US2945123A - Mass spectrometer - Google Patents

Description

5 Sheets-Sheet 1 J. R. PARSONS ETAL MASS -SPECTROMETER July 12, 1960 Filked April so, 1954 HMM f ym? ATTORNEYS t N m.

INVENTORS J R PARSONS D.A. FLUEGEL 5 Sheets-Sheet 2 July l2, 1960 J. R. PARSONS ETAL uAss SPECTROMETER Filed April 30. 1954 A T TOR/V575 n f A .M @le l... W. 9T.

July 12, 1960 Filed April 30. 1954 J. R. PARSONS ETAL MASS'SPECTROMETER 5 Sheets-Sheet 3 D.A.FLUEGEL BY 'l' A r TORNE rs 5 Sheets-Sheet 4 INVENTORS J R PARSONS D.A. FLUEGEL @ON OmN N Hicom h o@ AAAAAAA J. R. PARSONS ETAL MASS SPECTROMETER nun;

vvvvvv Ju'ly 12, 1960 Filed April 30. 1954 July 12, 1960 J. R. PARSONS ETAI- MASS SPECTROMETER Filed April 30. 1954 5 Sheets-Sheet 5 All HW f @my A T TORNE YS MASS SPECTRMETER Y r.irirnes R. Parsons and Dale A. Fluegel, Bartlesville, Okla.,

Filed Apr. 30, 1954, Ser. No. 426,768

19 Claims. (Cl. Z50-41.9)

This invention relates to mass spectrometers. In one speciiic aspect it relates to an ion velocity modulating selection type of mass spectrometer. In another aspect it relates to an electron emission regulator for ion sources. In another aspect it relates to a current measuring device incorporating a servo balance system. In still another aspect it relates to constant output oscillators.

ln recent years mass spectronieters have been developed from highly specialized academic research instruments for measuring the relative abundance of isotopes into analytical tools of extreme sensitivity and accuracy. At the present time applications are being found for the use of mass spectromcters in process monitoring and control. Mass spectrometry comprises, in general, ionizing a sample of material under investigation and separating the resulting ions according to their masses tofdetermine the relative abundance of ions of selected masses. The material to be analyzed usually is provided as a gas which is bombarded by a stream of electrons to produce the desired ions. Although both positive and negative ions may be formed by such electron bombardment, most mass spectrometers make use of only the positive ions. These positive ions are accelerated out of the region of the electron beam by negative electrical potentials applied thereto. Such potentials impart equal kinetic energies to ions having like charges such that ions of different masses have dierent velocities after passing through the electrical iield and consequently have different momenta.

The presently known mass spectrometers can be classilied into one of two general groups: the momentum selection types and the velocity selection types The momentum selection instruments sort the ions into beams kof different masses 'by'ineans of magnetic and/or electrical deecting elds.` lons of a selected'mass are allowed to impinge upon a collector plate to which is connected a suitable indicating circuit. The velocity selection instruments sort the ions according to the velocities imparted to the ions by electrical accelerating elds. The present invention is directed primarily toward providing animproved mass spectrometer of the velocity selection type.

ln United States Patent 2,535,032 there is disclosed a mass spectrometer which is provided with two sets of three equally spaced accelerating grids. Direct potentials are applied to the outer two grids and a radio frequency potential is applied between the center grid and the two outer grids of each set. lons which enter the space between the irst two grids in proper phase are accelerated through the fields between the first and second grids and the second and third grids. The ions subsequently pass through a field-free drift space and enter the second group of accelerating grids. The spacing between the grids, the frequency of the accelerating radio frequency voltage and the magnitudes of the accelerating potentials are such that ions of predetermined mass rares Patent O 2,945,123 Patented July 12, 1960 receive sufficient energy to overcome a potential barrier and impinge upon a collector plate. The mass spectrometer of the present invention is an improvement over the mass spectrometer disclosed in Patent 2,535,032. Pour sets of accelerating electrodes are employed to provide three separate drift spaces. This greatly irn-Y proves the resolution power of the spectrometer. The radio frequency applied to the accelerating grids is modulated by the output of a square wave audio frequency oscillator so that the output signal generated by the positive ions impinging upon the collector plate is modulated at anaudio frequency. This alternating signal can be measured more accurately than a direct current signal. The output signal is amplified by a temperature compensated tuned amplifier adapted to pass frequencies corresponding to the frequency of the audio oscillator. The

Vampliiied signal is then compared with a reference voltage and any difference therebetween is applied to a servo motor which adjusts the feedback in the tuned amplier, thereby varying the gain of the amplier. This adjustment of feedback is made in a manner such that the amplified output signal is equal to the reference voltage at all times. The degree of rotation of the servov motor needed to `accomplish this equalization is a measure of the output signal from the mass spectrometer tube.

In order that the output signal of a mass spectrometer represents the total ions of a preselected mass which are present in the gas sample under analysis, it is important to maintain the degree of ionization constant. is accomplished in accordance with the present invention by disposing a screen electrode between the source of electrons and the ionization chamber. The potential applied to this screen electrode is regulated in terms of the electron emission from a heated filament to maintain a constant flow of electrons from the filament into the ionization chamber.

Accordingly, it is an object of this invention to provide an improved mass spectrometer which operates upon the principle of velocity selection of ions of a predetermined mass.

Another object is to provide a mass spectrometer of the velocity selection type wherein an accelerating potential of a tirst frequency is modulated by a potential of a second lower frequency so that the resulting ion beam is modulated at the frequency of said second frequency.

Another object is to provide an electron emission regulator for an ion source.

A further object is to provide a current measuring de.- vice which includes a servo system to regulate the gain of an amplifier to maintain the output signal of the amplifier constant.

A still further object is to provide an oscillator having a voltage regulating circuit associated therewith to maintain the oscillator output constant.

Various other objects, advantages and features of this invention should become apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

Figure l is a schematic representation of the mass spectrometer of this invention;

Figure 2 is a schematic circuit diagram of the power supply circuit of Figure A1;

Figure 3 is a schematic circuit diagram of the emission regulator of Figure 1 Figure 4 is a schematic circuit diagram of the audio and radio frequency oscillators of Figure 1;

Figure 5 is a schematic circuit diagram of a tuned amplifier which is included in the detector circuit of Fig- Ure l;

This

Figure 6 is a schematic circuit diagram of the servo balance system of the detector and timer of Figure l; and

Figure 7 is a schematic circuit diagram ofthe ampliiier shown in Figure 6.

Referring now to the drawing in detail and to Figure 1 in particular, there is shown the mass spectrometer tube 10 which can comprise a glass envelope, the interior of which is maintained at a reduced pressure by a vacuum pump, not shown, which communicates with the interior of tube 10 through a conduit 11. A sample of the gas to be analyzed is supplied by a conduit 12 which has a normally open solenoid-operated valve 13 therein. A sec ond conduit 1-4 having a normally closed solenoid-operated valve 15 therein communicates with conduit 12 downstream from valve 1.3. Conduit 14 is connected to a source of reference gas. A branch conduit 16 having a restriction therein communicates with conduit 12 downstream from the junction with conduit 14. This conduit thus forms a viscous leak path for a sample of the gas being analyzed. A second branch conduit 17, having a narrow oriiice 18 therein, communicates between conduit 16 and tube 10. Orifice 18 defines a molecular leak path. Valves 13 and 15 are operated in unison by a timer circuit 19. The electrical circuitry associated with mass spectrometer tube 10 is energized from a source of alternating potential 20, the output terminals of which are connected to a power supp-1y circuit 21. Power supply circuit 21 is provided with a first output terminal 22 which is maintained at a constant positive potential and with a second output terminal 23 which is maintained at a constant negative potential. Output terminals 24 and 25, which provide a source of regulated alternating current of lower voltage than source 20, are connected to the respective end terminals of an electron emitting filament 26 disposed in one end of tube 10. The end terminals of a potentiometer 27 are connected to respective voltage terminals 2-4 and 25, and the contactor of potentiometer 27 is connected to one input terminal of an electron emission regulator circuit 29. VA second input terminal of emission regulator 29 is connected to negative potential terminal 23. The electrons emitted from iilament 26 are accelerated into an ionization chamber 30', which is de `lined by a pair of grounded spaced grids 31 and 32, by the potential diierence between grids 31, 32 and iilament 26. A pair of focusing grids 33 and 34 is positioned on the second side of ionization chamber 30.` Grid 33 is connected to the contactor of a potentiometer 35 and grid 34 is connected to the contactor of a potentiometer 36. First end terminals of potentiometers 35 and 36 are connected to negative potential terminal 23, the second end terminals of these potentiometers being grounded. The electron flow from iilament 26 is regulated by a screen electrode 37 which is connected to an output terminal of emission regulator 29. The gas sample to be analyzed is introduced into ionization chamber 30 and is there subjected to electron bombardment so that positive ions are formed. These positive ions are accelerated through tube 10 toward a collector plate 38 which is mounted at the end of tube 10 opposite filament 26.

