US2691108A - Mass spectrometry - Google Patents

Description

Oct. 5, 1954 c. E. BERRY MASS SPECTROMETRY Filed Feb. 25. 1947 2 Sheets-Sheet 1 24 H. u 23 POWER I SUPPLY PM I 2/ PULSE I r FORMER 1 I WIDE BAND PULSE DELAY A c AMPL 2 I -F0RMER NETWORK 29 CONSTANT 20 FREQUENCY PEAK OSC/I-LATOR kV4CUUM TuaE VOLTMETER /6 y m, m '77; m m m TI 7 A K 77 7 K v :2 k v x TARGET STARTING POSITION PULSE CONTROLLED GENERATOR AMPL/F/ER INVENTOR.

CLIFFORD E. BERRY C LZMm ATTORNEYS Oct. 5, 1954 c. 5 BE 2,691,108

MASS SPECTROMETRY Filed Feb. 25, 194'? 2 Sheets-Sheet 2 Rah 5R 23 SUPPLY Ii;

PULSE FORMER 2 L 40; W/DE BAND A. c, AMPL. g CONSTANT i/WEQUE/VC) 20 gasc/LLAroR PEAK VACUUM TUBE l/OL TMETER 4l WAVE FORM /A/ WAVE FORM //v SENDING C/RCU/T COLLECTING c/RcU/T 22 if; v n --0 t t T P4! .28

INVENTOR. CLIFFORD 5 BERRY Mar A T TORNEYS Patented Oct. 5, 1954 UNITED STAT' OFFICE Application February 25, 1947, Serial No. 7303718 11 Claims. 1

This invention is concerned with mass spectrometry and contemplates improvements in mass spectrometry involving a high frequency pulse forming instrument as described hereinafter.

In mass spectrometry, a gas sample bombarded by moving electrons to produce ions of various substances present in the sample, and the ions thus formed are separated into various components having different mass-to-charge ratios by subjecting the ions to the influence of electric or magnetic fields or both. The individual components are then directed upon an ion collector or target and discharged, and the intensity of the resulting ion current is measured. Thus, the several components may be caused to fallv successively upon the collector by varying the electric or magnetic fields, or by moving the collector. successively into the respective paths of the several components or by the method as hereinafter set forth.

In mass spectrometry as previously proposed, a

gas sample to be analyzed is bombarded by an electron beam to ionize the molecules thereof. The ions thus formed are projected by means of accelerating electrodes or the like through a semicircular path to an ion collector or target. Throughout this semi-circular course oftravel the ions are subjected to the effect of an electrical or magnetic field or both whereby those ions of different massi-to-charge ratio will assume different radii of travel and may therefore be successively focused on the ion collector.

Thepresent invention has as an object there of the provision of a method of mass spectrometry and an apparatus for the accomplishment thereof which involves the segregation of the ions of varying mass-to-charge ratios intospatially separated mass pulses which maybe selectively measured by an ion collector or target.

The above contemplated object isaccomplished according to my invention by the provision of an evacuated envelope, an electron gun to .ionize the molecules of the sample to be analyzed by the emission of an electron beam, pusher and accelerating electrodes to propel the ion beam toward a target or ion collector, means for applying time-spaced pulses to the electrodes to energize them in a pulsating fashion with the consequent formation of a pulse type ion beam collecting means comprising, an ion collector and electrical: measuring apparatus responsive to the currentat the ion c'ollector,.the collecting means being so synchronized with the ion beam forming circuit as to measure. only those mass pulses of ions of. a givenmass-to-charge ratio at a given voltage.

The invention may be more clearlyunderstood by reference to the following. detailed description thereof taken in relation to the accompanying-drawings in which Fig. l shows sthe mass spectrometer of the invention in diagrammatic form;

Fig. 2 showsa-modification-of the target or collecting-element ofFigi l in": which: a magnetic field has been'interposed between the ion source and the ion target or collector;

Fig 3 shows diagiamm'atioally the generat'ion of the ion pulses and certainofthe-mathematical functions involved therein;

Fig. at shows a modified arrangement of the pulse. forming and synchronizing system; and

Fig 5 showsa chart of the-Wave forms of the voltage to the sending and collecting circuits.

