US2463544A - Mass spectrometry - Google Patents

Description

March 8; 1949. R. v. LANGMUIR MASS SPECTROMETRY Original Filed April 30, 1943 F/Gl.

2 Sheets-Sheet l LINEAR AMP (48 NARROW FILTER BAND PASS AMP.

RECORD/NG GALVA/VOMETER I INVENTOR. ROBERT M LANGMU/R A "mm R. V. LANGMUIR MASS SPECTROMETRY Original Filed April 50, 1943 2 Sheets-Sheet 2 ION ACCELERA TING VOLTAGE FIG. 4.

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INVEN TOR. ROBERT l LA A/GMU/R r I 9 Q7466 A TTORNEVS Patented Mar. 8, 1949 MASS SPECTROMETRY Robert 'V. "Langmuir, Schenectady,

N. Y., assignor to Consolidatedllngineering Corporation, Pasadena,'Calif., a corporation of California Original application Ap 483,745. Divided ril 30, 1943, Serial No. and this application October 7, 1946,Serial No."701,738

Claims.

This invention is concerned with mass spec- :trometry and contemplates improvement therein involving the production .of pulsating ion current which can be amplified with A. C. instruments. The present invention 'is a division of my co-pending application Serial No. 483,745 filed April 30, 1943 (now United States Patent :No. 2,457,162).

In mass spectrometry, a gas sample is bombarded by moving electrons to produce ions of -various substances present in the sample, and the lions thus formed are separated into various components having difierent mass-to-charge ratios by subjecting the ions to the influence of :electric or magnetic .fields or both. The indi- -vidual components are then directed upon an ion collector and discharged, and the intensity of the resulting ion current is measured. Thus, the several components may be caused .to fall successively upon the collector by varying the electric or magnetic field, or by moving the collector successively into the respectivepaths of the several components.

Ina method of mass spectrometry'here-tofore proposed, the several components having different mass-to-charge ratios are formed of :posi- ,tive ions. When such components are directed to the io-n'collector and there discharged, a unidirectional current is set up in acircuitconnected to the ion collector and the intensityof this current is measured following amplification by a direct current amplifier. However, this method (especially when the samples of gasto be'treated in the mass spectrometerare small) is subject .to-certain inherent disadvantages arising princitpally out of nature of direct current amplifiers. The speed with which a mass spectrum maybe measured with a given degree of accuracy Ade- .pends upon the time constant of the input circuit of the D. C. amplifier. This is necessarily .long because of the large input resistance re- ;quiredto detect the small currents available at the collector. The time constant is the product of the input resistance times the input capacitance and may be several'seconds or even. ofthe order of minutes. Thus, employing a direct current amplifier, the rate at which'asmall sample of gascanbe analyzed in a massspectrometer is relatively slow and the-capacity of theiinstrument in terms of useful work is limited.

As disclosed in my co-pending application Serial :No. 294,346 filed September '11, 1939 (now .Unitedstates Patent No. 2370,6723), I have disrcovered that the speed of analysis can be "in- {creased markedly .by producing a pulsating ion the intensity of each beam 'manner. The pulsating ion beams thus formed are caused to fall successively at the collector izing particles (electrons) oscillating potential which is amplified with an A.-C. type amplifier and recorded or measured. For example, a plurality of separate ion beams of difierent mass-to-charge ratios may be generated or controlled in such fashion that varies'in a sinusoidal to produce a mass spectrum. Preferably, the

frequency of the ion beams should be smaller than the number of ions falling on the collector per second. Otherwise, statistical variations in the 'rate at which the ions fall onthe collector "may mask the pulsations in that rate, and it :is these *pulsations which should be detected.

The required pulsating ion beam may be formed in several ways, but inall cases the ions of a given mass-to-charge ratio strike the collector and there discharge at a rate which oscillates, pulsates or varies in a regular manner to produce a corresponding pulsating or oscillating electric current which may be amplified with an alternating current amplifier. The required oscillation maybe brought about ('1) by varying the intensity of the beam of ionizing particles (such as electrons) with which'the gas in the mass spectrometer is bombarded; (2) by varying the velocity (or energy) of these ionand (3) by varying 'the forces acting upon the ions during the separation of ion components having different mass-to-charge ratios, i.e. by varying the field forces which control the deflection of the ions during separation into the components.

