US 3742213 A
Apparatus and methods for detecting gases (such as those indicative of human activity) and for gas analysis, concentration, or purification. Negative ions formed by electron attachment to electronegative components of a gaseous sample are separated from neutral molecules and quantized. In one embodiment negative ions formed at atmospheric pressure are classified according to their mobility in an electric field and are collimated, accelerated, and detected in vacuo.
Claims available in
Description (OCR text may contain errors)
United States Patent Cohen et al.
[ June 26, 1973  APPARATUS AND METHODS FOR 2,810,075 10/1957 Hall 250/419 TF DETECTING, SEPARATING, 3,217,161 11/1965 Craig 250/419 CONCENTRATING AND MEASURING FOREIGN PATENTS OR APPLICATIONS ELECTRONEGATIVE TRACE VAPORS 314,171 2/1918 Gennany 55/101 Inventors; Martin J Cohen West Palm Beach; Great Britain in??? W. Crowe, Lake Worth, both OTHER PUBLICATIONS lon Source For Low Energy Collision Studies, D.  Asslgnee: Franklin Gno Corlmrauo! west Vance, Review of Scientific Instruments, Vol. 34 No.
Palm Beach, Fla. 8, August 19 1  Filed: Jan. 28, 1971 Primary Examiner-James W. Lawrence  Appl' 1102634 Assistant Examiner-C. E. Church Related Application Data Att0mey-Raphael Semmes  Continuation of Ser. No. 618,635, Feb. 27, 1967,
abandoned. [57 ABSTRACT 52 US. Cl. 250/419 TF, 250/419 s13, 250/419 0 'PW 9 methods for f ng gases (such as 51 Int. c1. H0lj 39/34, 1301a 59/44 mdcatlve human i 835  Field of Search 250/41 9 SB 41 9 TF concentration, or purificatlon. Negatwe ions formed by 250/41 9 d 9 electron attachment to electronegative components of r a gaseous sample are separated from neutral molecules  References Cited and quantized. In one embodiment negative ions formed at atmospheric pressure are classified accord- UNITED STATES PATENTS ingto their mobility in an electric field and are collimated accelerated and detected in vacuo a ton 2,854,583 9/1958 Robinson 250/419 18 Claims, 7 Drawing Figures IO .12 a 11.
MR IN AIR FLOW SAMPLE m OUT METER coNcENTRAToR Sample I l j I8 22 24 THERMAL ATTACHMENT NEGATIVE NEGATlVE NEGATIVE ii ggafif ELECTRON DRIFT ION BEAM 1014 IoN MASS MULTIPLIER GENERATOR CHAMBER COLLIMATOR ACCELERATOR SELECTOR DETECTOR 5" i I I 30 l I I I 5 ELECTRO- DIRECT I AuToMATIc METER 0 SAMPLE 1 CALIBRATOR SINGLE PULSE CONCENTRATION 1 l COUNTER READOUT L 28 as i VACUUM VACUUM PUM P PUMP 7 No. 1. NO. 2
PAIENTEDJUN 2-6 I975.
SNEHZUFS ROBERT W- CROWE' ATTORNEY PAIENIED JUN 26 I975 3. (42 2 1 3 SHEEF 3 BF 5 I2 66 78 x 78 76 39 I f l f 78 j l j M M M M M -M M M M M M M M M- M 48 Air 8 M M M e M M M e r 68 M 72 Negative HVGFHomem F 6 pp y I52 I50 I l [Signal 5 no i i: ii las I.
h I68 3 I84 1 Air In g; u :54 s A 1 ii I I68 is Multiplier E: Detector I it 156* Vacuum Pump A Q 6 v INVENTOR5 MARTIN J- COHEN ROBERT W. CROWE BY WW ATTORNEY PATENTEUJUNZB 197a NO v: mPSom 3.2 :5
0: m9 K 4 .532 3200mm 1 19.305 1 INVENTORS MARTIN J. COHEN ROBERT W. CROWE $323 3058 new 2 622 n n5m mmiom ATTORNEY APPARATUS AND METHODS FOR DETECTING, SEPARATING, CONCENTRATING AND MEASURING ELECTRONEGATIVE TRACE VAPORS This is a continuation of application Ser. No. 61 8,635 filed Feb. 27, 1967, now abandoned.
This invention relates to apparatus and methods utilizing negative ions formed by electron attachment to molecules of electronegative substances. More particularly, the invention is concerned with separating electronegative components of a gas and for sorting, detecting and measuring molecular quantities of electronegative substances in large gaseous samples.