Referring to the drawings, thereiis shoWn i-n Figs. 1 and 4 a diagrammatic form ofiaa mass spectrometer in enclosed in-anenvelope WA. The spectrometer is equipped with ionizing-means, for example' an electron gun H Whichfires a beam l2 of electrons'toia catcher ii. The beam passes between apusher electrode 'l land ai first by a. conventional oscillator it"which ordinarily operates at constant frequency. This oscillator powers 2. pulse former 21-311mm is used in television and radar. Such an apparatus 'is' described in detail in an article entitled -)Eaoran indicator circuit operation by" David Davidson, in Electronics Industries of" March 1946. 'Ihe pulse former produces an al'ternating current of thewave form shown at ZZ in Fig. 5. Itwillflbe observed that for'most of a cycle time 'I,: the voltage is zero with 'square'waves' at 'I intervals. These waves are B vol-ts high andof t seconds in duration. This form of pulse is supplied =to a high voltage power supply 23 which=in turn supplies -.awave ot-the same form-to the'aoceleratingequipment. ot the mass spectrometer. Thus thehigh voltage power supply is connected across a potentiometer 24. The pusher is connected. at one end, the mid-accelerating electrode 15 at a midpoint, and the"fin'al' accelerating electrode lfi 'at the end of the potentiometer;

In this apparatus, the accelerating voltage is supplied for 15 seconds at intervals of T seconds, the result being the propulsion of ions through the aperture for only the fraction t/ T of the cycle. Heavy ions will travel slowly since their rate of travel for a given voltage is inversely proportional to the square root of their mass. In consequence of this action, the ions tend to separate into bunches in traveling toward the target. For example, in one method of operation, a bunch of light ions 25 is formed each cycle and the geometry, i. e. the distance of ion travel is such that the instant a bunch of light ions 25A is leaving the slit of the accelerating apparatus, the preceding bunch 25 has just reached the target.

Referring to Fig. 4, when we consider the balance of the apparatus, it will be observed that the pulse former 2i is directly connected to both the discharge and pickup circuits in which case synchronization is obtained by spacing the target and the accelerating slit such a distance apart that the target picks up the preceding pulse 25 of a given mass-to-charge ratio as a second pulse 25A is being projected by the electrode toward the target.

In the alternative embodiment as shown in Fig. 1, the constant frequency oscillator 28 is also connected through the delay network 29 and a second pulse former 21a to a circuit 26, the purpose of which is to energize an amplifier in synchronism with the ion accelerating circuit so that the amplifier is only active during the intervals necessary to pick up the particular bunches of the mass 25, 25a to be measured. The delay network 29 in the pickup system synchronizes the energization of the collecting means with the accelerating apparatus so that the energization of the former lags behind that of the latter an interval suflicient to allow travel of the pulse of the desired mass to the target.

Between the pulse former and the circuit 26 a bias 2'! is inserted. This does not alter the wave form of the output of the pulse former but does shift it with respect to the zero voltage as shown at 28 in Fig. 5. Thus the voltage is shifted so that the base line is negative.

The voltage from the bias 2'! is as indicated before fed to the selection circuit 26 which comprises a diode 30 and a triode 3|, the latter being provided with a conventional biassing apparatus 32 comprising a bias battery in series with a resistance. When the diode becomes non-conductive the grid of the triode will be energized and the pulse created by the ion mass 25 impinging on the target will be transmitted through the triode 3! to the wide band amplifier 40 and the peak vacuum tube voltmeter 4| The selecting circuit thus comprises a diode 30, a triode 3|, and a conventional biassing arrangement for the triode comprising a bias battery in series with a resistor, the bias apparatus being indicated at 32. The target is connected to the plate of the diode and to the grid of the triode. The cathode of the diode is connected through a resistance to the lower end of R, and the cathode of the triode is connected to ground. The signal from the pulse former is applied to the cathode of the diode. Thus the diode 30 effectively shunts the resistor R. The plate of the triode is connected to a wide band amplifier 40 which in turn is connected to a peak vacuum tube voltmeter 4 I.

Ordinarily, when no pulse is sent out by the pulse former, the cathode of the diode is negative with respect to its anode, so that the diode is conductive and no signal is received by the amplifier, i. e. the current of the ions is shunted through the diode rather than producing a drop in resistor R. However, when a pulse is received, the cathode of the diode is made positive with the result that the diode becomes non-conductive and any ion current must flow through resistor R and produce a voltage to energize the grid of the triode.

To summarize, the amplifier circuit is synchronized with the voltage supply circuit so that selected separated quantum of the ions of a given mass-to-charge ratio are accepted by the amplifying circuit while the others are grounded.

The apparatus and its functioning will be understood more thoroughly in the light of the following mathematical analysis, wherein u is equal to the velocity of a given ion mass which in turn is proportional to the square root of the voltage V over the mass m.