The positive ions produced in chamber 3i) are accelerated toward collector plate 38 by a iirst accelerating grid 40 which is connected to a potential dividing network 41. Negative potential terminal 23 of power supply 21 is connected to one end terminal of voltage dividing network 41, the second end terminal of this network being grounded. The second accelerating grid 42 of tube 10 is connected to one output terminal 47 of a radio frequency oscillator 43, the second output terminal .of oscillator y43 being grounded. The output of oscillator 43 is modulated by the output signal from an audio frequency oscillator 44. An inductance coil 45 is connected between grids 40 and 42. A third accelerating grid 46 is connected to potential dividing network 41 at a point which is maintained at a negative potential of lesser magnitude than the negative potential applied to grid 40. Grids 40, 42, vand 45 thus form the rst set of velocity modulating grids. The spacing between grids '40 and 42 is equal to the spacing between grids '42 and 46. A second set of corresponding grids 40a, 42a and 46a is positioned within tube 1d in spaced relation with grid 4u, 42 and 46. Grid 40a is connected to grid 46, grid 42a is connected to oscillator output terminal 47, and grid 46a is connected to potential dividing network 41 at a point which is maintained at a negative potential of lesser magnitude than the negative potential applied to grids 46 and 43a. A third set of corresponding grids 40]), 42b and 46h is positioned in spaced relation with grids 40a, 42a and 46a. Grid '4Gb is connected to grid 46a, grid 42E) is connected to oscillator output terminal 47, and grid `4Gb is connected to a point on potential dividing network 41 which is maintained at a negative potential of lesser magnitude than the negative potential applied to grids 46a and 4Gb. A fourth set of corresponding grids 40e, "42C and '46c is positioned in spaced relation with grids 461i), 4212 and 46]; in tube 10. Grid 40C is connected to grid 46h, grid 42e is connected to output terminal 47 of oscillator 43, and grid 46c is connected to grids '46h and diie. First and second groups of grids 50 and 5lY are positioned between this last set of accelerating grids and collector plate 3S. Grids 50 are connected to the contactor of a potentiometer 52, one end terminal of potentiometer 52 being connected to positive potential terminal 22 and the second end terminal of potentiometer 52 being grounded. Gr-ids 51 are connected to negative potential terminal 23. Collector plate 38 is connected to one input terminal of a detector circuit 53, the second input terminal of which is grounded.

The positive ions produced within chamberr30 are accelerated toward collectorY plate 38 by the negative potential applied to grid 4t?. During one-half cycle of the output signal from oscillator 43, the electrical field between grids 40 and 42 is of such phase that the ions entering this eld are accelerated. Ions which enter the iield during a particular phase of this half cycle receive maximum energy. During the next half-cycle of the signal from oscillator L43, the field between grids 42 and 46 is of such phase that the ions are further accelerated.V These ions then drift through the held-free space between grids 46 and 40a. The masses of the individual ions determine their times of arrival at grid 40a. The ions which arrive at grid 40a at the proper time are again accelerated by the eld applied between grids 40a and l42a and receive additional energy. The same accelerating procedure continues as the ions pass through the next seven grids. Thus, the resulting ion beam is Velocity modulated so that ions of a particular mass receive maximum energy. The positive potential applied to grids 50 is adjusted such that only those ions having a velocity greater than predetermined value are able to pass through grids 50 to impinge upon collector plate 3S. The pur pose of negative grids 51 is to suppress any electrons which may be formed within tube 10 by the ions bombarding plate'38 or other elements of the tube. The ions imp inging upon collector plate 3S cause a current to flow through detector circuit 53 to ground, which current is proportional in magnitude to the number of ions impinging upon collector plate 33. The current'tlow is amplitude modulated at the same frequency as the frequency of audio oscillator 44. lt is this modulated component of the current that is measured in accordance with the present invention by detector circuit 53.

Power supply circuit 21 is shown in detail in Figure 2. Voltage source 2t) is applied across the end terminals of the primary winding 60 of a voltage regulating transformer 61 through a switch 62. lf desired, switch 62 can be a pressure responsive switch which is connected to a pressure gage 62 which communicates with the Ainterior of tube 10. Thus, if the vacuum system should fail for any reason, switch 62 opens when the pressure withintube a exceeds a predetermined value. This removes 'the operating potentials from filament 26 and the various grids to minimize the explosion danger should tube 10 be filled with gas from sample lines 13 and 15.

The end terminals of a first secondary winding 63 of transformer 61 are connected to the respective anodes of a double diode 64, the center tap of transformer winding 63` being grounded. The filament of double diode 64 is energized by a second secondary winding 68 on transformer 61. The cathode of double diode 64 is connected to a first positive output terminal 65 through a pair of series connected inductors 66 and 67. The junction between the cathode of double diode 64 and inductor 66 is connected to ground through a capacitor 70; the junction between inductors 66 and 67 is connected to ground throughha capacitor 71; and the junction between inductor 67 and potential terminal 65 is connected to ground through a capacitor 72. A resistor 74 and a pair of voltage regulating tubes 75 and 76 are connected in series relation between positive terminal 65 and ground. The junction between resistor 74 and voltage regulating tube 75 is connected to a second positive potential output terminal 77. A capacitor 78 is connected in parallel with voltage regulating tube 75, and a 'capacitor 79 is connected in parallel with voltage regulating tube 76. A resistor 81 andk a second pair of Voltage regulating tubes 82 and 83 are connected in series relation between positive terminal 65 and ground. The junction between resistor 81 and voltage regulating tube 82 is connected to a third positive potential output terminal 22. The junction between voltage regulating tubes 82 and 83 is connected to ground through a pair of series connected resistors 85 and 86, the junction between resistors 85 and 06 being connected to a fourth positive potential output terminal 87: Y i

The operation of the power supply circuit thus far described should now be apparent. The rectified voltage provided by double diode 64 is filtered by inductors 66 and 67 and capacitors "70, 71 and 72. The voltage dividing networks formed by the several voltage regulating tubes and resistors 74 and 81 provide output voltages of predetermined magnitudes to operate the circuits described in Adetail hereinafter.

In the lower portion of Figure 2 there is shown a second voltage regulating transformer 90 which has a primary winding 91 connected in parallel with transformer winding 60. The end terminals of the rst secondary winding 92 of transformer 90 are connected to the respective anodes of a double diode 93. The filament of double diode 93 is energized yby a second secondary winding 96 of transformer 90. The cathode of double diode 93 is Vconnected to the anode and to the screen grid of a pentode 94 through an inductor 95. A capacitor 97 is connected between the center tap of transformer winding 92 and the junction between inductor-95 and the cathode of double diode 93. A second capacitor 98 is connected between the center tap of transformer winding 92 and the anode of pentode 94. The supressor grid of pentode v94 is connected to the cathode thereof, and the cathode is connected to ground. The center tap of transformer winding 92 is connected to an output terminal 23 which is maintained at a constant negative potential.

A voltageregulating tube 101 and a resistor 102 are connected in series relation between terminal 23 and ground. The junction between voltage regulating tube 101 and resistor 102 is connected to the control grid of a triode 103 through series connected resistors 104 and 105. A thermistor 107, having a negative coeiiicient of thermal resistivity, is connected in parallel with resistor 104. The junction between resistors 104 and 105 is connected to terminal 23 through a resistor 108. A capacitor 109 is connected in parallel with resistor 108. The cathode of triode 103 is connected to terminal 23 throughy a -resistor 110 and to ground through a resistor ,111. The anode of triode y103 is connected to ground 6 through a resistor 1.1.2 and directly to the control grid of a triode 113. The cathode of triode 113 is connected to terminal 23 through a resistor 115 and to ground through a resistor 116. The iilament of triode 113 yis connected to the cathode of triode 113 through a resistor 117 and to ground through a capacitor 118. A capacitor 119 is connected between terminal23 and ground. The anode of triode 113 is connected to ground through a resistor 121 and to the control grid of pentode 94 through a resistor 122. Heating current for the filaments of triodes 103 and 113 and pentode V94 is supplied by a winding 124 Vof transformer 90 having output terminals ,r

and y.

The electron current flow through double diode 93 passes through pentode 94 and filter inductor 95. In this manner, the potential at terminal 23, which is con'- nected to the center tap of transformer winding 92, is maintained at a constant negative value. Voltage regulating tube 101 and resistor 102 form` a potential dividing network between terminal 23 and ground. Voltage regulating tube 101 is in turn shunted by a potential dividing network comprising resistors 104 and 108. Thermistor 107 is connected in parallel with resistorr 104 to compensate for any ambient temperature changes which may affect the voltage regulation of tube 101. It has been found that an increase in ambient temperature causes the voltage across the tube 101 to decrease. For example, a change from about 25 C. to 60 C. causes a decrease of about 0.2 volt across tube 101. If the ambient temperature of tube 101 increases, the voltage across the tube decreases and this in turn tends to make the voltage applied to the control grid of triode 103 become more negative. However, this same temperature increase causes the resistance of thermistor 107 to decrease so that the ratio of voltages across network 107, 104 and 108 is changed. This in turn tends to make the voltage applied to the control grid of triode 103 less negative so that the temperature changes have no effect on the voltage regulating action of the circuit including tube 101. if the temperature should decrease, the opposite regulation takes place. Tube 101 and thermistor 1 07 are maintained in thermal contact.

The normal voltage regulating action of this circuit can be explained by assuming that the potential at terminal 23 becomes more negative. This in turn makes the potential applied to thecathode of triode 103 more positive with respect to the control grid so that less current flows through triode .103 and anode resistor 112. This makes the potential on the anode of triode 103 more positive, which potential is also applied to the control grid of triode 113 to increase the current flow through triode 113 and through anode resistor 121. The potential at the anode of triode 113 is thereby made more negative. This more negative potential is applied to the control grid of tube 94 to increase voltage drop across tube 94, and this in turn makes the potential at terminal 2,3 more positive. The circuit thus operates to amplify voltage iiuctuation at terminal 23 to restore the potential to the desired value. If the potential at terminal 23 should tend to become more positive, the above-mentioned potential changes are reversed so that the voltage drop across tube 94 decreases to restore the potential at terminal 23 to the desired value.

