTION OF THE INVENTION The present invention is concerned with further embodiments of the Plasma Chromatograph-Mass Analyzer and is especially concerned with the simplification of the apparatus and with the analysis of discrete samples. It is accordingly a principle object of the invention to provide improved apparatus and methods of this type.
A further object is to provide improved Plasma Chromatograph apparatus and methods and improved mass analyzer apparatus and methods.
Briefly stated, in accordance with the invention as applied to discrete samples, a sample of known volume (or weight) is injected into a first chamber and volatilized. Trace components of this sample are ionized by ion-molecule reactions with reactant ions produced in the chamber. The ions are subjected to a drift field which causes them to migrate through the first chamber toward an aperture leading to a second chamber. During this migration conditions are maintained in the first chamber so that the mean free path of the ions is short relative to the dimensions of the chamber. The ions may be separated and focused before passing through the aperture to the second chamber, which is maintained under high vacuum. Certain groups of ions may be rejected in the second chamber and other groups permitted to enter a mass analyzer therein, which may be adjusted to permit ions of a particular mass to produce an output, through employment, for example, of an electron multiplier and a multichannel analyzer.
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. 1 is a diagrammatic longitudinal sectional view of a first form of the invention;
FIG. 2 is a similar view of a second form of the invention; and
FIG. 3 is a similar view of a third form of the invention.
DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 of the drawing, apparatus in accordance with the invention may comprise an envelope 10 having, in sequence, a Plasma Chromatograph chamber 12 and a mass analyzer chamber 14. The envelope may be formed of metal, for example, and the chambers may be separated by a wall 16 having a central aperture 18. Chamber I2 is provided with a pair of principal electrodes, one of which may be constituted by wall 16, and the other by a planar member 20 spaced from the wall 16. Electrode 20 has a central inlet opening 22 connected to an inlet manifold 24, which includes a section of insulating pipe 26 to isolate the electrical potential of electrode 20. Since the inlet manifold is open to the chamber 12, it may be considered as part of the chamber.
A discrete sample to be analyzed is injected into chamber 12 by means of a syringe 28, such as a Hamilton Model 7001N. The hollow needle 30 of the syringe passes through a silicone rubber septum 32. A reactant gas enters chamber 12 through an inlet pipe 34, and a host gas enters chamber 12 through inlet pipe 36. Shutoff valves may be provided and are indicated diagrammatically at 38. An ionizer 40, such as a radioactive material or other type as set forth in the copending applications, is provided adjacent to the opening 22. The inlet manifold 24 is provided with a heater 39 (shown diagrammatically) which may be an oven surrounding portions of the manifold, or even the entire envelope.
Supported within chamber 14 are an electrode 42, such as a planar member with a central aperture 44, a mass analyzer 46, and an ion-detector output device 48. The mass analyzer is preferably of the conventional quadrupole type, the quadrupole structure being illustrated diagrammatically by two of the rods 50. The ion detector preferably includes an electron multiplier, such as Bendix Channeltron, indicated diagrammatically by electrodes 52, 54 and 56. lons striking the input cone 52 produce electrons which are multiplied by secondary emission within the multiplier structure 54 to produce an amplified output at the anode 56. Electrodes 20, 42, 50, 52, 54, and 56 have leads which pass through the envelope wall by means of insulators 58. The lead for electrode 16 does not require an insulator, because this electrode is at the potential of the envelope. Chamber 14 is exhausted by means of a pipe 60 connected to a vacuum pump. Readout apparatus 62 is connected to anode 56 through a coupling capacitor 64 (to the opposite sides of which resistors 66 and 68 are connected). The readout apparatus may be a pulse counter with numerical display, an X-Y recorder, or other conventional apparatus.
