US2683811A

US2683811A - Mass spectrometer

Info

Publication number: US2683811A
Application number: US35135A
Authority: US
Inventors: Charles F Robinson
Original assignee: Consolidated Engineering Co Inc
Current assignee: Consolidated Engineering Co Inc
Priority date: 1948-06-25
Filing date: 1948-06-25
Publication date: 1954-07-13
Anticipated expiration: 1971-07-13

Description

July 13, 1954 c. F. ROBINSON MASS SPECTROMETER 3 Sheets-Sheet 1 Filed June 25, 1948 /O/V SOURCE EVACUAT/NG SYSTEM VOLTAGE SOURCE 24 AMPL lF/ER SENSOR INVENTOR- CHARLES F. ROBINSON FIG. 3.

A r romvsr VOLTAGE SOURCE y 3, 1954 c. F. ROBINSON 2,683,811

MASS SPECTROMETER Filed June 25, 1948 3 Sheets-Sheet 3 {30 VOL T4 GE SOURCE 1N VENTOR. CHARLES F. ROBINSON A 7 TOR/VE V Patented July 13, 1954 HTED STATES PATENT OFFICE MASS SPECTROMETER Application June 25, 1948, Serial No. 35,135

16 Claims.

This invention relates to mass spectrometers and particularly to a method of operation of a mass spectrometer to make it less sensitive to variations in power supply voltage. The invention also relates to apparatus for carrying out the method.

A mass spectrometer is an analytical apparatus which functions to sort and measure ions. Ordinarily, it includes an ionization chamber in which molecules of the sample to be analyzed are bombarded by a stream of electrons and thereby converted into ions. The ions formed are propelled by so-called propelling or accelerating electrodes into and through an analyzer chamber. During passage through the chamber, the ions are subjected to a transverse electric or mag-- netic field or both to separate them according to their mass-to-charge ratios into a plurality of diverging ion beams. Each beam is composed of ions of the same specific mass and differing from the ions in the other beams. The diverging beams are successively focused, by adjustment of the potential applied to the accelerating electrodes, on and discharged at an ion collector. The current produced by each beam as it strikes the ion collector is indicative of the amount of ions in the beam. The current thus becomes a measure of the partial pressure of the molecules (from which the ions were derived) in the sam ple being analyzed.

In another type of mass spectrometer the several ion beams are focused on the collector by variation of the field forces in the analyzer chamher. In this type of instrument the focusing is independent of the velocity of the ions propelled into the analyzer chamber and thus of the accelerating potential. The present invention is applicable to both types of mass spectrometers. Thus the invention is directed to mass spectrome tors in which the focusing of the ion beams is brought about by a lateral electric field (accelerating field) a transverse electric field or a transverse magnetic field.

For purposes of convenience in description, however, the invention is described with relation to the more conventional mass spectrometer in which the focusing of the ion beam on an exit slit adjacent the ion collector is determined by a lateral electric field, by adjustment of the voltage applied to the accelerating electrodes.

In conventional practice the current produced by ions discharge at the ion collector is amp1ified in either an A. C. or D. C. amplifier. The current magnitude is recorded as a continuous curve in a suitable recorder. The peaks in the record corresponding to the ions of differing specific mass are referred to as mass peaks.

An important industrial application of mass spectrometry is in the continuous monitoring or control of industrial processes. In this type of analysis a gas stream, as for example a plant feed stream or an eiiluent stream is continuously or intermittently sampled. The composition there of with respect to one or more, but generally less than all, of the components is determined on a substantially continuous basis. The continuity of analysis is, of course, a function of the frequency of sampling and becomes continuous when a continuous sample stream is introduced in to the mass spectrometer. Continuous sam ple "leaks have been developed for this purpose and form no part of the present invention.

Generally in this type of operation there is a certain previously selected composition which represents an optimum for the specific process involved. A mixture having such a composition may be referred to as a standard sample.

In the application of mass spectrometry to continuous monitoring, one or more of the components of the gas stream to be analyzed are analyzed and a continuous record of the mass speaks for each of the components is obtained. The primary function of the continuous monitor is to detect variations from the theoretical stand ard composition. As long as the composition of the sample remains standard the mass peak for a given component wil1 theoretically fall at the same point on the record scale each time the record is sensing on that mass. Any deviation of a given peak should be an indication of a change in relative abundance in the sample stream of the component represented by the peak.