As previously mentioned, it is important to direct an electron beam of constant magnitude into the ionization chamber ofthe mass spectrometer tube in order that the number of ions formed is a function only of the gas pressure in the tube. The electron emission regulator circuit of Figure 3 is provided for this purpose. Filament 26 is heated by the voltage applied between terminals 24 and 25 from transformer Winding 89 of Figure 2. The contactor of potentiometer 27 is connected to the junction between a resistor and the anode of a voltage regulating tube 131. The second terminal of resistor 130 is grounded. The cathode of tube 131 is connected lto 7 thecontrol grid of a pentode 132 and to one end terminal of a resistor 133, the second end terminal'of which is connected to a variable resistor 134. The contactor of variable resistor 134 is connected to negative potential terminal 23. The anode of pentode 132 is connected to ground through a resistor 135 and directly to the screen electrode 37 in tube 10. The cathode of pentode 132 is connected to terminal 23 through a resistor 136 and to ground through a resistor 137. The suppressor grid of pentode 132 is connected to the cathode of pentode 132, and the screen grid of pentode 132 is connected to ground.

The electron current emitted from heated filament 26 is supplied from negative terminal 23 through resistors 134 and 133 and voltage regulating tube 131 to the contactor of potentiometer 27. If the electron emission from filament 26 should tend to increase, the potential applied to the control grid of pentode 132 tends to become more positive because of the increased current ow through resistors 133 and 134. This change in control grid potential increases the current iiow through pentode 132 and anode resistor 135. The increased current flow through resistor 135 makes the potential at the anode of pentode 132 more negative and the potential applied to screen electrode 37 more negative. The change in po- -tential on screen electrode 37 tends to impede the electron ilow therethrough such that the electron flow into ionization chamber 30 tends to decrease by an amount suicient to compensate for the increased current emission from lilament 26. If the electron emission from iilament 26 should tend to decrease, the above-mentioned potential changes are reversed such that the potential applied to screen elect-rode 37 is made more positive. This in turn tends to increase the electron diow into 'ionization chamber 30. By the use of the emission regulator of Figure 3 it has been found that the electron flow into ionization chamber 30 can be maintained constant irrespective of small yfluctuations in power supplied to filament 26 and changes in the emitting properties of filament 26.

As previously described, the potential applied to grids 42, 42a, 42b and 42e is the sum of a D.C. voltage and a radio frequency voltage which is modulated by a frequency in the audio range. The circuit illustrated in Figure 4 is adapted to provide this modulated radio frequency potential. The audio frequency signal is supplied by oscillator 44 which has its frequency established by -a vibrating tuning fork 150 which is constructed of magnetic material. A irst inductance coil 151 is mounted adjacent one arm of tuning fork 150 on a permanent magnet 148, and a second inductance coil 152 is mounted adjacent the second arm of tuning fork 150 on a permanent magnet 149. One end terminal of inductance coil 151 is connected to the control grid of a triode 153, the second end terminal of inductance coil 151 being grounded, as is tuning fork 150. One end terminal of inductance coil 152 is connected to the cathode of a second triode 154, the second end terminal of inductance coil 152 being grounded. The anode of triode 153 is connected to positive potential terminal 77 through series connected resistors S, 156 and 157 and to the control grid of triode 154 through a capacitor 153 and a resistor 161 connected in series relation. The cathode of triode 153 is connected to ground through a resistor 159. The control grid of triode 154 is connected to ground through series connected resistors 161 and 160. The anode of triode 154 is connected to terminal 77 through series connected resistors 162, 156 and 157. The anode of triode 154 is also connected directly to the control grid of a triode 163. The anode of triode 163 is connected to terminal 77 through resistor 157, and

the cathode of triode 163 is connected to ground through a resistor 164. The cathode of triode 163 is also connected to the control grid of a triode 165 through a capacitor 166. 'The control grid of triode 165 is connected to ground through a resistor 167. The anode of triode is connected to terminal 77 throughseries connected resistors 169 and 157 and to the suppressor grid of a pentode 170 through a capacitor 171. The cathode of triode 155 is connected to ground Ithrough a resistor 172 which is shunted by a capacitor 173. A capacitor 175 is connected between ground and the junction between resistors 157 and 169, and a capacitor 168 is connected between ground and the junction between resistors 155 and 156. Heater current for the filaments of triodes 153, 157, 163 and 165 is supplied by terminals x and y of the power supply circuit of Figure 2.

The oscillator circuit thus far described is adapted to provide an output signal of frequency in the audio range. This output signal is of the same frequency as the frequency of vibration of tuning fork 150. Energy is supplied to tuning fork 150 by coil 152 and magnet 149. The output signal from triode 154 is further amplified by triode 163 connected as a cathode follower to isolate the oscillator from the following amplification stage. The output signal from triode 163 is amplified by triode 165 that is biased to operate as an overdriven amplifier whereby its output signal, which is applied to the suppressor grid of pentode 170, has substantially a square Wave form. Y

Pentode 170 and the circuit components associated therewith function to provide oscillations of a radio frequency. These oscillations are modulated Yby the audio frequency signal applied to the supressor grid of pentode 170. The control grid of pentode 170 is connected t0 ground through a crystal which regulates the frequency of oscillations. Crystal 180 is shunted by an inductor 131. The cathode of pentode 170 is connected to ground through a resistor 182 which is shunted byV a capacitor 183. The suppressor grid of pentode 170 is connected to ground through a resistor `184. The anode of pentode 170 is connected to a positive terminal 185 through a tuned circuit 186 which comprisesan inductor 187 and a capacitor 188 connected in parallel relation. Terminal is connected to ground Ithrough a capacitor 191. The screen grid of pentode 170 is connected to one terminal of a second tuned circuit 192 which comprises an inductor 193 and a capacitor 194 connected in parallel relation. The second terminal of tuned circuit 194 is connected to terminal 185 through a voltage regulating tube 195 which is shunted by a capacitor 196. The junction between the second terminal of tuned circuit 192 and voltage regulating tube 195 is connected to ground through a resistor- 197 that is shunted by a capacitor 198. The output signal from radio frequency oscillator 43 is removed from a terminal 189 which is connected to a point on inductor 187 through a capacitor 190. The output signal which appears between terminal 189 and ground thus constitutes a radio frequency voltage, the envelope of which represents the square wave output of audio oscillator 44.

For satisfactory operation of the mass spectrometer, it is important that the output signal from oscillator 43 be maintained at a constant value. This is accomplished by a voltage regulating circuit. The anode of pentode 170 is connected through a capacitor 202 lto the anode of a first diode 203 and to the cathode of a second diode 204.- The cathode of diode 203 is connected to one end terminal of a potentiometer 205. `The 4second end terminal of potentiometer 205 is connected to the first end terminal of a resistor 206, the second end terminal of resistor 206 being grounded. A pair of parallel connected capacitors 207 and 2018 is connected between the cathode of diode 203 and ground. The anode of diode 204 is connected to ground. The contactor of potentiometer 205 is connected to the control grid of a pentode 210. The cathode of pentode 210 is connected to ground through a voltage regulating tube 211, and the anode of pentode 210 is connected to positive potential terminal 65 through a resistor 212. A cathode of pentode 210 is also connected to terminal 185 through a resistor 213. The suppressor grid of pentode 210 is connected to the cathode thereof, and this cathode is connected to ground through a capacitor 215. A capacitor 216 is connected between terminal 185 and ground. The anode of pentode 210 is connected through a resistor 217 to the control grids of a pair of triodes 218 and 219. The cathodes of triodes 218 and 219 are connected to one another and to terminal 185. The anodes of triodes 218 and 219 are connected to one another and to terminal 65. Y

rl`he output signal from pentode 170 is rectified by the voltage doubling rectier circuit comprising diodes 203 and 204 and capacitors 202, 207 and 20:3. The rectified voltage is in turn applied across the potential dividing network comprising potentiometer 205 and resistor 206. If thisV rectified voltage should tend to increase, the potential applied to the control grid of pentode 21d is increased such that the current ow through pentode 210 and anode resistor 212 is increased, which decreases the potential applied to the control grids or" triodes 218 and 219. This decreases the current ow through triodes 21S and 219 such that the potential at terminal 185 also is decreased. This decrease of potential `is in turn applied through tuned circuit 186 to the anode of pentode 170 and through tuned circuit 192 to the screen grid of pentode 170 such that the output signal from pentode 170 is decreased in magnitude by an amount sucient to restore the amplitude of output signal to its original value. Conversely, if the rectified voltage applied across potentiometer 205 `and resistor 206 should tend to decrease, the above-mentioned potential changes are reversed so that the voltage -applied to the anode and screen grid of pentode 170 is increased. In this manner the ioutput signal of oscillator 43 is maintained at a constant value.