Typically, for positive or negative ion analysis the lettered terminals shown in FIG. 1 may have the following static potentials applied thereto (relative to ground) from a suitable DC power supply, such as a battery with a voltage divider and appropriate taps:
In the operation of the apparatus shown in FIG. 1, a discrete sample of unknown concentration, such as the types of organic materials referred to previously, is placed in the syringe 28, which may have a total sample volume of l microliter, for example, and is injected into the inlet manifold 24 of chamber 12. For explanatory purposes it may be assumed that the sample is g. of slightly soluble organic material, such as vanillin in water. The heater 39 raises the temperature of the apparatus to a level (e.g., l50 C.) sufficient to volatilize the liquid and solute. The injection may be made directly into the gas mixture flowing into chamber 12 from pipes 34 and 36. Gas flow may be provided from pressurized sources, for example. The primary reaction gas through pipe 34 may be oxygen and the host gas through pipe 36 may be nitrogen. Other reaction gases include methane or other hydrocarbons or other gases of higher electronegativity or higher ionization potential than the sample gas. Furthermore, either the host or primary reaction gas may contain a selected amount of a secondary reaction gas, typically a solvent such as water, lower alcohol, ammonia, or others depending upon the ion product desired.
The volatized sample mixes with the preheated gases, flows through the electrically insulated section of tubing 26 (such as quartz) and passes over the radioactive source 40, such as tritium or nickel 63. The mixture is irradiated by the beta rays from the radiation source. The initial ionization of the predominant population of nitrogen and oxygen occurs to form positive nitrogen and oxygen ions as well as electrons. Either sign of charge may be used.
Considering the negative sign, the free electrons quickly attach to form 0 ions, since the 0 population is predominant of the electronegative species. This reaction occurs within the inlet in the vicinity of the radioactive source 40 and in front of the inlet. A high negative potential (e.g., from l 00 to -l ,000 volts) is placed upon electrode 20. Assuming that chamber 12 has a i000 cm volume (the volume may be l0 cm to 2,000 cm", for example), the length of the chamber may be about 10 cm. and electrode 20 may be 8 cm. by 8 cm. The negative ions are repelled toward the grounded anode 16, which may be spaced from cathode 20 by about 8 cm. The 0 ions travel at a speed of 1.6 cm. per volt per cm. at one atmosphere (the assumed pressure in chamber 12) and 273 C. With an electric drift field between electrodes 16 and 20 of volts per cm., the drift time for such ions to reach electrode 16 is about 0.033 seconds. During this time the oxygen ions collide with other ions at the rate of 2.5 l0 per second and make l.3 l 0 collisions, the mean free path being much smaller than the chamber dimensions. If 10 gram of vanillin (molecular weight 152) is in the 1,000 cubic cm. volume, there are approximately 6X10 moles of vanillin in l/22 mole of air." This is a molecule ratio of 1.2Xl0 of vanillin molecules to air molecules. Thus each 0 ion has a l.5 10 chance of forming a negative ion of the trace material of the sample.
Typically, with a 0.001 diameter aperture 18 and a 1,000 liter per second pumping speed for the evacuation of chamber 14, which maybe maintained at 10" Torr, about 0.15 cm. /sec. of STP gas equivalent flows from the one atmosphere region of chamber 12 through aperture 18. This flow carries 10 ions/sec. into the mass analyzer chamber 14. With a probability of the trace population of LSXIO about 1,500 ions/sec. are trace and the difference O ions. With the use of pulse counting in the output, for example, this provides a very strong signal.
In an average case of a pure trace material, using a 100 second counting period, as low as one ion per second in a total of 10 per second is observable. Thus, in this example a trace detection level of less than 10' is achievable under optimum conditions.
After the ions pass through the aperture 18, they are focused by electrode 42 into the quadrupole rods 50 of the mass analyzer. An ion of a selected mass (determined by the potentials conventionally employed in adjusting the mass analyzer) strikes the cone 52 of the multiplier 48. The resultant electrons are multiplied and produce a pulse count at the anode 56.
It is now of interest to consider sample handling and counting time. Injection of the sample into the inlet manifold is considered instantaneous. The manifold is maintained hot enough to volatilize the sample completely, so that the sample is carried by the gas flow into the chamber 12. With a flow of 0. l5 cm./sec. leaving the chamber 12, the pressure in the chamber will remain essentially constant with a total inlet flow of 0.15 cm. /sec. In the elementary case it is simpler to make the injection into an evacuated manifold in order to permit effusive flow to distribute the sample all over the chamber. Then gases can be permitted to flow into the chamber until the pressure is above atmospheric pressure, say 1% atmospheres. A premeasured volume of the gases can be connected to inlets 34 and 36, so that opening valves 38 causes flow to a final pressure which has been previously determined. In any case, the pressure reaches the operating level in say 30 seconds. The inlet valves 38 are then shut. The gas pressure in the 1 liter chamber 12 then slowly falls as the sample enters the mass analyzer chamber 14.
