US2775708A - Mass spectrometer - Google Patents

Description

MASS SPECTROMETER 3 Sheets-Sheet l Filed Jan. lO. 1955 A TTORNEV Dec. 25, 1956 J. R. PARSONS ET AL MASS SPECTROMETER Filed Jan. l10, 1955 3 Sheets-Sheet 2 RECORDER TUNED AMPLIFI ER DETECTOR INVENTORS J. R. PARSONS D. A. FLUEGEL A TTORNEV Dec- 25, 1956 J. R. PARSONS ET AL 2,775,708

MASS SPECTROMETER 3 Sheets-Sheet 5 Filed Jan. l0. 1955 IBO INVENTORS J.R. PARSONS D.A. FLUEGEL HUM q' TIME n {TrZmPOm A T TORNE YS United States Patent() "ice MASS SPECTROMETER James R. Parsons and Dale A. Fluegel, Bartlesville, Okla.,

assignors to Phillips Petroleum Company, a corporation of Delaware Application January 10, 1955, Serial No. 480,699

14 Claims. (Cl. Z50-41.9)

This invention relates to mass spectrometry. In one specific aspect it relates to a method of and apparatus for adjusting a mass spectrometer to compensate for variations in accelerating potentials. t

In recent years mass spectrometers 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 spectrometers 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 to determine 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 diiferent velocities after passing through the electrical field and consequently have different moments.

In 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. Ions which enter the space between the first two grids in proper phase are accelerated through the fields between the first and second grids and the second and third grids. The ions sub* sequently 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 receive suicient energy to overcome a potential barrier and impinge upon a collector plate. In our copending application Serial No. 426,768, filed April 30, 1954, there is disclosed a mass spectrometer which is an improvement over that described in Patent 2,535,032. Four sets of accelerating grids are employed to provide three separate drift spaces. This greatly improves the resolving power of the spectrometer. The radio frequency potential applied to the accelerating grids is modulated by the output of a square wave audio frequency oscillator so that the output signal generated by positive ions impinging upon the collector plate is modulated at an audio frequency. This alternating signal can be measured more accurately than a direct current signal. l

One of the major problems encountered in operating a mass spectrometer to monitor continuously one or more mass peaks is the ditiiculty of maintaining the voltages sufficiently constant to keep the spectrometer focused 2,775,708 Patented Dec. 25, 1956 precisely on the desired mass peak. Failure to monitor the true peak results in variations of the output signal even though the number of ions entering the tube does not change. One of the chief difliculties is that of maintaining the accelerating potentials at constant value for selected ion masses. In accordance with the present invention an improved mass spectrometer is provided wherein the accelerating potentials applied to the spaced grids in the tube are modulated by an alternating signal, preferably in the audio range. This alternating signal is of diierent frequency than the audio oscillator which modulates the radio frequency oscillator. Thus, the output signal of the spectrometer contains a second cornponent of the same frequency as the second modulating frequency. This particular component of the output signal is measured by a separate detecting circuit which is tuned to .the second frequency. The magnitudes ot' the accelerating potentials are then adjusted automatically in reponse to variations in this detected signal to maintain the accelerating potentials at proper values to focus ions of a particular mass.

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 alternating potential modulates the accelerating potentials applied to the spectrometer so that changes in the magnitude of this modulated component in the output signal can be employed to adjust the accelerating potentials.

A further object is to provide a method of adjusting the accelerating potentials applied to a mass spectrometer so that ions of a predetermined mass can be measured continuously.

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 a mass spectrometer incorporating the modulation and adjustment features of this invention;

`Figure 2 is a detailed circuit drawing of the apparatus shown schematically in Figure 1;

Figure 3 is a graphical representation of the operation of this invention; and

Figure 4 is a second graphical representation of the operation of this invention.

Referringnow to the drawing in detail and to Figure l in particular, there is shown a mass spectrometer tube 10 which can comprise a gas envelope, the interior of l which is maintained at a reduced pressure by a vacuum pump, not shown, which communicates with tube 10 through a conduit 11. A sample of the gas to be analyzed is supplied to the interior of tube 10 by a conduit 12.