Mass spectrometer 10 is operated to limit the passage of ions therethrough to those ions having a predetermined mass. These selected ions impinge collector plate 38 and provide a current flow in detector circuit 53, the magnitude of which is a function of the ions impinging plate 38. In order to simplify the detection of this current, the radio frequency accelerating potential is modulated by audio oscillator 44 so that the output signal from plate 38 is also modulated at the frequency of oscillator 44. The magnitude of the components of the output signal `of the frequency of oscillator 44 is measured in accordance with this invention to determine the magnitude of the ion beam impinging collector plate 38. The circuit of Figure 5 comprises a tuned amplifier which forms the first stage of detector circuit 53. This amplier is tuned topass signals of the frequency of oscillator 44.

Collector plate 38 is connected to the control grid. of a pentode 225. This control grid is connected to ground through a tuned circuit 226 which comprises a capacitor 227 connected in parallel with a series connected inductor 228 and thermistor 229. The cathode of pentode 225 is connected to an input feedback terminal 230 through a resistor 232i, and the anode of pentode 225 is connected to positive terminal 65 through a resistor 232. The anode of pentode 225 is also connected to the control grid of a pentode 234 through a capacitor 235. The screenv grid of pentode 225 is connected to terminal 65 through a resistor 236 and to ground through a capacitor 237. The suppressor grid of pentode 225 is connected to the cathode thereof. The anode of pentode 234 is connected to terminal 65 through a resistor 23S,- and the ycathode of pentode 234 is connected to ground through a resistor 240 which is shunted by a capacitor 241. The control grid of pentode 234 is Aconnected to ground through a resistor 242, and the suppressor grid of pentode 234 is connected to the cathode thereof. The anode of pentode 234 is connected to the control grid of a triode 2,45 through a resistor 246. The cathode of triode 245 is connected tol ground through aresistor 247 and a potentiometer 248 which are connected in series relation. The contactor of potentiometer 248 is connected to the screen grid of pentode 234. The anode of triode 245 is corrnected to the anode of a second triode 250, and these two anodes are connected to terminal 65n through apre'- sistor 251 and to ground through a capacitor 252. The cathode of triode 245 is connected to the control grid of triode 250 through a capacitor 254 and a resistor 255 connected in series relation. The cathode of triode 245 is also connected to an output terminal 256 through a resistor 257. The cathode of triode 250 is connected to a feedback output terminal 260 through a resistor 261 which is shunted by a capacitor 262. The cont-rol grid of triode 250 is connected to ground through a resistor 263. Feed'- back terminal 260 is connected to one end terminal of a potentiometer 265. The second end terminal of potentionieter 265 -is connected to ground, and the contacter of potentiometer 265 is connected to the cathode of pentode 225 through feedback terminal 230.

The tuned ampliier circuit thus far described provides an amplified signal between terminal 256 and ground which is representative of the alternating component of the ion current which is of the same frequency as the frequency of oscillator 44', circuit 226 being tuned to this frequency. The gain of this amplifier is adjusted by feedback potentiometer 265 and potentiometer 24d. The output signal appearing between terminal 256 and ground is further amplied lby arsecond tuned ampliiier, also illustrated in Figure 5, which is generally similar to the ampliiier above-described and wherein corresponding components are designated by like primed reference numerals. A feedback network, which is illustratedl in Figure 6; is connected between terminals 260 and 230 in place of a potentiometer corresponding to potentiometer 265. A capacitor 263 is connected inA parallel with resistor 231.

The feedback network connected between terminals 260 and 230 is illustrated in Figure 6. Terminal 260' is connected to one end terminal of a potentiometer 270, the second end terminal of potentiometer 270' being oonnccted to the arm 271 of a relay 272. t vitch arm 271 normally engages a contact 273 which is connected to one end terminal of a. potentiometer 274. The second end terminal of potentiometer 274 is connected to' ground. A capacitor 283 is connected between the end terminals of potentiometer 270. When current is passed to the coil of relay 272, switch arm 271 engages a second contact 276 which is connected toy one end terminal of a potentiometer 277. The second end terminal of potentiometer 277 also' is connected to ground. The contacter of potentiometer 274' is connected to a terminal 279 which is engaged by a switch arm 280. Switch arm 2801s connected to terminal 230. In its second position switch arm 280 l engages a contact 281 which is connected to the com tactor of potentiometer 277 The contactor of potentiometer 270 is connected through` a capacitor 284 to the anode of a diode 235. Thev anode `of diode 285 isl connected to ground through a resistor 423. The cathode of diode' 285 is connected to ground through a resistor 286 which is shunted by a capacitor 287. The cathode of diode 285 is also connected to one input terminal 288 of a servo amplifier 290. The second input terminal 291 of amplifier 290 is connected to the cathode of a second diodo 292. The cathode of diode 292 is connected to ground through a resistor 293, and the anodevof diode 292 is connected to positive potential terminal 77 through` a resistor 295 and to ground through a resistor 424. One output terminal 297 of amplifrer 290 is connected to ground and the second output .terminal 298 is connected to the switch arm 3550 of a relay 301. In the absence of current being applied to the coil of relay 301, switch arm 300' engages a terminal 302 which is connected to one terminal `of the first coil 303 of a reversible two-phase induction servo motor 304. The second terminal of coil 303 is connected to ground. When current is applied to the coil of relay 301, switch arm 300 engages a terminal 305 which is connected to one terminal of the first coil 306 of a second reversible two-phase induction servo motor 307. The second terminal of coil 306 is connected to ground. The second coils 309 and 310 `of respective - motors 304 and 307 are connected -across voltage terminals 311 and 312 which are described in conjunction with Figure 7.

In normal operation of the feedback circuit of Figure 6, relays 272 and 301 are not energized such that switch arm 271 engages terminal 273 and switch arm 300 engages terminal 302. The voltage appearing between the contactor of potentiometer 270 and ground is rectified by diode 285 and applied to input terminal 288 of servo amplifier 290. This voltage is in turn compared with a constant reference voltage which is applied to input terminal 291. Any difference between the voltages applied to input terminals 288 and 291 results in an output signal from amplifier 290 Which drives motor 304 to adjust the position of the contactor of potentiometer 274, the contactor of potentiometer 274 being mechanically coupled to the drive shaft of motor 304. The contactor of potentiometer 274 is moved in a direction to eliminate the potential difference between terminals 288 and 291 ot amplifier 290. It can be seen that theposition of the contactor of potentiometer 274 regulates the feedback signal applied between terminals 260 and 230 of the amplier of Figure 5. This varies the gain of the second section of the tuned amplifier of Figure 5. The detector circuit is thus operated so that the gain of the tuned amplifier is varied whereby the output signal therefrom is of a constant magnitude. The degree of rotation of servo motor 304 is necessary to restore the tuned amplifier circuit to this condition of balance is a function of the magnitude of the ion current impinging upon collector plate 38. The rotation of motor 304 can be observed by a calibrated dial, not shown, associated with potentiometer 274 and by a telemetering transmitter which comprises a potentiometer 315 having one end terminal thereof connected to potential terminal 87 and the second end terminal conl nected to ground. The output terminals 316 and 317 of this telemetering transmitter are connected to the contactor of potentiometer 315 and to ground, respectively. The contactor of potentiometer 315 is also mechanically coupled to the drive shaft `of motor 304.

Servo amplifier 290 is illustrated in detail in Figure 7. Input terminal 288 is connected to the junction between iirst end terminals of respective primary windings 325 and 326 of a transformer 327. input terminal 291 is connected to a vibrating reed 328 which is moved by a coil 329 to engage periodically terminalsv 330 and 331 which are connected to the respective second end terminals of transformer windings 325 and 326. The end terminals of coil 329 are connected to the respective end terminals of a secondary winding 333 of a transformer 334. The primary winding 335 of transformer 334 is connected across the output terminals of voltage source 20. A potentiometer 336, having a grounded center tap, is connected in parallel with coil 329.

One end terminal of the secondary winding 337 of transformer 327 is connected to the control grid of a 4pentode 338. The second end terminal of transformer .winding 337 is connected to ground. A capacitor 339 is connected in parallel with transformer winding 337. The cathode of pentode 338 is connected to ground through a resistor 341 which is shunted by a capacitor 342. The anode of pentode 338 is connected to positive potential terminal 65 through Series connected rc- - sistors 343 and 344. A capacitor 345 is connected between ground and the junction between resistors 343 and 344. The suppressor grid of pentode 338 is connected to the cathode thereof, and the screen grid of pentode 338 is connected to the contactor of a potentiometer 346. One end terminal of potentiometer 346 is connected to terminal 65 through a resistor 347 and the second end terminal of potentiometer 346 is connected to ground through a resistor 348. The anode of pentode 338 is also connected to one end terminal of a potentiometer 350 through a capacitor 351. The second end terminal of potentiometer 350 is connected to ground, and the contactor of potentiometer 350 is connected to the control grid of a pentode 352. The cathode of pentode 352 is connected to ground vthrough a resistor 353, and the anode of pentode 352 is connected to terminal 65 through a resistor 354. The anode of pentode 352 is also connected through a capacitor 356 to the control grids of four triodes 357, 358, 359 and 360. The screen grid of pentode 352 is connected to terminal 65 througha resistor 362 and to ground through a capacitor 363.

