US3870881A

US3870881A - Method of analyzing output signals representing the mass spectrum from a scanning mass spectrometer

Info

Publication number: US3870881A
Application number: US260315A
Authority: US
Inventors: John Stephen Halliday; Brian Noel Green
Original assignee: Associated Electrical Industries Ltd
Current assignee: Kratos Analytical Ltd
Priority date: 1965-01-07
Filing date: 1972-06-06
Publication date: 1975-03-11
Anticipated expiration: 1992-03-11

Abstract

In a scanning mass spectrometer, electrical output including a series of time-related peaks representing an ion mass spectrum of an unknown sample material are produced along with a series of reference peaks derived from a reference material. The time positions of the peaks due to the unknown sample are thus identified. The electrical output may be recorded on a traveling magnetic medium along with marker signals indicative of time during a scan when a signal was produced. The marker signals may also be indicative of the amount of deflection provided by an analyzer, and hence of the mass/charge ratios of ions producing certain output peaks.

Description

United States Patent [191 Halliday et al.

[4 1 Mar. 11, 1975 Noel Green, Manchester, both of England [73] Assignee: Associated Electric Industries,

I London, England 22 Filed: June 6,1972

211 App]. No.: 260,315

Related U.S. Application Data [63] Continuation of Ser. No. 82,479, Oct. 20, 1970, abandoned, which is a continuation of Ser. No. 538,876, Jan. 6, 1966, abandoned.

[30] Foreign Application Priority Data Jan. 7, 1965 Great Britain 755/65 [52] US. Cl 250/283, 250/295, 250/296, 250/299 [51] Int. Cl. HOlj 39/34 [58] Field of Search... 250/419 G, 41.9 D, 41.9 ME

[56] References Cited UNITED STATES PATENTS 2,380,439 7/1945 l-Ioskins et al. 250/41.9

2,412,359 12/1946 Roper 250/419 3,103,582 9/1963 Morgan 250/419 X 3,154,747 10/1964 Kendall 250/419 X 3,244,876 4/1966 Kanda et a1 250/41.9 3,260,845 7/1966 Duncumb 250/419 X 3,318,149 5/1967 Varadi 250/419 X 3,342,991 9/1967 Kronenberger 250/419 OTHER PUBLICATIONS Introduction To Atomic And Nuclear Physics, by H.

Semat, Rinehart & Co., New York, pg. 55.

Data Recording On Magnetic Tape, by L. G. Killian from Electronic Industries & Electronic Instrumentation, April, 1948', pg. 3-5 & 31.

Magnetic Recorder for Nuclear Pulses," by P. E. Cavanagh et al., from The Review of Scientific Instruments, Vol. 27, No. 12, Dec., 1956; pgs. 1028-1033.

Use Of Magnetic Tapes and for for Nuclear Data Storage and Computation," by F. H. Wells, from Nuclear Instruments, Vol. 2, Feb., 1958; pgs. 165-168.

Magnetic Tape Simplifies Handling of Nuclear Data," by D. W. I-Ialfhill, from Nucleonics, Vol. 16, No. 12, Dec., 1958; pgs. 55-57 & 66.

Mass Spectrometry by K. Biemann, McGraw-Hill Book Company, 1962, pgs. 37-40.

Primary Examiner-William F. Lindquist Attorney, Agent, or. FirmWatts, Hoffman, Fisher & Heinke Co.

[57] 1 ABSTRACT In a scanning mass spectrometer, electrical output including a series of time-related peaks representing an ion mass spectrum of an unknown sample material are produced along with a series of reference peaks derived from a reference material. The time positions of the peaks due to the unknown sample are thus identified. The electrical output may be recorded on a traveling magnetic medium along with marker signals indicative of time during a scan when a signal was pro duced. The marker signals may also be indicative of the amount of deflection provided by an analyzer, and hence of the mass/charge ratios of ions producing certain output peaks.