The electrical circuits associated with mass spectrometer tube 10 are energized from a source of alternating potential 20, the output terminals of which are connected to a power supply circuit 21. Power supply circuit 21 has a first output terminal 22 which is maintained at a constant positive potential and a second output terminal to one input terminal of an emission regulator circuit 29. The second input terminal of emission regulator 29 is connected to negative potential terminal 23.

4 The electrons emitted from filament 26 are accelerated into an ionization chamber 30, which is defined by a pair of grounded spaced grids 31 and 32, by the potential difference between grids 31, 32 and filament 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 terl`rninals of potentiometers 35 and 36 are connected to negative potential terminal 23 of power supply circuit 21. The second end terminals of potentiometers 35 and 36 are grounded. The electron flow from filament Z6 is reg- 'ulated by a screen electrode 37 which is connected to an output terminal of emission regulator 29. Although such an emission regulator circuit is not essential to the operation of the spectrometer of this invention, it is desirable to maintain a constant ow of electrons into this ionization chamber 30. As described in detail in our abovementioned copending application, this emission regulator is adapted to adjust the potential on screen electrode 37 in response to any change in emission from filament 26 so as to maintain the flow of electrons into ionization chamber 30 constant. The gas sample to be analyzed is introduced into ionization chamber 30 from conduit 12 to be subjected to electron bombardment so that positive ions are formed. These positive ions are accelerated through tube toward a collector plate 38 which is positioned in the opposite end of the tube.

The positive ions produced in chamber 30 are accelerated toward collector plate 38 by focusing grids 33 and 34. A third grid 40 is connected to a potential dividing network 4l which is described in detail hereinafter. Negative potential terminal 23 of power supply circuit 21 is connected to one end terminal of network 41, the second end terminal of the network being connected to ground. The next grid 42 in tube 10 is connected to one output terminal of a radio frequency oscillator 43, the second output terminal of which is grounded. The output signal of oscillator 43 is modulated by the output signal of an audio oscillator 44, which preferably provides a signal of substantially square wave form. An inductance coil 45 is connected between grids 40 and 42. The next grid 46 in tube 10 is connected to a second point on potential dividing network 41 which is maintained at a potential of lesser negative magnitude than the potential applied to grid 40. Grids 40, 42 and 46 form the first set of ion velocity modulating grids in the spectrometer tube. 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 in tube 10 in spaced relation with grids 40, 42 and 46. Grid 40a is connected to grid 46, grid 42a is connected to the output of oscillator 43, and grid 46a is connected to potential dividing network 41 at a point which is maintained at a potential of lesser negative magnitude than the potential applied to grid 46. A third set of corresponding grids 4017, 4213 and 46b is positioned in spaced relation with grids 40a, 42a and 46a. Grid 4017 is connected to grid 46a, grid 42b is connected to the output of oscillator 43, and grid 46b is connected to a point on potential dividing network 41 which is maintained at a potential of lesser negative magnitude than the potential applied to grid 46a. A fourth set of corresponding grids 40C, 42e and 46c is positioned in spaced relation with grids 40h, 42b and 46b. Grid 40e is connected to grid 46b, grid 42C is connected to the output of oscillator 43, and grid 46c is connected to grids 46b and 40e.

First and second groups of grids 50 and 51 are positioned between grid 46c and collector plate 38. Grids 50 are connected to one another and to the contactor of a potentiometer 52. One end terminal of potentiometer 52 is connected to positive terminal 22 of power supply circuit 21, and the second terminal of potentiometer 52 is 4i 'coni'iected to ground. Grids 5i are connected to one another and to negative terminal 23. Collector plate 38 is connected to corresponding first input terminals of tuned amplifiers 53 and 54; the corresponding second input terminals of these amplifiers are grounded.