The control grids of triodes 357, 358, 359 and 360 are connected to ground through a common resistor 364. The cathodes of triodes 357, 358, 359 and 360 are connected to one another and to ground through a common resistor 365. The anodes of triodes 357 and 360 are connected to one another and to the rst end terminal of a secondary winding 366 of transformer 334. The anodes of triodes 358 and 359 are connected to one another and to the second end terminal of transformer winding 366. The center tap of transformer Winding 366 is connected to amplier output terminal 298. -A capacitor 368 is connected between terminal 298 and grounded terminal 297. `One output terminal of voltage source 20 is connected to output terminal 311 `and the second output terminal of voltage source 20 is connected to terminal 312 through a capacitor 369.

Reed 328 is vibrated between contacts 330 and 331 by coil 329 at the frequency of voltage source 20, which can be sixty cycles per second, for example. The input signal appearing between terminals 288 and 291 is thus applied alternately to transformer windings 325 and 326 so that a sixty cycle signal of magnitude proportional to the signal appearing between terminals 288 and 291 is applied to the input of pentode amplifier 338. The output signal from pentode 338 is in turn applied to the input of pentode 352 and the output of pentode 352 is applied to the input of the four triodes 357, 358, 359 and 360. V

The voltage from source 20, which is applied to the anodes of triodes 357 and 358 through transformer 334, results in these two anodes being positive during alternate half cycles of the applied voltage. In the absence of a signal being applied to the control grids of triodes .357 and 358 from pentode 352, the output of the two triodes 357 and 358 consists of two pulses of current per cycle of applied voltage from source 20. If these two pulses are of equal magnitude, which they normally are because of the balanced circuit, there is no sixty cycle component in the output signal which appears between terminals 298 and 297. However, if a sixty cycle signal, either in phase with or 180 out of phase with the operating voltage supplied by source V20, is applied to the control grids of triodes 357 `and 358, one of the output pulses from these tubes is increased and the other decreased to provide a sixty cycle component in the output signal between terminals 298 and 297. By providing triodes 359 and 360 in parallel with respective triodes 358 and 357, a safety factor is established because if one of the triodes should fail the other will continue to operate. 297 is applied across coils 303 and 306 of respective motors 304 and 307 depending upon the position of switch arm 300. The voltage appearing between terminals 311 and 312 is applied across coils'309 and 310 of respective motors 304 and 307. This latter voltage is out of phase with `the voltage applied across the The output signal between terminals 298 and first-mentioned coils so as to provide a rotating' magnetic iield. The direction of rotation of motors 304 and 307 depends upon the relative magnitude of the potentials applied to terminals 288 and 291 of amplier 290. If the. potential at` terminal 288 exceeds the potentialat terminal 291 the motors rotate in iirst directions, while if. the relative magnitude oi these potentials is reversed the. motors rotate in opposite directions.

r.It will bev noted that the gain of pentode 338 in ampliiier 290 is determined in part by the potential applied to the screen grid thereof from the voltage dividing circuit comprising resistors 347 and 348 and potentioemter 346. The contacter of potentiometer 346 is mechanically coupled to motor 384 and is adjusted by the output rotation or' this motor. As the contactor of potentiometer 2.74 (Figure 6) is moved from one end terminal to the other, the potential applied to the screen gridof pentode 338 is simultaneously varied. As the contactor of potentiometer 27 4 moves tov/ard the grounded endterrninal to reduce the feedback signal and increase thev amplier gain, the contactor of potentiometer 346 is moved to'reduce the potential applied to the screen grid of pentode 338 to decrease the gain of servo amplier 290'.v The importance of this variable gain potentialV dividing network should become apparent from the following examples. it is rst assumed that the output signalY from spectrometer tube 10 is large so that the position of the contacterV of potentiometer 274 is at its upper limit whichV provides maximum feedback. and minimum tuned ampliiier gain. itis next assumed that the output signal from tube liti drops to one-half its ini? tial value. Y r1`he tuned amplifier gain must then be .doubledV to maintain a constant output. The contactor orf-potentiometer 274 must, accordingly, move to its midpoint to double this gain. Thus, for a tWo-to-one gain variation, motor 304 must mover the contactor of potentiometerV 274V over fty percent of its total path. it isf next assumed that the output signal from tube 10 is only one-tenth'A ot'its full value and that the contacter of potentiometer 274 is' one-tenth of the distance between its; lower a-nd upper end terminals. Under this conditionV the amplifier gain is ten times the value initially assumed. If the output signal from tube 10 is againhalved, the contact'or of potentiometer 274 must be moved to a point one-twentieth of the distance between the lower and upperend terminals. For a two-to-one gain variation under these latter conditions, motor 304 moves-the contactorof` potentiometer 274 over only ve percent ofits total-path.-

From a comparison of the two examples it can be seen thatthedirect current voltage difference applied to servo amplifier 290 is some ten times as largev per degree of movement of the contactor of potentiometer 274 when thecontactor is at its lower end than at the upper end. This requires the servo ampliiier gain in the lirst situation to be approximately ten times greater than that required for the second situation. However, by varying the gain of pentode 338, the gain of amplifier 290 can be variedy in a non-linear fashion which nearly matches the nonlinear change in output signal per unit change of the position ofy thecontacter of potentiometer 274. The gain of amplifier 290 is not critical because this amplifier is employed merely to provide suflicient power to rotate motors 304y and 307m eliminate the voltage diierence between- terminals 288 and 291.

rlimer circuit 16 is illustrated in Figure'. The purpose of this timer is to pass a standard gas sample into ionization chamber 38 periodically in place ofthe gas sample under analysis so that the operation of the mass spectrometerv tube and circuitry can be checked and adjusted as necessary to retain the instrument properly calibrated. YThe timer is actuated by a synchronous motor 375 which is connected across voltage source 20. Motor 375 drives a pair of cams 376 and 377 at a con- 14 stant speed Vby suitable gearing, not shown. The coils of relays 272 and 301 are energized by a source of direct voltage appearing between terminals 378 and 379'. One output terminal of voltage source 20 is connected to ter"-l minal 378 through a rectifier 380 and the second output terminal of voltage source 20 is connected directly to terminal 379. A lter capacitor 381 is connected in parallelV with terminals 378 and 379. The coils of relays 272 and 301 are connected across terminals 378l and 379 through a switch 383 which is operated by cam 376. Solenoids 13a andy 15a, which operate respective valves 13 and -15 (Figure l), are connected across terminals 378 and.k 379 through a switch 384 which is operated by cam 377. During a predetermined portion of one cycle of rotation of cams 376 and 377, switches 385 and384 remain open. However, during a second predetermined portion oi one cycle of rotation of cams 376 and 377, switch 383 is closed so that relays 272 and 301' are energized. This moves switch arms 271 and 300 into engagement with respective contacts 276 and 305. Relay 301 thus terminates the rotation of motor 304 and. energizes motor 307 in place thereof from the output signal of amplifier 290. Potentiometer 277 isl connected in circuit with potentiometer 278 in place of potentiometer 274. by relay 272. At the same time,.solenoids 13a and 15a are energized by. closure of switch 384 to close valveV 13 and open valve 15. This allows areference gas to pass into ionization chamber 30.

If no change in the operation of the electrical circuitry associated with the spectrometer tube or the tube itself has. occurred since the previous standardization period, the voltage applied to. servo amplifier 290 remains zero so that motor 307 remains stationary. However, if` any drifthasoccurred inthe system the input voltage applied toamplilier 290results in rotation of motor 307 to vary the position` of the contacter of the potentiometer 270, which is mechanically coupledto the' drivev shaft` of motor 307, by an amount suiicient to correct for any driftv in the circuitry.

The overall operation oi the mass spectrometer ofthis invention shown now become apparent. With reference to- Figure l, electrons emitted from heated filament 26 are accelerated into ionization chamber 30. The electron lioW into ionization chamber 30'is maintainedconstant by emission regulator 29. The positive ions formed in chamber 30 are accelerated toward collector plate 38-by the-negative potentialsV applied to grids 33 and 34. Ions'.- of alparticular mass are further accelerated by means of the modulatedV radio frequency potential applied to gridsv 42 42u, 42b and 42e. The ionsv which acquire suficient energyy to overcome the positive potential barriermaintained at grids.V S'impinge collector plate 38 to actuate detector 53.

'Iheispacings s between individual grids ofeach'set are maintained equal, whereas the'spacings rbetween th'esets of grids. are represented'by the experssion.:

where? n2 is'A an integralV number and tV is' the thickness of each grid, `all dimensions being in inches. In one embodiment of this invention, the following spacings were utilized: grid 42 was 0.118 inch from grid 40,' grid' 46 was 0.118 inch from grid 42, grid 40a was 2.618 inches fromgrid 46, grid 42a was 0.118 inch from grid 40a, grid' 46awas 0.118 inch fromv grid 42a, grid 40b was 1.345 inches from grid 4de, grid 42h was 0.118finch froru grid` 40b, grid 46b was 0.118 inch from grid 42h, grid 40e was 1.982 inches from grid 4Gb, grid 42C was 0.118 inch from grid 40C, and grid 45C was 0.118 inch from grid 42C. In thisarrangement s is 0.118, t is 0.01, the n between grids 46 and 40a is nine, the n between grids 46a and 40b is five, and the n betweenv grids `4Gb and 40e is seven. Other values of n can be employed if desired, andi the number of drift spaces can be varied if desired.