However, this is not always the case since a variation in power supply voltage beyond a given tolerance will cause a variation in peak height independent of sample composition. Thus, in a conventional instrument the maximum tolerable variation is that change which will move the ion. beam away from the center of the exit slit by an amount equal to:

Where Xu=the distance of beam shift d=the width of the exit slit, and w=the width of the beam.

The most insidious feature of this inherent dependency on voltage uniformity is that there is no way of detecting the fact that a deviation in peak height is due to a voltage fluctuation and not to a change in simple composition. This is due to the fact that a uniform voltage is applied from the source to the focusing means during the period, say seconds, during which the instrument is sensing on a given mass. As a result the peak is determined by a straight line. A voltage variation at the source will produce a peak of similar shape in which the fiat top will lie above or below the correct point. Since the erroneous peak is of the same shape as the correct peak, the reason for the shift is not ascertainable. This requires constant checking of the instrument to verify that changes in peak height are due to actual changes in sample composition and not to changes in the accelerating voltage and hence materially reduces the utility of a continuously monitoring instrument.

I have now discovered that the application of mass spectrometry to continuous monitoring can be greatly improved by causing the voltage supplied to the focusing means to vary over a small range of either side of the theoretical peak voltage. By peak voltage, I mean the voltage which will focus a given ion beam in the center of the exit slit.

Thus, in one aspect, the invention contemplates a method of mass spectometry which comprises ionizing a sample to be analyzed, propelling the ionized sample toward an ion collector, separating the ionized sample into a plurality or diverging beams of ions, each beam being composed of ions of a given specific mass and differing from the ions in the other beams, subjecting the ions to a focusing electrical potential to focus one of the beams on an ion collector, periodically altering the focusing potential in increments to focus different ion beams on the collector, and superimposing on the focusing potential a slowly and continuously varying voltage which is small in magnitude compared to the focusing potential. The term superimposing as here used includes not only the actual superimposition of a linearly varying voltage but also a linear variation through a small range of the voltage supplied from a source.

The invention is also directed to apparatus for carrying out the foregoing method. In this aspect I propose in a mass spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber, and means for propelling the ions from the ionization chamberthrough the analyzer and for focusing the ions on the collector, the improvement which comprises means for periodically applying successively diiferent voltages to the focusing means to successively focus ions of a different specific mass on the ion collector and means for superimposing a slowly, continuously and preferably linearly varying voltage on each of the diiferent voltages applied to the focusing means.

In a mass spectrometer wherein the focusing of the ion beams is accomplished by adjustment of the potential applied to the accelerating electrodes, the invention contemplates applying a slowly and linearly varying voltage to each different incremental potential applied to the accelerating electrodes. In mass spectrometers of the type wherein the focusing of the ion beams is accomplished by variation in the electrical field in the analyzer, the invention contemplates applying such a slowly and linearly varying voltage to the potential applied to this focusing electrical field.

Consider in relation to the foregoing, a mass spectrometer having an exit slit width 11, beam width 10. where h is the distance between the peaks belongin to adjacent ion masses. Now if we superimpose on the D. C. focusing voltage a slowly and continuously varying voltage sufllcient to move the beam by a maximum distance it, the following requirements are established:

(1) At some time during the scanning of a single peak, all of the peak of interest must pass through. the exit slit.

(2) At no time during the scanning of the peak of interest may more than one half of an adjacent peak pass through the exit slit. This requirement has nothing to do with the relative intensities of the two beams under discussion but is established so as to insure that the beam of interest will be in the exit slit for a markedly longer time than adjacent beams and thus make impossible a mistake in peak identification.

Under the above conditions the maximum permissible voltage variation is that which will shift the ion beam away from the center of the exit slit by a distance (X0 equal to:

obtains:

ZmV T e where 1'=the radius of the ion path in the magnetic field,

B=the magnetic field strengh,

=the mass-to-charge ratio of the ions in a given 6 beam, and V=the total accelerating voltage.

Also,

and

X =2AV=TIVAV (5) Since we shall be interested in the possible interference of adjacent masses in which case dm=l, the term dm will be omitted in subsequent discussion.