The positive ions produced within chamber 30 are accelerated toward collector plate 38 by the negative potentials applied to grids 33, 34 and 40. During one half cycle of the radio frequency signal from oscillator 43, the electrical field between grids 40 and 42 is of such phase that ions entering the field are further accelerated. Ions which enter this field during a particular phase of the alternating field receive maximum energy. During the following half cycle of the output signal from oscillator 43, the field between grids 42 and 46 is reversed such that the ions are again accelerated. The ions then drift through the field-free space between grids 46 and 40a. The masses of the individual ions determine their respective arrival times at grid 40a. The ions which arrive at grid 40a at the proper time with respect to the output signal of oscillator 43 are accelerated by the field applied between grids 40a and 42aand thereby receive additional energy. The same accelerating procedure continues as the ions pass through the next seven grids so that the resulting ion beam is velocity modulated whereby ions of a particular mass receive maximum energy. The positive potential applied to grids 50 is adjusted so that only those ions having a velocity greater than a predetermined value are able to pass through grids 5i) to impinge upon collector plate 38. The purpose of negative grids 5i is to suppress electrons which may be liberated within tube 10 by the ions bombarding plate 38 or other elements of the tube.

The ions impinging upon collector plate 38 create a current in the input circuit of tuned amplifier 53. The magnitude of this current is proportional to thc number of ions impinging upon collector plate 3S per unit time. This current is amplitude modulated at the same frequency as the frequency of audio oscillator 44. Amplifier 53 is tuned to pass only signals of the frequency of oscillator 44. The output signal from amplifier 53 is applied to a detector circuit 56 which preferably has an output that is a linear function of the input signal applied thereto. The output signal of detector 56 is applied to an indicating circuit, such as a recorder 57. While the particular circuits of oscillators 43 and 44, power supply circuit 21, amplifier 53, detector 56 and recorder 57 do not form a part of the present invention and conventional circuits can be used, it is preferred that these circuits be of the types described in our above-mentioned copending application.

In accordance with the present invention, voltage dividing network 41 includes a transformer 60. The prmary winding 61 of transformer 60 is energized by the output signal of an oscillator 62, which preferably has a frequency in the audio range. The frequency of oscillator 62 must necessarily be different from the frequency of oscillator 44 and should not produce harmonics or be a harmonic of the frequency of oscillator 44. Negative potential terminal 23 of power supply circuit 21 is connected to ground through a resistor 63, a secondary winding 64 of transformer 61, series connected variable resistors 65, 66, 67 and 68 and a potentiometer 69. Resistors 70, 71 and 72 are comfeeted in series relation with one another and in parallel with variable resistor 66. The junction between resistors 70 and 71 is connected to grids 46 and 40a of tube 10; the junction between resistors 71 and 72 is connected to grids 46a and 40b; the junction between resistors 65 and 66 is connected to grid 4t): and the junction between resistors 66 and 67 is connected to grids 46b, 40C and 46c.

Amplifier 54 is tuned to pass signals of the same frcquency as oscillator 62. The output signal of amplifier 54 is applied to first input terminals of a phase detector circuit 75, which is also energized by the output of oscillator 62 through a secondary winding 73 of transformer 60. The output of lphase detector is applied to the inputof a direct Vcurrent ampliiier 476. The output voltage of amplifier 76 is applied to the contactor of potentiometer 69 to adjust the magnitude of the accelerating potentials applied to the grids of'tube 10 in the manner described hereinafter in detail.

`With reference to Figure'Z, oscillator 62 comprises a first pentode 80 having the cathode thereof connected to ground through a resistor 81. The control grid of pentode 80 is connected to ground through a resistor 82 which is shunted by a capacitor 83. The screen grid of pentode 80 is connected to ground through a resistor 84 and to a positive potential terminal through series connected resistors 86 and 87. The vanode of pentode 80 is connected to terminal 85 through series connected resistors 88 and 87. The anode of pentode 80 is also connected to the control grid of a second pentode 90 through a capacitor A91. The control grid of pentode 90 is connected to ground through a resistor 92. The suppressor grid and cathode of pentode 90 are connected to one anotherand to ground through a resistor '93 which is shunted by a capacitor 94. The screen grid of pentode 90 is connected to ground through a capacitor 96 and to terminal 85 through series connected resistors 97 and 87. The anode of pentode 90 is connected to terminal 85 through series connected resistors 98 and 87 and to the cathode of pentode 80 through a feedback resistor 100. The anode of pentode 98 is also connected to the control grid of pentode 80 through a resistor 101 and a capacitor 102 which are connected in series relation. A capacitor 103 is connected between ground and the junction between resistors 97 and 87.