24 Claims, 5 Drawing Figures PATENTEDHARI 1 I 3.870.881

sz-imlp g E INVENTORS JOHN STEPHEN HALLIDAY BRIAN NOEL GREEN PATENTEDMARHIEIYS' 3,870,881 sum 2 ggg "MAMA AAAAF v'nv- 'VYV FIG. 3

FIG. 5

INVENTORS JOHN STEPHEN HALLIDAY BRIAN NOEL GREEN METHOD OF ANALYZING OUTPUT SIGNALS REPRESENTING THE MASS SPECTRUM FROM A SCANNING MASS SPECTROMETER This is a continuation of application Ser. No. 82,479, filed Oct. 20, 1970, which in turn was a continuation of Ser. No. 538,876, filed Jan. 6, 1966, both prior applications of which are now abandoned.

This invention relates to improvements in mass spectrometers and mass spectrometry and more particularly to the provision of an improved apparatus for, and method of, presenting the output data from a mass spectrometer.

Mass spectrometry isa known method of testing a material to ascertain its composition. This method is applicable to the improvement or control of manufacture since it enables a manufacturing process to be monitored and modified from time to time to control the nature, quality, and consistency of a material being produced.

In a mass spectrometer, a material being analyzed is first ionized. In one class of mass spectrometers, the ions are accelerated and then passed through an electrostatic field to a monitor collector. The ions to be analyzed pass through the monitor collector and then through a magnetic field. After the ions pass through the magnetic field, they are received by a collecting device.'The coaction of the acceleration, the electrostatic field, the magnetic field and the collecting device separates undesired ions from those which are to be analyzed. Analysis of the information gathered by the collecting device can identify both the quantity and the nature-or quality--of the ions which reach the collector.

The physical principle upon which mass spectrometers operate is that each ion has a characteristic mass charge ratio. As the ion passes through the electrostatic and magnetic fields it is deflected by them. The amount of deflection is a function of the mass of the ion. The amount of the deflection is also a function of the speed at which the ion is travelling and the strength of its electrical charge.

Prior mass spectrometers have been used for the control of certain manufacturing processes. While prior mass spectrometer systems have been used in this manner, the characteristics of these systems have imposed some limitations on their use.

One shortcoming of prior mass spectrometers has been that it has been very difficult to produce a fast, truly high-resolution mass spectrometer. By a highresolution mass spectrometer, it is meant one which is capable of resolving two ion beams-which differ by less than one part in three thousand in mass charge ratios of the ions in them. Since the output signals produced by ions are somewhat triangular in shape, it will be recognized there may be some overlap between the signals. Accordingly, if the two beams are of equal peak intensity, they are said to be resolved when the minimum intensity between them is ten percent or less than ten percent of the intensity of either beam.

With prior mechanisms, when two differing ions have substantially identical mass charge ratios, it has often been substantially impossible to distinguish one from the other, where the analysis must be at high speeds. The present mechanism retains high resolution with great facility.

Another difficulty which has been experienced in monitoring given manufacturing processes with priorart mass spectrometer systems, has been the speed and accuracy with which thecollected information can be collected, analyzed, and thenutilized for the control of a manufacturing process. Fast scanning conditions are essential when the composition of a sample is changing rapidly. For example, when the sample is the effluent from a gas chromatograph and the components are separated in the chromatograph and then analyzed in turn as they are discharged. The present invention has overcome this problem by providing a system which will collect and analyze information with increased accuracy and speed.

According to the present invention in a basic aspect thereof, a method of molecular analysis of a sample of matter comprises the steps of introducing into a high resolution mass spectrometer both the sample to be analyzed and a reference sample of known material, operating the mass spectrometer in scanning mode to obtain an electrical output including a series of time-related peaks representing an ion mass spectrum of the sample having interspersed throughout the spectrum a series of reference peaks derived from the reference sample and corresponding to known ion masses (mass/chargeratios), the reference sample having been chosen to provide this series of reference peaks, identifying from the output the time positions of the sample and reference peaks in the spectrum represented thereby, and establishing in accordance with the mass/time relationship determined from the time positions and known masses of the reference peaks, the masses represented by the peaks attributable to the analyzed sample.

Whereas the mass spectrometer output may be fed directly to a computer programmed to perform to effect the identification of the peak time positions and the calculation of the masses corresponding to the peaks attributable to the analyzed sample, there is advantage, as will hereinafter become apparent, in first magnetically recording the output either in analogue or ditigized form on magnetic tape or other traveling magnetic recording medium.