Thus, in a mass spectrometer having means for bombarding a gas sample with ionizing particles to produce ions of substances present in the sample, means for separating the ions thus produced according to their mass-to-charge ratios into a plurality of ion beams, means for pulsating the ion beams, and means for collecting ions of a beam to produce an alternating =or -pulsa'ting current, my inventioncontemplates the combination which comprises an electrode disposed adjacent the path of the electron stream, means for supplying a potential to the electrode, and means for varying the potential at said electrode in a pulsating manner, whereby the ion beams are caused to pulsate. The electrode supplied with the pulsating potential may be an anode adapted to draw electrons from a filament or other electron source, or it may be an electrode disposed between such an anode and such an electron source. Likewise the electrode may be disposed in the region where the electron stream bombards the gas sample, for example it may be one of a pair of plates (one of which has an aperture through which the electrons pass into the analyzing portion of the apparatus). Another manner for causing the ion beam to pulsate is to supply a pulsating potential to a solenoid surrounding the analyzing portion of the apparatus, whereby the radius of curvature of the ion beam is caused to vary in a pulsating manner. The ion beam may also be caused to pulsate (a) by causing the electron beam to pulsate at its source, for example a filament, by varying the temperature at that point and hence the rate of electron emission or (b) by means of a shutter which periodically is interposed in the path of the ionizing particles.

These and other features of my invention will be more thoroughly understood in the light of the following detailed description taken in conjunction with the accompanying figures, in which Fig. 1 is a cross section of one form of mass spectrometer constructed in accordance with my invention (provided with an amplifier, a recorder and a potential source, all shown schematically) and adapted to bring about pulsation of the ion beam through modulating the beam of the electrons employed to bombard the gas from which the ions are formed;

Fig. 2 is a longitudinal view, partly in section, taken through the mass spectrometer of Fig. 1 along the line 2-2;

Fig. 3 is a graph showing the relationship of ion accelerating voltage to the voltage in the resistance 20 of the collector circuit for the type of mass spectrometer illustrated in Fig. 2; i. e., in Fig. 3 and A. C. voltage component is superimposed on the D. C. voltage that would be present across resistor 30A if the beam intensity did not pulsate, the A. C. component being proportional to the D. C. voltage at each point; and

Fig. 4 illustrates the relationship of the ion A. C. voltage to the amplitude of the A. C. voltage in the output of the amplifier circuit in the spectrometer illustrated in Fig. 2.

Referring to the drawings and particularly to Figs. 1 and 2, it will be observed that the mass spectrometer has an envelope 2| which may be evacuated through vacuum lines 22, 23 connected to the envelope and maintained at low pressure by one or more vacuum pumps (not shown) connected to the vacuum lines.

A capillary tube 24 is connected to the envelope and through it a sample of gas to be analyzed is admitted. Within the envelope adjacent the entrance of the capillary there is disposed a pusher plate 25 and a plate 26 having a slit 21. The plates are disposed respectively on either side of the entrance of the capillary tube so that a gas sample entering the envelope flows into the space between the plates and is there bombarded by a unidirectional electron stream, the strength of which is varied or modulated in a pulsating manner. The bombardment of the gas molecules results in the formation of gaseous ions in the space between the plates, the amount of ions formed in unit time being correspondingly varied in a pulsating manner.

A small negative voltage (relative to the voltage on the pusher plate 25) is established on the plate 26. In consequence, positive gaseous ions formed in the space are drawn toward the plate 26. Some of these ions pass through the slit 21 in this plate. A semi-cylindrical case 28 is disposed within the envelope adjacent the plate 26. A flat plate 29 forming one side of the case is disposed substantially parallel to the plate 26 and is provided with a second slit 3!] which matches the slit in plate 26. A high accelerating potential is maintained between the plate 26 and the case 28. This potential provides an electric accelerating field which causes the positive ions that pass through the first slit 21 to be highly accelerated towards the case. Some of these accelerated ions enter the case 28 through the second slit 30.

A solenoid 3| is disposed around the envelope and produces a magnetic field therein, so that the ions entering the case through the second slit at high velocity follow curved paths within the case.

