US2378936A - Mass spectrometry - Google Patents

Description

June 26, 41945.

R. v. LANGMUIR MASS SPEGTROMETRY Filed July 15, 1943 P05/fa? aen-R005 /P/ 4 Sheets-Sheet 2 June 26, 1945.

R. V. LANGMUIR MASS SPECTROMETRY 4 Sheets-Sheet 3 Filed July 15, 1943 INVENToR. l Rom-RTM AA/@Mum 'BY @QJ W .4

,"ATTORNEYS Gmo 5/,45

311116,26, .1945- y R. V. LANGMUIR l 2,378,936

MASS SPECTROMETRY Filed July 15, 1945 4 Sheets-Sheet 4 Afm INVENTOR. ROBERT K AA/@Mum ATTORNEY Patented June 26, 1945 UNITED, sTATEs PATENT OFFICE MASS SPECTBUMETBY to Consolidated ltobert V. Langmuir, Schenectady, N. Y., assigner Corno ration,

Pasadena, Calif., a corporation of California Application July 15, 1943, Serial No. 494,807

l 4 claims. (cl. '1s-1s) 'I'his invention is. concerned with mass specample a mixture of hydrocarbons), determination of isotope ratios, etc. It may include means such as an ionization chamber in which molecules of the mixture to -be analyzed are converted into ions, means such as an analyzer tube in which a beam of the ions is separated (under the influence of an electric or magnetic ield or both) into a plurality of diverging beams of ions of dinerent specific mass (the ions of the same specific mass being included in a.v single beam), an ion collector,l means for bringing the several beams successively into contact with the collector (which means is referred to hereinafter as the beam sweeper") and means for indicating separately (and preferably recording) the currents collected from the several beams.

' The record thus obtained represents the mass spectrum. It may take any one of several forms, but conveniently is a, graph on which, the magnitudes of the currents corresponding to the several collected ion beams are represented by the heights in the spect of a series of peaks projecting from the abscissa axis. Such a record can be produced on a, 1ater` ally moving sheet, for example a photographic paper by a recording galvanometer or the like. For some purposes, such a recorded mass spectrum is entirely adequate, but in other cases, for example in the quantitative analysis of a'mixture of. hydrocarbons containing a number of dierent kinds of molecules, some of which'upon being bombarded with ionizing particles crack to form a plurality of different kinds of ions, it is necessary to determine the specific mass of the ions to which each peak corresponds. In accordance with my invention, the interpretation of a mass spectrum is facilitated by indicating in a coordinated manner and preferably simultaneously with the indication of the peaks (ion currents), the specific masses of the ions to which the respective peaks (currents) correspond. Thus my invention contemplates, in a mass spectrometer, the combination which comprises Imeans for, marking on a record a series of peaks representing the relative abundance of ions of different specific mass separated from each other rometer and means for indicating simultaneously with the marking (and preferably i on the same record adjacentthe respective peaks) the specific masses of the lons to which said peaks correspond.

.- In one form of theapparatus of my invention the specific mass indicator is operatively connected with the beam sweeper and is actuated by it. For example, a spectrometer constructed lin accordance with my invention may comprise an ionization chamber or the like wherein molecules of a mixture are bombarded with an electron beam and converted into ions, an ion propelling device such as a plurality of spaced electrodes which impress upon the ions a potential to propel them through' an analyzer or sorting chamber in which the ion beams are separated, i. e., caused to diverge from each other, an ion. collector, a beam sweeper for causing the separated ion beams to sweep successively over and impinge upon the collector, a recording galvanometer connected to the collector to record the currents collected from successive beams separately on a chart, and a specific mass indicator operatively associated with the beam sweeper and actuated in response to it' for marking on the chart successively during the recording of the currents and 'adjacentthe respective peak positions the specific mass of ions to which each current corresponds.