Since for a given instrument and given magnetic field, X0 and AV are linearly related, we can illustrate the relationship between AV for a fixed voltage and AV for a slowly changing voltage as:

AV X0 (6) Referring again to Equations 1 and 2, keeping in mind that h is always greater than d, it is apparent that the method of the invention results in a greater tolerance to variations in the source voltage without disturbing the analysis. The following table illustrates the magnitude of this difference for various values of h, d, and w. In interpreting the table reference should be had to Another advantage of the method of the invention is that it yields information on the peak shape. Thus due to the small varying voltage applied to the focusing voltage the recorded peaks are curved rather than fiat at their maximum point. For example, if the source voltage is at the correct value the recorded curve over the voltage range applied will extend over an appreciable part of the entire peak profile. If the source voltage is in error, the recorded curve will shift toward one side or the other of the peak. If the curve peaks, 1. e. reaches a maximum during the sensing period, the error in source voltage will be immaterial. At the same time the shift of the curve will indicate the existence and magnitude of the error. If the source voltage is in error by more than the tolerable limits, the recorded curve will not peak, and this fact will immediately apprise the operator that the supply voltage error is excessive. On the other hand, a change in sample composition will afiect the magnitude only and not the shape of the curve. Thus a change in peak configuration either with respect to magnitude or symmetry will indicate immediately whether the change is due to an actual change in sample composition or is due to an error in the supply voltage.

It is important to be able to ascertain the magnitude of the induced variation which can be tolerated Without interference by adjacent ion beams. The maximum distance it that the ion beam can be shifted by the varying voltage is a function of h, d, and w as follows:

k=h/2d/2+w/4 (7) k is also a function of r and V as follows:

dr 1* IV 2 AV- AV (8) Substituting the value for it given in Equation '7 for k in Equation 8 and transposing, the followin relationship is established:

rie-er] Equation 9 serves as a yardstick by means of l where (AV)m=the permissible voltage variation for a mass m. Transposition of Equation 11 in other words, since m mon is approximately a constant according to the fol lowing relationship:

Equation 13 can be used generally in place of Equation 9 to determine permissible Values of AV. However since Equation 13 does, not take into account the geometry of the apparatus nor its inherent limitations (beam width) it is Well to check it against the more accurate Equation 9 for each instrument.

Various apparatus for accomplishing the method of the invention are illustrated in the accompanying drawing in which:

Fig. l is a diagrammatic illustration of a con ventional magnetic analyzer mass spectrometer;

Fig. 2 is a circuit diagram of one form of voltage supply circuit in accordance with the invention;

3 is a circuit diagram of a modification of a circuit in Fig. 2;

Fig. 4 is a circuit diagram of another modification of the voltage suppl circuit;

Fig. 5 is a circuit diagram of another form of circuit in accordance with the invention;

Fig. 6 is a circuit diagram showing a modification of the circuit in Fig 5;

Fig. 7 is a circuit diagram of still another circuit for carrying out the invention.

In the drawings the invention is shown in association with a mass spectrometer of the type wherein the ion beams are focused by varying the potential applied to accelerating electrodes. As pointed out above and as will be apparent from the following description, the invention is equally adapted to incorporation in a mass spectrometer wherein the ion beams are focused by variation in applied field forces.

Referring to Fig. 1, it will be observed, that it shows a mass spectrometer having an ion source ii, an analyzer tube l2, and an ion collector Is disposed Within an envelope 14 which is kept at low pressures during the operation of the instrument. The analyzer chamber I2 is provided at the end adjacent the ion collector It with an exit slit If: through which the ion beams are focused onto the collector IS.

The spectrometer is provided with a pumping system or envelope exhaust line 16 which may be connected with a mercury diffusion pump, molecular pump, or any appropriate evacuating sys tem not shown. The analyzer tube l2 may be provided with ports Ii b means of which the ion source and th analyzer tube are evacuated through the envelope. Alternatively. the envelope may be omitted by making the analyzer tube 52 gastight and attaching the pumps to it by means which are well known.