At the leakage rate of 0.15 em /sec. of STP gas, a time period of about 55 minutes is required to reduce the pressure from 1 /4 to A atmospheres. Thus, the pressure is sensibly constant for a measurement period of 100 seconds or more. A correction for pressure and differential diffusion through the aperture can readily be made for highest accuracy. Even if the chamber volume is reduced to 100 cm, sufficient measurement time of 5.5 minutes is provided. This reduction may be appropriate where another factor of ten of increased concentration is desired.
The ion-molecule reactions proceed from the vicinity of the radioactive source within the inlet opening 22 and toward the electrode 16. A planar source facing electrode 16 would also be suitable.
In the embodiment just described no pulsing is employed in the reaction region. There is a steady direct current through the aperture 18 into the mass analysis chamber 14. Space charge produces a limit to the number of charges that can be moved through the aperture into the mass analyzer. The effect of space charge is to limit the ion current that can be drawn between two electrodes in a steady state ion current case. Typically, for a single ion of mobility [.L, the current density j that can be drawn between two electrodes d cm. apart is:
where V is the applied voltage.
For an applied field of 100 volts per cm. and the spacing between electrodes 16 and 20 of 8 cm.,j=2.5 l amps for .=2 cmI /sec. volt. This is equivalent to a charge density of about l0 ions/cm". With a gas flow of 0.2 cm."/sec., about 2X 10 ions/sec. enter the mass analyzer.
Operating in a direct current mode, with no separation of ions by mobility as taught in the aforesaid copending applications, an increase in voltage will increase the number of ions in the signal if an adequate ionization source is present. The number of signal or product ions is proportional to the time available for reaction as long as the total number of reactant ions is small (less than l0 percent). The product or trace ions j formed relative to the reactant ions j,, is given by the following relationship:
1 amp/cm. l
Thus, the number of the trace ions increases as the potential is increased and the distance decreased. Subject, therefore, to background noise, measurement time, and other factors in the instrumentation, an optimum value of the reaction region parameters, V and d, exists. It is also apparent that further calculations can be made to determine reaction rates and the like from observed signals with the simpler reactions.
Aperture 18 coupling the Plasma Chromatograph to the mass analyzer permits both neutral gas and charged ions to enter the mass analyzer chamber. With space charge saturation, a small fraction of trace ion at the threshold of sensitivity is buried in the larger number of reactant ions. As long as these ions are intermixed the limitations of the mass spectrometer apply. The rule of thumb is that ions differing in several mass units can be distinguished if the trace population is or better. In other words, one ion in a million can be seen. Obviously, a higher fraction is desirable. Thus, apparatus and methods which improve this ratio (subject to obtaining a sufficient signal) are highly desirable.
Focusing of ions toward an aperture does not serve a significant purpose in principle if the ions are already crowded together as tightly as permissible. If the ions can be separated, then the local space charge of the product is considerably below saturation and focusing techniques will be advantageous. The manner in which ion separation and focusing may be achieved will be described hereinafter in conjunction with further embodiments of the invention.
In the embodiment of FIG. I, which employs a two-electrode or diode Plasma Chromatograph, only certain features of the gated Plasma Chromatograph (described in the aforesaid copending applications) are present, namely, reactant ion formation and ion-molecule reaction to form a product ion. Since there are no ion gates (e.g., shutter grids) as in the gated Plasma Chromatograph, ion drift time separation or resolution, as in the gated apparatus, does not occur. The additional control provided by a third electrode (grid), as will now be described in conjunction with FIG. 2, permits the separation of the ions by a drift time or mobility difference. For simple mixtures containing a single trace, mobility differences may be sacrificed and sensitivity enhanced by the use of the diode embodiment of FIG. I. The DC signal of this embodiment gives more trace current than ingated embodiments, and the mass spectrometer provides adequate mass resolution. A triode arrangement, as will now be described in conjunction with FIG. 2, provides ion drift time resolution (although not as great as in the tetrode arrangement set forth in the aforesaid copending applications). The resolution improvement is, however, obtained at the expense of decrease in current.