Due to the combined action of the aforementioned accelerating field maintained between the plate 26 and the case and the magnetic field produced by the solenoid, the ions entering the case are separated into ionic beams of different massto-charge ratios. Each beam follows a semicircular path, the radius of which is determined by the strength of the electric accelerating field, the strength of the magnetic field and the mass-tocharge ratio of the ions in the particular beam. Thus, each beam in effect, originates at the slit 30 and follows its own curved path to focus on the flat plate 29 opposite the slit 30.

A third slit 32 is provided in the flat plate of the case in the region where the beams tend to focus. Ions having a predetermined mass-tocharge ratio follow a substantially semi-circular path 33 from the second slit 30 to the third slit 32, pass through the third slit and fall upon an ion collector 34 which is protected by a shield 34A having therein an aperture 35 matching the third slit.

The ions discharge on the ion collector and produce through a resistor 36A of a linear amplifier 36 connected to the ion collector a pulsating electric current that corresponds in frequency and intensity to the collected ion current.

The voltage which results in the resistor 36 is amplified with an A. C. amplifier combination discussed in detail hereinafter and the amplified current is recorded by a, galvanometer 31, so that the intensity of the ion beam being discharged at any instant is indicated.

The several ion beams of different mass-tocharge ratio are caused to pass successively through the slits so, 32, 35 onto the collector 34 by varying the intensity of the accelerating electric field between the plate 26 and the case, this being accomplished by moving a slider 38 on a potentiometer 397 In short, the separate beams existing within the case are successively brought to a focus at the slit 32, so that the intensities of the respective beams are measured successively.

To consider one embodiment of my invention for varying the ion current in a pulsating manner, reference is made to Fig. 2. As shown in this figure the electrons employed to bombard a gas sample in the space between the plates 25 and 26, originate at an ionizing source represented by a filament 4!], an anode 4|, and a control electrode 42 with associated batteries 42A, 46 and 6|. The

filament, the anode structure and the control electrode are disposed within an extension 2IA of the envelope at one end thereof. A D. 0. potential difference between the anode and control electrode is maintained by a battery 6| which also supplies current to the plate 25.

The control electrode is disposed between the filament and the anode structure and has an aperture therein in line with two other apertures in the anode structure so that the electron beam is directed into the space between the plate and the plate 26 approximately at right angles to the capillary tube through which the gas sample is admitted.

The electrode and anode preferabl are maintained at positive potentials with respect to filament.

The anode 4| is connected to an alternating current generator 43 connected between the filament .and the aperture anode 4! in series with direct current battery iii. The alternating current provided from the alternating current source 43 serves. to vary the velocity of the electrons passing through the aperture anode 4| into the ionizing region between the pusher plate 25 and the plate 26. The plate 26 is maintained at ground potential and the pusher plate is maintained at a positive voltage by the batery 6 I.

In this form of my invention the potentials supplied by the battery 46 preferably are sufilciently large to provide the electrode 42 with a potential that is adequate to draw oiT the maximumpossible number of electrons from the filament and project them through the aperture in the. control electrode 42 so that an electron beam of constant intensity is provided upon this point. Under these circumstances, the alternating voltage provided by the A. C. source 43 serves to vary both the rate at which the electrons enter the ion region and the velocity of the entering electron. Both of these effects combine in an additive manner to vary the rate of production of ions in the ionization region, with the resulting production of a pulsating ion beam. Ion

beams of given mass-to-charge ratio are su-ccessively focussed at the collector 34 shown in Fig. 1 and produce an alternating current through the resistor 36A as described hereinafter.

Some of the electrons of the beam or stream may pass all the way between the plates 25 and 25 to be picked up by an electron collector or cage M disposed in line with the apertures in the control electrode and the anode structure. The vagrant electrons thus picked up by the collector M flow to ground through a battery 45 connected to the collector.

It will be apparent that the concentration of ions in the space between plates 25, 26 will be substantially proportional to the intensity of the electronbeam if the velocity of the electrons is constant.

