US2587555A - Mass spectrometer - Google Patents

Description

Feb. 26, 1952 Filed Oct. 26, 1948 E, A. WEISS MASS SPECTROMETER 4 Sheets-Sheet l FIG.

AMPLIFIER PM M AMPLIFIER PHASE ADJUSTER INVEN TOR. ERIC A. 14 5/55 BY dag? ATTORNEY v Feb. 26, 1952 WEISS 2,587,555

MASS SPECTROMETER Filed Oct. 26, 1948 4 Sheets-Sheet 2 76 as P 84 8O F/GIZ F FIG. 4'

1 INVENTOR. ERIC A, wuss ATTORNEYS Feb. 26, 1952 I w ssw 2,587,555

MASS SPECTROMETER Filed Oct. 26, 1948 4 Sheets-Sheet 3 174 INVENTOR. ERIC A, WEISS ATTORNE S Feb. 26, 1952 w ss 2,587,555

} MASS SPECTROMETER Filed Oct. 26, 1948 4 Sheets-Sheet 4 FIG. 7.

INVENTOR.

ATTORNEYS ERIC .A. WEISS Patented Feb. 26, 1952 I v MASS SPECTBOMETER Eric A. Weiss, Springfield Township, Delaware County, Pa., assignor to Sun Oil Company, Philadelphia, Pa., av corporation of New Jersey Application October 26, 1948, Serial No. 56,571

7 Claims. (o1. 250F413) This invention relates to mass spectrometers and has particular reference to the provision of means for increasing stability and amplification of the output with attendant effective suppression of noise.

Aspresently constructed the ion stream of a mass spectrometer gives riseto a potential on the. collector plate which is subjected to direct current amplification to operate an indicator or recorder. The potential signal appearing at the collector plate contains a quite substantial noise component arising in the ionizationchamber and to this noise component there is contributed further noise in the amplifier with. the result that the relatively weak desired signals at the collector plate are submerged in such a considerable noise background as to be difficult to observe accurately at the output of the apparatus. The amount of amplification which may be effectively usedis also. relatively'small. The mass spectrometer further is somewhat unstable in operation and this instability makes accurate measurements with the conventional system quite difficult.

In accordance with the present invention the ion stream is chopped to produce pulses of ions or a modulated stream of ions instead of a continuous stream thereby giving rise to a pulsating signal at the collector plate. The alternating component of this pulsating signal is subjected to alternating current amplification and is then subjected to synchronous rectification, the synchronism being with respect to the frequency of chopping or modulation. A resulting direct. signal is thus obtained, this being of an integrated nature involving very substantial suppression of noise'and smoothing out of the unstable fluctuations which appear in the ionization apparatus. Preferably, the synchronous rectification of the signals. is effected electrically and the chopping or modulation is also effected electrically to avoid other difficulties which arise if mechanical systems for these purposes are used. Various alternative systems, however, may be used with attendant advantages within the scope of the invention.

The broad objects of the invention will be evident from the foregoing. These and other objects particularly relating to details of construction and operation of the apparatus will become apparent from the following description read in conjunction with the accompanying drawings in which:

Figure 1 is a diagram illustrating a preferred embodiment of the invention which involves modulation of=the.. ion beam through the use. of

. 2- V a -magnetic field and a synchronousrectifying system of a preferred type; I

Figures 2, 3 and- 4' are diagrams illustrating alternative synchronous rectifying systems of preferred types;

Figures 5, 6 and '7 are further diagrams illustrating other synchronous rectifying systems which may be employed to attain the objectsof the inventiomand.

Figures 8; and 9 are diagrams illustrating certain other modes of modulation of the ion stream, these specifically illustrating modulation of the ion stream through the modulation of the electron stream which gives rise to the ions.

