US2780728A

US2780728A - Mass spectrometry

Info

Publication number: US2780728A
Application number: US408879A
Authority: US
Inventors: Robert V Langmuir
Original assignee: Consolidated Electrodynamics Corp
Current assignee: Consolidated Electrodynamics Corp
Priority date: 1954-02-08
Filing date: 1954-02-08
Publication date: 1957-02-05
Anticipated expiration: 1974-02-05

Description

Feb. 5, 1957 Filed Feb. 8,

R. V. LANGMUIR MASS SPECTROMETRY 2 Sheets-Sheet l /NPUT GA T/NG S/GNAL H. v. 7 GA TED POWER SUPPL r AMPL/F/ER 7 PULSE PULSE 7 0 SENS/N6 FORMER FORMER MEA NS v. F. DELAY OSC/LLATOR L /NE /2 I LL? I:

IN V EN TOR.

ROBERT M. LANGMU/R BY yw /7%% A T TORNE V Feb. 5, 1957 R. v. LANGMUIR 2,780,728

MASS SPECTROMETRY Filed Feb. 8, 1954 2 Sheets-Sheet 2 AMPLIFIER h. 1 POWER SUPPLY PULSE 1 F FORMER OSCILLATOR F G. 5. F/ 6 6.

IN V EN TOR. ROBERT V. LANGMU/R ATTORNEY MASS SPECTROMETRY States Patent Robert V. Langmuir, Altadena, Califl, assignor, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Application February 8, 1954, Serial No. 408,879

3 Claims. (Cl. 250-413) This invention relates to mass spectrometry and more particularly to an improved instrument of the type generally referred to as a time of flight mass spectrometer.

A mass spectrometer is an analytical instrument which functions to ionize a sample to be analyzed, to spatially separate these ions in accordance with their mass-tocharge ratio and to selectively sense the relative abunis the time of flight mass spectrometer. Instruments of this type as described in the art generally comprise an ion source, a drift tube and a collector electrode. Means are provided in the ion source to project ions into and through the drift tube in mass pulses whereby ions of differing mass segregate themselves in the drift tube into spatially separated ion bunches with each bunch com prising ions of a given mass-to-charge ratio and dilfering from the mass of ions forming the other bunches. These ion bunches strike and discharge at the collector electrode.

One conventional way of resolving the ion bunches is to gate the amplification and sensing system connected to the collector electrode so that it becomes sensitive only to pulses formed by the successive discharge of ion bunches of a given specific mass. This can be done by interconnecting the pulsing means by which the ions are projected from the ion source and the amplification and sensing system of the collector electrode to determine the proper time sequence of sensitivity of the collector system. A second means of resolving the discharges of the collector electrode is by feeding the amplified output of the collector electrode to a conventional cathode ray tube and adjusting the sweep of the tube so that only the output pulses developed by successive discharge of ion bunches of a given specific mass are displayed on the 1 By reason of the pres-' abundance of the ions in any one bunch. The present invention is directed to improvements in a mass spectrometer of this type whereby this problem of induced currents is overcome. To this end the invention contemplates in a mass spectrometer the combination comprising an evacuable envelope, an ion source, means associated with the ion source for propelling ions from the .source in mass pulses, a collector electrode disposed in the envelope at a distance from the source and in the path of ions propelled from the source, means operatively associated with the collector electrode to permit resolution of ion masses discharged at the collector electrode,

and an electrostatic shield disposed closely adjacent the collector electrode.

The invention will be clearly understood with reference to the following detailed description taken in conjunction with the accompanying drawing, in which:

Fig. 1 is a diagrammatic illustration of a mass spectrometer in accordance with the invention;

Fig. 2 is a diagram of a simple electrostatic circuit illustrating the problem to which the invention is directed;

Fig. 3 is a graph of current flow with time in the electrostatic circuit of Fig. 2;

Fig. 4- shows the mass spectrometer of Fig. 1 provided with a cathode ray display means and without the gating circuit;

Fig. 5 shows a type of signal which may be obtained on a cathode ray display means without the electrostatic shield of the invention; and

Fig. 6 shows the type of signal produced on a cathode ray tube by the illustrated mass spectrometer provided with the electrostatic shield.