An inlet line l8 provides means for introducing a sample to be analyzed either continuously or intermittently into the ion source. An electron gun (not shown) is mounted in or adjacent to the ion source and accelerating electrodes 28, 2| are disposed with relation to the source so as to propel the ions formed therein through the analyzer tube. Representation of this particular ion source is for illustrative purposes only, there being many modifications thereof, equally applicable to this invention.

As above described, the ions propelled from the source are formed in the analyzer tube into diverging beams of ions of a given specific mass. These diverging beams are successively focused on the ion collector l3 through the exit slit l5 by varying the potential applied to the accelerating electrodes. The ion collector i3 is linked with the amplification and sensing circuit by a lead 23 sealed through the wall of the 7 envelope [4. The lead 23 connects the collector to an amplifier 24 which is in turn linked to a sensing means 25. The sensing means 25 is conveniently a recorder but may be any type of sensing device adapted to indicate either the amplifier output or deviations from a normal peak condition.

The accelerating electrodes 20, 2| are connected through leads 26, 21 respectively to a voltage supply circuit shown in the following figures.

Referring now to Fig. 2 which shows one embodiment of the voltage supply circuit in accordance with the invention it will be observed that a plurality of adjustable tap potentiometers Pl, P2, and P3 are connected in parallel to a voltage source 30. Voltage source 30 is representative of a source of theoretically constant voltage applicable through the potentiometers Pl, P2 and P3 to the accelerating electrodes associated with the ion source. One side of each of the potentiometers is connected to the accelerating electrodes in the ion source and

adjustable taps

32, 33 and 34 of the potentiometers Pl, P2 and P3 respectively are each connected to one or more junctions of a stepper switch 36. Switch arm 36A is connected by lead 31 to the ion source and may be rotated either manually or by means of a cam arrangement familiar in the art to intermittently contact the junctions corresponding to the

taps

32, 33 and 34. The rotation of the switch arm 36A is stepwise so that it remains at each junction for a short period sufiicient to permit proper recording of the peak focused on the collector electrode by the potential applied through that junction.

Since different potentials are applied to the accelerating electrodes 20 and 2|, the leads from the circuit shown in Fig. 2 to the ion source generally are provided with a voltage divider network (not shown) for applying the potentials to the electrodes.

To cause the voltage applied to the electrodes 28 and 2! to vary slowly and linearly over a small range (as compared to the applied voltage) a rheostat 40 is connected in series in a common lead from the voltage source to the ion source. Rotation of the arm 40A of the rheostat 45! is synchronized with the rotation of the switch arm 36A of the stepper switch 36 so that for each position of the stepper switch arm the arm 40A of the rheostat will go through one complete revolution. Conventional mechanical equipment for producing this stepped synchronism may be employed and is represented diagrammatically by linkage 42 in the drawing.

In operation, with the switch arm 36A in contact with the junctions of the adjustable tap 32, voltage determined by the setting of the tap 32 will be delivered to the voltage source 39 through the accelerating electrodes 2!} and 2|. During the period, say 30 seconds, that the switch arm 36A is at this junction the arm 40A of the rotating rheostat will complete a revolution so as to vary the voltage applied to the ion source over a range determined by the resistance or" the rheostat 40. As hereinbefore described the tolerable magnitude of this voltage variation may be established by the equation:

V r d w AV 1 [2m 2 4= The cycle is repeated when the switch arm 36A is rotated through one step to contact the junction of the tap 33 of the potentiometer P2. At this stage a difierent voltage is applied to the accelerating electrodes 20 and 2| which also is varied by rotation of the potentiometer arm 40A. As each of the potentiometers Pl, P2 and P3 are connected through the switch 38A to accelerating electrodes 20 and 2! a different ion beam will be focused through exit slit 15 on the ion collector l3, and a different mass peak will be sensed by sensing means 25.

The circuit shown in Fig. 2 is set up to permit analyses of three components of a gaseous mixture. It is obvious that by inclusion of additional parallel coupled potentiometers substantially any number of components may be analyzed by applying additional accelerating potentials to the accelerating electrodes.