Heretofore, gas chromatography has been the best available solution to the problems of discriminating between organic gases and the detection of trace vapors. While gas chromatography will resolve many trace gases with high sensitivity, the apparatus is limited to small samples per unit time. Moreover, the apparatus is complex, expensive, bulky, and requires skilled operators. A need has existed for greatly improved instrumentation, and the problem has been intensified by the demand for an instrument which is capable of reliably detecting human activity in a seclusive environment, such as a jungle.
It is accordingly a principal object of the invention to provide such improved instrumentation and more particularly to provide improved systems, apparatus, components, and methods for the detection, separation, analysis, and measurement of molecular quantities of trace substances or impurities in large gaseous samples.
A further object of the invention is to provide apparatus and methods for purifying gaseous environments and for the conversion and qualification of test substances.
Another object of the invention is to provide apparatus and methods for concentrating molecular quantities of electronegative gases.
Briefly stated, the apparatus and methods of the invention utilize the phenomenon of electron capture by electronegative gas molecules to form negative ions. The ions may then be concentrated, separated, detected, and measured. Mass separationis preferably accomplished by utilizing the difference in mobility of ions of different mass in anelectric field applied to a gas. In a preferred form of apparatus, a cloud of negative ions is formed by subjecting a gas at atmospheric pressure to a pulse of low energy electrons. Thereafter,
a drift field is applied to cause the ions to migrate toward an ion gate or shutter, which is opened at and for a predetermined time in order to pass ions of predetermined mobility. These ions are then collimated, accelerated, and detected in vacuo. The apparatus of the invention has rapid response, very high sensitivity and mass resolution, and high signal-to-noise ratio. It is simple and compact, analyzes large quantities of gas continuously, without requiring the collection of isolated samples, and performs its functions without substantial fragmentation or decomposition of the test material.
The invention will be further described in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and wherein:
FIG. 1 is a block diagram of one form of system in accordance with the invention for separating, classifying, and measuring electronegative materials in a gaseous sample;
FIG. 2 is a diagrammatic perspective view, partially broken away, of apparatus for performing the functions of the block diagram of FIG. 1;
FIG. 3 is a diagrammatic sectional view of apparatus for concentrating electronegative gas molecules;
FIG. 4A is a partially schematic block diagram of a system utilizing the ion mobility principle for separating and measuring electronegative components of a gas;
FIG. 4B is a waveform diagram illustrating the operation of the system of FIG. 4A;
FIG. 5 is a block diagram of a control system for the apparatus of FIG. 4A; and
FIG. 6 is a diagrammatic sectional view of a gas analyzer cell in accordance with the invention.
Most complex gas molecules have a strong natural tendency to form negative ions upon interaction with free electrons of low kinetic energy. These ions become relatively stable when produced at moderate gas pressures. Gases having this tendency are known as elec tronegative gases. Most toxic materials which can be introduced into the air contain chemical groups that are known to be electronegative.
Negative ions of such electron-attaching gases may be produced by exposing the gas to a cloud or a beam of free electrons at moderate pressures. The attachment process is most efficient when the electrons are near thermal energies. The electrons may be produced by photoemission from a photosensitive cathode, by electron beam techniques, by a radioactive source, by corona discharge, by thermoemission from a hot filament, or by avalanche. The utilization of two of these techniques will be described in greater detail hereinafter.
There are three important mechanisms for the production of negative ions by electron impact. These may be summarized as follows:
1. Direct capture A free electron may be captured by a molecule with vibrational excitation of the molecular ion and its subsequent stabilization in a collision with another molecule.
2. Charge transfer process This is a special case of the first mechanism in which stabilization is enhanced by an electron exchange process between the vibrationally excited molecular ion and another molecule of the same gas in a time short compared with the mean lifetime of the ion. It is also possible to exchange the negative electron charge between unlike molecules. In such a case, the exchange favors the species with the lower ground state and longer lifetime.
3. Dissociative attachment In this process a free electron is captured by a molecule, the excess energy going into dissociation of the molecule into two or more fragments.
At moderate pressures, all of the above mechanisms are commonly observed. Furthermore the attachment cross section associated with all of the mechanisms has its maximum value when the energy of the electron is well below one electron volt. By the use of a very low D.C. potential gradient between the cathode and the anode of an attachment chamber (to be described), essentially all of the electrons in the distribution will be in this energy range. Consequently,,attachment of electrons to the electronegative gas molecules is quite efficient. Since the probability of producing ions of identical mass by attachment of electrons to the molecules of different gases is highly remote, the detection of several electronegative gases can be accomplished by mass analysis, as will be seen hereinafter.