Oscillator 62 thus provides an output signal at the an ode of pentode 90 of frequency determined by the values of resistors 101 and 82 and capacitors 102 and 83. These values are adjusted to provide a desired 4frequency of the order of 285 cycles per second, `for example. Oscillator 44 can have a frequency of the order of 1000 cycles per second, for example. These frequencies are merely by way of example because any desired frequencies can be employed.

The anode of pentode 90 `is connected to the control grid of a triode 105 through a capacitor 106. The control grid of triode 105 is connected to ground through a i resistor 107. The cathode of triode 105 is connected to ground through a resistor 108 which is shunted by a capacitor 109. The anode of triode 105 is connected to a positive potential terminall 110 through the primary winding 61 of transformer 60.

A one stage pre-amplifier in the form of a triode 112 can be positioned between collector 38 and ampliers 53 and 54. Collector plate 38 is connected directly to the control grid of triode 112 and is connected to ground through a resistor 113. The anode of triode 112 is connected to a positive potential terminal 114, and the cathode of triode 114 is connected to ground through a resistor 115. The cathode of triode 112 is also connected through a resistor 116 to `one input terminal of amplifier 53 and through a capacitor 117 to the `control grid of a triode 118 which forms the rst stage of amplifier 54.

The control grid of triode 118 is connected to ground through a resistor 119. The cathode of triode `118 Vis connected to ground through a resistor 120 which is shunted by a capacitor 121. The anode of triode 118 is connected to a positive potential terminal 123 through series connected resistors 124 and 125. The junction between resistors 124 and 125 is connected to ground through a capacitor 126. The anode of triode 118 is alsoconnected to the control grid of a triode 127 through a capacitor 128. The control grid of triode 127 is connected to ground through a resistor 129. The anode of triode 127 is connected to potentialterminal 123 through resistor 125. The cathode of triode 127 is connected directly to the cathode of a triode 131 and is connected to ground through a` resistor 132. The anode of triode 131s connected to potential terminal 123 through series connected resistors 134 and 125. The anode of triode 131 is also connected to one terminal of a capacitor 135. The second terminal of capacitor 135 is connected to one end terminal of a potentiometer 136. The second end terminal of potentiometer 136 is connected to ground. The contactor of potentiometer 136 is connected to the control grid of a triode 138. The cathode of triode 13S-is connected to ground through a resistor 139, and the anodeof triode 13.8 is connected to terminal 123 through a resistor 140.

A tuned parallel-T feedback network is connected between the second terminal of capacitor 135 and the control grid of triode 131. This network comprises a pair of series connected resistors 151 and 152 having a pair of series connected capacitors 153 and 154 connected in parallel therewith. A capacitor 156 is connected be* tween ground and the junction between resistors 1`51 and 152. A resistor 158 is connected between ground and the junction between capacitors 153 and 154. Network 150 is tuned to the frequency of oscillator 62 so that amplifier 54 passes only signals of Vthis frequency.

The anode of triode 138 is connected through a capacitor 160 to the control grid of a triode 161 which forms a part of `phase detector '75. The control grid of triode 161 is connected through a resistor 162 and a capacitor 166 to ground, The junction between resistor 162 and capacitor 166 is connected through transformer winding 73 to the cathode of triode 161. The anode of' triode 161 is connected to the control grid of a triode 167 which forms -a part of direct current amplier 76. The control grid of triode 167 is connected to the junction between resistor 68 and potentiometer 69 through a resistor `168. The anode of triode V167 is connected to ground, and the cathode of triode 167 is connected to the contactor of potentiometer 69.