The frequency of the electron beam modulation, which is controlled by the frequency of the A. C. potential supplied by the A. C. generator, preferably is less than the average number of ions falling on the ion collector per second, so that statistical variations in the rate at which the ions reach the collector will not obscure the pulsations that are to be measured. In this manner, the intensity of the ion current varies in an easily controllable manner. However, some of the advantages of my invention may be retained if the ion beam is modulated at a frequency higher than the average number of ions falling on the collectorper second.

To facilitate'amplification ofthe output of the mass spectrometer, it is preferable toproduce pulsating ion currents that vary in a sinusoidal man.- ner, this being accomplish-ed by employing an electron beamthat pulsates in this fashion.

The alternating portion of the ion: current'appearing at the collector 34 is amplified in a circuit including the first linear amplification stage 36, a second linear amplification stage 41, a narrow band pass filter 48, a logarithmic amplifier 49 andv indicated at the recording galvanometer 31. The two amplification stages are coupled through the coupling and blocking condenser "A, which filters out the D. C. component of'the signal appearing in the output of the, first stage.

The amplifier stage 36 has, as indicated above, a linear characteristic and comprises one or more tubes. Thus, the first amplifier stage may com prise an amplifier tube 50 of the pentode type provided, as shown in Fig. 1, with a control grid 5!, a screen grid 52 anda suppressor grid.53;to.- gether with a cathode 54 and an anode 55. Suitable resistors, including the resistor 36A and a second resistor 56 are connectedto suitable batteries for maintaining the control grid and the anode respectively at correct operating potentials. The input capacity between the control gridand the cathode is represented by the dotted structure 57.

The alternating portion of the signal voltage appearing between the screen grid 52 and the cathode 54 may be calculated according to the following formula where i is the alternating portion of the ion'current falling on the collector 33' and C is the input capacitance 51 of the tube 50. As indicated For this purpose a resonant circuit having a-very high Q may be employed. For example, Q obtainedfrom a resonant circuit utilizing electromechanical units such as magneto-striction oscillators or tuning forks is desirable. The use of a narrow band pass filter or a sharp resonant circuit permits an increase in the signal-.tor-noise ratio of the circuit so that smaller ion currents than would otherwise be possible can be detected, This will be apparent from the following:

Due to thermal agitation within the resistor 36A, noise will be impressed on the control grid 5| and amplified by the system. If'only asmall range of frequencies is passed by the filter "the magnitude of the input noise is given by E m; %Fdf approx. (2) where T is temperature in degrees Kelvin, K is Boltzmanns gas constant, and d is the effective band width of filter48.

Hence the signal-to-noise ratio inthe output is Inasmuch as the current i is to be measured may be as low as ampere, I prefer to utilize as large a resistance 36A as possible, of the order of 10 or 10 ohms, and the narrow band pass filter 48, and in this manner achieve high ultimate sensitivity and. provide high signal-to-noise ratio.

Following the filter 48, I prefer to utilize an amplifier the output of which is proportional to the logarithm of the input, thus making possible the recording of a wide range of ion intensities on a recording medium of limited width. Logarithmic amplifiers suitable for this purpose are well known to those skilled in the art.

In the form of my invention illustrated I utilize a common control connection 58 to coordinate the movement of the recording medium within the recorder 31 with the magnitude of the accelerating or analyzing electric field provided between the plate 26 and the case 28 by the potentiometer 39. In this manner, I am able to produce a continuous record in which one coordinate indicates the mass-to-charge ratio and the other coordinate measures intensity of ion current. Thus, the control connection is connected not only tothe potentiometer and the galvanometer but also to the case 28. It is to be understood that the recording speed necessary to attain a predetermined resolving power varies as an inverse function of the width (df) of the band passed by the filter 48, the term resolving power being employed to define the ability to differentiate and accurately measure the relative intensity of neighboring peaks on the final record obtained.

It is clear that the ultimate sensitivity, resolving power, and recording speed are interrelated with the characteristics of the filter 48. In the preferred form of my invention the recording speed is made as rapid as possible without causing undue loss of sensitivity or resolving power.