In one form of my apparatus the beam sweeper automatically and gradually alters either the potential impressed between the ion propelling electrodes or the magnetic eld, thus causing the paths of the beams propelled through the analyzer to change sufdciently to sweep the beams successively over the collector. The specific mass indicator is connected in a circuit with the beam sweeper andthe propelling electrodes, and the change in potential in this circuit actuates the mass indicator. For example, the mass indicator may be a galvanometer calibrated in terms of the ion masses or it may be a, vacuum tube circuit which is tripped at intervals at successive values of ion accelerating potential, means being provided to mark the spectrogram each time the cirfcuit trips. In this way the beam sweeper and the mass indicator are synchronized and the peaks on the spectrogram correlatedwith the specific masses of ions to which they correspond.

'I'he foregoing and other features of my inventionwill be more thoroughly understood in the light of the following detailed descriptiontaken in conjunction with the accompanying drawings in which: V

-collector.

Fig. 1 is a diagram of a mass spectrometer provided with a mass marker controlled by a beam sweeper and adapted to identify the various peaks corresponding to various ion beams on a recorded mass spectrogram.

Fig. 2 is a wiring diagram of a preferred form of mass marker of my invention applied to a mass spectrometer and actuated by a beam sweeper of the type described and claimed in the co-pending application Serial 499,055, led August v18, 1943 by Raymond C. Olesen;

Fig. 3 is a graph illustrating the manner in which the ion accelerating voltage decays during the recording of the mass spectrogram; and

Fig. 4 is a graph illustrating the grid bias of the mass marker circuit and its utilization to indicate speciiic mass in correlation with the peaks on the mass spectrogram.

Referring now to Fig. 1, it will be observed that the apparatus comprises an ionization chamber connected to a tubular semi-circular analyzer both adapted to be'malntained under high vacuum, for example, by enclosure in an evacuated envelope (not shown). A sample of gas to be analyzed is admitted to the ionization chamber through a gas sample inlet conduit and the molecules of the gas are bombarded in the chamber by an electron beam, so that they are ionized. The ions are forced by electrical potential out of the ionization chamber and through the adjacent analyzer. I'his is accomplished by impressing a potential between a pusher electrode P disposed in the ionization chamber and a rst propelling electrode E1, which forms one wall of the chamber. Thus an electrical potential is impressed across 4the region through which the electron beam passes and ions formed in this region are pushed through an exit slit S1 ln the rst propelling electrode. The ions are accelerated further by voltseparated by blank spaces. Each peak corre'- sponds to a specific mass, `but it is diiiicult to identify these masses without the mass marker of my invention.

To consider the balance of the apparatus of Fig. l, it will be observed that it comprises a high voltage D. C. potential supply which is connected to the lon accelerating electrodes P, E1 and Ea of the spectrometer through a beam sweeper and a potential divider. The function of the beam sweeper is to change gradually the potential appliedvto the ion accelerating electrodes. thereby causing the beams to move successively past the exit slit S3. The potential divider is employed to distribute a relatively small potential between the pusher electrode and the rst propelling electrode, and a relatively large potential between the two propelling electrodes. A mass marker control, which will be described in greater detail hereinafter, is electrically coupled with the beam sweeper through the potential divider. The mass marker is provided with a marking relay and the changing accelerating potential applied to the spectrometer by the beam sweeper effectively actuates the mass marker contro1 so that the spectrogram is marked periodically, these marks being correlated with the peaks on the spectrogram, for purposes of identifying the masses of the respective peaks.

n the preferred form of my apparatus, the ion accelerating voltage (which is the voltage output of the beam sweeper and hence the voltage that appears across the potential divider) decreases as an exponential function of time, this eiect being produced by a beam sweeper mechanism, for example, that described and claimed in the aforementioned co-pending application Serial No. 499,- 055, flled August 18, 1943 by Raymond C. Olesen. Inasmuch as the virtual specific mass of the ages of higher potential impressed between the 4010115 being detected at any moment Varies i11- electrode E1 and a second propelling electrode Ez, which has a slit Sz matching that in the irst electrode. The two slits act as a collimator.

