US2921198A - Mass spectrometer - Google Patents

Description

Jan. 12, 1960 N. WARMOLTZ ETAL 2,921,198

MASS SPECTROMETER Filed Feb. 10, 1954 6 2 ,7 Me 30 l6 OSCILLHTOR FREQUENCY METER ELECTRON GUN / BY LSRAELPELCHOWITCH United States Patent MASS SPECTROMETER Nicolaas Warmoltz, Eindhoven, Netherlands, and Israel Pelchowitch, Paris, France, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Application February 10, 1954, Serial No. 409,448

Claims priority, application Netherlands February 13, 1953 6 Claims. (Cl. 250--'41.9)

This invention relates to devices for analysing substances or indicating the presence of a small amount of a substance, in which the gasified substance is ionised, and the ions under the action of an electric field are caused to traverse a path, at the end of which they strike a collector. This process takes place in mass spectrometers.

Mass spectrometers of this kind are known in which the ions, after being given a certain speed, are subject to the action of a magnetic or electric field, or both, with the result that they are deflected out of their path and this more strongly according as the mass of the ions is smaller and their charge is higher. As a rule, this kind of mass spectrometer is constituted by a semi-circular tube, one extremity of which contains an ionisation device and the other extremity contains the members required for separating the ions according to their masses. In such mass spectrometers in which a geometric separation between the ions having different masses takes place use is made of a plurality of screens limiting the ion beam. Said screens bring about a loss of intensity of the ion beam.

Furthermore, mass spectrometers are known which operate without magnetic fields and in which differences inthe transit timesv of the ions are utilised for separating the ions having different masses. They operate with gauze-like grids, which alsoresults in a loss of intensity of the mean ion-current. In such mass spectrometers ion-current pulses at the collector are measured, the measurement being eifected only during a small part of the available time, which also involves a certain loss.

The object of the invention is to provide a device which does, not exhibit some disadvantages of known devices and which nevertheless is simple and easy to operate. Theinvention consists in that provision is made made of means for modulating the ion current and of means for measuring the phase diflerence of the modulation at two points of the ion path located at a given mutual distance.

The ion current is preferably modulated sinusoidally in intensity, since for this case devices are available by which therelative phase may be determined in a simple manner. The modulation may be effected by means of a griddike electrode provided in the vicinity of the ion source or, which ismore favourable, by means of a grid modulating the electron current of a cathode, which electron current brings about ionisation of the gas molecules. The phase difference of the modulation may best be measuredbetween the modulating electrode and the collector, which electrodes for this purpose are connected to the input circuit of the device for measuring the phase difference.

It has been found that use may successfully be made of. an electronic device for measuringthe relative phase as described in a. copending US. application, Serial No. 369,821, filed July 23, 1953. The device concerned comprises a closed circuit in which two voltages. are produced, thefrequencies of which differ from those of the voltages between which the phase is to be determined. Said circuit comprises in succession a first modulator in which one of the voltages provided by the device is modulated by one of the other voltages, a filter which passes only voltages of sum or difference frequencies of the, modulating voltages, a second modulator in which the passed voltage is modulated by the second of the voltages provided by the device, and a second filter which passes a voltage of difference or sum frequency of the voltages supplied to the second modulator and of which the output. voltage is supplied again to the first modulator. In general, then, the two voltages or signals whose phase difference is being measured are each modulated in their own modulating circuit by the output signal from the. other modulator.

The frequency or the frequency variation in at least one of the voltages occurring in the circuit is measured and from this the phase difference may be deduced by a single calculation. Thus, the measurement actually consists in measuring a frequency or a variation in frequency.

A device which is even simpler is obtained if the voltage set up at the collector, after being amplified is an amplifier which passes a broad range of frequencies, is supplied to the modulating grid. It is thus possible to obtain a self-oscillating device in which the frequency of the generated oscillation is connected with the transit time of the ions in the tube.

In order that the invention may be readily carried into effect, it will now be explained with reference to the accompanying drawing showing diagrammatically, by way of example three embodiments of devices according thereto.

Fig. 1 is a cross-sectional view of the tube in the axial direction.

Fig. 2 shows a similar cross-section along the axis of, the tube with the connections between the electrodes shown.

Fig. 3 shows a second embodiment.

Fig. 4 shows a modification of the embodiment illustrated in Fig. 2.