Referring to the drawings, FIG. 1 illustrates a system for the production, classification, identification, and measurement of gaseous electrophilic materials in accordance with the invention. A sample of gas, such as air containing electronegative trace materials, ispassed through a conventional flow meter to a sample concentrator 12, which supplies a concentrated sample to an attachment drift chamber 14. The gas in the chamber 14 is subjected to low energy electrons from a thermal electron generator 16. The resulting negative ions migrate through the drift chamber to a negative ion beam collimator 18, the resulting ion beam being accelerated by a negative ion accelerator 20. The ion beam then passes to a negative ion mass selector 22, the output of which is fed to the secondary electron multiplier and detector 24. The output of the detector is passed to an electrometer or single pulse counter 26, which is connected to an automatic calibrator 28 and to one input of a direct sample concentration readout 30. Another input to the readout is applied from the output of air flow meter 10. Vacuum pumps 32 and 34 provide successively greater evacuation in the collimator 18, accelerator 20, and mass selector 22.
A representative apparatus for carrying out the functions of the system of FIG. 1 is shown in FIG. 2. Reference numerals employed in FIG. 1 have been annexed to the correspondingportions of the apparatus of FIG. 2. The thermal electrons are produced by photo emission by allowing ultraviolet radiation from a mercury discharge lamp 36 to pass through a 50 percent semitransparent conducting metallic film cathode 38 evaporated on a quartz window 42. For improved electron emission the UV radiation is reflected from the highly polished surface of an anode 40 to impinge again upon the cathode 38. The discharge lamp 36 may be mounted in a reflector 40' and the ultraviolet radiation may pass through quartz window 42 and cathode 38 at the end of an envelope 44, in which the various electrodes of the apparatus are mounted. Air is drawn in through the inlet hose 46, the air flow meter 10 and the concentrator 12 (to be described hereinafter) by an exhaust fan 48. The concentrated sample passes through a slit inlet 50 into the drift chamber portion of envelope 44 adjacent to cathode 38.
Photoelectrons drift across the interelectrode space between the cathode 38 and the anode 40 under the influence of an applied D.C. potential. Electrode guard rings not shown may be used to keep potential gradient linear. Some of the photoelectrons become attached to electronegative molecules in the drift space, which are converted into negative ions ofcharacteristic mass. An attachment proceeds, a fraction of the negative ions pass through a narrow slit 52 in the anode 40. These ions are collimated through small apertures 54 in a series of collimating electrodes 56 at a region of the envelope in which the pressure has been decreased by differential pumping.
After the negative ion beam has been collimated, it passes through apertures 58 of accelerating electrodes 60 to the ion mass selector 62. The latter can be one of a number of types used in mass spectrometers, such as the 180 mass spectrometer described in The Review of Scientific Instruments, Vol. 35, No. 10, October, I964, pages l,265-l,267. A magnet 64 for providing the magnetic field required by the spectrometer is shown in phantom lines. Selection of the ion mass to be detected is controlled by the proper selection of the accelerating potential, as is well known in the art of mass spectrometry.
After mass selection has been made, the ions enter the secondary electron multiplier-detector 24, the output of which may be fed to an electrometer or single pulse counter, depending upon the concentration of the ion selected. A suitable electron multiplier and detector is the Model 306 sold by the Cincinnati Division of the Bendix Corporation, Cincinnati, Ohio. A continuous comparison of the ion current with the volume rate of flow of air makes possible direct measurement of sample concentration. By using standard techniques, this information can be displayed on a direct sample concentration readout meter.
Variations in the quantum efficiency of the attachment chamber cathode can be avoided by the use of automatic calibrating techniques. Such techniques may be based on the knowledge that some of the. oxygen molecules in the air form negative ions upon electron impact. By monitoring the oxygen ion current as well as the ion current of the substances to be detected, calibration can be made automatic and continuous.
FIG. 3 illustrates a negative ion concentration 12 of the invention and demonstrates the use of an electron beam source for increasing the magnitude of the electron current by many orders of magnitude. The con centrator comprises a duct 66 through which a gaseous sample for example, air containing an electrophilic test substance is drawn by fan 48. Since the electron beam is generated in a vacuum and used at atmospheric pressure, an electron-permeable but gaseous opaque wall is required. Such a wall consists of a metallic grid covered with a thin metallic foil and is indicated at 68 in FIG. 3.