I'he ionization chamber and the analyzer are both mounted between the poles of a large electromagnet (not shown) with the result that the heterogeneousbeam of ions issuing into the analyzer through the slit Sz pursues a curved path toward an exit slit Ss at the far end of Vthe analyzer tube. 'I'he magnetic field acts upon the ions and separates them in accordance with their specic masses into a plurality of separated diverging beams B1, Bz, Ba, B4, Bs etc., each composed of ions of a different given specic mass. By varying the strength of the magnetic eld or the accelerating potentials, or both, the radii of curvature of the several beams may be altered simultaneously with the result that the beams are swept successively over the exit slit S3. As a beam passes through the exit slit it impinges on an ion The current carried by the beam is thus discharged. This current is amplified and recorded as a series of peaks on a moving record or spectrogram' in an automatic ampller and recording system. Automatic recorders i'or such mass spectrograms are described in a co-pending application Serial No. 437,922 filed April 6, 1942, by Edmund E. Hoskins and myself, and in my co-pending application Serial No. 444,491, filed May 25, 1942. I

Since the ion beams that are separated from each other in the analyzer are spaced substantially apart at the far end of the analyzer adjacent the ion collector, the collected currents versely as the accelerating voltage, the virtual specic mass of ions that may be detected at any moment during thesweep of the beams across the collector increases as an exponential function of time. By "virtual specific mass of ions, I mean a mass-to-charge ratio of ions that can :follow a particular line path in themass spectrometer, say, the path B3 from a point of ionization to a point on the collector. I employ this relationship of virtual specific mass to ion accelerating voltage in the operation of the mass marker which automatically marks the mass spectrogram at the moment that ions of a predetermined specic mass are impinging on the ion collector, thus facilitating the determination of the actual speciiic mass of ions which produce a particular peak on the y vdivider comprises a relatively small resistance 32A followed by a relatively large resistance 32B, the pusher electrode being tapped in ahead of the small resistance, the electrode E1 being tapped in between the two resistances and the electrode Ez being connected .back to the power source through the beam sweeper and through a rotary stepping appear on the spectrogram as a series of peaks relay 33- The beam sweeper comprises a vacuum tube SIA connected acrossthe high voltage power supply 30. 'I'he grid of this tube isconnected to an RC circuit comprising a resistance 35 in parallel with a condenser 54. An auxiliary direct current power supply 36 is connected in parallel with the RC circuit, a switch 31 being connected between the resistance of the circuit and -the auxiliary power supply. 'I'hebeam sweeper is also provided with resistors 38, 39 connected in one side oi.' the circuit, respectively, between the tube and the potential divider andV between the power supply and the tube. A

In the operation of the beam sweeper, a large negative voltage is applied to the grid and across the condenser when the switch 31 is closed. Consequently, the tube resistance is high and a high voltage is developed across it. When the switch is opened the condenser discharges with a corresponding decay in the resistance of the tube and of the voltage in the circuitl so that the voltage applied to the ion accelerating electrodes decreases gradually and brings about a sweep of the ion beams successively across the exit slit to impinge upon the collector, the result being a recording of a mass spectrogram in a series of` current peaks, each corresponding toI an ion beam of a given specific mass.

As indicated above, the mass marker apparatus l of my invention employs a rotary stepping relay connected in series with the potential divider.

The other major portions of the apparatus comprise a D. C. amplifier 40, a control relay 4I, a pulsating relay 42, and a marker means 43, together with suitable power sources for separating the ampliiier relays, etc.

The rotary stepping `relay conveniently is the type designated by Guardian Electric Manufacturing Company as Series R stepping relay. Such a relay comprises a rotary contact arm 44 which upon rotation may contact any one of a series of equally spaced circumferentially positioned xed taps T1', T2, Ta T25. A disc or cylinder rigidly secured to the same shaft as` the arm has numbers thereon which are position beneath a pointer and become illuminated when the arm contacts the corresponding tap. Thus, as shown, the numeral 3 is illuminated on a tap indicator 45 when the rotary arm contacts tap T3.

The rotary relay also includes a stepping coil 45 and reset coil 41. AWhen an electrical impulse is applied to the stepping coil, the rotary contact moves to the next tap in sequence. When current is applied to the reset coil,the rotary arm returns to the iirst tap position T1.