According to another aspect of the mechanism, the travelling medium is simultaneously provided with marker signals which serve to identify the recorded signals accurately in terms of their mass charge ratio. A computer facility can be arranged to compare recorded signals indicative of a peak in the output from the output device with idealized peak information supplied to the computer. This is used to ascertain the best fit of the idealized peak to the recorded signal peak and thereby indicate the time position or centroid of the recorded signal peak.

As suggested above, with the mechanism and method of this invention, it is possible to use very high speeds of spectrum scanning. For example, it will be appreciated that this method of recording is appreciably faster than with a galvonometer type of recorder using a beam of ultraviolet light to mark sensitive paper. With the galvonometer type of recorder, the writing speed is limited by the inertial lag of the galvonometer mirror and its suspension. For fast writing speeds, the dwell of the light beam at each point of its trace on the sensitive chart is too short to produce a good record.

In addition, with the mechanism and method of this invention no processing of the record is necessary, as

p is necessary with a film record of the trace on an oscilloscope.

Another advantage of this invention is that once the information is obtained it is readily and permanently available in a form in which it may be subsequently read out for processing. In particular the information on the tape can easily be read out in a form suitable for feeding to an analogue or digital computer programmed to process the data and set out the final, analytical results.

The preferred embodiment of the invention may be used to analyze, qualitatively, a sample such as an organic compound. In this instance, it is important to be able to specify for each and every peak in the spectrum two numbers. The first number indicates the intensity or height of a peak and the other number indicates the mass charge ratio of the ion corresponding to that peak.

For simple identification purposes, it is often sufficient to record the mass charge ratio only to unit mass; i.e., in accuracy of about one part in one thousand. The intensity, however, needs to be measured accurately so that comparison can be made between the peaks of unknown compounds and those of a series of compounds contained in a catalogue of spectral data. Again, a digital computer can readily be arranged to check the results of an analysis with the peaks of each of a series of catalogue compounds and to indicate the compound or compounds present in the specimen substance.

In that situation where the structure of the compound is complex and/or the spectra of the likely possibilities are not available in a catalogue, it may be necessary to analyze the peaks in the spectrum in more detail to find its composition. In this event, it is very important to be able to ascertain the mass charge ratio very accurately and the intensity information is not as important as in the previous example. The mechanism and method of this invention are ideal in this circumstance because they have the necessary ability to produce peaks which are sufficiently sharp so that their position on the scan can be measured accurately.

In the past, it has been necessary, in order to achieve high accuracy of measurement, such as ten parts per million or better, to use one of two cumbersome techniques. The first technique is the use ofa decade potentiometer to compare mass charge ratios. The second and alternate technique is to use a travelling microscope in conjunction with a photoplate on which a spectrum produced by the whole of the ion beam is recorded. With the magnetic travelling medium of this invention these difficulties are overcome because output ion signals from the known and reference samples are recorded with simultaneous applied time markers. With this technique, the mass charge ratio can be very accurately evaluated. The difficulties of feeding the output from the microscope examining a photoplate to an analogue or digital computer are avoided.

According to another aspect of the invention, the speed of analysis of the information can, when desired, be increased still further. This is true because during the scanning of a mass charge spectrum, the percentage of the time during which peaks produce an output from the output device is quite small. Accordingly, a considerable portion of a recording tape has no useful record except the time markers. The tape can be edited if the on the scan. Thus, instead of signals being used simply as time markers on the tape, the time from the beginning of the scan can be measured by a clock device.

' carried out without any other processing. This is true playback speed is too slow. By this editing the blank parts are eliminated and the tape is still quite useful for the desired purpose so long as the time markers indicate not just time intervals but also mark actual times because the play-back information as to deflecting forces and the information from the output device provides all the required information.

. Editing can be avoided, if the scan is relatively slow, by starting and stopping the tape so that recording starts only when the output reaches a sufficiently high level. The inertia of the tape and its associated mechanism limits the use of such arrangement, obviously. Furthermore, no record at all will be made of small peaks which are of magnitude less than the threshold of the start and stop arrangement.

Another advantage of the method and mechanism of this invention, and example of the flexibility of it, is that it can be used to compare two peaks by peak switching. This is known as direct mass measurement. It is accomplished 11y suitable modifications to record the voltage or flux used in the mass spectrometer.