The grid supports the foil to resist implosion by the difference in gas pressure between the duct 66 and an evacuated chamber 70 on the opposite side of wall 68.
A typical grid structure may be a foil of either beryllium, aluminum, or copper on a grid of the same materials. A beryllium foil of 5 X 10 centimeters thickness which is attached to a mesh grid with a 0.05 centimeter spacing is typical. Such a structure can be made many square centimeters in area and will transmit approximately 20 percent of a beam of 18 to 20 kilovolt electrons. The energy on the side exposed to the air is, on the average, reduce to 5 kilovolts or less by energy absorption in the foil. Since grid cooling is available a power dissipation of 1 watt per square centimeter is easily achieved. A net current of up to 2 X 10" amperes per square centimeter can be obtained from an initial beam current of l X 10 amperes per square centimeter. Such a current is from 10 to 10 times that available from a photoelectric or tritium source and makes feasible the forming of negative ions from a substantial part of a gas present at low concentration for the purpose of increasing its relative concentration.
Electrons e from a source within the evacuated chamber 70, such as a heated f lament 72 supplied from a suitable power supply 74, pass through the wall 68 toward an anode 76 supported upon the opposite side of the duct 66, as by insulators 78. A suitable D.C. potential for example, 15 kv) is applied between the anode 76 and the filament cathode 72 by means of the power supply 74. As the electrons e move toward the anode of the concentrator, a certain percentage of them will be captured by the electronegative molecules M to be detected. The negative ions M so produced drift to the anode, release the attached electrons, and become neutral molecules again. Under the combined influence of diffusion and dynamic air flow, the molecules tend to move back into the air stream to a position near the exit end of the concentrator. More electrons are emitted, and attachment is repeated.
The net effect of this process, operating throughout the air volume, is a concentration of M or M in the vicinity of the anode as the air approaches the exit of the concentrator. The depleted air is exhausted back into the atmosphere, while the more concentrated sample is pumped through a passage 80 adjacent to the anode 76 at the exit of the duct. In the apparatus of FIG. 2, the concentrated sample enters the drift chamber 14 through the slit 50.
The apparatus of FIG. 2 utilizes a conventional mass selector 62 in order to separate and identify different electronegative components of the gaseous sample. In accordance with another concept of the invention, which will be termed plasma chromatography, negative ions formed by electron capture are classified in a drift chamber by the use of pulse techniques which produce a prearranged timing sequence for the introduction of electrons, the application of the drift voltage, and the opening of an ion gate. By virtue of this concept of the invention, a high degree of sensitivity and mass resolution of considerable accuracy are possible at atmospheric pressure. As willbe seen more fully hereinafter, an essential difference between this concept of the invention and the schemes of the prior art resides in the sequence between the pulsing of the electron source and the application of the drift voltage to the attachment chamber.
In FIG. 4A a drift chamber is shown at 82 and comprises an envelope 84, a photocathode 86, an anode 88, and a coplanar duel grid constituted by grids 90 and 92. One or more guard rings 94 may be employed to ensure uniformity of drift field. The entire drift chamber is connected to a vacuum system by pipe 96, and a quartz window is provided at 98 so as to pass ultraviolet light from a source 100 to the photocathode 86. Samples may be admitted through pipe 96. Continuous sample admission and exit through the pipe 96 may be used as an alternate method.
With the exception of the quartz window, the envelope 84 of the drift chamber may be constructed of Pyrex glass. The elements within the chamber may be separated electrically by the use of quartz supporting rods and quartz tubing, which determine the relative position of the various elements within the chamber. The entire internal structure may be held in its proper position by the use of loading springs whose physical behavior is unaffected by repeated heating to temperatures approaching 450 C.
Basic forms of drift tubes are well known in the art. For example, reference is made to the article in Physical Review, Volume [28, No. 1, Oct. 1, 1962, pages 219-230. In the instruments of the prior art, which employ drift tubes for the investigation of electron attachment in electronegative gases, the drift voltage between the photocathode and the collector-anode has been applied continuously throughout the entire experiment. This technique results in a steady state distribution of electrons and ions throughout the drift distance. This method provides a considerable amount of information regarding the attachment characteristics of an electronegative gas, but it lacks the ability to provide maximum resolution of different electronegative ions by mobility.