Referring to FIG. 2, the apparatus illustrated is essentially the same as that shown in FIG. 1, except that an additional electrode 70 is added. The purpose of this electrode is to permit turn-on and turnoff of the radioactive source current without changing the motion of the ions in the region between this electrode and electrode 16. Electrode 70, which may be an isopotential conventional grid of parallel wires, is located adjacent to electrode 20 and constitutes with electrodes 20 and 16 a triode structure. The DC operating potential" for the electrodes described in connection with FIG. 1 may have the values given previously, in which event the potential applied to terminal G of electrode 70 from the power supply may be +1 ,200 volts for operation with positive ions or 1 ,200 volts for operation with negative ions. In addition, pulsating potentials are applied to electrode 20. For example for positive ions a square wave having a base line at grid potential and a positive peak of +l50 volts may be superimposed upon the DC operating potential at terminal A, while for negative ions a negative square wave may be applied.
Assuming negative ions, when the negative-going pulses of the square wave are applied to the cathode 20, the negative ions will be repelled toward and past the grid 70. Between such pulses, when the cathode is at its static potential, there will be no difference of potential between the cathode and the grid 70, and the ions will not be so moved. Thus, bunches or groups of ions will start out toward the anode 16 separated in time and space. During the drift between electrodes 20 and 16, the ions of each bunch will become separated in accordance with their drift velocity, and the faster reactant ions will reach the aperture 18 before the shower product ions. For a drift field of volts per centimeter, a negative reactant ion with a reduced mobility of L6 cmF/volt sec. will travel at the rate of 240 cm./sec. and cross the 8 cm. drift space in 0.033 seconds. A slower ion of reduced mobility of 2.4 cmF/volt sec. will require three-halves of the time or about 0.40 seconds. There is thus 7 milliseconds difference in ion transmit time across the drift space.
The fast ion, which has the greater population, may be removed from the beam before it reaches the mass analyzer and detector by gating the focusing electrode 42 in the proper delayed time phase. Thus a negative square wave (which may be identical to that previously described) may be superimposed at terminal C upon the static potential applied to electrode 42 to repel the negative fast ions. Alternatively, a high positive pulse voltage may be applied to this electrode to collect the fast negative ions, particularly if it is desired to measure the ion current. A further alternative is the use of a shutter grid (as described in the said copending applications) or a deflection plate arrangement. In any event the fast ions are prevented from reaching the mass analyzer. Only the wanted slower ions produce a readout. This increases the signal-to-noise ratio very significantly.
Typically, the square waves employed may have an ON (and OFF) time equal to the difference in transit time for the fast and slow ions to be separated. Thus, in the example given above, the square wave may be ON for 7 milliseconds and OFF for 7 milliseconds. The delay in applying the square wave gate to electrode 42 relative to the time of application to electrode 20 may be equal to the drift time of the fast ion less an integral multiple of the difference in transit time between the slow and fast ion. Thus the delay may, in the sample above, be computed as: 33-[4X(4033)]=5 milliseconds. The delay between the aperture 18 and electrode 42 is neglected.
With the trace and reactant ions separated as set forth with respect to FIG. 2, it is now possible to employ ion focusing or concentration productively. The trace ions in the arrangement of FIG. 2 will be well under the maximum concentration near the limits of detection thresholds. Thus, a convergent electric field with its center at the aperture 18 will result in an increase of signal compared to the linear case set forth in FIG. 1. The embodiment shown in FIG. 3 illustrates this concept.