Reference will be made first to Figure 1 which illustrates the preferred embodiment of the invention. The mass spectrometer tube is of conventional type comprising a gas-receiving tube 2 which is provided with an opening communicating with the source of gas which is to be subjected to analysis. As is conventional this tube has a curved form and is located between magnetic poles indicated atl which produce a magnetic field in a direction parallel to the axis of curvature of the tube for the purpose of imparting curved paths to the ions which originate in the ionization chamber at one end of the tube. The ionization chamber is of conventional type,

' indicated diagrammatically, and need not be described in detail. It includes an electron source illustrated as a filament 6 associated with an electron beam-defining' electrode 8, an anodeto to which the electron beam flows and electrodes l2 and I4 for the formation of the ion beam which enters the slit IS in the curved tube -l8 and emerges throughrthe exit slit 2!! to impinge upon a collector. plate 22. Other factors being con; stant there may be secured, by control of the magnetic field; the selection, from all of the ions entering tube l8 at IS, the particular ions which it is desired to measure, these emerging through the slit20 and producing a potential at the anode 22.

In accordance with conventional practices the signals appearing as potentials at the collector plate 22 would be subject to direct current amplification to control an indicator or recorder. These signals include a high level of noise to which further noise is added in the direct current amplifier; thedegree of amplification attainable by this system is limited by the total noise appearing at its output. 1

In accordance with the invention the theoretically steady ionbeam ordinarily produced is chopped or modulated by the provision ofa mag,-

netic field produced by a pair of coils 24, this field being preferably at right angles to the field provided by the poles at 4 and serving to deflect the beam upwardly and downwardly at right angles to the deflections produced by the main magnetic field. By the application, preferably, of square waves of current to the coils 24 the beam during alternating half cycles is caused to emerge from the slit 20, while during intermediate half cycles it is so deflected as to strike the walls of the curved tube l8 and so fail to emerge from the slit. There will now be described a typical apparatus for providing this type of modulation or chopping and also to provide a synchronous rectification of the signals produced at the collector plate 22.

There is provided at 26 an oscillator which may be, of any conventional type but is illustrated as an oscillator of phase shift type having an output at any frequency which it is desired to use. There is a wide choice for this frequency and the only considerations involved in this choice are those, for example, of avoiding frequencies which may possibly synchronize with disturbing frequencies arising from power lines or other sources of disturbance or frequencies which are so high as to involve periods comparable to the transit times of ions in the beam. Generally speaking, frequencies in the usual audio range are very suitable. The output of the oscillator 26 is amplified by a tube 28 which feeds a phase splitting circuit comprising the triodes 30 and 32 and condenser 34, the push-pull output of which circuit is delivered through the connections. 36 and 38. A limiter system is provided by a. pair of dual diodes 40 and 42 having connections as illustrated to a series of resistances 43 supplied with direct current. This system supplies a, rectangular wave of substantially constant amplitude through the condenser 44 to the amplifier 46 which is desirably of the automatic volume control type to maintain constant amplitude of its output. The output of the amplifier 46 is fed through a conventional phase adjusting network 48 to energize the coils 24. It may be here'remarked that this phase adjusting network may be located elsewhere in the system as, for exextremely high discrimination of desired signals against noise and against alternating signals difample, following the amplifier whichreceives the signals from the collector plate 22, or at the output of the oscillator 26, or of the amplifier tube 28.

The amplifier 45 for the signals from the collector plate 22 may be of conventional alternating current type. This amplifier delivers its output through line 41 and through the pair of resistances 5 2 and 54 of the anode of a diode 49 and to the cathode of a diode 50, respectively. The cathode of the diode 49 and the anode of the diode 50 are respectively connected through condensers 5'6 and '58 to the lines 36 and 38, from which they receive rectangular waves, and to them respectively are connected the resistances and 62, the remote ends of which are connected together to the indicating or recording meter 66 shunted by the condenser 84. 'For best operation the resistance 52 should be equal to the resistance 54 and the resistance 60 should be equal to the resistance 62. As disclosed and discussed in greater detail in the application of Elbert Neil Shawhan, Serial Number 46,225, filed August 26, 1948, for Selective Circuits, Patent No. 2,559,173 patented July 3, 1951, the system provided by the diodes 49 and 50, the resistances 52, 54, 60 and 62, and the condenser 64 constitutes a synchronous rectifying system which has an fering only slightly in frequency from the frequency of the desired signals. The condenser 64. by virtue of an integrating action over a period exceeding that of a modulating cycle, causes the meter 66 to indicate accurately those signals which enter the rectifier through the signal line 41 and are of the frequency and in proper phase relationship with the synchronous lock-in signal introduced into the system through the lines 36 and 38.