In accordance with the method of the invention pulse techniques are applied so that high resolution mass selection can be achieved. Instead of supplying a constant drift voltage, the voltage is pulsed in a particular relationship with the production of the electrons. More specifically, no drift voltage is applied during the time that electrons are being produced at the cathode surface. This allows attachment to proceed in the vicinity of the cathode while the electrons are at thermal energy, where attachment is more efficient. Furthermore, it permits the build up of a well defined ion pulse at a known location between the electrodes of the drift chamber, because in the absence of an applied electric field, photoelectrons move from the cathode surface only by diffusion processes.
In a typical apparatus of the invention the well defined pulse or cloud of ions at the photocathode 86 is formed by exposing the cathode to a pulse of ultraviolet light from source 100 to produce thermal energy electrons and in the absence of drift voltage. The pulse may be produced by the use of a conventional light chopper disc 102 driven by a chopper motor 104 under the control of a motor control 106. The pulses UV in line(a) of FIG. 48 represent repetitive pulses of ultraviolet light applied to the photocathode 86.
An electrometer 108 is connected to the anode 88 and its output is recorded by a recorder 110. The system ground is located in the anode circuit so asto allow electrometer 108 to operate at ground potential. All drift tube voltage are negative with respect to ground. One adjustable voltage DC source in the power supply 112 establishes a grid average potential for both the grids and 92, which is negative with respect tothe potential of the anode 88. Additional circuits within the power supply 112 control the instantaneous potential of each grid with respect to the grid average potential. Another adjustable voltage DC source within the power supply applies a potential to the cathode 86 negative with respect to the grid average potential. Additional circuits control the instantaneous potential of the cathode with respect to the grid average potential.
During the time when ultraviolet light impinges upon the cathode, the cathode potential is switched to the grid average potential, as indicated by the pulses C in line(b) of FIG. 4B. Hence no drift voltage is applied to the tube. Upon termination of the pulses C, as indicated by the intervals D, the cathode returns to cathode average potential, and hence a drift voltage is applied. At a predetermined time following each pulse of light UV on the cathode of the drift tube, the grids 90 and 92 are switched to the grid average potential for a short period of time, as indicated by pulses G in line(c) of FIG. 4B. (The pulse widths are not shown to scale.)
The dual control grid, which consists of thin parallel wires with alternate wires connected electrically, is normally kept closed with respect to transmission of both ions and electrons by the application of an equal and opposite bias voltage (with respect to grid average potential between adjacent wires. However, during the selected spaced time intervals G the grid is madetransmitting by the application of rectangular voltage pulses whichreduce the field between adjacent wires to zero and allow most of the ions and electrons to pass through to the anode.
It can be shown that the mobility of a negative ion in a drift field is a function of its mass. By Langevins theory, mobility is approximately proportional to the reciprocal ofthe square root of the mass.) By virtue of the application of drift voltage only after the formation of the negative ions, a well defined cloud or pulse of negative ions is produced, the position of which (adjacent to the photocathode86) is predetermined. Hence by opening the ion gate constituted by grids 90 and 92 a predetermined time delayed with respect to production of the ions and the application of drift voltage, ions of predetermined mobility are passed to the anode 88. The net result of this pulsing technique is that the electrometer will record the arrival of negative ions of different mobilities at the control grid, depending upon the delay time of pulses G with respect to pulses UV, permitting separation of electronegative components in the mixture according to the drift velocities of the ions produced. By this process high. sensitivity and high mass resolution are achieved.
While the pulsing sequence of FIG. 4B can be accomplished completely electronically, as by the use of an electronically pulsed ultraviolet lamp and by the use of electronic pulse generators and pulse delay circuits, a simple electromechanical scheme is illustrated in FIG. 4A.'Thus, pulses C are produced by employing a pair of permanent magnets 114 and 116 mounted upon the light chopper disc 102 and coupled to a pickup coil 118 of the type employed in tape recorder heads. The grid control pulses G- are produced by another such magnet 120'coupled to another coil 122. 7
Typical cathode and grid drive channels for producing the dynamic cathode and grid potentials are illustrated in FIG. 5. As each magnet passes its pickup coil, a positive and a negative going waveform is produced in the coil in synchronism with the pulse of ultraviolet light. The cathode drive channel utilizes two magnets, 114 and 1 16, one for actuating the control channel just prior to the generation of the light pulse, and the other for actuating this channel directly after the light pulse. The grid channel, however, uses only a single magnet 120. In both channels the input signal from the pickup coil is amplified by a trigger amplifier 124, which fires a Schmitt trigger circuit 126 into one of its two stable states. In the cathode drive channel the output of the Schmitt trigger is used to trigger a control binary circuit 128, which provides a count-down by two. The output of the control binary is amplified by driver amplifier 130, driver 132, and output stages 134 to generate a cathode pulse which will drive the cathode from the cathode average potential to the grid average potential during the application of ultraviolet light to the cathode. In the grid drive channel, however, the output of the Schmitt trigger 126 is applied to a one-shot multivibrator 136, the output of which is applied in parallel with the output of the Schmitt trigger 126 to the control binary 138. At the completion of the one-shot timing the output of the multivibrator will trigger the control binary and thereby reset it for the next pulse applied directly from the Schmitt trigger. The output of the control binary 138 is amplified by stages including the driver amplifier 140 and the driver 142, which produces a push-pull output for providing power to switch the two control grids from equal and opposite potentials to the grid average potential.