15 In this same embodiment of the invention the circuit components of power supply 21 were as follows: resistor 74, 4000 ohms; resistor 81, 33,000 ohms; resistor 85, 75,000 ohms; resistor 86, ohms; resistor 122, 1000 ohms; resistor 121, 560,000 ohms; resistor 112, 500,000 ohms; resistor 116, 15,000 ohms; resistor 117, 2.7 megohms; resistor 115, 15,000 ohms; resistor 110, 10,000 ohms; resistor 111, 30,000 ohms; resistor 105, 100,000 ohms; resistor 108, 1 megohm; resistor 104, 8,200 ohms; resistor 102, 120,000 ohms; capacitor 70, 4 microfarads; capacitor 71, 40 microfarads; capacitor 72, 60 microfarads; capacitors 78 and 79, 0.02 microfarad each; capacitor 97, 30 microfarads; capacitor 98, 40 microfarads; capacitor 118, 0.1 microfarad; capacitor 119, 30 microfarads; capacitor 109, 0.1 microfarad; inductors 66, 67 and 95, 13 henries each; diodes 64 `and 93, type 5V4; tubes 75 and 76, type CA2; tubes 82, 83 and 101, type 5651; pentode 94, type 6Y6; and triodes 113 and 103, type 6SL7. The voltage supplied by source 20 was 115 volts, 60 cycles. The voltage between terminals 24 and 25 was approximately 6.3 volts. The voltages at the output terminals were: 65, 370 volts; 77, 300 volts; 22, 174 volts; 87, 10 millivolts; and 23, negative 330 volts.

In Figure 3, potentiometer 27 had `a total resistance of 1000 ohms; resistor 130, 50,000 ohms; resistor 135, 560,- 000 ohms; resistor 137, 20,000 ohms; resistor 133, 50,- 000 ohms; potentiometer 134, 50,000 ohms; resistor 136, 20,000 ohms; pentode 132, type 6AU6; and tube 131, type 5651.

In Figure 4, tuning fork 150 vi'brated at 1000 cycles per second; resistors 155 and 162 were 300,000 ohms each; resistors 157, 160, 167 and 206, one megohm each; resistor 159, 1200 ohms; resistor 161, 300,000 ohms; resistor 156, 220,000 ohms; resistor 164, 100,000 ohms; resistor 169, 10,000 ohms; rresistor 172, 6,800 ohms; resistor 157, 10,000 ohms; resistor 184, 100,000 ohms; re-

. sistor 182, 180 ohms; resistor 212, 560,000 ohms; re-

sistor 214, 820,000 ohms; resistor 217, 470 ohms; resistor 213, 50,000 ohms; resistor 197, 18,000 ohms; capacitor 158, 0.01 microfarad; capacitor 168, 0.1 microarad; capacitor 166, 0.1 microfarad; capacitor 173, 0.001 microffarad; capacitor 175, 4 microfarads; capacitor 171, 0.1 microfarads; capacitor 188, 200 micro-microfarads; capacitor 191, 0.002 microfarads; capacitor 194, 300 micro-microarads; capacitor 183, 0.002 microfarad; capacitor 202, 0.0015 microfarad; capacitor 190, 0.0015 microfarad; capacitor 207, 0.0015 microfarad; capacitor 208, l microfarad; capacitor 198, 0.002 microfarad; capacitor 215, 0.02 microfarad; capacitor 216, 20 microfarads; inductor 81, 2.5 millihenries; circuits 186 and 192, each tuned to 3.7 megacycles; triodes 153, 154 and 163, 165, type 12AX7; pentode 170, type 6AS6; diodes 203, 204, type 6AL5; pentode 210, type 6AU6; triodes 218, 219, type 12AU7, tube 195, type CB2; and tube 211, type 5651. Crystal 180 had a resonant frequency of 3.7 megacycles. be varied from approximately 175 to approximately 205 volts.

With reference to Figure 5, resistors 232 and 232' were 510,000 ohms each; resistors 236 and 236', 2.7 megohms each; resistors 238 and 238', 15 megohms each; resistors 242 and 242', 1 megohm each; resistors 263 and 263', l megohm each; resistors 231 and'231', 2,200 ohms each; resistors 246 and 246', 560 ohms each; resistors 240 and 240', 51,000 ohms each; resistorsv255 and 255', 560 ohms each; resistors 251 and 251', 10,000 ohms each; resistors 247 and 247', 27,000 Ohms each; potentiometers 248 and 248', 25,000 ohms each; potentiometer 265, 50 ohms; capacitors 237 and 237', 0.03 microfarad each; capacitors 268 and 268', 10 microfarads each; capacitors 235 and 235', 0.002 microfarad each; capacitors 241 and 241', 50 microfarads each; capacitors 254 and 254', 1.0 microfarad each; capacitors 262 and 262', 0.002 microfarad each; capacitors 252 and 252', 4 microfarads each; capacitor 227, 0.001 micro- The voltage at terminal 185 could farad; capacitor 227', 0.05 microfarad; capacitor 239, 10 microfarads; inductor 228, 25 henries; inductor 228', 500 millihenries; potentiometer 265, 50 ohms; pentodes 225 and 225', type 5879; pentodes 234 and 234', type 6AU6; and triodes 245, 250 and 245', 250', type 5687.

In Figure 6, resistor 295 was 440,000 ohms; resistor 293, 100,000 ohms; resistor 286, 100,000 ohms; resistors 423 and 424, 10,000 ohms each; capacitor 287, 20 microfarads; capacitor 284, 0.22 microfarad; capacitor 283, 0.002 microfarad; potentiometer 270, 1,000 ohms; potentiometers 274, 277 and .315, 50 ohms each; and diodes 285, 292, type 6AL5.

1n Figure 7, resistor 343 was 510,000 ohms; resistor 344, 100,000 ohms; resistor 354, 470,000 ohms; resistor 362, 1 megohm; resistor 364, l megohm; resistor 365', ohms; potentiometer 336, 1,000 ohms; resistor 353, 1,800 ohms; resistor 341, 2,200 ohms; resistor 347, 180,- 000 ohms; resistor 348, 5,100 ohms; potentiometer 346, 10,000 ohms; capacitor 339, 0.0033 microfarad; capacitor 345, 1 0 microfarads; capacitor 351, 0.05 microfarad; capacitor 356, 0.05 microfarad; capacitor 342, 50 microfarads; capacitor 363, 0.1 microfarad; pentode 338, type 5879; pentode 352, type 6AU6; and triodes 357, 358 and 359-, 360, type 12AU7.

In the operation of the mass spectrometer tube of this invention it is important that a tixed relationship between the radio frequency voltage and the negative accelerating potentials be maintained. In this regard it has been found advantageous to modulate the radio frequency signal by a square -wave audio frequency signal. 1f the radio frequency signal were sine wave modulated, then the radio frequency signal envelope would varypin sinusoidal fashion. Under such circumstances, the radio frequency voltage for which the tube -is designed to operate lat would be realized for only a very sho-rt interval at the peak of the envelope. The use of square wave modulation, however, permits the radio frequency envelope to remain at the proper level for substantially onehalf of the modulating period. This permits an output signal of greater magnitude fromthe collector plate than could be obtained with sine Wave modulation of the radio frequency voltage. The amplitude of the modulating voltage appears to be less critical for `a square wave than -for a sine Wave.

The collector current, which `depends primarily upon the number of positive ions of preselected mass that have impinged collector plate 38, is returned to ground through the parallel resonance circuit 226 of Figure 5. This circuit is tuned to the modulating frequency of 1000 cycles per second, for example. The input voltage applied to the 'amplier of Figure 5 is, therefore, the product of the collector plate current and the impedance of circuit 226. Obviously, the higher the input impedance the -higher is the voltage applied to the control grid of pentode 225. The resonant impedance of circuit 226 can be expressed as ZzwLQ Where w=2vrf, f is the resonant frequency. L is the inductance of coil 228,

and R. is the series resistance of coil 228. In the particular circuit Q=40, L=25 henries, f=1000 cycles per second and Z=6.28 10s ohms. The actual Q of coil 228 is approximately 100. However, to compensate for impedance variations resulting from ambient temperature variations it has been `found desirable to include thermistor 229 in series with coil 228. This, of course, increases the effective resistance of coil 228 and lowers the Q to the indicated value. The use of such a tuned circuit in the amplifier input network provides at least two decided advantages. A tuned circuit restricts the band Width of the system and thereby improves the signal-to-noise ratio. Also, the stray shunt capacitance of collector plate 38, the input capacitance of pentode 225 and the capacitance of the cable connecting plate 38 to the control grid of pentode 225 are utilized as part of the tuning capacitance in circuit 226. If an input resistor were employed in conjunction with pentode 225, this stray capacitance would shunt the input resistance and lower the effective input impedance which could be obtained. For example, if the total shunt capacitance were 50 microfarads, then the reactance at 1000 cycles per second is approximately three megohms. Thus, the highest possible input impedance is approximately three megohms as compared with the considerably higher input impedance resulting from circuit 226.