As another example of the flexibility of the device, the output from the recorded magnetic tape during playback can be converted into digital form and the digital information recorded, if desired, on another magnetic tape or on punch cards.

As further examples of the flexibility of the device, analogue records obtained on a magnetic tape in the various ways mentioned can subsequently be displayed in analogue form by playback at the same or slower speeds using oscilloscope display systems. Alternately, a multi-channel galvonometer recorder may be used. As a still further alternative, the analogue record and the magnetic tape can be played back and digitized by a system that would consist of a multiplexer, an analogue-to-digital converter, a memory device, either core or delay line, and again recorded on magnetic tape in digital form for processing by a computer.

From the foregoing it will be recognized that magnetic tape recording gives at least the following advantages:

l. A record that is immediately permanent (i.e. no developing or fixing, as is required with photoplate or U.V. sensitive paper).

2. Higher frequency response than chart recorder or galvonometer records so that fast scan high resolution spectrum can be obtained without serious distortion (peak clipping).

3. Multi channel recording:

a. Several levels of sensitivity from one output. b. Several outputs. c. Other features:

i. time, flux, voltage parameters ii. monitor output.

4. Signals can be recorded in analogue or digital form, and the latter can be made suitable for direct input to computer.

5. Since the output information is immediately and permanently stored on a magnetic tape, it can be duplicated and/or shipped to computer ata remote location. i 6. In many instances, the output information pro- I vided'by the mass spectrometer is at a rate which is so high it cannot be directly fed into a computer. l-Ieretofore, expensive equipment has been used to enable that information to be converted into binary form and then fed to a digital computer as it is derived. With the use of a magnetic tape, one may simply play the tape back at a reduced speed to feed the information to the computer, thereby avoiding the very expensive converting equipment.

One problem which is present in applying the output of a mass spectrometer to a magnetic tape is that the output signal may vary in intensity. A weak output signal requires a large amplification to provide a suitable recorded signal. On the other hand, if strong output signal is amplified to the same extent, distortion will occur either in the amplifier or in the recording of the magnetic tape which is magnetically saturated. This problem is overcome by the provision of a plurality of amplifying recording channels which are operated at different gains. Accordingly, no matter how weak or strong the output signal, one of the amplifiers will produce a useful record. The outputs of each of the channels are simultaneously recorded, in side-by-side relationship, on the tape.

The preceding discussion will make apparent the solution of yet another inherent problem in the use of the tape. When a tape is fed from a coil, and/or wound onto a coil, the translational speed of the tape will vary. Unless extremely expensive mechanism is used, it is not possible to provide a uniform speed of tape movement in this circumstance. Moreover, because of tape stretch and the like, even a tape which has been fed at relatively uniform speeds past the recording head may well produce distortions in the play-back. Accordingly, the novel use of simultaneously recorded time indicia completely obviates the problem of non-uniform tape travel speeds and produces extremely reliable and dependable results.

Other objects and a fuller understanding of the invention may be had by referring to-the following description and claims taken in accordance with the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of a mass spectrometer installation;

FIG. 2 is a graphical representation of the output from an electron multiplier shown in FIG. 1;

FIG. 3 is a graphical representation of the output from a time marking unit shown in FIG. 1;

FIG. 4 is a graphical representation of the variation of the selected mass charge ratio with time when the current of a magnetic analyzer shown in FIG. 1 is caused to vary in a predetermined manner; and,

FIG. 5 is a front elevation of a recording head shown in FIG. 1, as viewed in the direction indicated by the Arrow A.

Referring first to FIG. 1, the mass spectrometer includes an ion source chamber 1. The chamber 1 is preferably one of the type described and claimed in greaterdetail in U.S. Pat. No. 3,158,740, issued Nov. 24, 1964, to Craig et al. for Mass Spectrometer Sample Insertion Devices. With such a chamber, a specimen carrying probe 3 can be inserted and ions can be liberated from a specimen inserted via the probe or a gas inlet or otherwise. An electrode 5 to which an accelerating 6 voltage of 8 kilovolts is applied'serves to repel these ions as a beam. The beam passes first through an electrostatic analyzer 7 which includes opposed conductive plates 7? between which a potential difference is maintained. The beam then passes through a slit of a monitor collector 9 into a magnetic analyzer 11. In magnetic analyzer 11, an electromagnet coil 11C establishes a strong magnetic field directed in a direction transverse to the path of the ions, and since the ions are charged particles their paths will be curved in the magnetic field. The deflected ions, or certain of them if different groups of ions are deflected to different degrees, pass through a slit in a member 13 and are picked up by a collector electrode 15. The collector electrode is associated with an electron multiplier 17. The output from the electron multiplier 17 is used, after amplification, to provide a record of the ions passing through the slit in the slit member 13.