The overall operation of the system will now be evident. The oscillator, through the limiting system, amplifier 46 and phase adjuster 48, gives rise to waves (preferably of square form but not necessarily so) which, by giving rise to correspending variations of the magnetic field produced by the coils 24, will modulate or chop the ion stream emerging from the slit 20, this modulation or chopping taking place at the frequency of the oscillator. The signals from the collector plate 22 at this frequency are then rectified or detected in the synchronous rectification system which is provided with a synchronizing voltage also having this same frequency and the correct fixed phase relationship. The output of the synchronous rectifier is of direct voltage type due to the integration effected by the condenser 64 and the meter 66 responds to this. It may be here pointed out that the meter 66 may be of either indicating or recording types and if desired the magnetic field produced at thepoles 4 may be varied continuously to secure a. sweep action to provide a continuous record of the ionic spectrum. The frequency of the variation of the magnetic field produced by the poles 4 must, of course, be low, as compared with the frequency of the chopping and synchronizing actions but the latter frequency might be quite high. It will be evident that a high' rapidity of scanning of the spectrum may be effected.

The phase adjuster, whether located as shown or in the line 41 as an alternative, may be manually set to secure the maximum output of the system. This adjustment is, of course, necessary in. view of phase changes which may be introduced through the amplifiers and the modulating coils or such other modulating devices as will be hereafter referred to.

Through the use of the described system there may be secured a tremendous reduction of noise with respect to the desired signals as will be evident when it is considered that the noise in general does not have any particular frequency or phase. Such definite frequency components as might be introduced through power connections or the like may be segregated from the output merely by a choice of an oscillator frequency which does not correspond to any such frequency or any of its harmonics.

Additionally, unstable fluctuations which may arise in the ionization chamber are effectively suppressed due to the integrating action of the synchronous rectifier. Short period surges, for example, can produce no substantial deflections of the output meter.

As will be evident from the Shawhan application mentioned above the synchronous rectifier may'take numerous forms. One of these is illus-. trated in Figure 2 in which a pair of diodes 68- and 10 are connected through a pair of equal resistances l4 and 16 to the input signal terminals 12, the connections of these resistances being respectively to the anode and cathode of these avenue diodes. The cathode of the first diode and the anode of the second are respectively connected through equal resistances 85 and 88 to the parallel arrangement-of the condenser 9Iiand meter 92. The synchronizing potential is applied through the terminals 18- and as and resistances 82 and 84 in push-pull fashion with the center grounded. It will be noted that the synchronous rectifying system shown in Figure 2 differs from that in Figure 1 only in that the synchronizing potential is applied to the input rather than the output sides of the diodes. Figure 3 illustrates still another synchronous rectifier of the type disclosed in said Shawhan application in which a pair of diodes 94- and 96 have the respective anodeand cathode connections through resistances I and I02 and a common' resistance 93 to one of the signal terminals 99. A synchronizing pots ntial is'applied through the resistances I M and I56 from the terminals I03 and I05. The terminals of the diodesnot mentioned are grounded through resistances I08 and I III. In this case the output is taken through the line H2 and resistance IIII across the terminals of the condenser Hit by the meter II'I.

Figure 4 illustrates still another'synchronous rectifying system which may be used, this also being illustrated in said Shawhan application. A pair of diodes I I8 and I20 are connected as shown through equal resistances I24 and I25 and I23, the other being grounded. The electrodes other than those so connected are connected through equal resistances I28 and I3Ilto ground. n

The output is taken across the condenser I3 4 by meter I35 which is connected through resistance I32 to the junction of resistances I22 resistances I24 and I26.