Let it be assumed a pulse of ions is sent out at a voltage V. The pulse lasts for 15 seconds. The physical length of a pulse for a mass m is therefore U t t where K is a constant depending upon the physical units used. After a time T, each mass pulse has gone a distance of If T is large enough compared with t, the ion pulses will become spatially separated according to mass, 1. e., as shown diagrammatically in Fig. 3 wherein the spatially separated quantum of ions of dilfering masses m1, m2, m3, m4, m5, and me are illustrated as traveling between the starting position [6 and the target IT. The pulse generator comprising the oscillator 28 and the pulse former 2! and the amplification system 30 are also shown schematically in Figs. 1 and 4.

The following expressions can be developed by reference to the mathematical functions illustrated in Fig. 3:

Target distance d: KJ T At the target, the distance between trailing edges of adjacent bunches of masses equals Distance between adjacent edges of the adjacent masses equals KT K -Kt K mg m 7711 For purposes of resolution, the distances between the adjacent edges must be greater than the length of the bunch of mass m2; i. e.

are I 77742 wherefrom the following expressions may be developed:

aeeraa expresses the relation that must exist between the frequency as represented by T and the duration of the pulse t in order to obtain resolution between masses m1 and 1122. Thus for resolving ions of mass 50 and mass 51 The velocity at mass .53 is, for V=500 v.

: 4.4 X 10 cm./sec.

Suppose T=l sec., i. e. a frequency of 1 million cycles/sec, then the mass 50 pulse moves out 4.4 cm. in sec. If there is a target at this point, and the ion pulses are t seconds long in time, where (since m m T as from the point I 6 for the time delay network can beadjusted to provide a lag sufficient to accommodate substantially any distance of pulse travel.

If a wide range of masses is present, an ion of mass me which is four times as heavy as another ion of mass me will travel at exactly half the speed of ion 1). Such an ion is called a harmonic ion, and it is plain that the apparatus of Fig. 4 cannot distinguish between two such ions.

As one way of eliminating the dilliculty due to harmonic ions, it may be desirable to insert a lowresolution magnetic analyzer 58 between the ion accelerating apparatus and the target. In such a case as illustrated in Fig. 2 the mass pulses of harmonic ions will be deflected less by the magnetic field so that the harmonic ions are angularly separated from the desired ions.

Another method of eliminating the diificulty is shown in Fig. 1. Here, if the structural dimensions are the same as in Fig. 4, we may cut the frequency of operation in half, that is increase the period to a value T1=2T where T is the original period. A delay network 29 supplies the pulse to the selecting circuit 26 at a time T seconds after the sending pulse. Thus in efiect pulses of the same length and speed as before are sent out, but only half as often. If other harmonic masses are present, such that ma=mnu then a value T1= /1fi must be used, where ma is the heaviest ion present and ms the lightest.

I claim:

1. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the envelope, collecting means including a collector electrode, amplifier and sensing means for measuring ions impinging on the collector electrode, means for projecting the ions toward the collector electrode in mass pulses, and means for relating the operation of the projecting means and the collecting means to render the collecting means sensible only to ion pulses of a given mass-tocharge ratio.

2. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the evacuated envelope, collecting means including a collector electrode, amplifier and sensing means for measuring ions impinging on the collector electrode, accelerating electrodes disposed in the envelope for projecting ions toward the collector electrode in mass pulses, and means for relating the operation of the accelerating electrodes and the collecting means to render the collecting means sensible only to ion pulses of a given massto-charge ratio.

3. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the envelope, collecting means including a collector electrode, amplifier and sensing means for measuring ions impinging on the collector electrode, accelerating electrodes disposed in the envelope for projecting ions toward the collector electrode, a high frequency oscillator, a pulse former and a source of high voltage in circuit with the acoeler ating electrodes to project the ions in a pulsating beam toward the collector electrode, and means for relating the operation of the accelerating electrodes and the collecting means to render the collecting means sensible only to ion pulses of a given mass-to-charge ratio.

4. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the envelope, collecting means including a collector electrode, amplifier and sensing means for measuring ions impinging on the collector electrode, acceleratingelectrodes disposed in the evacuated envelope, a high frequency oscillator, a pulse former and a source of high voltage in circuit with the accelerating electrodes whereby the accelerating electrodes may be caused to project ions in a pulsating beam toward the collector electrode, the collecting means being in circuit with the high frequency oscillator and said pulse former whereby operation of the accelerating electrodes andcollecting means is related so as to render the collecting means sensible only to ion pulses of a given mass-to-charge ratio.

5. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the envelope, collecting means including a collector electrode, amplifier and sensing means for measuring ions impinging on the collector electrode, means for projecting ions toward the collector electrode in mass pulses, means for establishing a transverse magnetic field within the space separating said means for projecting ions and the collector electrode, and means relating the operation of the projecting means and the collecting means to render the collecting means sensible only to ion pulses of a given mass-to-charge ratio.

6. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the envelope, collecting means including a collector electrode, amplifier and sensing means for measuring ions impinging on the collector electrode, accelerating electrodes disposed in the evacuated envelope, a high frequency oscillator, a pulse former and a source of high voltage in circuit with the accelerating electrodes so that the accelerating electrodes will be caused to project ions in a pulsating beam toward the collector electrode, means for establishing a transverse magnetic field in the space separating the accelerating electrodes and the collector electrode whereby ion pulses of a given mass-to-charge ratio may be deflected toward the collector electrode, and means for relating the operation of the accelerating electrodes and the collecting means to render the collecting means sensible only to ion pulses of a given mass-to-charge ratio.

7. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the envelope, collecting means including a collector electrode, amplifier and sensing means for measuring ions impinging on the collector electrode, accelerating electrodes disposed in the evacuated envelope, a high frequency oscillator, a pulse former and a source of high voltage in circuit with the accelerating electrodes whereby the accelerating electrodes may be caused to project ions in a pulsating beam to the collector electrode, means in association with the collecting system to retard the energization thereof with respect to the accelerating electrodes for an interval sufficient to permit a given ion pulse to travel from the accelerating electrodes to the collector electrode.

8. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the envelope, a collector electrode for measuring ions, accelerating electrodes disposed in the evacuated envelope, a high frequency oscillator, a pulse former and a source of high voltage in circuit with the electrodes whereby the electrodes may be caused to project ions in a pulsating beam toward the collector electrode, the collector electrode being in circuit with the high frequency oscillator, a time delay network, a second pulse former, an amplifier and sensing means whereby operation of the amplifier and sensing means and the accelerating electrodes are related so as to render the sensing means sensible only to ions of a given mass-to-charge ratio.

9. A mass spectrometer comprising means for ionizing a sample, a substantially straight tube associated with the ionization means, collecting means comprising an ion collector electrode disposed in the straight tube opposite the ionizing means, and an amplifier and sensing means, accelerating electrodes disposed adjacent the ionizing means for projecting ions toward the collector electrode in mass pulses, and means for relating the operation of the accelerating elec trodes and the collecting means so as to render the collecting means sensible only to those ion pulses of a given mass-to-charge ratio.

10. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the evacuated envelope, collecting means including a collector electrode, an amplifier and sensing means for measuring ions impinging on the collector electrode, accelerating electrodes for projecting ions from the means for ionizing a gas sample toward the collector electrode, means for impressing a pulse-type potential on the accelerating electrodes so as to project the ions toward the collector electrode in mass pulses, means for making the collecting system sensible to ion discharge on the collector electrode for spaced periods corresponding in duration to the period of the pulses impressed on the accelerating electrodes, and means for relating the accelerating electrode and collector periods so as to render the collecting system sensible only to ion pulses of a given mass-to-charge ratio.

11. In a mass spectrometer the combination which comprises an evacuated envelope, means for ionizing a gas sample introduced to the avacuated envelope, collecting means including a collector electrode, an amplifier and sensing means for measuring ions impinging on the collector electrode, accelerating electrodes for projecting ions from the means for ionizing a gas sample toward the collector electrode, means for impressing a pulse-type potential on the accelerating electrodes so as to project the ions toward the collector electrode in mass pulses, means for making the collecting system sensible for spaced periods corresponding in duration to the period of the pulses impressed on the accelerating electrode, and means for relating the periods of the accelerating electrodes and the collecting system so that the instant a pulse of given mass-tocharge ratio is propelled from the accelerating electrodes the immediate preceding pulses of ions of the same mass-to-charge ratio will strike the collector electrode and be sensed by the collecting system.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,948,384 Lawrence Feb. 20, 1934 2,331,189 Hipple Oct. 5, 1943 2,387,550 Winkler Oct. 23, 1945 OTHER REFERENCES SmythAtomic Energy for Military PurposesPrinceton University Press 1945, pp. 197, 198. (Copy in Div. 54.)

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