Mass spectrometers as described above are well known in the art. It will be recognized from this and the ensuing discussion that the invention may be utilized with either a single or double focussing mass spectrometer so long as the output is in the form of electrical signals.

The angular deflection of an ion in passing through the magnetic analyzer 11 will depend upon the accelerating voltage, since that determines the speed of the ions, the intensity of the magnetic field in the analyzer 11, the electrical charge on the ion, and the mass of the ion. One method of scanning a range of a mass spectrum is to maintain the voltages used in the electrostatic analyzer 7 and on the accelerating electrode 5 constant, and scanning by decreasing the current used in the electromagnetic coil 11C of the magnetic analyzer 11. This progressively changes the deflections of all the ions passing through the magnetic analyzer. If the output from the electron multiplier 17, indicative of the number of ions passing through the slitted member, is presented on a cathode ray tube as the vertical deflection with a horizontalscanning speed corresponding to the decay of the magnetic field in the magnetic analyzer 11, the trace shows peaks where ions are present having such a mass charge ratio that they are deflected to pass through the slitted member 13.

By way of example, for high resolution scans a useful scan speed is 10 seconds for a factor of ten in mass. When only a narrow band of mass charge ratios is to be scanned; for example one per cent in mass, the main magnet current can be kept constant, and the fast output saw-tooth voltage'of the oscilloscope time base can be applied to an auxiliary magnet 21 added at the entrance to the magnetic analyzer.

Another method of scanning a range of a mass spectrum is to maintain the current in the magnet coils constant, and to vary in a progressive manner the voltage applied to the electrode 5. This will need a similar modification of the voltage applied between the plates 7? of the electrostatic anaylzer 7 to keep the ratio of the two voltages constant.

Associated with the mass spectrometer is a tape recording unit 31 including a magnetic tape 33. The tape 33 is originally wound on a spool 35.

As the tape is fed from the spool 35, it is pressed by a roller 37 against a capstan 39. The capstan 39 is driven at a constant speed by a motor 41. The spool 35 is suitably braked to maintain the part of the tape between the spool and the capstan taut. The tape after leaving the capstan 39 is wound onto a take-up spool 43 which is motor driven in the appropriate direction.

three

parts

45A, 45B, 45C respectively of the recording head 45, FIG. 5. The gains of the three

amplifiers

47A, 47B, 47C are typically 1:10:100.

The output from a very stable sine wave oscillator 49, operating at a frequency of 100,000 cycles per second and serving as a time marking unit, is applied to part 45D of the recording head, FIG. 5. The waveform applied to part 45D is shown in FIG. 3.

The output from the monitor collector 9 is applied through an amplifier 51 to part 45E of the recording head. The output from a magnetic-flux-measuring device 53 disposed in the magnetic field of the magnet 11 is applied through an amplifier 55 to part 45F of the recording head, FIG. 5.

In use of the apparatus described above, the specimen to be analyzed is applied to the probe 3. The probe is inserted into the ion source chamber 1, and ions are liberated from the material of the specimen, for example by the action of an electron beam. At the same time, ions of a known substance are produced in the ion source chamber 1 either by inclusion of that substance with the specimen to be analyzed, or by admission of the known substance as a gas through a gas inlet 56 to the chamber 1.

The ions produced in the chamber 1 are repelled by the electrode 5 and are discharged as a beam through a slit outlet from the chamber 1 into the electrostatic analyzer 7. The ions then pass through the slit of the monitor collector 9 into the magnetic analyzer 11. In the analyzer 11 the ions follow a trajectory which depends upon their initial velocity, their electrical charge, their mass, and the intensity of the magnetic field in the analyzer ll. Ions of different mass charge ratios therefore diverge from one another to form, at the entrance to the member 13, a spectrum in which ions of different mass charge ratios are laterally displaced from one another. Because of this lateral displacement at any instant only ions having mass charge ratios within a relatively small range of mass charge ratios can pass through the slit in member 13 to be picked up by the collector electrode 15.