144, the position of which in the cycle of rotation of the magnet may be adjusted by a scan motor 146 controlled by a motor control 148. (For clarity, slip rings, etc. are not shown.) Thus the time delay of each pulse G relative to the preceding pulse UV may be adjusted. After a number of pulse cycles (during which the electrometer integrates the output current for a particular time delay of the pulses G) the delay time may be adjusted to select ions of different mass. By successive adjustments of delay time a recording may be obtained illustrating the relative amounts of different electronegative components of the gas sample. Pulses UV may have a duration of a fraction of a millisecond and a repetition rate about SO/sec, for example. The drift period D may be of the order of 15 milliseconds, while the grid pulses G may have a duration of a few milliseconds. The drift voltage (grid to cathode) may be about 150 volts and anode-cathode spacing about 10cm.
Instruments have been proposed recently for detecting humanbeings by monitoring certain chemical effluents, such as pyruvic acid. Studies have been conducted in sufficient detail to make possible an accurate indication of the products given off by man under a variety of conditions. Other trace materials may result from interaction of these products with the environment or from clothing, cooking, food carried by humans, and similar sources. In general, presence of the material in the atmosphere in concentrations exceeding some threshold level must be a positive indication of human activity. The average residual background level of the material in the atmosphere should be very low. FIG. 6 illustrates diagrammatically an analyzer cell 150 of high sensitivity and mass resolution which may be used for operation'upon continuous large-volume samples to detect man-related or other trace materials. The envelope 152 of the cell has an air inlet 154. A differential vacuum pump- 156 exhausts air through an exit 158 from pipes 160, 162, and 164 connected to successive regions of the envelope. An ultraviolet source, such as a flash lamp 166, is positioned at one 'end of the envelope and supplies pulses of ultraviolet light to a semi-transparent photocathode 168 adjacent to the source. A negative ion drift region is provided between the photocathode and an ion shutter or gate 170, which may be constituted by a coplanar. grid of the type previously described. By employing the pulse techniques described in connection with FIGS. 4A and 4B, mass resolution can be achieved.
In order to permit the thermal energy negative ions to enter an evacuated ion detector region, a'gas leak is provided by a set of apertures 172, 174, and 176, formed in corresponding disc anodes 178, 180, and 182.-The negative ions which migrate to the gate 170 during the application of a drift field between the grids and the cathode 168 (after flashing of lamp 166 and which are selectively passed by the gate 170 are formed into a beam directed through the apertures 172, 174, 176 to the secondary electron multiplier 184, which may be of the type previously described. The neutral molecules which pass through the apertures are removed by the vacuum pump 156; A set of three apertures 0.2 millimeter in diameter and spaced one centimeter apart will separate the negative ions from the neutral particles efficiently. The pressure in the drift chamber may be atmospheric 760 Torr between electrodes 178 and 180 10 Torr, between electrodes 180 and 182 10" Torr, and in the multiplier-detector region less than l Torr.
Conical grids 186-196, which are permeable to neutral particles but negative ion reflective, produce a convergent field to direct the ions to the next aperture. Typically, this field will not be effective until the gas flow velocity has reached at least 100 centimeters per second about one centimeter from the aperture. The magnitude of the focusing field is limited by the requirement that any electrons which may be present must not ionize the medium. (It is unlikely that electrons are present because any remaining unattached will be collected by the closed grid. This criterion is roughly that a field of less than 10 volts per mean free path exists at a pressure at 10* Torr between the second and third apertures. This mean free path is about 0.05 centimeters. Thus, the maximum permissible field is 200 volts per centimeter. The convergent ion gun regions between the successive apertures overcome space charge effects and separate the ions from the neutral molecules. A Pierce ion gun electrode configuration is used to obtain maximum convergent fields. The outer grids 186, 188, and 190 can be used to maintain a reflective electric field and to recover ions which pass the inner grids.