In the figures, reference numeral 1 indicates the vessel of the mass spectrometer which comprises a lateral space 2. The vessel contains, at one extremity, a collector electrode 3 which may be connected to earth by way of an impedance 18. In front of the electrode 3 there may be provided a grid 20 which is supplied with a low volt age with respect to earth by a voltage source 22. The other extremity contains, at the area of the portion 2, an accelerating electrode 4 which is supplied with a very constant positive voltage with respect to earth by a voltage source 10, which voltage is preferably adjustable. Furthermore there is provided at this area, on the side facing the collector electrode 3, a grid 5 which is conductively connected to the electrode 4. The space between the said electrodes is the ionisation chamber in which ions are produced, which ions possess a positive charge. The ions herein also are accelerated and thus acquire a certain speed to move towards the collector 3 by reason of the attractive forces exerted by the grounded electrodes 19 and 3. The portion 2 contains a cathode 8 having the capacity of emitting electrons, a control grid 7 for the electron current emitted by the cathode, and two screen-like electrodes 6. The control grid 7, as a rule, requires a low negative potential with respect to the cathode, which is provided by either a voltage source 21 or the parallel combination 23 of a] resistor and a capacitor in the cathode lead. The screen electrodes 6, the number of which may be chosen comparatively at will, are connected to tappings on a potentiometer 9 having connected in parallel with it1a voltagesource 11, of which the positive terminal is connectedito.

a terminal ofthe source 10 and the negative terminal is connected to the cathode, so that the said electrodes acquire a positive potential with respect to the cathode. They exhibit slit-shaped apertures, the slits being directed at right angles to the axis of the tube, so that a flat electron beam enters into the space between the electrodes 4 and 5 and ionisation can herein take place over a large surface area thereof. The tube 1 also comprises couplings 35 and 36 for the supply and discharge of the diluted gas which is to be examined to the ionization space between the electrodes 4 and 5.

The electron current provided by the cathode 8 is modulated by means of an oscillator 17 in which a highfrequency alternating voltage of, for example, 10 c./s. is produced. This also results in modulation of the ion current propagating from the space between the electrodes 4 and Stewards the collector 3. The oscillator 17 is preferably so constructed as to provide a sinusoidal voltage.

T 'Due to finite transit-time of the ions in the tube 1, the sinusoidal alternating voltage set up at the collector 3 when the ions are collected is shifted in phase with respect to the alternating voltage at the grid 7 and the modulation of the ion beam at the area of the electrode 5. Y The difference in phase is dependent upon the transittime of the ions between the electrodes 5 and 3 and hence is a measure of the mean mass of the ions.

The difference in phase between the voltages at the electrodes 7 and 3 is measured with the use of an electronic phasemeter 12, such as described in the aforementioned US. application, Serial No. 369,821. It has been found that for this purpose use may excellently be made of a phasemeter comprising a closed circuit which comprises in succession amodulator M 15, a filter F if desired combined with an amplifier 13, a second modulator M 16, and a filter F 14 which may likewise be combined with an amplifier. The phasemeter operates as follows. An input signal from the collector 3 is fed to an input of the modulator M and the other input signal from the grid 7 is fed to an input of the modulator M Since the elements 13 to 15 are arranged in a circulatory fashion, the assembly constitutes an oscillator circuit which oscillates at a frequency different from the frequencies of the input signals. Thus, present in each modulator M and M is this new frequency. The filter F is tuned to pass the sum-frequency of the modulated output signal of modulator M and the filter F is tuned to pass the difference-frequency of the modulated output signal of modulator M If desired, the filters F and F may be interchanged without affecting the operability of the circuit. Thus, each of the input signals is modulated with a different modulation component of the output signals. These modulation components are oscillatory signals having frequencies which vary in accordance with variations in the phase difference between the two input signals. When the phase difference varies, the frequencies being generated also vary. The phase angle or difference may thus be determined by utilizing a frequency meter 30 connected to measure the frequency of one of the modulation components produced.