Referring first to Fig. 2, the circuit there shown comprises a battery it) connected between electrostatic plates 11 and 12. A charged particle 13 is shown in the region between the plates 11 and 12 and, as indicated by the accompanying arrow, is considered to be in motion toward plate 12. To move the particle 13 a distance x in time t the rate of work W done by the battery 10 is given by the expression W=iV 1 where W=work done by battery;

V=the voltage between the plates 11 and 12; and i=the current flow in the circuit.

The rate of work done on the particle is given by the expression v=the velocity of the particle;

d=distance between the plates (in cm.);

x=a linear coordinate measured between plates 11 and t=the time coordinate; and

V=the voltage between the plates.

Equations 1 and 2 above must be equal so that i=qv/d where v=dx/dt This current i as given'by Equation 3 actually flows in the circuit while the particle is crossing the gap. This current flow is a result of the induced charges on the two plates, which change with time as the particle moves across the intervening region. The shape of the current pulse in this simple example can be shown to be as illustrated in Fig. 3, i. e. substantially triangular because of the uniform increase in velocity as the particle traverses the gap. This subject is discussed in detail in Vacuum Tubes, K. R. Spangenberg, McGraw-Hill, 1948.

The foregoing description illustrates the problem encountered in a time of flight mass spectrometer. Thus, a sharply defined bunch of charged particles will not give a sharp current pulse on arrival at the collector electrode. This effect will be appreciated with reference to the following description of a mass spectrometer in accordance with the invention as illustrated diagrammatically in Fig. l. The description is based upon the motion and detection of positive ions. Obvious variations in potentials within the instrument will permit operation with negative 'dotted" lines since the construction of theenvelope-and the usual evacuating and sample inlet'conduits are unimportant to an understanding of the present invention. An

ion source is disposed adjacent one, end of the envelope and includes a conventional electron gun for directing an electron beam 16 transversely of the ion source region to anelectron target 17. .A number of

electrodes

18, 19. and 20 aredisposed serially in the tube with the of the electron beam 16. A collector electrode 21 'is disposed adjacent theopposite end of the tube. In accordance with the present invention an electrostatic shield 22 is disposed immediately in front of the collector electrode .21 and, as .illustrated,-is biased with a battery 23.

The external circuitry of the instrument is shown diagrammatically as comprising a variable frequency oscillator 24,.a pulse former 25 and a high voltage power supply. 26 connected serially, with the output of the high electrodes 18 and 19being. oriented on opposite sides,

voltage power supply being connected across a voltage divider 27. The

ion source electrodes

18, 19 and Marc connected to the voltage divider 27 so that a pulsed aecelerating potential is applied to these electrodes whereby ions are propelled from the source in mass pulses in synchronism with the applied pulse.

The variable frequency oscillator 24 is connected 7 through a delay line 28 and a second pulse former 29 to deliver a gating signal to a gated amplifier 31. Collector clectrode 21 is connected to the amplifier 31, the output of the amplifier being connected to' a conventional sensing system (not shown). The purpose of gating amplifier 31 by means of the oscillator 24, delay line 28 and pulse former 29 is to match the amplifier with the ion accelerating circuit so that the amplifier is sensitive only during the intervals necessary to pick up successivebunches of. ionsofagiven mass.

Ions propelled from the ion source in mass pulses segregate in the drift tube, this being the region between the source and the collector electrode into spatially separated bunches, each bunch consisting of ions of a given mass- 7 all of the other ion bunches. To resolve the given mass,

cnergization of amplifier 31 is matched through the'described circuit withthe pulsed operation of the ion source so that the amplifier is rendered sensitive only in those intervals at which successive bunches of the ion mass of interest strike the collector.

Delay line'28 causes theperiods of amplifier energization to lag behind the pulses applied to the ion source by an interval suflicient to allow travel of the ion bunch of thedesired mass to the collector.

