US2551637A - Self-calibrating mass spectrometer - Google Patents

Description

y 1951 c. F. ROBINSON 2,551,637

SELF-CALIBRATING MASS SPECTROMETER Filed March 8, 1948 2 Sheets-Sheet l ION/ZAT/ON CHAMBER T0 T0 AMPL/F/CAT/ON & 144 CUAT/NG CAL/BRAT/O'V C/RCU/T SYSTEM 1 AMPL/F/ER INVENTOR. CHARLES E ROBINSON ATTORNEY y 1951 I c. F. ROBINSON 2,551,637

SELF-CALIBRATING MASS SPECTROMETER Filed March 8, 1948 2 Sheets-Sheet 2 AUX IL IARV AMPL IF /E R E M4 ZERO ADJUSTMENT ON D. C AMPL/F/ C A T/ON IN VENTOR. CHARLES E ROBINSON ATTORNEY Patented May 8, 1951 SELF-CALIBRATING MASS SPECTROMETER Charles F. Robinson, Pasadena, (la-iii, assignor to Consolidated Engineering Corporation, Pasadena, Caiif., a corporation of California Application March 8, 1948, Serial No. 13,687

This invention relates to mass spectrometry and more particularly to a mass spectrometer which is self-calibrating with respect to a gas or vapor sample of known composition.

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 converted into ions. These ions are propelled by so-called propelling or accelerating electrodes into and through an analyzer chamber. During passage through the analyzer chamber the ions are subjected to a transverse electrical or magnetic field or both to separate them according to their mass-to-charge ratio into a plurality of diverging beams of ions, each beam being composed of ions of the same specific mass and differing from the ions in the other beams. The diverging beams are successively focused and discharged on an ion collector. The current produced by each beam is indicative of the amount of ions in each beam and is thus a measure of partial pressure of the molecules (from which the ions were derived) in the sample being analyzed.

An important potential industrial application of mass spectrometers, heretofore unrealized, is in the continuous monitoring or control of industrial processes. In this type of analysis a gas stream is continuously or intermittently sampled and the composition thereof 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 i introduced into the mass spectrometer. Means for continuously introducing this sample stream into a monitoring mass spectrometer form no part of the present invention.

One of the problems encountered in the construction of a mass spectrometer for continuous monitoring is the achievement of suificient stabil-- ity that the instrument, once calibrated, will retain its calibration with suflicient accuracy for a period of hours, days or weeks. This problem becomes particularly acute in applications in refineries, chemical industries and the like where the instrument may be exposed to explosive, corrosive or other atmospheres which necessitates enclosure of the instrument in a sealed housing. Such a housing, or the inaccessible location of the instrument, may make access thereto for calibration purposes difficult and tedious.

The present invention is directed to a mass 22 Claims. (Cl. 250-415)) spectrometer which is self-calibrating with respect to a gas or vapor sample of known composition, and in which the effects of slow drifts in the sensitivities of the various components are eliminated. When using the mass spectrometer of the invention for monitoring or other purposes, a gas sample of known composition may be periodically admitted in place of the unknown stream whereupon. the instrument recalibrates itself with substantially no loss in time from the monitoring operation.

The mass spectrometer of the invention may include a conventional ionization chamber with associated electron gun and accelerating electrodes, an analyzer chamber having a collector electrode in the path of the ions discharged from the ionization chamber, means for indicating or recording the current striking the collector, which may include a current amplifier for increasing the collector currents to convenient values, means for subjecting the ions in the analyzer chamber to a transverse electrical or magnetic field or both, means for evacuating the apparatus, and a selfcalibrating circuit includingmens for recording the collector discharge values for the unknown sample.

The utility of the self-calibrating circuit of the instant invention is not limited to the particular type of mass spectrometer outlined above. Any mass spectrometer having means for delivering output currents responsive to the proportion of one or more constituents in the sample undergoing analysis will require periodic calibration. The present calibration circuit provides means for automatically calibrating any such mass spectrometer.