By progressive variation of either the accelerating voltage on electrode 5 together with the voltage across the plates 7P, or the magnetic field in the analyzer 11, the spectrum incident upon the member 13 can be shifted laterally to change the range of mass charge ratios for which the ions can pass through the slit in member 13. In this manner, the output from the electron multiplier 17 varies with time to correspond with the number of ions passing through the slit in member 13.

FIG. 2 illustrates, by way of example, how the output from the electron multiplier 17 might appear after amplitication and application to a cathode ray oscilloscope as the Y-deflection signal while the X-deflection or time base was varied in a regular manner with time. On the oscilloscope, the trace includes peaks marked respectively R1, Ul, R2, U2, R3, U3, R4. For the purposes of the present example, R1, R2, R3, R4 are the peaks which would be produced by the known substance when supplied by itself to the ion source chamber 1. Peaks U1, U2, U3 are the peaks produced by the unknown substance undergoing analysis.

The recording head 45 records simultaneously on tracks passing respectively the

parts

45A, 45B and 45C separate signals representative of the same intelligence, namely the output from the electron multiplier 17, but at different power levels. By using three otherwise similar amplifiers acting in parallel, any possibility of phase shift between the three signals can be reduced or eliminated.

It will be appreciated that a weak output signal from the electron multiplier requires a large amplification to provide a suitable recording signal. On the other hand, when the same amplification is applied to a strong output signal from the electron multiplier, distortion will occur either in the amplifier, the output of which is overloaded, or in the recording on the magnetic tape, which is magnetically saturated. On playback, this will produce an output which is too blurred to provide accurate information as to the location of the recorded peak. By the use of three recording channels operating at different gains, a useful record is made (on different tracks) of both weak output signals from the output device (i.e. the electron multiplier 17) and strong output signals from the output device, despite magnetic saturation of the recording means and/or the tape track associated with the higher power levels by strong output signals.

The recording head 45 also simultaneously records on the track passing the part 45D time markers having the waveform indicated in FIG. 3. It also records on the track passing the part 45E the total ion current through the mass spectrometer, as indicated by the monitor collector 9. The head also records on the track passing the part 45F the instantaneous value of the magnetic flux in the magnetic analyzer, as measured by the device 53.

It may not be necessary in any particular analysis to use all the inputs to the recording head 45 which have been included above by way of example, and on the other hand it may on occasion be desirable to use a further track or tracks to record further information. One item of further information which may be desirable is the magnitude of the accelerating voltage applied to the ion gun, but in many instances this voltage is maintained constant at a known value and therefore in the preferred embodiment described the necessary recording means has not been shown. When the voltage is to be recorded, it can be recorded in digital form or as the level of modulation of a carrier signal.

FIG. 3 illustrates in schematic form how the mass charge ratio (m/e) for deflected ions intercepted by the collector 15 might vary with time (t) when the current in the magnetic analyzer 11 is caused to vary with time. Ideally, the relationship between the mass charge ratio and the magnet current could be the dashed curve 61. In practice, the curve would depart from curve 61 and would tend towards the curve 63. To illustrate the nature of the likely departures of the curve 63 from the ideal curve they are exaggerated in FIG. 4.

If one could be certain that (l) the speed of the tape 33 during recording were exactly even, (2) the stretch in the tape as it passes the capstan 39 were also exactly even, and (3) the scanning law for the magnetic analyzer were precisely known, then the position of the peaks U1, U2, and U3 along the length of the tape 33 would indicate the mass charge ratio at each of these peaks. However the actual flux deviates from say the tape 33, the effects of uneven recording speeds and uneven tape stretch during recording are substantially completely overcome. This is because in interpretation of the tape recorded signals the operator or a computer measures not the linear distance between marks on the tape but the time interval as indicated by the time markers. Since interpretation of the tape is effected through running the tape past a playback head, the effects of uneven and improper playback speed and uneven stretch during playback are also substantially completely overcome.