In the embodiment shown in FIG. 3, the structure is essentially as described in connection with FIG. 2, except that the electrodes 20 and 70 have now been made spherical and designated by reference numerals 20 and 70. Each electrode is a section of a sphere with the center at the aperture 18. Between the electrode 70' and the aperture, the spherical electric field is preserved by a resistive envelope electrode 72 which has a conical configuration and a special change of resistivity per unit radial distance. The resistance is designed so that when a potential is applied between the base of this electrode and the apex at the aperture 18 (which may be at ground or Zero potential), the potential along the inner surface of the conical electrode is the same as the concentric equipotential which would exist there in the absence of this electrode. This potential is given by I'Vo r V R where V,, is the voltage on electrode 20, located at R cm. for the aperture and V,. is the potential at a distance r cm. from the aperture. The potential applied to terminal K may be +1 ,000 volts for negative ions or l,000 volts for positive ions with the voltages previously mentioned for the other electrodes.
The resistive cone may be replaced by a set of spaced guard rings of progressively smaller diameter corresponding to successive elements of the cone, the guard rings being connected to taps on a suitable voltage divider potential supply to provide the potentials corresponding to those at the associated elements of the cone. Alternatively, electrode 16 may itself produce the convergent field by making this electrode a conical resistive electrode (like electrode 70') or by constructing this electrode of a series of guard rings and insulating separators to form a cone. Guard rings may also be used in the chamber 12 to maintain the uniformity ofthe drift field.
Although the readout apparatus 62 may be of the types previously set forth, it is highly advantageous, especially in gated Plasma Chromatographs, such as the triode versions of FIGS. 2 and 3 or the tetrode (shutter grid) versions of the aforesaid copending applications, to employ a multichannel analyzer, such as Instrument Computer Model No. 1072-S sold by Fabri-Tek Instruments, Inc., as the readout apparatus. The combination of multichannel analyzer and electron multiplier provides very wide band, high gain amplification, which permits the detection of each and every single ion passing through the mass analyzer. Thus, as fast as the ions traverse the aperture 44. and impact upon the multiplier, they produce a readout which is sorted into the proper time delay channel by the multichannel analyzer (which may be synchronized, for
example, with the square wave applied to electrode 20 or 20' This effect permits a mass analysis of all of the ions. The count rate in a selected mass channel is proportional to the original amount of the corresponding trace. An emperical calibration may be performed with known samples under given operating conditions.
If desired, the usual Plasma Chromatograph ion drift time spectrum can be obtained in the embodiment of FIGS. 2 and 3 by setting the quadrupole resolution to zero to permit all of the ions to pass through and produce a signal at the multiplier. The high gain of the multiplier yields a high signal level. The multichannel analyzer may be employed with tetrode (shutter grid) Plasma Chromatographs of the type described in the aforesaid copending applications. The use of a multichannel analyzer in conjunction with a Plasma Chromatograph makes possible the avoidance of shutter grids and movable delay gates, however. The electron multiplier multichannel analyzer combination can be used as a readout f or a Plasma Chromatograph whether or not a mass analyzer is used.
The selectivity and sensitivity of the Plasma Chromatograph-Mass Analyzer technique is believed to result from the following characteristics:
1. A method of detecting trace gas, which comprises admitting a discrete quantity of trace gas to a first chamber, forming primary ions at a localized region of said first chamber, forming product ions of different mobility at a further region of said first chamber by ion-molecule reactions involving said primary ions and molecules of said trace gas, applying a drift field to said ions and causing them to drift toward a second chamber, maintaining the pressure in said first chamber at a high enough level such that the mean free path of the ions in said first chamber is very small compared to the dimensions of said first chamber and such that said ions reach substantially constant statistical terminal velocity in said first chamber dependent upon their mass, passing said product ions to said second chamber, maintaining the pressure in said second chamber at a substantially lower level than that in said first chamber such that the mean free path of the product ions in said second chamber is substantially longer than the mean free path in said first chamber, analyzing product ions in said second chamber in accordance with their mass, and detecting at least some of the analyzed ions.
2. A method in accordance with claim 1, in which said discrete quantity of trace gas is produced by volatilizing a discrete sample of a volatile material.
3. A method in accordance with claim 1, in which the primary ions are formed by ionizing a reactant gas.
4. A method in accordance with claim 1, in which said first chamber is evacuated when said discrete quantity of trace gas is admitted thereto, and thereafter a quantity of a host gas is admitted to said first chamber to raise the pressure thereof to a level at which the mean free path of the ions in said first chamber is compared to the dimensions of said first chamber.