In essence, the self-calibrating circuit comprises a separate output channel for the current developed at the collector by one or more of the constituents of the known sample and a separate source of known standard voltage associated with each of the output channels or circuits. Means are provided for mixing the known voltage in opposition to the voltage in its associated output channel. The comparison or opposition voltages between the output channels and the respective known voltage sources are amplified and applied to means, say motors, for adjusting the output channels, to equalize or to bear some other known and fixed relationship to the known voltages.

The number of components of the known sampie to be calibrated, and as a consequence the number of output channels andthe number of components of the unknown sample to he monic tored may be arbitrarily selected and need not be the same. A well designed mass spectrometer will preserve accurately the relative responses to the various masses although the absolute magnitudes of the ion currents corresponding to different masses may change due to changes in sample manifold pressures, changes in over-all amplification of the output amplifier if employed, or other causes. In such an instrument adequately reliable operation, for many purposes, may be obtained by self-calibrating only the zero (when no ions are hitting the ion collector) and a single arbitrarily chosen component of the known sample, and then scanning as many component of the unknown sample as desired. However, for greatest accuracy, it is desirable to calibrate with respect to two or more components of the known sample in addition to the zero value.

The invention will be more clearly understood by reference to the following detailed description thereof, taken in relation to the accompanying drawings, in which:

Fig. l is a diagrammatic illustration of a conventional mass spectrometer; and

Fig. 2 is a circuit diagram of the calibration circuit of the invention as applied to the mass spectrometer of Fig. 1.

Referring to Fig. 1, it will be observed that it shows a mass spectrometer having an ionization chamber I i, an analyzer tube 52, and an ion collector l3 disposed within an envelope it which is kept at low pressures during the operation of the instrument. This spectrometer may be of any appropriate design, there being numerous modifications particularly adapted to specific uses.

The spectrometer is provided with a pumping system or envelope exhaust line l5 which is connected with a mercury diffusion pump, molecular pump or another appropriate exacuating system not shown. An inlet line it provides means for introducing the sample to be analyzed either continuously or intermittently into the ionization chamber. An electron gun (not shown) and accelerator electrodes IT, i8, H) are disposed with relation to the ionization chamber so as to pro pel the ions formed therein through the analyzer. The representation or" this particular ionization chamber is for illustrative purposes only, there being many modifications thereof equally applicable to this invention.

The ion collector l3 is linked with the amplification and calibration circuit (Fig. 2) by the lead sealed through the wall of the envelope I 4. The lead 20 may connect the collector i3 through an amplifier 22 to the terminals Ti and T2 of the calibration circuit (see Fig. 2).

In the calibration circiut of Fig. 2, 81a, Sit and Sic are three levels of a stepper-switch Si which may be operated manually or by a slowly rotating cam or other suitable device (not shown). The three levels sla, S1!) and Sic, normally superimposed one upon the other, have been separated in Fig. 2 for purposes of clarity.

Pi, P3, and P5 are potentiometers connected through relays R2, R3 and R4 respectively to the output of amplifier 22 (Fig. 1) as applied to terminals Ti and T2. In this manner the output from the amplifier 22 may be fed into any of the three circuits represented by the potentiometers Pi, P3 and P5. The output voltages of the potentiometers Pi, P3 and P5 are controlled by adjustable taps 24, 25 and 25 mechanically linked to motors Mi, 1 2 and M3 respectively which determine the positioning of the taps.

This particular embodiment of the invention employs two-phase alternating current motors so that an auxiliary source oi power is required to drive one phase; this source, not shown, is connected to T7, T8. Other motors, for example D. C. motors, might not require such a source and in any case the particular type of motor employed is not an essential feature of this invention.

A calibrated source of E. M. F. 28 is connected across adjustable tap potentiometers P2, P4, P0 having adjustable taps 25, 39 and 3i respectively which may be manually adjusted to deliver a preselected calibrated voltage.