In the circuit of Fig. 3 the rheostat 4B is included in series in the output of the stepperswitch 36. Here again, the operation of the rheostat and the stepper switch is synchronized. For each step of the stepper switch arm 33A the rheostat arm 40A will go through a complete revolution. Linkage 42 illustrates diagrammatically the means of accomplishing this synchronization. The accelerating electrodes do not draw any appreciable current and the rheostat 40 in the output of the stepper switch 36 will not accomplish the desired variation in the voltage applied unless means are provided for drawing a small amount of current through the rheostat. This is accomplished in the embodiment shown in the Figure 3 by grounding the rheostat arm MA at 44 through a resistor 45. Small current leakage through the resistor 45 will cause the rheostat 40 to vary the voltage applied through line 46 to the ion source. In a practical situa' tion, resistor 45 may represent the above mentioned divider network (not shown).

In the circuit shown in Fig. 4 each of the parallel coupled potentiometers Pl, P2 and P3 is provided with a rheostat 50, 5|, 52 respectively. The adjustable taps 32, 33 and 34 are connected through a stepper switch 36A to the ion source and the stepper switch 38 is synchronized in its operation with the several rheostats 50, 5!, and 52. When the switch arm 36A is at the junction of the adjustable tap 32, the rheostat arm 50A will undergo a complete revolution. As the switch arm 36A is stepped to the junction of the adjustable tap 33 of the potentiometer P2 and the arm 5IA of the rheostat 5! will undergo a complete revolution. A similar result is obtained with respect to rheostat 52 when the switch arm 36A is stepped to the junction of the adjustable tap 34. In this figure the synchronized operation of the stepper switch 36 and the several rheostats is indicated by means of gang lines. Since the mechanical linkage for accomplishing this synchronization is conventional and forms no part of the present invention, the dotted line repre sentation thereof is suflicient.

An advantage of the circuit of Fig. i is that the variation in the applied voltage brought about by the several rheostats is proportional to the voltage applied through the potentiometer with which the particular rheostat is linked. Thus, although the resistances of the several rheostats, 50, 5! and 52 are the same, the eifective voltage variation resulting from the rotation of these rheostat arms will be a function of the voltage applied through the associated potentiometer. In this manner we may more nearly approach the maximum permissible voltage variation for each mass focussed on the ion collector electrode.

In the circuit of Fig. 5 potentiometers Pl, P2 and P3 are again connected in parallel across the voltage source 39. Adjustable taps 32, 33, 34 are connected through separate junctions of a stepper switch 36. A stepper switch arm 36A is connected to the anode 60 of a vacuum tube 62. The cathode 83 of the vacuum tube is connected, together with the common line 64 from the potentiometers, to the mass spectrometer. A battery 66 coupled across a condenser 6'! supplies a voltage to grid 65 of the vacuum tube 52 through a resistor E59. A switch 15 is .connected across capacitor e1. The switch is synchronized in its operation with the stepper switch '36 so that with each step of the stepper switch arm 3A the switch it is momentarily closed to short the condenser 5?. In this manner the voltage applied to the grid 53 varies over a small range depending upon the capacity of the capacitor, the battery voltage, and the magnitude of resistor 69. Variation in grid bias in turn varies the voltage at the cathode 53 as applied to the mass spectrometer provided there is some current flow in lines 84, E3 to the mass spectrometer as for example in the customary divider network (not shown).

A modification of the circuit of Fig. '5 is shown in 8. In this embodiment the switch arm 36A is connected through a fixed resistance 72 directly to the mass spectrometer. The vacuum tube 62 having a grid bias circuit including the battery 58, capacitor 6?, resistor 59 and switch '56 is connected to the grid 68 of the vacuum tube 62. In this circuit the cathode B3 of the vacuum tube is grounded at it. Again the switch ill is synchronized with the switch arm.38A of the stepper switch so that the capacitor 61 is shorted at the beginning of each step. The voltage applied to the grid 63 through the circuit shown will determine leakage from the resistor 12 to ground and will vary the voltage applied to the mass spectrometer accordingly.