5. A method in accordance with claim 1, in which a reactant gas is admitted to said first chamber and in which said primary ions are produced by ionizing said reactant gas in said first chamber.
6. A method in accordance with claim 1, in which said sample is volatized by applying heat thereto.
7. A method in accordance with claim 1, in which a host gas is admitted to said first chamber in a quantity sufficient to compensate for loss of gas from said first chamber to said second chamber.
8. A method in accordance with claim 1, in which the ions passed to said second chamber are separated in accordance with their mobility in said first chamber.
9. A method in accordance with claim 8, further comprising rejecting certain ions before said analyzing.
10. A method in accordance with claim 1, further comprising focusing the ions passed to said second chamber before said ions are analyzed.
11. A method in accordance with claim 1, in which said detected ions are detected by producing electrons in response thereto and multiplying said electrons.
12. A method in accordance with claim 1, in which the ions in said first chamber are focused before being passed to said second chamber.
13. Apparatus for performing ion measurements, which comprises an envelope having successive first and second chambers, means for injecting a discrete volatile sample into said first of said chambers, means for volatilizing said sample, means for admitting a reactant gas to said first chamber, means for ionizing said reactant gas at a localized region of said first chamber to produce primary ions which react with molecules of the volatilized sample at a further region of said first chamber to produce different mobility product ions in said first chamber, means for applying a drift field to said ions in said first chamber to cause them to drift toward said second chamber, means for admitting ions to said second chamber, means for analyzing ions admitted to said second chamber in accordance with their mass, and means for detecting at least some of the analyzed ions, said second chamber having means for evacuating the same to a pressure at which the mean free path of product ions therein is long compared to the dimensions of said second chamber, and said first chamber having means for admitting a host gas thereto to maintain the pressure therein at a value at which the mean free path of the product ions in said first chamber is very short compared to the dimensions of said first chamber and such that said ions reach substantially constant statistical terminal velocity in said first chamber dependent upon their mass.
14. Apparatus in accordance with claim 13, said first chamber having valve means for controlling the admission of reactant gas and host gas thereto.
15. Apparatus in accordance with claim 13, said means for injecting said sample comprising a syringe.
16. Apparatus in accordance with claim 13, said means for volatilizing said sample comprising a heater.
17. Apparatus in accordance with claim 13, said detecting means comprising an electron multiplier.
18. Apparatus in accordance with claim 17, said apparatus having a multichannel analyzer connected to the output of said electron multiplier.
19. Apparatus in accordance with claim 13, said means for admitting ions to said second chamber comprising an aperture.
20. Apparatus in accordance with claim 19, further comprising means for focusing ions from said aperture to said analyzing means.
21. Apparatus in accordance with claim 19, further comprising means for focusing ions in said first chamber toward said aperture.
22. Apparatus in accordance with claim 21, said focusing means comprising an arcuate electrode.
23. Apparatus in accordance with claim 22, said focusing means comprising tapered electrode means converging from the direction of said arcuate electrode toward said aperture.
24. Apparatus in accordance with claim 23, in which said tapered electrode means has means for producing progressively different potentials therealong.
25. Apparatus in accordance with claim 13, further comprising means for segregating the ions in said first chamber in accordance with their velocity, and means for rejecting certain ions before said analyzing means.
26. Apparatus in accordance with claim 25, said drift field applying means comprising a pair of electrodes spaced apart in said first chamber, said means for segregating said ions comprising a grid between said electrodes adjacent to one of said electrodes and means for applying pulsating potentials between said grid and said one electrode.
27. Apparatus in accordance with claim 26, said means for rejecting ions comprising an electrode and means for applying pulsating potentials to the last-mentioned electrode in synchronism with the first-mentioned pulsating potentials.
28. Apparatus in accordance with claim 27, wherein said pulsating potentials are square waves. 5
29. Apparatus for performing ion measurements, which comprises an envelope, means for forming primary ions at a first localized region of said envelope, means for producing product ions at a second region of said envelope by reaction of a sample gas with said primary ions, means for causing said ions to drift toward a third region of said envelope, means for maintaining the pressure at said first and second regions high enough such that the mean free path of the ions at said regions is very much smaller than the dimensions of said regions and such that said ions reach substantially constant statistical terminal velocity in said regions dependent upon their mass, means for maintaining the mean free path of said ions at said third region long relative to the dimensions of said third region, and means for producing an output from at least some of the ions reaching said third region, said output producing means comprising a mass analyzer feeding a multichannel analyzer.