The overall operation of the mass spectrometer of this invention should now become apparent. With reference to Figure l, electrons emitted from heated filament 26 are accelerated into ionization chamber 30. The electron flow into ionization chamber 30 is maintained constant by emission regulator 29. The positive ions formed in chamber 30 are accelerated toward collector plate 38 b'y the negative potentials applied to grids 33, 34, 40, 46, 40a, 46a, 40b, 46h, 40e and 46c. Ions of a particular mass are further accelerated by means of the modulated radio frequency `potential applied to grids 42, 42a, 42b and 42c. The ions which acquire sucient energy to overcome the positive potential barrier maintained at grids 50 impinge collector plate 38 to actuate detector 53.

The spacings s between individual grids of each set are maintained equal, whereas the spacings r between the sets of grids are represented by the expression:

where n is an integral number and t is the thickness of each grid, all dimensions being in inches.

Network 41 is designed such that changes can be made in the potentials applied to the grids in tube 10. The potential applied to grid l40 is referred to as the accelerating potential, and the potentials applied to grids 46, 40a, 46a, 40h, 46b, 40C and 46c are referred to as step-back potentials. The accelerating potential can be varied by ganged variable resistors 65 and 68. One of theseresistors is increased as the other is decreased. Adjustment of resistors 65 and 68 does not vary the relative values 'of the step-back potentials, which are adjusted by ganged ` resistors 66 and 67. Resistors 65 and 68 Vare varied to focus ions of selected masses on collector plate 38. Adjustment of resistors 66 and 67 provide correct step-back potentials. The step-back potentials decelerate the ions suiciently to enable them to travel at `proper yvelocities to remain in phase with the alternating accelerating potentials from oscillator 43.

The direct potentials applied to grids 40, 42, 46, 40a, 42a, 46a, 40h, 42b, 4Gb, 40C, 42C, and 46c are modulated by the alternating output potential oscillator 62. The ion beam collected at plate 38 is, therefore, also modulated at the frequency of oscillator 62. The magnitude of this frequency component of the output signal from tube is a function of the extent to which the accelerating potential difers from the correct value. This alternating signal is amplified by amplifier 54 and applied to the control grid of triode 161. The output signal from oscillator 62 is applied to the cathode of triode 161 by transformer winding 73. Curve 175 of Figure 3 represents a typical output signal from the mass spectrometer when the accelerating potential is scanned over a particular mass peak. It is assumed that the negative accelerating potential represented by numeral 176 is the proper value to focus these ions. From an inspection of the drawing it can be seen that the magnitude of the spectrometer tube output signal decreases rapidly if the accelerating potential either increases or decreases from this value. Curve 177 represents the modulating voltage from oscillator 62 which is superimposed upon the direct current accelerating potential provided by network 41. During the first half cycle of the output signal from oscillator 62, the accelerating potential applied to grid 40 is increased in the positive direction so that the output signal from tube 10 decreases to a minimum value indicated by point 178 on curve 175. During the next half cycle of modulating voltage, the tube output signal decreases to a minimum value represented by point 180 on curve 175. The spectrometer tube output signal which is applied to the input of amplifier 54 thereby decreases periodically at a frequency twice the frequency of oscillator 62. The wave form of this signal is illustrated by curve 181. Signals of the frequency of curve 181 are not passed by amplifier 54. Thus, when the accelerating potential is at exactly the correct value, a zero signal is applied to phase detector 75 from amplilier 54.

If the accelerating potential should drift in the positive direction to a value indicated by numeral 182, for example, the tube output signal is decreased to the magnitude indicated by point 183 on curve 175. If the output alternating potential of oscillator 62 is superimposed upon the direct current accelerating potential at this point, the amplitude of curve 175 decreases to point 184 during the first half cycle of applied signal. During the second half cycle of applied signal the spectrometer tube output signal increases to a point 186. The spectrometer tube output signal applied to amplifier 54 thus varies periodically at the frequency of oscillator 62. This signal is represented by curve 187. lf the accelerating potential should drift in the negative direction to a value indicated by numeral 190, the spectrometer output signal is decreased to a value indicated by point 191 on curve 175. During the first half cycle of the applied signal from oscillator 62, the amplitude of the tube output signal increases to a point 192. During the second half cycle of the output signal from oscillator 62, the tube output signal decreases to a point 193. The tube output signal thus varies periodically at the frequency of oscillator 62. This periodic signal is indicated by curve 194. Curves 187 and 194 are 180 out of phase with one another.