Voltage dividing network 41, which is illustrated in Figure l, is employed to maintain the proper negative potentials on the grids of tube 10. Negative potential terminal 23 of power supply circuit 21 is applied to one end terminal of a first resistor 400. The second end terminal of resistor 400 is connected directly to a switch terminal 401 and to switch terminals 402'and 403 through respective resistors 404 and 405. A switch arrn 407, which selectively engages terminals 401, 402 and 403, is connected to the end terminal of a variable resistor 408. The contactor of variable resistor 408 is connected to grid 40 and to first end terminals of resistors 409 and 410. The second end terminal of resistor 409 is connected to the contactor of a variable resistor 411. The end terminal of resistor 411 is connected to the first end terminal of a potentiometer 412 and to the first end terminal of a resistor 413. The second end terminals of potentiometer 412 and resistor 413 vare connected to one another. The contactor of potentiometer 412 is connected to the end terminal of a variable resistor 414. The contactor of resistor 414 is connected directly to a switch terminal 403a and tolswitch terminals 402a'and 401a through respective resistors 404a and405a. A switch arm 407a, which is connected to ground, is mechanically coupled to switch'arm 407 and selectively engages terminals 401g, 40261 and 403a at the same timeV switch arm 407 engages respectively terminals 401,402 and '403. The second end terminal ofresistor 410 is connected to the iirst'end terminal of a resistor 416, and the second end terminal of resistor 416 is connected to vthe first end terminal of a resistor 417. The second end terminal of resistor 417 is connected to the junction between variable resistor 411 and potentiometer 412. This latter junction is also connected to grids 46c, 40C and 46b. The Vjunction between resistors 410 and 416 is connected t` grids 46 and 40a, and the junction between resistors 416 and 417 is connected to grids 46a and 40b.

In the previously mentioned embodiment of the mass spectrometer, the circuit components of voltage dividing network 41 .were as follows: resistors 410, 416 and 417, 120,000 ohms each; resistors 404 and 404a, 8,000 ohms each; resistors 405 and 40511, 16,000 ohms each; potentiometers 408 and 414, 10,000 ohms each; resistor 40.9, v'2,000 ohms; potentiometers 411 and 412, 5,000 ohms each; and resistor 413, 360,000 ohms. f

Network 41 is designed such that changes can be made in the potentials applied to the various grids. 'I'he potential applied to grid 40 is referred to as the accelerating potential, whereas the potentials applied to grids 46, 40a, 46a, 40b, 46h, 40C and 46c are referred to as step-back potentials. These step-back potentials suiiciently retard .acceleration of the ions sothat selected ions retain proper velocities to receive maximum energy from each radio frev q'uency accelerating field through which they pass. The

accelerating potential can be varied by either ganged 412.y A motor 420 is provided to adjust the accelerating Vpotentials automatically .to scan a sample for the presence ,p of ions,A of various masses.- Motor 420 adjusts resistors Y4,08 and v414 to vary the acceleratingy potential over approximately one hundred volts. Additional variance is -pbtained by manual adjustment of switches 407 and 407a.

Motor 420 can drive the recorder chart associated with detector 53 such that the output signal is correlatedwith the accelerating potentials.

From the foregoing description, it should be apparent that there is provided in accordance with this invention an improved mass spectrometer. This instrument is particularly useful for process analysis and control because of its small size. The spectrometer does not require a magnetic deiiecting field, and as such is less bulky and less expensive to operate than conventional mass spectrometers. While this invention has been described in conjunction with a present preferred embodiment, it should be evident that the invention is not limited thereto.

What is claimed is: v v

1. A massfspectrometer comprising a gas impermeable envelope enclosing an ion source; a collector plate spaced from said soure; a plurality of groups of grids spaced between said ion source and said collector plate, each of said groups comprising three equally spaced grids, the spacings between adjacent groups being n-0.3183-(2s|t) inches, where n is an integral number, s is the spacing between adjacent grids in each group, and t is the thickness of each grid, all of said dimensions being in inches; first and second spaced grids positioned between said collector plate and the group of said grids adjacent'said collector plate; a first source of alternating potential of a first frequency, one terminal of said first source being connected tol the center grid in each of said groups, the'second terminal of said lfirst source being connected to a point of reference potential; a second source of alternating potential of a second frequency which is lower than said'lirst frequency; means to modulate said first source by said second source; means to apply a potential of polarity opposite the polarity of the ions being analyzedto said first grid; and means to `apply a potential of polarity the same as the polarity ofthe ions being analyzed to said second-grid..

2l The combination in accordance with claim 1 further comprising means connected to said collector plate to measure the magnitude of the component of the l ion 'stream impinging said collector plate of the frequency of said second frequency.

' 3. A mass spectrometer comprising a gas impermeable -envelope enclosing an ion source; a collector plate spaced from said ion source; twelve grids spaced consecutively between said ion source and'said collector' plate, thefirst `ofsaid grids being adjacent said ion source and r the twelfth of Vsaid grids being adjacent said collector plate,

the spacings between said first and second, said second and third, said fourth and fifth, said fifth and sixth, s aid seventh andY eighth, said eighth and ninth, said tenth and eleventh, andsaid eleventh and twelfth grids being equal,

veach of said twelve grids having the same thickness, the Vspacing between said third and fourth grids vbeing 's is the spacing between said first and second grids and t ytenth grids being n."0.3183(2s|-t) inches, where n" is -an integral number; a thirteenth grid positioned between said twelfth grid and said collector plate; means for apply- `ing a potential to said thirteenth grid of polarity opposite 4the polarity of the ions being analyzed; a source of alternating potential, one terminal of said source being connected to said second, fifth, eighth and eleventh grids, the -second terminal of said source being connected to a point ,of reference potential; and means for applying potentials of polarity opposite the polarity of the ions being analyzed to said first, third, fourth, sixth, seventh, nineth, tenth and twelfth grids comprising a potential dividing network, and a source of potential of polarity opposite the polarity of the ions being analyzed applied across said network, said first grid being connected to a first point -on said network which is maintained at a first potential,

said third and fourth grids being connected to a second point kon said network which is maintained at a second potential of lesser magnitude than said first potential, said sixth and seventh grids being connected to a third point on said network which is maintained at a third potential of lower magnitude than said second potential, and said ninth, tenth and twelfth grids being connected to a fourth point .0n said network which is maintained at a fourth potential of lesser magnitude than said third potential.

il The combination in accordance with claim 3 further comprising means to vary the magnitude of said first, second, third and fourth potentials simultaneously.

5. An ion source comprising, in combination, an ionization chamber including inlet means to receive material to be ionized, an electron emitting filament, means for directingelectrons emitted from said filament to said ionization chamber, -a screen electrode positioned inthe path Gf' 4Said electrons, a potential dividing network, a voltage source applied across said network, an electron tube having` at least a cathode, an anode and a control grid, an impedance element' having one terminal connected to said anode, said cathode being connected to one terminal of said voltage source, the second terminal of said impedance element being connected to the second terminal of saidvoltage source, said anode being connected to said vscreen electrode, means connecting said filament to a point on said network intermediate the end terminals thereof, .and means connecting said control grid to a second point on said network intermediate the end terminals thereof.

l 6,.; An ion source comprising, in combination, anionpation chamber including inlet means to receive material .to be ionized, an electron emitting filament, means Ifo directing electrons emitted from said filament to said io ,zativon chamber, a screen electrode positioned in the path of said electrons, an electron tube having at least a cathode, an anode, and a control grid, a firstV resistor having one terminal connected to said anode, a voltage sc mrceV .applied between said cathode Vand the second termittalv of said first resistor, the positive terminal of said voltage source being connected to said first resistor, a second resistor having one terminal connected to the positive terminal of said; voltage source, a voltage regulat- ,ing4 tube having the anode thereof connectedr to the secterlninal of said second resistor, and a third resistor `having one terminal thereof connectedto the cathode of sai'dyvoltage regulating tube, the second' terminal of said 'third` resistor being connected to the negative terminal of voltage source, the anode of said voltage regulating Itube being connected? tol said; filament, .and the anode of said-electron tube being connected to said' screen electrode.

7. Circuit means for measuring alternating lcurrent signale comprising, in combination, an alternating current amplifier having a variable feedback networkincluded therein; to vary` the gain of the ampliiiensaid' amplifier comprising an; electron tube having an anode, a cathode and a control grid, means to apply a potential between said anode and saidY cathode, an input terminal connected Ito said control grid, a capacitor connected between said control grid and `a point of reference potential, anfinductorand a resistance element having a negative temperature coefficient of resistivity connected in series relation, said series connected inductor and resistance element being connected in parallel with said capacitor;

means to establish a rst voltage of magnitude proportionalto the magnitude of the output signal from said amplifier; a source of reference voltage; means to compare said first voltage with said reference vol-tage; means responsive to said comparing means to Yvary said feedback network untill ther-eis a zero difference between said voltages being compared; and means to measure the magnitude of variance of said feedback network.

" 8. Means .for measuring alternating current signals comprising, in combination, an alternating current amplifer having a variable feedback networkinclndedtherein to vary the gain of the amplifier, means to establish a first direct. voltage of magnitude proportional to the magnitude ofthe output signal from said amplifier, a source of reference direct voltage, means to compare said reference'voltage with said first voltage to establish a voltage difference, means to amplify said voltage difference, a servo motor actuated by said amplified voltage difference, the gain of said servo amplifier being :adjustable, means connecting said servo motor to said feedback network so that the gain o-f said amplifier is adjusted by said servo motor until there is Ia zero diiference between said voltages being compared, and means under control of said servo motor to vary the gain of said servo ampliiier so that the gain of said servo vamplifier has a irst value when the amplitude of the signal being measured has a Ifirst value and has a second value when the amplitude of the signal being measured has a second value.