By the use of a reference substance to produce reference peaks R1, R2, R3 and R4, the effects of the departures of the curve 63 from the ideal curve 61 are also minimized. Thus, the assumption that the ideal curve 61 is followed over the part or segment of the scan between peaks R1 and U1, over the part of the scan between peaks R2 and U2, and over the part of the scan between peaks R3 and U3, does not introduce any serious errors.

In practice, it is sufficient to assume that the scan law is followed properly in a segment of a scan between two convenient reference peaks, for example the peaks R1 and R2, typically to 14 mass numbers apart if the reference substance PERFLUROKEROSENE is used, in evaluating the intermediate unknown peak, in this case the peak U1. Thus, in this example the assumption described in the preceding paragraph is extended from R1 to R2. Obviously, another example can be given with respect to U2. Thus, in practice it is sufficient in the identification of U2 to assume the scan law is followed between the reference peaks R2 and R3.

The tape 33 on which the spectrum is recorded can be used to supply an input to a standard chart type recorder, when played back at a speed lower than the speed used for recording. In this case it can be caused to produce two traces on the chart representing respectively the peaks and the time markers, shown respectively in FIGS. 2 and 3.

For fast scanning, it is often very convenient to use electrical scanning, in which the voltage applied to the accelerating electrode 5 is varied. However, when electrical scanning is used the important peaks near the parent peak (that produced by the molecular ion) region may produce too small an output from the electron multiplier, since the sensitivity of the apparatus, as well as the deflection, varies with the applied voltage. It is therefore desirable to use magnetic scanning, with which the sensitivity remains substantially constant during scanning. However, at high speed the resolution (which is essential to accurate mass charge ratio measurement) is reduced.

It has been found that the resolution can be improved if the ion beam is displaced in the magnetic analyzer, relative to the magnetic field, from a first optimum region suited to a slow scanning speed to a second optimum region suited to a fast scanning speed. The actual scanning is effected at the fast scanning speed mainly by variation of the current in the main magnet coil 1 1C.

The non-uniformity in the magnetic field, besides causing a general loss of resolution which can be corrected by repositioning the main magnet, can cause a variation of resolution during the time of the scan. This temporal variation can be reduced by a small corrective scanning displacement produced either by variation of the current in an auxiliary magnetic scanning coil, such as the coil of the auxiliary magnet 21, or by variation in the voltage applied to a deflecting electrode arranged near the point where the ion beam enters the magnetic analyzer. By way of example, the magnet of the magnetic analyzer 11 may be shifted laterally through a distance of about 0.040 inch, the analyzer may produce a deflection of and the auxilliary magnet 21 produce during the scan a corrective shift of say 2.

Digital recording on the tape may be used. One advantage of such a system is that subsequent data processing is more convenient. In digital recording, a number of tracks are used on the travelling recording medium, one for each digit, and therefore a wide strip of magnetic recording tape is used. If the digital recording is carried out on the binary system, it might be convenient to use a magnetic drum instead of a magnetic tape as the travelling medium. In the case of a digital system, a digital conversion system must be used to transform the analogue electrical signal from the electron multiplier 17 into digital signals applied to the separate recording heads acting on the several tracks on the recording travelling medium.

When it is desired to use a magnetic tape to produce an analogue record of the peaks and the record of the time markers this tape can subsequently be passed through a suitable tape reader which will read out the information impressed on the tape, and with ancilliary equipment convert the information into digital form, and punch out a tape with this information. The punched tape can then be fed into a digital computer in orthodox manner.

In the interpretation of the signals recorded on the tape, when a computer facility is available it can be used to ascertain the time position or centroid of a given peak. With large peaks, it is possible to find the time position or centroid of the peak by arranging that the peaks shall be quite sharply pointed and applying the signal indicative of the peak to an electronic device arranged to sense the instant at which the signal ceases to increase and begins to decrease. With relatively smaller peaks, representing the existence of a relatively small number of ions of that particular mass charge ratio, statistical variations in the shape of the peak occur due to the fact that the number of ions arriving at the collector electrode 15 from instant to instant, with the scan checked, can vary. Thus, the record produced during scanning by the ions may not produce a simple unambiguous pointed peak in the recorded signal. By use of the computer facility, to compare an idealized peak with the recorded peak, the best fit of the idealized peak to the recorded peak can be found, so providing (from the position of the tip of the idealized peak) an indication of the time position or centroid for the recorded peak.