From an inspection of Figure 2 of the drawing it can be seen that triode 167 forms a variable resistance in parallel with the right-hand portion of potentiometer 69. The potential applied to the control grid of triode 167 determines the effective resistance of triode 167 and thus the effective parallel resistance of triode 167 and the righthand portion of potentiometer 69. Any change in the potential applied to the control grid of triode 167 changes the potential at the contactor of potentiometer 69, which changes the accelerating potential applied to grid 40 of tube 10. The output signal of oscillator 62 is applied through transformer winding 73 to the anodecathode circuit of triode 161. Curve 196 of Figure 4 represents the potential applied to the cathode of triode 161 as a function of the potential on the anode thereof- The polarity of transformer winding 73 is adjusted such that the potential on the cathode of triode 161 becomes more negative at the same time the potential applied to grid 40 becomes more positive, assuming the accelerating potential has drifted from 176. It is assumed that amplifier 54 transmits signals therethrough without any phase shift. If this is not true, a suitable phase correcting network can be incorporated to eliminate phase shift.

As previously mentioned, there is no output signal from amplifier 54 if the accelerating potential is exactly at the center 176 of curve 175 of Figure 3. If the accelerating potential shifts toward the value indicated by numeral 182 the output signal applied to amplifier 54 is represented by curve 187. This results in a potential being applied to the control grid of triode 161 of the phase shown by curve 198 of Figure 4. The gain of amplifier 54 is adjusted so that triode 161 passes a predetermined current during alternate half cycles of the voltage applied to the anode-cathode circuit thereof from transformer winding 73. When the signal represented by curve 19S is applied to the control grid of triode 161, the current iiow through triode 161 is increased because the bias on the control grid during the half cycles that the tube can conduct current is decreased. The magnitude of this bias potential is a function of the amount the accelerating potential has shifted from the proper value. This increase in current flow through triode 161 results in the control grid of triode 167 becoming more negative. Triode 167 thus conducts less current so that the potential at the contactor of potentiometer 69 and the potential applied to grid 40 become more negative. This serves to adjust the accelerating potential applied to grid 40 to a more negative value to focus ions of the desired mass.

If the direct current accelerating potential becomes more negative as indicated by numeral 190, the signal applied to the control grid of triode 161 is of the phase represented by curve 197. This potential results in the control grid of triode 161 becoming more negative during the half cycles that the triode conducts. The decreased conduction through triode 161 results in the control grid of triode 167 becoming more positive. This increases the current ow through triode 167 so that the potentials at the contactor of potentiometer 69 and at grid 40 become more positive to restore the accelerating potential to the correct value.

While the 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:

l. A mass spectrometer comprising an ion source, an ion collector electrode spaced from said ion source, an

ion permeable electrode, means applying a first potential to said ion permeable electrode to accelerate ions in a direction away from said source, means to direct said ions which have a predetermined mass to said collector electrode, means to vary the acceleration of said ions at a first frequency, second ion detecting means connected to said collector electrode, said second detecting means being responsive solely to the impingement of ions on said collector electrode at said first frequency, and means responsive to said second detecting means to vary the magnitude of said first potential until the output signal of said second detecting means is a minimum.

2. The combination in accordance with claim l wherein said means to vary the acceleration of said ions comprises means to apply a potential of said first frequency to said ion permeable electrode.

3. A mass spectrometer comprising an ion source, an ion collector electrode spaced from said ion source, an ion permeable electrode, a potential dividing network including a variable resistance, a source of first V,potential applied across said network, means connecting apoint on said network to said ion permeable electrode to'accelerate ions in a direction away from said source, means to direct said ions which have a predetermined mass to said collector electrode, means to vary the acceleration of said ions at a first frequency, rst ion'detecting means connected to said collector electrode, second ion detecting means connected to said collector electrode, said second detecting means being responsive solely to the impingement of ions on said collector electrode at said first frcquency, and means responsive to said second detecting means to adjust said variable resistance until the output signal of said second detecting means is a minimum.