For a trace partial pressure of 10 parts, it is estimated that 10 ions per second pass through the apertures. With vacuum in the multiplier-detector region of Torr, the mean free path is more than one centimeter. For an electrode spacing of one centimeter, the accelerating potential can be made quite high, in excess of 200 volts based on the well known Paschen curve to obtain a substantial amount of kinetic energy in the negative ions. These ions now strike the multiplier cathode ejecting about 100 secondary electrons with 2,000 ev energy.
Resolution can be improved, with loss of sensitivity,
by interposing a simple type of mass spectrometer between the last aperture, 176, and the multiplierdetector 180. The loss of sensitivity arises from the restriction placed on the solid angle through which the ions must pass in order to be focused properly in the mass spectrometer.
The apparatus of FIGS. 4A and 6 can be calibrated automatically by periodically scanning the entire range of mobility. At a known point determined by the pressure of the chamber and the operating voltages there should always be evidence of the characteristic oxygen negative ion response. By electronically arranging for a pedestal or window in the detection system, the proper position of the oxygen-ion response can be indicated. If the electronics were to drift for any reason, the position of the oxygen response would vary relative to the pedestal or window. The error signal produced by this drift can be used to servo the drift potential of the system or other appropriate variable to return the oxygen response to the proper delay with respect to the light pulse.
With the photoemissive source described with reference to FIGS. 4A and 6, electrons for use in the electron capture process are emitted in the zero to 3 ev energy range and are moderated within less than one centimeter to thermal energy (0.04 ev )by collision. Various other electron sources may be employed, as alluded to earlier. For example, a suitable radioisotope source can be made from the radioactive isotope of hydrogen (tritium). A standard commercial source is tritiated metal foil, which emits low-energy beta rays with energies up to a maximum of L8 X 10 electron volts. The electrons may be moderated in air to thermal energy, and may be pulsed into the attachment region by opening an electron gate or grid, for example.
The invention is especially useful in detecting complex gas molecules having, many vibrational modes, where electronegative attachment is quite efficient. Large molecules are detected intact without fragmentation, because the electrons attach naturally to the molecules at thermal energy, the process being most efficient at moderate pressure, such as between atmospheric and 10 mm of mercury. In conventional mass spectrometers, the electrons are accelerated to energies where positive molecular ion fragments are formed, leaving no parent molecule ion left for identification. Accurate mass resolution is difficult because of the presence of the many positive ion fragments.
The invention is capable of a sensitivity level of the order of 10" parts of trace material, with millisecond response time. Explosive gases in air can be detected at concentrations well below explosion level and an appropriate warning given. The concentrating apparatus of FIG. 3 may also be employed for gas purification by concentrating and removing undesirable electronegative components. Similarly, gas purification may be achieved by disposing of components passed by the ion gate of the drift tube of FIG. 4A, for example, and retaining the gas in the drift space. Solid or liquid analyses may be carried out by first converting the sample to gas. Moreover, by virtue of its small size and light weight, the invention can readily be employed in aerial reconnaissance.
While preferred embodiments of the invention have been shown anddescribe'd, it will be apparent to those skilled in the art that changes can be made in these embodimentswith'out departing from the principles and spirit of the invention, the scope of which is defined in the appended claims. Accordingly, the foregoing embodiments are to be considered illustrative, rather than restrictive of the invention, and those modifications which come within the meaning and range of equivalents of the claims are to be included therein.
The invention claimed is:
1. Apparatus for detecting electronegative components of a gas, comprising an electron attachment drift chamber for receiving said gas and having a source of thermal electrons for forming negative ions from the molecules of said components, means for maintaining the pressure in said chamber at substantially atmospheric pressure, means for collimating said negative ions to form a beam, means for accelerating said beam, said collimating and said accelerating meanscomprising a series of electrodes with apertures for passing said ion beam, mass spectrometer means for receiving said beam after passage through said electrodes and for passing ions of predetermined mass, and detecting means including means for producing electrons in response to said ions of predetermined mass and means for multiplying said electrons to produce an output.
2. The apparatus of claim I, said collimating means being located in a chamber evacuated with respect to said drift chamber, said chambers being connected by an apertured interface.
3. The apparatus of claim 1, further comprising means for concentrating said components in a sample of said gas and supplying the concentrated sample to said drift chamber.