The voltage developed by the various output channels represented by the potentiometers P1, P3, and P5 is mixed with the respective known voltages developed by the potentiometers P2, Pi, and Pa through relays R6, R7, R8, respectively and may be fed into an auxiliary amplifier The amplifier 34 is connected through the stepperswitch level Sib to the motors M1, M2, and M3 which control the taps of the potentiometers Pi, Pa, and P5 as above described. The relays R2, R3, R4, R5, R6, R7, R2, and Re which control the distribution of the output of the amplifier 22as introduced into the circuit through the terminal Ti are operated through the switch Sic by a source of curent (not shown), connected to the terminals T9 and T10.

The voltages from the sliding contacts 24, 25, 2B of potentiometers Pi, P3, P5, are fed through the upper contacts of relays R2, R3, R4, to a measuring or recording device such as a strip-chart recorder 36 which may be of conventional type. A conventional switch valve controls the fiow of sample into the ionization chamber H and is in turn controlled by the switch level Sic.

A motor Mi energized from a current source (not shown) connected to terminals T11 and T12 is linked through the switch level Slb to the auxiliary amplifier 34. The motor M4 is in turn connected to the zero adjustment 62 on the D. C. amplifier 22 (see Fig. i

The terminals T5, T6 are connected (connecting wires are not shown) to electrodes i? and it (see Fig. 1) which are controlled through the switch level Sla. The terminals T3, and T4 are connected to a source of fixed voltage (not shown) and through potentiometers P7, P3, and P9 and the terminals of the switch level Sla to the accelerating electrode terminals T5, T6. By means of the relay Ri which is controlled by the switch level Sic the terminals T5, T6, may be connected across the terminals T3, T4, through the intermediate switch S111, and through the resistor Rio or the potentiometer Pie.

The invention, and particularly the calibration circuit, will be more clearly understood from the description of the operation thereof.

With the switches Sla, Slb, and Sic, in the zero position the terminal T5 is shorted to the terminal To and through Rio, or Pic to T4. In this condition there is no potential impressed on the accelerating electrodes i1, i8 (Fig. 1) of the mass spectrometer. The relay R5 is energized through Sic connecting the output of the amplifier 22 to the strip chart recorder. At the same time the relay R9 is energized and the output of the amplifier is connected therethrough to one side of ,the auxiliary amplifier the other side of which is grounded at 59 through the second pole of the relay R9. With this orientation, if the output voltage of the amplifier 22 is not zero (there being no ions discharging at the ion collector) a i Mi through the switch Slb.

signal is impressed on the auxiliary amplifier and this signal amplified therein, is fed to the motor The signal energizes the motor M4 which operates the zero adjustment on the D. C. amplifier so that the D. C. amplifier output is brought to zero. At the same time, the switch Sic feeds a signal to the sample switch valve 48 through relay Ri which serves to change the inlet system (It of Fig. l) of the mass spectrometer from the unknown sample to the standard sample.

With each of the switches Sla, Sit, and Sic in the 1 position a standard sample is flowing into the inlet system and an accelerating voltage, determined by the setting of P7, is impressed on the accelerating electrode terminals T5, T6, through the switch 819.. The relay R2 is energized through the switch Sic whereby the output of the amplifier Z2 is fed through the relay to the potentiometer Pi which as above indicated is controlled by the motor M1. Output of the potentiometer P1 is fed, together with the output of the potentiometer P2, which output has been previously selected, into the auxiliary amplifier through the relay Re which is simultaneously operated by the switch Sic. The voltage difference or comparison voltage between the outputs of the potentiometers Pi, and P2, is amplified in the auxiliary amplifier 3eand fed through the switch slb to the motor M1. The motor is so linked to the adjustable tap 24 of the potentiometer Pi as to move it in the direction necessary to bring the outputs of Pi and P2 to equality or to a pre-selected relationship. This process is repeated for each mass peak of the known sample which is to be calibrated through separate ones of the circuits controlled by the relays R2, R3, and R4. These relays are in turn operated by the settings 1, 2 and 3 of the switch 81b. The accelerating potentials applied to the accelerating electrodes are varied by the alternate connection thereof through switch Sic to P7, P8 and P9 simultaneously with selection of relays R2, R3 and R4 by switch S10.