4. The combination in accordance with claim 3 wherein said variable resistance comprises an electron tube having a cathode, an anode and a control grid, the anode of said tube being connected toone point in said network and the cathode of said tube being connected to a second point in said network whereby the resistance of said tube between the cathode and anode thereof forms a part of said network, and wherein said means to adjust said variable resistance comprises means to adjust the potential applied to the control grid of said tube.

5. A mass spectrometer comprising an ion source, an ion collector electrode spaced from said ion source, an ion permeable electrode, means applying a first potential to said ion permeable electrode to accelerate ions in a direction away from said source, means to direct said ions which have a predetermined mass to said collector electrode, a source of alternating potential of a first frequency, means including said source of alternating potential to vary kthe acceleration of said ions at said first frequency, first ion detecting means connected to said collector electrode, a phase sensitive detector, means applying said alternating potential to the first input of said phase detector, means connecting said collector plate to the second input of said phase detector, and means responsive to the out put of said phase detector to adjust the -magnitude of said first potential applied to said permeable electrode.

6. A mass spectrometer comprising an ion source, an ion collector electrode spaced from said ion source, an ion permeable electrode, means applying a first potential to said ion permeable electrode to accelerate ions in a direction away from said source, means to direct said ions which have a predetermined mass to said collector electrode, a source of alternating potential of a first frequency, means including said source of alternating potential to vary the acceleration of said ions at said first frequency, first ion detecting means connected to said collector electrode, a phase sensitive detector, means applying said alternating potential to the first input of said phase detector, an amplifier tuned to pass signals of said first frequency, means connecting one input terminal of said amplifier to said collector electrode and means connecting the second input terminal of said amplifier to a point of reference potential, means connecting the output terminals of said amplifier to the second input terminals of said phase detector, and means responsive to the output of said phase detector to adjust the magnitude of said first potential applied to said permeable electrode.

7. A mass spectrometer comprising an ion source, an ion collector electrode spaced from said ion source, an ion permeable electrode spaced from said ion source, a potentiai dividing network including a variable resistance, a source of direct voltage applied across said network, a source of alternating voltage applied across said network, means connecting a point on said network to said permeable electrode to accelerate ions from said source in a first direction, means to direct said ions which have a predetermined mass to said collector electrode, first ion detecting means connected to said collector electrode, a phase sensitive detector, means connecting said collector electrode to one input of said phase sensitive detector, means connecting said source of alternating voltage to the second input of said `phase sensitive detector, and means responsive to the output of said phase sensitive detector to adjust said variable resistance.

8. The combination in accordance with claim 7 further comprising means to energize said means to direct ions periodically at a second frequency different from the frequency of said source of alternating voltage, Vand wherein said first ion detecting means is responsive solely to variations in ion impingement on said collector electrode at said second frequency.

S. A mass spectrometer comprising a gas impermeable envelope enclosing an ion source, a collector plate spaced from said ion source, a plurality of groups of grids spaced in a line between said source of ions and said plate, each of said groups comprising three grids in spaced relation with one another, the spacings between adjacent grids being equal, the spacings between adjacent groups of said grids being substantially IIwOSlSS-(ZS-l-t) inches, where n is an integral number, s is the spacing between adjacent grids in each group and t is the thickness of the grids, all of said dimensions being in inches, and a second grid positioned between said collector plate and said groups of grids; means applying steady potentials to the two end grids in eaclrof said groups of grids; means applying a potential to said second grid of polarity opposite the polarity of the ions being detected; means applying an alternating potential of a first frequency to the center grid in each of said groups of grids; means applying an alternating potential of a second frequency to at least one grid positioned between said ion source and said collector plate; means connected to said collector plate to measure ions impinging thereon at said rst frequency; means connected to said collector plate to measure ions impinging thereon at said second frequency; and means responsive to said last-mentioned means to vary the magnitudes of said steady potentials to reduce the magnitude of the ion impingement on said plate at said second frequency to a minimum.

l0. The combination in accordance with claim 9 wherein said alternating potential of said second frequency is applied to the two end grids in each of said groups of grids.