4. The apparatus of claim 1, further comprising means for providing a sample of said gas to said drift chamber in proportion to a flow of said gas, means for determining the flow rate of said gas, and means for modifying said output in accordance with said flow rate to produce a readout of the concentration of said components in said gas.
5. The apparatus of claim 1, sad pressure maintaining means comprising means opening said chamber to the atmosphere.
6. Apparatus for separating electronegative components from a gas, comprising an envelope, means for introducing said gas into said envelope, means for pro ducing a pulse of low energy electrons at a first region of said envelope to form negative ions of said components at said first region, means applying a drift field to said ions, only after their formation, for causing said ions to drift from said first region toward a second re gion of said envelope, ion gate means between said regions,means for opening said gate means for a predetermined interval during the application of said drift field to pass ions of predetermined mobility to said second region, means for producing an output in response to the ions at said second region, means for maintaining substantially atmospheric pressure in said envelope in said first region before said gate means, and means located between said gate means and said output means for focusing and accelerating the ions passed by said gate means, said accelerating means comprising a series of electrodes defining successive chambers in said envelope coupled by apertures in said electrodes, said successive chambers having means for forming successively higher vacuums therein.
7. A method of detecting selected electronegative components of a gas, which comprises applying a pulse of low energy electrons to said gas in a region of moderate pressure to form a pulse of ions by electron attachment to the molecules of said components, thereafter applying a drift field to said ions to cause them to drift along a predetermined path, opening an ion gate, which normally blocks said path, during application of said field for an interval commencing a predetermined time after formation of the ion pulse in order to pass ions of predetermined mobility, maintaining the pressure in the region of ion formation, ion drift, and ion gating at substantially atmospheric pressure, and producing an output in response to the last-mentioned ions.
8. The method of claim 7, wherein the production of said output comprises forming said last-mentioned ions into a beam and producing an output signal in response to said beam.
9. The method of claim 7, further comprising varying the time at which opening of said gate is commenced in order to pass ions of different mobility.
10. Apparatus for separating electronegative components from a gas, comprising an envelope, means for introducing said gas into said envelope, means for producing a pulse of low energy electrons at a first region of said envelope to form negative ions from said components at said first region, means for maintaining the pressure in said envelope at substantially atmospheric pressure, means applying a drift field to said ions, only after their formation, for causing said ions to drift from said first region toward a second region of said envelope, ion gate means between said regions, and means for opening said gate means for a predetermined interval during the application of said drift field to pass ions of predetermined mobility to said second region.
11. The apparatus of claim 10, said gate means comprising a pair of interdigitated substantially coplanar grids having adjacent elements normally maintained at equal and opposite potential with respect to a reference potential, and said means for opening said gate means comprising means for placing said grids substantially at said reference potential.
12. The apparatus of claim 10, further comprising means for varying the time between the opening of said gate means and the production of said ions.
13. The apparatus of claim 10, further comprising means for producing an output in response to the ions at said second region. 4
14. The apparatus of claim 13, further comprising means located between said gate means and said output means for focusing and accelerating the ions passed by said gate means.
15. The apparatus of claim 14, said output means comprising means for producing electrons in response to the last-mentioned ions and means for multiplying the produced electrons.
16. The apparatus of claim 10, said pressure maintaining means comprising means for opening said envelope to the atmosphere.
17. Apparatus for separating electronegative compo nents from a gas, comprising an envelope, means for introducing said gas into said envelope, means for producing a pulse of low energy electrons at a first region of said envelope to form negative ions of said components at said first region, means applying a drift field to said ions, only after their formation, for causing said ions to drift from said first region toward a second region of said envelope, ion gate means between said regions, means for opening said gate means for a predetermined interval during the application of said drift field to pass ions of predetermined mobility to said second region, means for producing an output in response to the ions at said second region, and means located between said gate means and said output means for focusing and accelerating the ions passed by said gate means, said accelerating means comprising a series of electrodes defining successive chambers in said envelope coupled by apertures in said electrodes, said successive chambers having means for forming successively higher vacuums therein, said focusing means comprising conical gas-molecule-permeable,negative-ion-reflective grids between said electrodes.
18. A method of detecting molecular quantities of electronegative substances in a gas, which comprises concentrating said substances by attaching electrons to the molecules of said substances to form negative ions and collecting and neutralizing said ions to form new tral molecules, forming an ion cloud from the neutral molecules by electron attachment, converting said ion cloud to an ion beam, resolving said ion beam into components in accordance with the ion mass, and producing an output in response to at least one of said components.
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