'It is alternatively possible to utilize a grid-like electrode, for example, in which the ions during their passage induce a voltage and to measure the phase difference between the said electrode and the electrode arranged at the beginning of the path of ions. This is illustrated schematically in Fig. 4, wherein the output signal is derived from a grid located in front of the collector 3, the signal appearing across an impedance 37 coupled to the grid 20'. In this case also, the ion current itself, rather than the electron current, is modulated. This is done by coupling a sinusodial oscillator 17' to a grid 19' mounted in front of the electrode 5. The transit time is-"measured by connecting a phasemeter 12 between the input and output signals available at the grids 19""and '4 20', respectively. The electrode structure producing the electron stream is shown schematically in this figure. Other elements in this figure, which remain unchanged, bear the same reference numerals as before.

In the described devices each ion contributes to the total phase angle. If the constituents of a gaseous mixture are to be identified, the whole of the mass spectrum can be found by carrying out several measurements. It

- with correct phase and strength back to the electrode 7,

' the device may be made self-oscillating at a frequency which is connected with the transit-time of the ions. Fig. 3 shows a device which operates on this principle. This tube may be designed in the same manner as in the embodiment shown in Figs. 1 and 2, but now the voltage set up at the electrode 3 is supplied to either the grid 7 or a grid provided in the path of ions, preferably the grid 19, by way of an amplifier 24 which passes a broad range of frequencies and which is so designed that the phase difference between the input and output voltages is sub-.' stantially not dependent upon frequency. By suitablechoice of the amplification it may be ensured that oscilla-'.

tions are produced by the device itself. Since in a closed circuit the total phase shift is invariably required to be a whole multiple of 360, the frequency produced is dependent upon the transit-time of the ions between the electrodes 5 and 3. At constant accelerating voltages the frequency is thus dependent upon the kind of the gas and it will be possible to ascertain the kind of the gas by measuring the frequency by means of a frequency meter 31. By carrying out again several measurements at different accelerating voltages at the electrode 4, it will be possible to determine the composition of the gas in the tube. Since the entire system oscillates at a frequency determined by the transit time of the ions, a

separate oscillator, such as 17 in Fig. 2, is unnecessary,

since any instability in the system would set it into oscillation. It is in this case desirable that the pressure of the gas should be maintained constant, since it determines the intensity of the ion current and hence the amplitude of the voltage, at the electrode 3. The phase is substantially independent of the intensity of the ion current. In order that the alternating voltage at the electrode 7 may be maintained substantially constant, it is alternatively possible for the amplifier 24 to be provided with an intensity control known as such.

The devices described can be used not only for analysing gases, but also as leak detectors.

What is claimed is:

1. A mass spectrometer comprising means for ionizing a substance at one end of a path, means for collecting the thus-produced ions at the opposite end of the path, means for trajecting the ions along the path thereby to establish a current of ions, means for modulating the intensity of the ion current including means producing a modulating signal which varies continuously about an average level and in which the time intervals of the portions of the signal on opposite sides of the average is about the same and means utilizing said signal to modulate the ion current, and means for measuring the phase 'ditference between the modulated ion current at two spaced points along the path.

2. A mass spectrometer as set forth in claim 1 wherein Y the modulating means produce sinusoidal modulation of the ion current.

trons, means for sinusoidally modulating said electron beam, means for impinging said beam upon an ionizable gas to ionize the same, means for trajecting the thusproduced ions along a given path, means at the other end of the enclosure for collecting said ions, and phasemeasuring means connected between the modulating and collecting means.

4. A mass spectrometer as set forth in claim 3 wherein the trajecting means for the ions are adjustable whereby the ions may be propelled with different velocities along the given path.

5. A mass spectrometer comprising means for ionizing a gas at one end of a path, means for trajecting the ions along the path, means for collecting the ions at the end of the path, means for modulating the intensity of the ion current at the beginning of the path, means coupling the collecting and modulating means for causing the spectrometer to oscillate at a frequency determined by the phase difierence between the modulated and collected ion current and means for measuring the oscillating frequency.

6. A mass spectrometer comprising means for ionizing a gas, means for trajecting a beam of said ions along a given path, means for modulating the ion beam, means for deriving from the modulated ion beam an output signal, positive feedback means coupling together the deriving and modulating means thereby to cause the spectrometer to oscillate at a frequency determined by the transit time of the ions along the path, and means for determining said oscillating frequency and thus determining the mass of the ions.

References Cited in the file of this patent UNITED STATES PATENTS 2,582,216 Koppius Jan. 15, 1952 2,612,607 Stephens Sept. 30, 1952 FOREIGN PATENTS 20 1,024,654 France Jan. 10, 1953

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