30. Apparatus in accordance with claim 29, in which said output producing means comprises an electron multiplier and said multichannel analyzer arranged in sequence.
31. Apparatus for performing ion measurements, which comprises an envelope having first and second successive chambers separated by a wall having an aperture through which said chambers communicate, means for forming primary ions at a localized region of said first chamber and for forming different-mobility product ions of a sample gas at a further region of said first chamber by reaction of the gas with said primary ions, means for subjecting said ions to a drift field and causing them to drift toward said aperture, means for maintaining the pressure in said first chamber at a high enough level such that the mean free path of the ions in said first chamber is very much smaller than the chamber dimensions and such that said ions reach substantially constant statistical terminal velocity in said first chamber dependent upon their mass, means for maintaining the mean free path of the product ions in said second chamber substantially longer than the dimensions of said second chamber, means for separating the product ions in said second chamber in accordance with their mass, and means for detecting at least some of the last-mentioned product ions.
32. Apparatus in accordance with claim 31, said means for subjecting said product ions to a drift field comprising a pair of electrodes in said first chamber.
33. Apparatus in accordance with claim 33, further comprising a grid between said electrodes and adjacent to one of said electrodes, said product ions being formed adjacent to said one electrode and before said grid, and means for applying gating potentials between said grid and said one electrode for permitting groups of ions to drift from said one electrode toward the other electrode.
34. Apparatus in accordance with claim 33, further comprising means for preventing some of said ions from reaching said separating means in said second chamber.
35. Apparatus in accordance with claim 34, said preventing means comprising an electrode in said second chamber, and means for applying pulsating potentials to that electrode.
36. Apparatus for performing ion measurements, which comprises an envelope having first and second successive chambers, means within the first of said chambers for forming product ions of a sample gas by reaction of the gas with primary ions, means for maintaining the pressure in said first chamber at a high enough level such that the mean free path of the ions in said first chamber is very much smaller than the chamber dimensions and such that said ions reach substantially constant statistical terminal velocity in said first chamber dependent upon their mass, said chambers being separated by a wall having an aperture through which said ions may pass from said first chamber to said second chamber, means in said first chamber for segregating ions in accordance with their mobility, means in said first chamber for focusing segregated ions upon said aperture, means for evacuating said second chamber, and means including a mass analyzer in said second chamber for detecting ions passed through said aperture.
37. Apparatus in accordance with claim 36, said focusing means comprising arcuate electrode means for forming a field which converges toward said aperture.
38 Apparatus in accordance with claim 37, said focusing means further comprising tapered electrode means converging toward said aperture from said arcuate electrode means.
39. Apparatus in accordance with claim 38, said tapered electrode means being resistive.
40. Apparatus in accordance with claim 39, further comprising means for forming said product ions into groups adjacent to said arcuate electrode means.
41. Apparatus for performing ion measurements, which comprises a first chamber having means for forming product ions at a first region thereof by reaction of a sample gas with primary ions, means for gating said ions in groups toward a second region of said chamber, means for maintaining the pressure in said first chamber at a high enough level such that the mean free path of the ions in said first chamber is very much smaller than the chamber dimensions and such that said ions reach substantially constant statistical terminal velocity in said first chamber dependent upon their mass, means for causing the gated ions to move from said first region to said second region and to separate in accordance with their velocity between said regions, a second chamber adjacent to said second region, ion detecting means in said second chamber, means for evacuating said second chamber, aperture means for passing ions and molecules from said second region to said second chamber, and means in said second chamber for preventing certain ions from reaching said detecting means.
42. Apparatus in accordance with claim 41, said means for gating said ions comprising an isopotential grid and means for applying pulsating potentials thereto.
43. Apparatus in accordance with claim 42, said preventing means comprising an electrode having an aperture therein through which ions may pass and having means for applying pulsating potentials thereto for repelling ions or for collecting ions.