When the switches Sla and S11; are in the w position, relay R1, which may be of latch type, is operated by Sic which feeds a signal to the sample switch valve 3!] which causes it to stop the flow of known sample and to begin admitting the sample of unknown composition to the instru-' ment. The accelerating electrode terminals T5, T6 may be connected together through Rio, or P to T4.

With switches Sla and Sic in the 4: position a voltage corresponding to the setting of potentiometer P1 is impressed on the accelerating electrode of the mass spectrometer. At this stage the relay R2 is closed through the switch Sic and the output of potentiometer Pi is fed through the second pole of the relay R2 to the recorder and through the relay R6 to the input of the auxiliary amplifier. The output of the auxiliary amplifier is open circuited by the switch slb and the motors Mi, M2, M3, and M4 do not operate. With this orientation, the instrument is recording the ion current corresponding to the mass selected by the setting of the accelerating electrodes through the potentiometer P7, this ion current having been automatically adjusted by the events described above in accordance with the setting of potentiometer P2.

The sequence of events described above is repeated with the switches S121, and Sic, in the 5 and 6 positions corresponding to the two other masses selected by the settings of Pa, and P9.

The mass of the unknown sample focused on the ion collector with the potentials on the accelerating electrodes established by potentiometer P; will be the same as the mass of the known sample focused on the collector by the same accelerating potential (with $19. in position 1). Although these mass calibration channels are shown (represented by settings 1, 2 and 3 of the stepper switch) and a like number of unknown mass peak channels (represented by switch settings 4, 5and 6'), the circuit configuration may be altered by inclusion of additional accelerating electrode potentiometers and corresponding additional sequential stations in the switch levels Sla and Sic. ,In this manner additional known or unknown mass peaks or both may be recorded.

It is evident from the foregoing descriptio that any number of mass peaks may be calibrated and that the same or difierent number of masses of the unknown sample may be measured. The number of output channels may be increased by the addition of relays serving the functionof the relays R2 to R5, potentiometers serving the function of potentiometers P2 to P6 and motors serving the function of the motors M1 to M3 together with appropriate switch stations. Similarly, as mentioned above, additional masses of the unknown sample may be scanned by increasing the number of potentiometers P7, P8 and Pa so as to impose additional incremental voltage changes on the accelerating electrodes 11, I8 of Fig. l.

The frequency of the self-calibration may be arbitrarily chosen by the proper configuration of the switches Sla, Sit and Sic. Thus as illustrated in Fig. 2 the settings 4, 5 and 6 of the switches sis, and Sic, are repeated prior to return to the zero position. This sequence of settings may be repeated substantially any number of times before returning the switch to the zero position. As above indicated the switches Sla, slb, Sic, may be operated by a master switch either manually controlled or automatically controlled by means of a cam or the like.

To this point emphasis has been placed upon the use of the circuit of the invention for selfcalibration of the mass spectrometer. However, the circuit also finds application as an automatic computing circuit. For this purpose the circuit may be operated in a number of ways.

One such method of utilizing the circuit of the invention as a computing circuit comprises analyzing the standard sample on which the mass spectrometer is to calibrate itself and setting-in r the peak heights of the various masses to be monitored in the voltage sources P2, P4, P6. In this manner the voltage sources P2, P4, P6 bear the same relation relative to each other as the peak heights of interest, and the output circuits P1, P3, P5 are adjusted by the calibration circuit to correspond to these fixed voltages. This method of computing may be called direct proportioning.