The operations of all of the synchronous rec.- tifiers so far described will be found in said Shawhan application and need not be described in greater detail herein. In common, all of them provide a high degree of segregation of signal from noise. It may be here pointed out that, as discussed in said Shawhan application. symmetrical arrangements are highly desirable and it is also desirable to utilize high vacuum diodes rather than crystal diodes for achievement of a maximum separation of signals from noise though, if desired, crystal rectifiers may be used.

Instead of the synchronous rectifiers so far described there may be used other synchronous rectifying systems such as will be found in the prior art. For example, there may be used the system illustrated in Figure which will be found fully described in an article by Michels and Curtis in the Review of Scientific Instruments, September 1941, volume 12, No. 9, page 444. In this system a signal, such as from the amplifier 45, is introduced through terminals I46 and transformer I48 to a phase adjusting network I50 with the I '30 a resistance I22 to one of the input terminals ergi-ze' the heatingcoils I56 and I58 or the thermocouples I and I62, the difference of theouting a bridge arrangement of rectifiers illustrated at I89 is shown in Figure 7. The signal input to this is through the terminals I 82 while synchronizing potential is applied through a transformer I83 from the terminals I84. The meter I86 indicates or records the signals. Dry rectifiers of copper oxide or germanium may be utilized in this bridge though, if desired, high vac' uum diodes may be substituted for these elements.

While the last three synchronous rectifiers, or other synchronous rectiflers of mechanical (lye. commutating or vibrating) type, may be used effectively to suppress noise, their ability to ac'-' complish this end is far inferior to that of the synchronous rectifiers of Figures 1 to 4, inclusive, and others described in said Shawhan application.

Magnetic modulation or chopping as described in connection with Figure 1 is particularly.desirof a type which may also be used at 48 in the arrangement of Figure 1. This phase adjusting network delivers the signals to the control grids of pentodes I36 and I38, the screens of which receive synchronizing signals through the transformer I42 from the terminals I 44 which may correspond to the lines 36 and 38 of Figure 1. Self bias is provided by the resistance I49. The anodes of the pentodes I36 and I38 are connected to the primaries of transformers I52 and I54 which are joined to a centering potentiometer. The secondaries of these transformers en able since through its use there is no interference with the formation of the. ion beam. Furthermore, with the magnetic field disposedas described there is no interference with the deflect ing action of the poles 4. Before describing other methods of modulation it may be here noted that magnetic modulation may be effected in various other ways. For example, the electron beam rather than the ion beam may be modulated or chopped, by being magnetically deflected to'such' path as to diminish or substantially otherwise affect the formation of the ion stream. Furthermore, it will be evident that the coils such as'24 for producing modulation may be added to or form part of the coils which produce the magnetic field through the poles 4. This, however, is not so desirable since then the deflection involvesthe' possible throwing of ions other than those desired through the slit 20 with some possible contribu tion to the deflections produced in the meter.

It will also be evident that deflection of the ion beam for chopping or modulating action may be produced in various ways through the rotation by a motor of a permanent magnet located so that its magnetic field will exist across the ion beam path. In such case, of course, a synchronizing potential for synchronous rectification must be produced through mechanical, optical or electromagnetic means from the shaft which drives the rotatin magnet. I

The synchronous rectification systems which have been described are all electrical. While such systems have substantial advantages it will be evident that synchronous rectification as Well as the chopping may be effected through commutating systems of known mechanical type, these, by switching, producing alternating waves and also the synchronous rectifying action. Similar results may be secured through the use of vibrating switches, also of known types. 4

Modulation systems other than magnetic or mechanical are indicated in Figures 8 and 9. .In Figure 8 an ionization chamber I88 is provided similar to that illustrated in Figure 1 with the additionof a grid l 90 provided for the purpose of modulating the electron beam which produces ionization, thereby producing corresponding modulation of intensity of the ion stream. The grid I90 may be energized through an amplifier such as 46 in Figure 1 combined with a phase adjuster in the position illustrated in Figure 1 or in another suitable position in the system. Other than for this change the system may be identical with that illustrated in Figure 1.