Still another means of varying the voltage appli d to the mass spectrometer linearly and over a comparatively small range is shown in the circuit of Fig. 7. In this embodiment-potentiometers Pl, P2 and P3 are again connected in parallel across the voltage source 30. Adjustable 32, 34 of the several potentiometers are connected to diiierent junctions of the stepper switch 35. Stepper switch arm 35A is connected directly to the mass spectrometer (generally through a voltage divider network as described a above). Common lead 64 from the several potentiometers is connected to adjustable tap it or" a potentiometer ll. The potentiometer T! is connected across a battery 78 and one side of the potentiometer is connected through lead 86 to the mass spectrometer. The stepper switch 3% and the adjustable tap 16 of the potentiometer are synchronized so that for each step of the stepper switch arm 38A the adjustable tap '55 will slide around the entire circumference of the potentiometer W. In this manner a slowly and linearly varying voltage is actually superimposed on the voltage supplied to the mass spectrometer. As above indicated, the term superimposed is used in this application to include any means of slowly and linearly varying the voltage applied from the voltage source to the mass spectrometer.

The voltage variation circuits illustrated in each of Figs. 2-7 may be applied to vary the voltage applied to transverse magnetic or electric focusing fields to accomplish the same slow scan brought about by application thereof to the ccelerating electrodes.

I claim:

1. In a spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber, means for propelling the ions from the ionization chamber through the analyzer, and means for focusing the ions on the collector, the improvement which comprises a source of substantially constant voltage for energizing the focusing means, means for periodically applying successively different voltages from the source to the focusing means to successively focus ions of different specific mass on the ion collector, and means for superimposing a slowly varying unidirectional voltage on the voltage applied from the source to the focusing means.

2. In a mass spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber, and accelerating electrodes for propelling the ions from the ionization chamber through the analyzer and for focusing the ions on the collector, the improvement which comprises a source of substantially constant voltage for impressing a potential on the accelerating electrodes, means for periodically changing the voltage level applied from the source to the accelerating electrodes to successively focus ions of different specifio mass on the ion collector, and means for superimposing a slowly varying unidirectional voltage on each voltage level applied from the source to the accelerating electrodes.

3. In a mas spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber, and accelerating electrodes for propelling the ions from the ionization chamber through the analyzer and for focusing t e ions on the collector, the improvement which comprises a source of substantially constant voltage, a plurality of potentiometers connected in parallel across the source, means for periodically connecting a diferent one of said potentiometers to the accelerating electrodes so as to periodically change the potential on the accelerating electrodes and means for superimposing a slowly varying voltage on the voltage delivered through each of the potentiometers to the accelerating electrodes.

4. Apparatus according to claim 3 wherein the means for connecting separate ones of said potentiometers to the accelerating electrodes com prises a stepper switch connected in series between the taps of the plurality of potentiometers and the accelerating electrodes.

5. In a mass spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber, and accelerating electrodes for propelling the ions from the ionization chamber through the analyzer and for focusing the ions on the collector, the improvement which comprises a source of substantially constant voltage, a plurality of potcntiometers connected parallel across the source, selecting means for periodically connecting a different one of said potentiometers to the accelerating electrodes, and a rheostat connected in series between the source and the accelerating electrodes for slowly varying the voltage applied to the accelerating electrodes by the plurality of poten tiometers.

6. Apparatus according to claim 5 wherein the selecting means and the rheostat are synchronized so that for each selection made by the selection means the rotating potentiometer will be caused to vary throughout its complete range.

7. In a mass spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber means for separating ions into diverging beams each composed of ions having a specific mass differing from the specific mass of ions of each of the other beams, and accelerating electrodes for propelling the ions from the ionization chamberthrough the analyzer and for focusing the ions on the collector, the improvement which comprises a source of substantially constant voltage, a plurality of potentiometers connected in parallel across the source, selecting means for periodically connecting a different one of said potentiometers to the accelerating electrodes, a separate rheostat connected in series with each of said plurality of potentiometers, said rheostats being synchronized with the selecting means so as to vary over their complete range for each selection of said selecting means.

8. In a mass spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber means for separating ions into diverging beams each composed of ions having a specific mass differing from the specific mass of ions of each of the other beams, and accelerating electrodes for propelling the ions from the ionization chamber through the analyzer and for focusing the ions on the collector, the improvement which comprises a source of substantially constant voltage, a plurality of potentiometers connected in parallel across the source, selecting means for periodically connecting a different one of said potentiometers to the accelerating electrodes, a vacuum tube having an anode, cathode and control grid, the output of said selecting means be ing connected to one of the anode and cathode, the other of the anode and cathode being connected to the accelerating electrode, a variable grid bias circuit and means for causing said circuit to deliver a slowly and continuously varying voltage to the grid throughout the period that each potentiometer is connected through the selecting means to the accelerating electrodes.