Another method of employing the circuit for computation comprises adjusting the known voltages P2, P4, P6, with respect to the peak heights to be monitored so that the output voltages from each of these sources are all equal. In this condition the mass spectrometer output channels Pi, P3, P5, will also adjust themselves in operation of the circuit as above described so that all peaks of interest will have the same height when the standard sample is being measured. Hence any difference in composition between the standard sample and the sample being monitored will reveal itself by the failure of the tops of the peaks to lie on a straight line when the unknown sample is being analyzed. This method of computa tion is a more sensitive test of chemical disparity between the unknown and the standard sample than is aiforded by the above computation method and further may be more easily used by an untrained operator since no mathematical computations are required.

Still a third method of applying the circuit of the invention for computing purposes comprises adjusting the sources of known voltage P2, P4, P6 so that the voltages or currents corresponding to some of the peak heights of the standard sample are maintained in direct proportion while peaks of inconveniently small or great height are multiplied by appropriate factors such as 2, 10, etc. In this method the peaks of inconveniently small height are multiplied by integers 2, 5, 10 etc., while the peaks of inconveniently great heights are multiplied by fractions etc.

By way of example of this method of computation, assume an unknown sample in which it is desired to continuously determine the content of constituents A, B, and C. Now if all of A, B, and C are present in amounts which will produce mass peaks falling at a conveniently measurable height on the recording scale, the second computing procedure outlined above will be satisfactory. However, as is often the case, constituent A may be present in the sample in amounts which will develop a mass peak only scale height (referring to the recorder scale), and constituent C may be present in an amount which will develop a mass peak 100 times scale height. In such a case I propose, in accordance with the third procedure outlined above, to analyze or sense a sam ple of known composition containing constituents A, B, and C in amounts of the same order of magnitude encountered in the unknown sample. By suitable adjustment of the known voltage sources, say P2 and P6, in the circuit, the height of the peak corresponding to the constituent A in the known sample may be multiplied by a factor of 50 and the height of the peak corresponding to the constitutent C may be multiplied by a factor of ,5 or etc. In this manner these two mass peaks will lie, along with the mass peak corresponding to constituent B, at a conveniently readable height on the recording scale. Now if the unknown sample is analyzed the same sensitivity adjustment factors will be applied by potentiometers P2 and P6 to the constituents A and C therein. By comparing the peaks obtained from the known and unknown sample with respect to each constituent, the content of each of the constituents can be determined directly, bearing in mind the sensitivity adjustment factors applied to the A and C mass peaks.

Although the foregoing description emphasizes a particular type of self-calibrating mass spectrometer, it is understood that the invention con templates the arrangement, in substantially any mass spectrometer, of sensing and recording circuits in relation to each other, so that the mass spectrometer will be self-calibrating with respect to one or more mass peaks. This is accomplished by an arrangement which compares the output of the mass spectrometer sensing on these peaks with permanently stored controllable parameters which may be set in advance. In the drawing these parameters are the voltages from the sliding contacts 29, 38, (H of potentiometers P2, P4, P6, respectively. Alternatively, and withill in the contemplation of the invention, these parameters may be currents from a regulated source, perforations in a punched card, the physical length or mass of a mercury column, or such other memory devices as are well known in the computing art. The essence of the invention is that the mass spectrometer periodically senses on a sample of known composition and automatically adjusts its sensing with respect to one or more mass peaks in such a way as to correspond in a previously selected way to the composition of the known sample and, having done this, reverts to sensing a sample of unknown composition.

I claim:

1. A mass spectrometer comprising means for self-calibrating with respect to at least one mass peak of a standard sample, means for self-calibrating with respect to a zero mass peak, means for automatically introducing the standard sampericdically into the mass spectrometer to cause it to sense on the standard sample, means for automatically adjusting the measured value of said mass peak of the standard sample in accordance with a pre-selected fixed value, and means for causing the mass spectrometer to sense on a sample of unknown composition in the intervening periods.