The ldirect current amplifier is of conventional design and includes a switch 61 for disconnecting its anode power supply battery 49 from its amplifier tubes 50, 5l, 52. In the grid circuit there "is a potentiometer 53 for determining normal grid bias. The sliding contact 53A of this potentiometer is connected directly with the rotary arm 44 of the stepping relay 33.

'I'he grid circuit is also connected to means including a battery 10 and the pulsing relay 42 for applying a negative biasl to the grid across a resistance-condenser network 54 connected between a grid leak resistor -55 and ground. This additional negative bias is applied" to the grid momentarily when the contacts of the pulsing relay are closed.

The time constant of the resistance-condenser network 54 in the grid circuit of the rst stage oi the D. C. amplifier should be small compared to the time interval between `successive steps so that the condenser becomes completely discharged before the firing instant is reached. In this way, no appreciable residual charge remains on the condenser to affect the gridv bias at the ring instant. i

'I'he control relay includes a relay coil lli and two pairs of normally open contacts 51, 5l. The

-control relay coil is connected in the output oi.'

the D. C. ampliiler and operates to close the two pairs of contacts when the D. C. amplifier output current exceeds a predetermined level corresponding lto some voltage threshold in the gridlcircuit. That is, when the bias on the grid of the input tube falls below a predetermined level hereinafter called the firing threshold, the control relay is actuated and its contacts are closed.v

One pair of control relay contacts 5B is connected in one side of a -power line 50 which supplies power to both the stepping coil oi the rotary relay and the coil of the pulsing relay 42. A panel 'light (not shown) and the marker light 4I areA connected in series with the other pair of contacts 51 of the control` relay. Power isapplied to both lights'when the contacts of the control relay are closed. i

The complete cycle of operation of the mass marker during the recording of a mass spectrogram may be understood from the following explanation:

Initially, the rotary contact is set on tapTi by applying power to the reset coil 41, and at the same time the beam sweep circuit output voltage is set at its maximum value by charging the condenser. of its R--C circuit. Initially the contacts of the control relayand the pulsing relay are open and the panel light and mass marker light are oi. l

'I'he initial bias on the D. C. amplier input tube grid circuit is equal to the voltage between the cathode of the rst tube and the sliding icontact 53A of the potentiometer plus the voltage between the rotary arm and ground. As long as the sum of these voltage is greater than the ring threshold, the contacts of .the control relay and the pulsing relay remain open.

When recording'a mass spectrogram, thev switch 31 is opened in the beam sweeper circuit, thereby causing the accelerating voltage across the potential divider to decrease gradually as a funcv contact with the rotary arm and ground. Ac`

cordingly. the total bias on the D. C. amplifier input tube grid decreases as the accelerating voltage. As soon as this total bias reaches the firing threshold, the control relay in the output of the D. C. amplifier is actuated and its contact closed.

The closing of one pair of contacts causes the panel light and the marker light to illuminate. The closing of one pair of contacts causes current 'to flow in the stepping relay thereby moving the rotary arm from the rst tap T1 to the second tap T2. Simultaneously, the closing of these contacts causes current to flow in the coil of the pulsing relay 42. When the contacts of this relay are closed, a negative. pulse is applied to the D. C. ampliiier input tube grid thereby causing a sharp decrease in the 'D. C. ampliiler output When the marker light flashes, it exposes the light-sensitive recording medium momentarily,

thereby producing on the mass spectrogram an indication corresponding to a predetermined value of accelerating voltage and hence a prede- Vtermined'value of virtual specific mass.

It should be noted that the fraction of the accelerating voltage that is included in the grid bias is greater when the rotary contact is on tap Tz than when itis on tap T1. Accordingly. the contacts of the control relay remain open until the grid bias again reaches the ilring threshold. At this time, the control relay closes again, making another mark on the mass spectrogram and moving the rotary arm to tap Ta.