Resolution of'ion masses in this type of an instrument is exceedingly diflicult if theelectrostatic shield 22 is absent. The reason for this difiiculty is made apparent in the discussion 'of Figs. 2'and'3. The movement of ions in theldrit't tube of the spectrometer induces a current in collector electrode which increases in magnitude as the ions approach this electrode. Because of the fact that; there are a number of ion bunches traversing the drift tube at a given time, this induced current is often unresolvablc. I have found. that collector electrode 21-can be conveniently masked from the effects of such induced currents by meansof the shield electrode 22 disposed immediately adjacent thereto. If the shield is kept at "approximately the same potential as the collector electrode the current induced by the movement of charged particles in the drift tube is absorbed in effect by the mask, and the induced current on the collector electrode will be extremely small until a particular bunch traverses the shield 22. Immediately upon traversal of the shield an ion pulse willinduce a largeand sharp current in the collector as it traverses the narrow region between the shield-and theicollector. For this reasonit is desirable that theshield be spaced from the collector a distance less than the spatial separation between adjacent ion bunches, 'A spacing of approximately .005 of ani nch is satisfactory for this purpose and is practical from a fabrication standpoint.- W

The mass spectrometer of Fig. 1 is shown diagrammatically in Fig. 4 connected to a cathode ray tube display system.- Like reference characters are used inFig. 4 to 'designateelements' common to Figs. 1 and 4. In the apparatus of Fig, 4, collector electrode 21 is connected 'directlyfthr'ough; an amplifier 32 to one set of deflecting platesofa cathode ray. tube 40. The illustrated connectionshows the output of amplifier 32 connected across the. vertical deflection plates of the cathode ray tube.

The horizontaldeflection plates are connected across f pulse former 29 so that the horizontal sweep of the tube is matched with the pulsed acceleration from the :ion source. .A' sequence of ion bunches A, B, C and D is shown in its approach to the collector electrode.

Without the-electrostatic shield 22, these ion bunches 'wiIlproduceat collector electrode 21 a signal which, whendisplayed on acathode ray tube, constitutes the aggregate ofthe several signals A, B, C and D in the drawing. lFor purposes of illustration it is assumed that ion bunches A, B, Cand D are all present in thesame proportions,. which causes the respectively developed diselectrostatic shield 22. The actual current signal on the tube.(curve '33) is the sum of the illustrated component signals. p

Fig. .6 is another view of a cathode ray tube showing by-curve 34 the type of signal obtained in the apparatus .ofIFig. 4 .in..the presence of the electrostatic shield 22.

By eliminatingove'rlapping induced currents, the shield permits display of a resolved signal as displayed in Fig. 6

wherein the individual peaks produced by discharge of successive ion masses on the collector electrode are observed separately.v

I claim:

1. In a mass spectrometer the combination comprising an evacuable envelope, an ion source, a collector electrodedisposed in the envelope and spaced from the ion source, means associated with the ion source for propel: ling ions from Ithesource toward the collector electrode .in masspulses, means operatively associated with'the collector electrode tosense ion masses discharged at the collector electrodeand an electrostatic shield'disposed adjacent the face of thecollectorelectrode at a distance ther efrornnotin excess of the approximate spacing of adjacent ion masses inthe region of the shield. I

2. Ina mass spectrometer thc'combination comprising an evacuableenvelope, an ion source, a collector electrodedisposed in the envelope and spaced from the ion source, means "associated with the ion source for propelling. ions from the source toward the collector electrode in masswpulses, means operatively associated with the collector electrodeto sense ion massesdischarged at'thc collector electrode, an electrostatic shield disposed adjacent the face of the collector electrode and spaced therefrom such that at no time shall more than one mass pulse have a capacity coeflicient to the collector substantially greater than zero, and means for maintaining the electrostatic shield at substantially the potential of the collector electrode.

3. In a mass spectrometer the combination comprising an evacuable envelope, an ion source, a collector electrode disposed in the envelope and spaced from the ion source, means associated with the ion source for propelling ions from the source toward the collector electrode in mass pulses, means operatively associated with the collector electrode to sense ion masses discharged at the collector electrode, an electrostatic shield disposed adjacent the face of the collector electrode and spaced therefrom such that at no time shall more than one mass pulse have a capacity coeflicient to the collector substantially greater than zero, and means for maintaining the electrostatic shield at a potential which is negative with respect to the potential of the collector.

References Cited in the file of this patent UNITED STATES PATENTS