2. A mass spectrometer comprising means for calibrating with respect to at least one mass al: of a standard sample, means for automaticaliy introducing the standard sample periodicaliy into the mass spectrometer to cause it to sense on the standard sample, means for automatically adjusting the measured value of said mass peak of the standard sample in accordance with a pie-selected fixed value, and means for causing the mass spectrometer to sense on a sample unknown composition in the intervening periods.

3. In a mass spectrometer adapted to sense at least one mass peak in an unknown sample, the improvement which comprises means for periodically sensing at least one mass peak of a known sample, and means for automatically adjusting the response to the known sample to bear a fixed relationship to the composition of the known sample.

a. A circuit for a mass spectrometer adapted to be coupled to the output amplifier of the mass spectrometer and comprising at least one output channel adapted to carry the output voltage from the amplifier, an adjustable tap potentiometcr in output channel, adjusting means for adjusting the tap of the potentiometer responsive to voltages applied to the means, a separate source of determinable voltage for each output channel and linked to the circuit so that the determinable voltage will oppose the output voltage and means for feeding the resultant voltages to the respective adjusting means.

5. A circuit for a mass spectrometer adapted to be coupled to the output amplifier of the mass spectrometer and comprising a plurality of output channels adapted to carry the output voltfrom the amplifier, an adjustable tap potentiometer in each of said output channels, adjustmeans for adjusting the taps of the potentiometer responsive to voltages applied to the means, a separate so rce of determinable voltfor each output channel and linked to the c nnel so that the determinable voltage will oppose the output voltage, means for amplifying the resultant opposition voltage and means for feeding the amplified opposition voltages to the respective adjusting means.

6. A calibration circuit for a mass spectrometer adapted to be coupled to the output amplifier of the mass spectrometer and comprising a plurality of output channels adapted to carry the output voltage from the amplifier, an adjustable tap potentiometer in each or" said output channels, adjusting means for adjusting the taps of the potentiometer responsive to voltages applied to the means, a separate source of determinable voltage for each output channel and linked to the channel so that the determinable voltage Will oppose the output voltage, means for feeding the resultant voltages to the respective adjusting means, and switch means for feeding the amplifier output through different ones of said output channels.

'7. In a mass spectrometer having an ionization chamber, an analyzer, rreans for ionizing molecules in the ionization chamber, accelerating electrodes for propelling ions from the ionization chamber to the analyzer, and an ion collector, the improvement which comprises a calibration circuit comprising a plurality of output channels carrying output voltages from the collector, a separate source of determinable voltage coupled with each of the output channels and in opposition thereto, and means operable by the resultant opposition voltage in each of the output channels for adjusting the output voltages in each of the output channels to bear a fixed relationship to the respective determinable voltages.

8. In a mass spectrometer having an ionization chamber, an analyzer, means for ionizing molecules in the ionization chamber, accelerating electrodes for propelling ions from the ionization chamber to the analyzer, and an ion collector, the improvement which comprises a calibration circuit comprising a plurality of output channels carrying output voltages from the collector, a separate source of determinable voltage coupled with each of the output channels and in opposition thereto, means operable by the resultant opposition voltage in each of the output channels for adjusting the output voltages in each of the output channels to bear a fixed relationship to the respective determinable voltages, and switch means for feeding the output from the ion collector through difierent ones of said output channels.

9. In a mass spectrometer having an ionization chamber, an analyzer, means for ionizing molecules in the ionization chamber, accelerating electrodes for propelling ions from the said output channels, means operable by switch for changing the potential applied to the accelerating electrodes simultaneously with a change in output channel selected.