This sequence of events is repeated cyclically to produce a series of marks on the mass spectogram corresponding to each tap T1, T: Tas, and adjacent the respective peaks.

It can be shown that if the virtual specic mass is an inverse function of the accelerating l voltage, the virtual specic mass which may be recorded at the instant of firing while the rotary arm is on any of the taps Ti, T2 T25, is proportional to the value oi the total resistance connected between that tap and ground. If for example, it is desired to place marks on the mass spectrogram at points corresponding to ions having specic masses of 10, 14, 18, 24, 28, 32, 38, 42, 46, etc., when the rotary arm is in contact with taps 'I 1,"I,'2Ta, T4, Ts, To, -T'1, Ts, T9, etc., respectively, the values of the resistance between each ofl said taps and ground should be in the proportions 10, 14, 18, 24, 28, 32, 38, 42, 46, etc., respectively. If the sliding contact of the D. C. amplier is set to yield a suitable ring threshold for any one of these masses, this same firing threshold will cause the mass marker to operate properly for each of the other specic masses.

The manner in which the total grid voltage appliedl to the ilrst tube of Athe D. C. ampliiler changes as a function of time during the recording of a mass spectrogram, when the ion accelerating. voltage decays exponentially, may be understood from the following explanation taken in commotion with Figs. 3 and 4.

When the accelerating voltage decays exponentially, a straight line may be obtained by plotting the logarithm of the specic mass ask ordinate against time, as abscissa. Such a relationship is indicated by the graph plotted in Fig. 3. In this graph the numbers 10, 14, 18, etc., represent the values of specific mass for which it is desired to produce marks on the mass spectogram. On the right side of the vertical axis the corresponding/tap numbers are given 1, 2, 3, etc. If the accelerating voltage is initially at a value suitable for the detection of ions of speciiic mass I at the ion collector, and the voltage decays at a rate vcorresponding to the slope of the straight line in Fig. 3, ions of specic mass 14, 18, etc., will be recorded at the corresponding times rep resented by the corresponding abscissa of points on the line. Thus, the time at which the ions in question are impinging the collector are given by the values in the following table:

Table "www" Recording Tap specic .m time i4 l. 45 i8 2. 55 24- 3. 8 28 4.45 32 5. 02 38 5. Tl 424 6. 2l 46 6. l

etc.

n the initial bias on the inputmte cf the D. c. ampliiler has just exceeded the firing threshold,

while the rotarycontact was on tap T1, the system f rotary arm moves to tap T2, thereby increasing the grid bias to a new value as shown in Fig. 4. The grid bias then decreases along a curve marked El until it reaches the firing threshold. At this moment, a new mark is made on the mass spectrogram directly above a peak corresponding to the yamount of any ions of specic mass I4 being recorded, .and the rotary arm of the rotary relay moves to tap Ts. Again the grid bias increases and then gradually decays along the curve E:

until it again reaches the ring threshold. This process is repeated sequentially for each tap. Each of thesecurves represents an exponential type of grid voltage decay between firing instants. It can be shown that the following relationship holds:

Vr= Vr(1 where m=virtual specic mass of ions which may be recorded at ring instant.

Am=virtual specific mas at ring instant subtracted from the virtua specific mass corresponding to the next firing instant.

VT=component of firing threshold voltage which exists between the rotary contact and ground.

Vr=component of grid bias existing between the vrotary tap and ground immediately after the rotary contact has ,moved to the next tap in sequence.

The grid bias curve coresponding to the sequence of events occurring while ions of speciiic mass I0 to 46 are being recorded are shown in Fig. 4.

In the explanation given hereinabove, it has been assumed that the ion accelerating voltage decays as an exponential function of time during the recording. The ring instants and the intervals between them are determined by the rate of decay. If the rate of decay varies during the recording, thev time intervals between ring'instants vary in a corresponding manner. For example, if the rate of voltage decay is less for specic masses of high value than it is for those of low value, the time intervals between firing instants will be longer than those indicated in Fig. 4.