Figure 9 illustrates an ionization chamber I92 again similar to that illustrated in Figure 1 with the exception of the provision of deflector plates I94 for the electron beam. These deflector plates through modulation of position of the electron beam may be utilized to produce modulation or chopping of the ion stream. The deflector plates will, of course, be energized from an amplifier such as 46, the remaining parts of the system being the same as illustrated in Figure 1. It may be here noted that corresponding deflector plates may be used for producing electrostatic deflections of the ion beam similar to those produced magnetically by the coils 24.

It will be evident from the above illustrations that the invention may be embodied in practice in numerous fashions and accordingly it will be understood that it is not to be construed as limited except as required by the followin claims.

What I claim and desire to protect by Letters Patent is 1. A mass spectrometer comprising means for producing a deflected ion stream, means for modulating said ion stream to provide alternating signals therefrom, and synchronous rectifying means receiving said signals and synchronous signals from the modulating means to provide an output integrated over a time interval exceeding the period of the modulation and indicative of the intensity of the stream.

2. A mass spectrometer comprising means for producing a deflected ion stream, means for magnetically modulating said ion stream to provide alternating signals therefrom, and synchronous rectifying means receiving said signals and synchronous signals from the modulating means to provide an output indicative of the intensity of the stream.

3. A mass spectrometer comprising means for producing a deflected ion stream, means for modulating said ion stream to provide alternating signals therefrom by producing deflections of said stream transverse to its principal deflection, and synchronous rectifying means receiving said signals and synchronous signals from the modulating means to provide an output indicative of the intensity of the stream.

4. A mass spectrometer comprising means for producing a deflected ion stream, means for magnetically modulating said ion stream to provide alternating signals therefrom by producing deflections of said stream transverse to its principal deflection, and synchronous rectifying means receiving said signals and synchronous signals from the modulating means to provide an output indicative of the intensity of the stream.

5. A mass spectrometer comprising means for producing a deflected ion stream, means for modulating said ion stream to provide alternating signals therefrom, said modulating means effecting modulation of the ion stream by modulation of an ionizing electron beam, and synchronous rectifying means receiving said signals and synchronous signals from the modulating means to provide an output integrated over a time interval exceeding the period of the modulation and indicative of the intensity of the stream.

6. A mass spectrometer comprising means for producing a curved ion stream, means for modulating said ion stream to provide alternating signals therefrom by producing deflections of said stream transverse to its principal deflection, and means for measuring said alternating signals.

'7. A mass spectrometer comprising means for producing a curved ion stream, means for magnetically modulating said ion stream to provide alternating signals therefrom by producing deflections of said stream transverse to its principal deflection, and means for measuring said alternating signals. 7

ERIC A. WEISS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,222,429 Brierbrecker Nov. 19, 1940 2,413,788 Sargeant et al. Jan. 7, 1947 2,424,933 Kalmus July 29, 1947 2,442,298 Liston May 25, 1948 2,457,162 Langmuir Dec. 28, 1948

Claims (1)

1. A MASS SPECTROMETER COMPRISING MEANS FOR PRODUCING A DEFLECTED ION STREA, MEANS FOR MODULATING SAID ION STREAM TO PROVIDE ALTERNATING SIGNALS THEREFROM, AND SYNCHRONOUS RECTIFYING MEANS RECEIVING SAID SIGNALS AND SYNCHRONOUS SIGNALS FROM THE MODULATING MEANS TO PROVIDE AN OUTPUT INTEGRATED OVER A TIME INTERVAL EXCEEDING THE PERIOD OF THE MODULATION AND INDICATIVE OF THE INTENSITY OF THE STREAM.

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