iii. In a spectrometer havin an ionization chamber, an analyzer, means for ionising molecules in the ionization chamber, accelerating electrodes for propelling ions from the ionization chamber to the analyzer, an ion collector, and an amplifier connected to the ion collector, the improvement which comprises a calibration circuit comprising a plurality of output channels carrying output voltages from the amplifier to an auxiliary amplifier, each output channel comprising an adjustable tap potentiometer separated from the amplifier and the auxiliary amplifier by a pair of relays, a separate source of determinable voltage coupled with each of the output channels and in opposition thereto, the opposition voltage thus produced being amplified in the auxiliary amplifier, means operable by the resultant amplified opposition voltage in each of the output channels for adjusting the taps of the potentiometers in each of the output channels to bear a fixed relationship to the respective determinable voltages 11. Apparatus according to claim 10 wherein the separate source of determinable voltage for each output channel comprises a single source of calibrated E. M. and a plurality of adjustable tap potentiometers connected in parallel therewith, the tap of each of the potentiometers being connected in opposition to a separate one of said output channels,

12. In mass spectrometer having an ionization chamber, an analyzer, means for ionizing molecules in the ionization chamber, accelerating electrodes for propelling ions from the ionization chamber to the analyzer, an ion collector, and an amplifier connected to the ion collector, the improvement which comprises a calibration circuit comprising a plurality of output channels selectively operable to carry output voltages from the amplifiers, a separate source of determinable voltage coupled with all but one of the output channels and in opposition thereto, an auxiliary amplifier, means for selectively feeding the resultant voltages developed in each of said output channels to the auxiliary amplifier, means operable by the resultant amplified opposition voltage in each of the output channels for adjusting the output voltages in each of the output channels to bear a fixed relationship to the respective determinable voltages, and means operable by the amplified voltage of said one output channel to adjust the zero setting of said amplifier.

13. A calibration circuit for a mass spectrometer adapted to be coupled to the output amplifier of the mass spectrometer and comprising a plurality of output channels adapted to carry the output voltage from the amplifier, an adjustable tap potentiometer in of output channels, adjusting means for adjusting the taps of the potentiometer responsive to voltages applied to the means, a separate source of determinable voltage for each output channel and linked to the circuit so that the determinable voltage will oppose the output voltage, an auxiliary amplifier for amplifying the resultant opposition voltage, means for feeding the resultant amplified voltages to the respective adjusting means, a recorder, a by-pass circuit for by-pass ing the output of the output amplifier to the recorder and the auxiliary amplifier, and means responsive to the amplified voltage of said bypass circuit to adjust the zero setting of the output amplifier.

14, In a mass spectrometer having an ioniza tion chamber, sample inlet means, accelerating electrodes, an ion collector and an amplifier for amplifying the current developed at the ion collector, the improvement which comprises a calibration circuit adapted to be coupled to the amplifier of the mass spectrometer and comprising a plurality of output channels adapted to carry the output voltage from the amplifier, an adjustable tap potentiometer in each of said output channels, a separate motor linked to each potentiometer for adjusting the taps of the po tentiometer responsive to voltages applied to the motor, a separate source of determinable voltage for each output channel and linked thereto so that the determinable voltage will oppose the output voltage, an auxiliary amplifier for amplifying the resultant opposition voltage means, for feeding the resulting amplified voltages to the respective motors, a sample switch valve for controlling the sample flow through the sample inlet means, a selector switch for feeding the output of the mass spectrometer amplifier through difierent ones of said output channels and for simultaneously controlling the sample switch valve and the potential applied to the accelerating electrodes.

15. A method of calibrating a mass spectrometer which produces an output voltage related to the number of ions of a given mass striking an ion collector, which method comprises comparing the output voltage produced by ions of a given mass in a sample of known composition with a known voltage to produce a resultant voltage, and applying this resultant voltage to adjust the output voltage so that it will bear a fixed relationship to the known voltage.

16. A method of calibrating a mass spectrometer which successively produces output voltages related to the quantity of ions of a given mass successively focussed on an ion collector, which method comprises comparing the output voltages produced by each successive quantity of ions of diilerent mass with a separate known voltage to produce a resultant voltage, and applying this resultant voltage to adjust the respective output voltage so that it will bear a fixed relationship to the known voltage.