The mass marker circuit shown in Fig. 2 includes a plurality of coupled three-position switches 65, 66, 6l. Position I on each switch is olf position. Switch 65 is connected in the power line when on positions 2 and 3. Switch 66 if oil in both positions l and 2, but in position 3 completes the circuit of the lower contacts of the control relay. In positions 2 and 3, the switch 81 connects the anode battery 49 to the amplifier tubes of the D. C. amplifier.

The three coupled switches are thrown simultaneously from a common control lever, respectively, to positions l, 2 or 3. When the coupled switches are in position I, the circuits including the switches are open. In position 2, power is supplied to the laments of the D. C. amplifler and the other electrodes of the D. C. amplifier tubes. In this position power is also available to the panel light and marker light through a transformer if the upper contacts of the control relay should be closed. In position 3, the connections between the lower contacts of the control relay and the stepping coil of the rotary relay, and the coil of the pulsing relay are completed.

An auxiliary push button switch B8 is connected in series with switch 65 and the power lines which supply power to the reset coil 41 of the rotary relay.

Of course there may be an excess voltage over that required to just cause the rotary arm to move to the tap. However, if the voltage is such that the rotary arm just moves to a tap, that is so that there is no excess bias over the flring grid bias,

the mass marker will indicate the specific mass of ions which can be detected-when the aforesaid constant voltage is applied to the potential divider.

I claim:

,1. In a mass spectrometer provided with an analyzer, in which a heterogeneous mixture of ions of diilerent specific charge are proiected as a beam by an electrical potential and separated into a plurality of beams of ions, each of the separated beams being composed of ions of the same lspeciiic mass, an ion collector, and a beam sweeper for sweeping the plurality of ion beams successively over and in contact with the collector by causing said potential to decay gradually, the combination which comprises a recording galvanometer for recordingseparately a series ofl currents collected from the successive beams, and a mass indicator operatively associated with the beam sweeper for indicating, simultaneously with the recording of each current. the speciiic mass of the ions from which that current was derived.

2. In a mass spectrometer provided vwith means for converting molecules into'ions, a plurality o! spaced electrodes for applying a potential upon the ions to propel them in a beam and an analyzer wherein .the beam of ions under the iniiuence of a neld may be separated intoaplurality of ion beams of diii'erent specific mass, eachbeam being composed of ions of the same specific mass, the combination which comprises anion collector, a

beam sweeper which causes said potential to, decay the speciflc charge of the ions of the collected beam from which that current is derived.

3. In a mass spectrometer provided with means for converting molecules into ions, a plurality of spaced electrodes for impressing a potential upon the ions to propel them in a heterogeneous ion beam, an analyzer wherein said beam is propelled, and means for producing a eld inthe analyzer to separate said beam into a plurality of homogeneous beams composed of ions of different specific masses, the combination which comprises an ion collector, a beam sweeper connected to the electrodes for progressively decreasing the potential impressed thereby on the ions so that the path of the plurality of ion beams is altered and they sweep successively over and impinge upon the collector, a galvanometer connected to the collector for indicating successively the currents collected from the successive beams, and a mass indicator operatively associated with and actuated by the beam sweeper for indicating simultaneously with the indication of each current, the specic mass of the ions of the collected beam from which that current is derived.

4. In' a mass spectrometer provided with means for converting molecules into ions, at leest three lspaced electrodes for impressing a, potential upon the ions to propel them as a. heterogeneous ion beam, an analyzervwherein said beam is propelled. and means for separating said beam in the analyzer into a plurality of homogeneous beams composed of ions of di'erent specific masses, the

combination which comprises anion collector, a

beam sweeper for altering the potential applied to the electrodes to cause the homogeneous ion beams to sweep successively over and in contact with the collector, a potential divider for distributing the potential supplied by the beam sweeper between the electrodes, a galvanometer connected to the collector for indicating successively .the currents collected from the successive beams, and a mass indicator connected to the beam sweeper through the lpotential divider and actuated by the sweeper, for indicating simultaneously with the indication of each current the specific mass ot the collected beam from which that current is derived? ROBERT V. IANGMUIR.

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