ll. A mass spectrometer comprising a gas impermeable envelope enclosing an ion source, a collector plate spaced from said ion source, a plurality of groups of grids spaced in a line between said source of ions and said plate, each of said groups comprising three grids in spaced relation with one another, the spacings between adjacent grids being equal, the spacings between adjacent groups of said grids being substantially n'0.3l83-(2s-jt) inches, where n is an integral number, s is the spacing between adjacent grids in each group, and t is the thickness of the grids, all of said dimensions being in inches, and a second grid positioned between said collector plate and said groups of grids; means applying a potential to said second grid of polarity opposite the polarity of the ions being detected; means applying an alternating potential of a first frequency to the center grid in each of said groups of grids; a potential dividing network including a variable resistance; a source of steady potential applied across said network; a source of alternating potential of a second frequency applied across said network; means counecting the two end grids in each of said groups of grids to points on said network; means connected to said collector plate to measure ions impinging thereon at said first frequency; means connected to said collector plate to measure ions impinging thereon at said second frequency; and means responsive to said last-mentioned means to adjust said variable resistance to reduce the amplitude of the ion impingement on said plate at said seco-nd frequency to a minimum.

l2. ln combination in accordance with claim l1 wherein said variable resistance comprises an electron tube having a cathode, an anode and a control grid, the anode of said tube being connected to one point in said network i 1l and the cathode of said tube being connected to a second point in said network whereby the resistance of said tube between the cathode and anode thereof forms a part of said network, and wherein said means to adjust said vari.- able resistance comprises means to adjust the potential applied to the control grid of said tube.

13. A mass spectrometer comprising a gas impermeable envelope enclosing an ion source, a collector plate spaced from said ion source, a plurality of groups of grids spaced in a line between said source of ions and said plate, each of said groups comprising three grids in spaced relation with one another, the spacings between adjacent grids being equal, the spacings between adjacent groups of said grids being substantially 1z0.3l83-(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 the grids, all of said dimensions being in inches, and a second grid positioned between said collector plate and said groups of grids; means applying steady potentials to the two end grids in each of said groups of grids; means applying a potential to said second grid of polarity opposite the polarity of the ions being detected; means applying an alternating potential of a first frequency to the center grid in each of said groups of grids; means applying an alternating potential of a second frequency to at least one grid positioned between said ion source and said collector plate; means connected to said collector plate to measure ions impinging thereon at said first frequency; a phase sensitive detector; means applying said alternating potential of said second frequency to one input of said phase senstive detector; means applying said collector plate to the second input of said phase sensitive detector; and means responsive to the output of said phase sensitive detector to adjust the magnitudes of said steady potentials.

14. A mass spectrometer comprising a gas impermeable envelope enclosing an ion source, a collector plate spaced from said ion source, a plurality of groups of grids spaced in a line between said source of ions and said plate, each -of said groups comprising three grids in spaced relation with one another, the spacings between adjacent grids being equal, the spacings between adjacent groups of said grids being substantially 11O.3183-(25+t) inches, where it is an integral number, s is the spacing between adjacent grids in each group, and tis the thickness of the grids, all of said dimensions being in inches, and a second grid positioned between said collector plate and said groups of grids; Imeans applying steady potentials to the two end grids in each of said groups of grids; means applying a vpotential to said second grid of polarity opposite the polarity of the ions being detected; means applying an alternating potential of a rst frequency to the center Vgrid in each of said groups of grids; means applying an alternating potential of a second frequency to at least one grid positioned between said ion source and said co]- lector plate; means connected to said collector plate to measure ions impinging thereon at said rst frequency; a phase sensitive detector; means applying said alternating potential of said second frequency to one input of said phase sensitive detector; an amplifier tuned to pass signals of said seco-nd frequency; means connecting one input terminal of said amplifier to said collector electrode and means connecting the second input terminal of said ampliiier to a point of reference potential; means connecting the output terminals of said amplifier to the second input terminals of said phase sensitive detector; and means responsive to the output of said phase sensitive detector to adjust the magnitudes of said steady potentials.

No references cited,

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