17. In the operation of a mass spectrometer which comprises ionizing an unknown sample to be analyzed and successively discharging separate quantities of ions on an ion collector, the ions of each quantity being of the same mass and of different mass than the ions of other quantitles, the improvement which comprises ionizing a sample of known composition, discharging the resultant ions of at least one mass on the ion collector, amplifying the resultant current, comparing the amplified current with a known voltage and applying the resultant voltage to adjust the output voltage so that it will hear a fixed relationship to the known voltage.

18. A mass spectrometer comprising means for introducing an unknown sample, sensing means for sensing at least one mass peak in the unknown sample and delivering an output signal in proportion to the magnitude of the mass peak, means for automatically and periodically int-crrupting the sensing means so that it senses on a zero mass peak, and means for automatically adjusting the sensing means to a zero output signal when sensing on the zero mass peak.

19. In mass spectrometry the method of determining the concentration of at least one constituent of an unknown sample, which comprises ionizing a sample containing said constituent in known amounts to produce an output voltage, mixing a portion of said output voltage with a known voltage to produce a resultant voltage,

adjusting said proportion of the output voltage so that the resultant voltage will bear a predetermined relationship to the known voltage, ionizing a second sample containing said constituent in unknown amounts to produce a second output voltage, mixing the same proportion of said second output voltage with said known voltage to produce a second resultant voltage, whereby the ratio of said first and second resultant voltages will be equal to the ratio of the concentration of said constituent in the known and unknown sample.

20. In mass spectrometry, the method of determining the concentration of at least two types of ions in an unknown sample after ionization of the sample which comprises ionizing a known sample containing constituents from which said ions are produced in known amounts to produce a separate output voltage for each type of ion, mixing a proportion of each of said separate output voltages with a different known.

voltage to produce separate resultant voltages, adjusting said known voltages so that the respective resultant voltages will be of equal value, ionizing the unknown sample containing said constituents in unknown amounts to produce a separate second output voltage for each of said types of ions, mixing a proportion of each of said second output voltage with the corresponding one of the adjusted known voltages to produce a sep arate second resultant voltage, the respective proportions of each of said second output voltages being equal to the corresponding proportions of each of said first output voltages, whereby the ratio of said first and second resultant voltages for each type of ion will be equal to the ratio of the concentrations of said types of ions in the known and unknown samples after ionization.

21. In mass spectrometry, the method of determining the concentrations of a plurality of constituents in an unknown sample which comprises ionizing a sample containing said constituents in known amounts to produce a separate output voltage for each constituent, mixing a proportion of each of said separate output voltages with a separate known voltage to produce a separate resultant voltage, adjusting the several known voltages and thereby adjusting the magnitude of each of said resultant voltages so that the respective resultant voltages will be of the same order of magnitude, ionizing a second sample containing said constituents in unknown amounts to produce a second separate output voltage for each constituent, mixing a proportion of each of said second output voltages equal to the proportion of the corresponding first output voltage with the corresponding one of the adjusted known voltages to produce a second resultant voltage for each constituent, whereby the ratio of the magnitude of said first and second resultant voltages of each constituent will be equal to the ratio of the proportion of the constituent in the known and unknown samples.

22. In mass spectrometry, the method of determining the concentrations of a plurality of constituents in an unknown sample which comprises ionizing a sample containing said constituents in known amounts to produce a separate output voltage for each constituent, mixing a proportion of each of said separate output voltages with a separate known voltage to produce a separate resultant voltage, adjusting the several known voltages and thereby adjusting the magnitude of each of said resultant voltages so that the respective resultant voltages will be the same,

ionizing a second sample containing said constituents in unknown amounts to produce a second separate output voltage for each constituent, mixing a proportion of each of said second output voltages equal to the proportion of the correspondingfirst output voltage with the corresponding one of the adjusted known voltages to produce a; second resultant voltage for each constituent, whereby the ratio of the magnitude of said first and second resultant voltages of each constituent will be equal to the ratio of the proportion of the constituent in the known and unknown samples.

CHARLES F. ROBINSON.

REFERENCES CITED The following references are of record in the file of this patent:

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