9. Circuit means for measuring alternating current signals comprising, in combination, an alternating current amplifier having a variable feedback network included therein to vary the gain `of the amplifier, means to establish a direct voltage of magnitude proportional to the magnitude` of the output signal from said amplifier, a source of reference direct vol-tage, a converter to establish an alternating signal of magnitude proportional to the difference between said direct voltage and said reference voltage, an alternating current servo amplifier to amplify said converted signal, the gain of said servo amplierbeing adjustable, a servo motor connected to said feedback network, said servo motor being energized by the output ofv said servo amplifier so that said feedback network is adjusted until there is a zero diiference between said reference voltage and said direct voltage, and means under control of said servo motor to vary the gain of said servo amplifier so that the gain of said servo ampliiier has a first value when the amplitude kof the signal being measured has a first value and has a second value when the amplitude of the signal being measured has a second value.

l0. The combination in accordance with claim 1 furtherincluding means to measure the magnitude of .the ion stream impinging said collector plate comprising, in combination, means Ito establish a first voltage ,of magnitude proportional to lthe magnitude of the output signal' from said amplifier, a source of reference voltage, means to compare said first voltage with said reference voltage, means responsive to said comparing means to vary said feedback network until there is a zero rdilferf ence between said voltages being compared, =and means to measure the magnitude of variance of said feedback network.

11. A constant output oscillator comprising, in combination, an electron tube including an anode, a cathode and -a control grid, a source of potential, a tuned circuit comprising an inductor and a capacitor connected in parallelrelation, a variable resistance element, said anode being connected to one terminal of said tuned circuit, the second terminal of said variable resistance elem-ent being connected to said source of potential, circuit means connected` to said control grid to sustain oscillations in said .tuned circuit, and means to vary the resistance of said resistancetelement in response to potential changes at the anode of said tube -whereby the output signal of said oscillator 4remains constant.

12. A constant output oscillator comprising, in combination,l an electron tube including an anode, a cathode and a control grid, aV source of potential, a tuned circuit comprising an inductor and a capacitor connected in parallel. relation, a variable resistance element, said anode being connected. to one Iterminal of said tuned circuit, the second terminal of said tuned circuit being connected tov one` terminal of said variable resistance element, the second terminal of said variable resistance element being connectedto saidisource of'potential, circuit meansl connec'ted to said control grid to sustain oscillations in said tuned circuit, rectifying means connected to said anode to establishV a direct voltage of magnitude proportional to the magnitude of oscillations appearing at said anode, and means under control of said rectified voltage to vary the resistance of said resistance element whereby the output signal of said oscillator 'remains constant. 13. The combination in -accordance with claim 12 wherein said resistance element comprises an electron tube having at least an anode, a cathode fand a control grid, .the resistance ibetween said cathode and said anode constituting said resistance element, and means to apply said rectified voltage to the control grid of said vacuum tube to regulate the current yflow therethrough.

14. 'Ihe combination in accordance with claim 12 wherein said rectifying means comprises a voltage doubling rectifier circuit.

15. f` An oscillatorcomprising, in combination, a first electron tube having an anode, a cathode and at least two grids, a source of potential, a first tuned circuit comprising an inductor and a capacitor connected in parallel relation, a second tuned circuit comprising an inductor and a capacitor connected in parallel relation', a second electron tube having an anode, a cathode and a control grid, the anode of said second tube being connected to the positive terminal of said source of potential, the cathode of said second tube being connected to one terminal of said first tuned circuit, the second terminal of said lfirst tuned circuit being connected to the anode of said first tube, circuit means connected to the second of said grids to sustain oscillations in said first tuned circuit, means connecting said second tuned circuit between the first of said grids in said first tube and the cathode of said second tube, voltage doubling rectifying means connected between the anode of said 4first tube and the negative terminal of said source of potential, and amplifying means energize-d by the rectified voltage, the output of said amplifying means being applied to the control grid of said second tube.

16. A mass spectrometer comprising, in combination; a gas impermeable envelope enclosing an electron emitting filament, an ionization chamber spaced from said lament, a screen electrode positioned between said 'filament and said chamber, an accelerating electrode spaced between said filament and said chamber, a collector plate spaced from said chamber, and .a plurality of groups of grids spaced between said chamber and said plate, each of said groups comprising three equally spaced grids, the spacings between adjacent groups being n -O.3l83- (2s|t) inches, where n is an integral number, s is the spacing between adjacent grids in each group, and t is the thickness of each grid, all of said dimensions being in inches; lfirst and second spaced grids positioned between said collector plate and the group of said grids adjacent said collector plate; a source of voltage applied across said filament, means to measure the current flow from said filament, and means responsive to said means to measure current to apply a potential to said screen electrode to maintain the electron flow into said ionization chamber constant, an oscillator tuned to a first frequency, a second oscillator tuned to a second lower frequency, means to modulate the output of said first oscillator by the output of said second oscillator, means connecting one output terminal of said first oscillator to the center grid in each of said groups, and means connecting the second output terminal of said first oscillator to a point of reference potential; means to apply potentials to the two outside grids in each of said groups of polarity opposite the polarity of the ions being analyzed; means to apply a potential of polarity opposite the polarity of the ions being analyzed to said first grid; means to apply a potential of polarity the same as the polarity of the ions being analyzed to said second grid; an amplifier tuned to the frequency of Said second oscillator, said amplifier including an adjustable feedback network, the input of said amplifier being connectedvto said collector plate; means to establish a first voltage of magnitude proportional to the output of said amplifier, a reference voltage, a servo amplifier to amplify any difference between said first voltage and said reference voltage; a servo motor coupled to' said feedback network, said servo motor being actuated by the output of said servo amplifier to vary said feedback network until said voltage difference is zero; and means to measure rotation of said servo motor.

17. The combination in accordance with claim 3 wherein said potential dividing network comprises first and second terminals across which said source of potential is applied; a first switch having a first movable arm and rst, second, and third contacts engageable selectively by said first arm, said first cont-act being connected to said first terminal; a first resistor connected between said second contact and said rst terminal; a second resistor connected between vsaid third contact and said first -terminal; a second switch having a second movable arm and fourth, fifth, and sixth contacts engageable selectively by `said second arm, said second arm being connected to said second terminal; a first variable resistor having one terminal connected to said first switch arm; a second van'- able resistor having one terminal connected to said fourth contact; a third resistor connected between said one termin-a1 of said second variable resistor and said fifth contact; a fourth resistor connected between said one terminal of said second variable resistor and said sixth contact; a third variable resistor; means connecting one terminal of said third variable resistor to the second terminal of said first variable resistor; a fourth variable resistor connected between the second terminals of said second and third variable resistor; potential dividing means connected between the second terminal of said first variable resistor and the second terminal of said third variable resistor; means to vary said first and' second switch arms in imison so that said first switch arm is in contact with said first contact when said second switch arm is in contact with said sixth contact, said first switch arm is in contact with said second contact when said second switch arm is in contact with said fifth contact, and said first switch arm is in contact with said third contact when said second switch arm is in contact with said fourth contact; means to vary said first and second variable resistors simultaneously; and means to vary said third and fourth variable resistors simultaneously. t

18. A potential dividing network comprising first and second terminals; a source of potential applied across said terminals; a first switch having a first movable arm and first, second, and third contacts engageable selectively by said first arm, said first contact being connected to said first terminal; a first resistor connected between said second contact and said first terminal; a second resistor connected between said third contact and said first terminal; a second switch having a second movable arm and fourth, fifth, and sixth contacts engageable selectively by said second arm, said second arm being connected to said second terminal; a first variable -resistor having one terminal connected to said first switch arm; a second variable resistor having one terminal connected to said fourth contact; a third resistor connected between said one terminal of said second variable resistor and said fifth contact; a fourth resistor connected between said one terminal of said second Variable resistor and said sixth contact; a third variable resistor; means connecting one terminal of said third variable resistor to the second terminal of said first variable resistor; a fourth variable resistor connected between the second terminals of said second and third variable resistors; potential dividing means connected between the second terminal of said first variable resistor and the second terminal of said third variable resistor; means to vary said rst and second switch arms in unison so that said first switch arm is in contact with said first contact when said second switch arm is in contact with said sixth contact, said rstswitch arm is in contact with said second contact when said second switch arm is in contact with said ifth contact, and said rst switch arm is in contact with said third contact when said second switch arm is in contact with said fourth contact; means to varyrsaid rst and second variable resistors simultaneously; and means to vary said third and fourth variable resistors simultaneously. p Y

19. The combination in accordance with claim 1 ,wherein said first source of alternating potential provides an output signal having a sinusoidal wave from, and wherein said second source of potential provides an output signal having a square wave form.

Lederer May 8, 1934 2,068,112 Rust Jan. 19, 1937 Travis Dec, 13, 1938 .24 y2,369,030 Edwards s Fe b`6; 19.45 2,400,557 Lawlor May 21,` 2,449,072 Houghton ..1 Septr14, 1'948 2,487,279 Stalhane Nov. f8, 1949 2,535,032 Bennett Dec. 26, 1950 2,563,626 Stein et al. `Aug. 7, v19151 2,598,478 Worchester May 27, 1952 2,598,734 Washburn June 3,1952 2,602,898 Inghrarn et al. July 8, 19,52 '2,617,843 Houghton Nov. 11,` 1952 2,790,945 Chop'e Apr. 30, 1957 2,854,629 Thirup Sept. 30, 195

OTHER REFERENCES Pearson: Thermistors, Their vCharacteristics and Uses, Bell Laboratories Record, December 1940, pages 106 to 111.

Bennett, Journal of Applied Physics,

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