Apparatus according to claim 8 wherein the grid bias circuit comprises a condenser connected between the grid and the cathode, a battery connected across the condenser, and a switch adapted to short the condenser each time the selecting means selects a different potentiometer.

10. In a mass spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber means for separating ions into diverging beams each composed of ions having a specific mass differing from the specific mass of ions of each of the other beams, and accelerating electrodes for propelling the ions from the ionization chamber through the analyzer and for focusing the ions on the collector, the improvement which comprises a source of substantially constant voltage, a plurality of adjustable tap potentiometers connected in parallel across the source, selecting means for periodically connecting the adjustable tap of a different one of said potentiometers to the accelerating electrodes, a common lead from each of said plurality of adjustable tap potentiometers, a separate adjustable tap potentiometer, the common lead being connected to the separate potentiometer, a battery connected across the separate potentiometer, means connecting the separate potentiometer to the accelerating electrodes, the selecting means and adj ustable tap of the separate potentiometer being synchronized so that the tap is moved through 12 its full scale for each setting of the selecting means.

11. In a mass spectrometer having an ionization chamber, an analyzer, an ion collector, means for ionizing molecules in the ionization chamber means for separating ions into diverging beams each composed of ions having a specific mass differing from the specific mass of ions of each of the other beams, and accelerating electrodes for propelling the ions from the ionization chamber through the analyzer and for focusing the ions on the collector, the improvement which comprises a source of substantially constant voltage, a plurality of potentiometers connected in parallel across the source, selecting means for periodically connecting a different one of said potentiometers to the accelerating electrodes, a common line connecting all of said potentiometers to the electrodes and a leak circuit adapted to vary the voltage applied to the electrodes by each of the potentiometers.

12. Apparatus according to claim 11 wherein the leak circuit comprises a vacuum tube having an anode, cathode and grid, means connecting the common line to the anode, means connecting the cathode to the other side of the voltage source, a variable grid bias circuit connected between the grid and cathode, and means for synchronizing variation in grid bias with operation of said selecting means.

13. In a mass spectrometer having a collector, means for separating ions into diverging beams each composed of ions having a specific mass differing from the specific masses of the other beam, and means for imposing a potential on the beams to propel them in the general direction of the collector, the combination which comprises means for causing the potential to vary gradually and unidirectionally over a first range in which one of the ion beams is focused on the collector, means for abruptly varying the potential from the first range to a second range in which a second ion beam is focused on the collector, and means for varying the potential gradually and unidirectionally through the second range.

14. In a mass spectrometer having a collector, means for separating ions into diverging beams each composed of ions of a given specific mass differing from the specific mass of ions of each of the other beams, and means for imposing a potential on the ions to propel them in the general direction of the collector and to focus a given ion beam on the collector, the combination comprising means for periodically and abruptly altering the potential in increments of sufficient magnitude to focus different ion beams on the collector, and means for varying each successive potential through a unidirectional range which is small as compared to the potential variation required to focus an adjacent ion beam on the collector.

15. In a mass spectrometer having a collector, means for separating ions into diverging beams each composed of ions of a given specific mass differing from the specific mass of ions of each of the other beams, the combination comprising means for impressing on the ions a focusing electrical potential varied stepwise to successively focus different beams on the collector, and means for superimposing on each focusing potential a unidirectional voltage varying continuously through a range which is small compared to the focusing potential variation required to focus an adjacent ion beam on the collector.

16. In a mass spectrometer having a collector, mean for separating ions into diverging beams each composed of ions of a given specific differing from the specific mass of ions of each of the other beams, the combination comprising means for impressing on the ions a focusing electrical potential varied stepwise to successively focus different beams on the collector, and means for superimposing on each focusing potential a unidirectional voltage varying continuously through a range proportional to the focusing potential and small as compared to the focusing 10 potential variation required to focus an adjacent ion beam on the collector.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,331,190 Hipple Oct. 5, 1943 2,373,151 Taylor Apr. 10, 1945 2,457,162 Langmuir Dec. 28, 1948