In the apparatus of Fig. 2, the alternating current in the outlet circuit, in resistor 35A, is of the same frequency as that generated in the alternating voltage source 43. In the operation of the apparatus, the ion accelerating voltage provided by the potentiometer 39 is varied gradually. As each beam passes by the exit slit, some of the ions impinge upon the plate 29 of the case, The rate at which ions from a given beam pass through the slit 32 pulsates or varies periodically, so that the rate of collection of charges from the ions at the collector 34 is likewise modulated.

The manner in which the rate of collection at the collector 34 varies is shown by Fig. 3. The alternating current voltage across the resistance 36A is, of course, proportional to the rate of ion collection, so the voltage across the resistance 35A will likewise vary as illustrated by Fig. 3. However, the capacitance effect of the condenser A (Fig. 1) blocks any D. C. component of the voltage generated in the plate (cathode) circuit of the tube 52, so that the voltage appearing at the output of the amplifier 36 will be of the alternating current type and will vary in amplitude as the voltage supplied by the potentiometer 39. For ions of a given mass-to-charge ratio, the voltage amplitude in the output of amplifier 36 is is a single peak as shown in Fig. 4.

The height of the single peak of Fig. 4 is proportional to the concentration of a gas from which ions of the particular mass-to-charge ratio are being produced, the constant proportionality depending upon the individual gas, the mass-tocharge ratio of the ions in question, and naturally upon the constants of the apparatus.

As described hereinbefore sources of square wave voltages may be provided, in which case the intensity of the beam striking the collector 34 is periodically changed between two values for a given setting of the potentiometer. This results in a production of square wave voltages across resistance 36A. The square wave voltages contain numerous sinusoidal components and with this apparatus a logarithmic, or other variable sensitivity, amplifier can be provided which amplifies weak signals more than strong signals, so that signals of widely varying strength may be accurately recorded with a conventional recordin galvanometer 31.

I claim:

1. In a mass spectrometer having means for bombarding a gas sample with an electron stream to produce ions of the substances present in the sample, means for separating according to their mass-to-charge ratios the ions thus produced into a plurality of ion beams, and means for collecting ions of a beam, the improvement comprising means for producing said electron beam, said last named means comprising an electron emanating cathode, an anode adapted to form the electrons into a beam and means for pulsating the potential on said anode so as to vary the velocity of said electron beam.

2. In a mass spectrometer having means for bombarding a gas sample with an electron stream to produce ions of the substances present in the sample, means for separating according to their mass-to-charge ratios the ions thus produced into a plurality of ion beams, and means for collecting ions of a beam, the combination which comprises means for producing said electron beam, said last named means comprising an electron emanating cathode, an anode adapted to form the particles into a beam, an electrode disposed between said electron emanating cathode and said anode and adjacent the path of said electron stream and means for uniformly pulsating the potential on said anode so as to vary the velocity of said electron beam.

3. In a mass spectrometer having means for bombarding a gas sample with electrons of an electron stream to produce ions of the substances present in said sample, means for separating according to their mass-to-charge ratios the ions thus produced into a plurality of ion beams, and means for collecting ions of a beam, the combination which comprises an electron producing means, an anode for forming the electrons thus produced into said stream, means for supplying a potential to said anode and means for varying said potential in a pulsating manner.

4. In a mass spectrometer having means for bombarding a gas sample with electrons of an electron stream to produce ions of substances present in the sample, means for separating according to their mass-to-charge ratios the ions thus produced into a plurality of ion beams, means for pulsating the ion beams and means for collecting ions of a beam to produce an alternating current, the combination which comprises an electron producing means, an anode for forming the electrons thus produced into said electron stream, a direct current source, and means for supplying an alternating potential to the anode, whereby said electron stream is caused to pulsate.

5. The method of mass spectrometry which comprises generating electrons, forming said electrons into an electron beam of uniformly pulsating rate and velocity, ionizing the sample to be analyzed by bombardment thereof with said 10 electron beam of varying rate and velocity, form- REFERENCES CITED ing the resulting ions into an ion beam causing the ions of said ion beam having a predetermined g gg fs are of record m the mass-to-charge ratio to impinge on an ion 001- p lector, and measuring the pulsating charge ap- 5 UNITED STATES PATENTS pearing on said ion collector. Number Name Dat e ROBERT LANGMUIR- 2,370,673 Langmuir Mar. 6, 1945

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