In an example mentioned above of the effluent from a gas chromatograph, it is necessary to perform the analysis to identify a chromographic peak occupying only a fraction of a minute. The carrier gas from the gas chromatograph and the entrained sample are applied to a molecular sieve which is chosen to pass the molecules of the sample in preference to those of the carrier gas. The molecules passed by the sieve are applied to the ion source chamber of the mass spectrometer. If there is any variation in the amount of the sample passing the sieve, during the scanning period, a record of the variation of the sample can be made on a further track of the magnetic recording tape. If the variation tends to be.

too large, then automatic gain control can be applied to the amplifier 47 (or alternatively to the electron multiplier 17) in order to maintain the output from amplifier 47 constant, despite variations of the quantity of the sample.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What we claim is:

l. The method of mass measuring a sample with a mass spectrometer including means to ionize substances, an analyzer, and collector means comprising:

a. simultaneously ionizing the sample and a reference compound;

b. concurrently passing ions from both the sample and the reference compound through the analyzer to the collector means while scanning the mass spectrum to collect data as to the mass spectrum of the sample and the reference compound with the collector means;

c. producing electrical outputs with said collector means representative of the data collected;

d. digitizing signals derived from said collection means outputs;

e. evaluating the digitized signals with a computer and in so doing evaluating a first peak of the sample with respect to a pair of reference compound peaks of different positions on the mass spectrum to locate the position of the first sample peak with respect to a segment of the spectrum established by said reference peaks and with respect to one of the reference peaks whereby to identify said first sample peak with respect to its position relative to said one peak, said segment thereby minimizing the effects of deviations in the results of the scan producing said first sample peak and said first and second reference peaks from an ideal scan;

f. said evaluation of said first peak of the sample being made according to assumed scan conditions established by said pair of reference compound peaks to compensate for variations from an ideal scan due to hysteresis and the like and whereby to produce a mass analysis of a sample without need for a visible spectrum.

2. The method of mass measurement in accordance with claim 1 further comprising the steps of comparing a part of said collected data produced by said ions of selected mass/charge ratios with characteristic output signals for particular mass/charge ratios of ions and determining from said comparison the centroid of said output signals whereby the relationship of said output signals from said substance to be analyzed may be more accurately compared with said output signals used as reference markers.

3. A method of mass measuring in accordance with claim 1 further including recording the collected data by magnetic recording means and a travelling medium and concurrently recording the output from a clocking device on said travelling medium whereby a time standard is obtained for obtaining the location of peaks and thereafter identifying equivalent data obtained by playback of said tape and thereby derived from said output.

4. A method of mass measuring in accordance with claim 3, further comprising the steps of producing electrical output signals as a portion of said data collection, amplifying said electrical output in parallel with a plurality of amplifying devices having differing gains, and recording the amplified output.

5. The method of claim 3 further including subsequently utilizing results recorded during the scanning of the mass spectrum by playing the recording medium over a total time period different than the elapsed time occuring during the recording of the results.

6. The method of claim 5 including causing said playback time period to be shorter than the recording time by editing of the medium.

7. The method of claim 5 including causing said playback time period to be shorter than the recording time by moving the medium at a playback speed which is faster than the recording speed.

8. The method of mass measuring a sample with a mass spectrometer including means to ionize substances, an analyzer and collector means comprising:

a. simultaneously ionizing the sample and a reference compound;

d. evaluating said outputs and in so doing evaluating a first peak of the sample with respect to both a first reference compound peak of a higher position on the mass spectrum than the sample peak and a second reference compound peak of a lower position on the mass spectrum than said first reference peak to locate the position of the first sample peak with respect to a segment of the spectrum established by said first and second reference peaks thereby identifying said first peak from its position on said segment with respect to said first and second reference peaks thereby minimizing the effects of the deviations in the results of the scan producing said first sample peak and said first and second reference peaks from an ideal scan;

. evaluating said outputs and in so doing evaluating a second peak of the sample with respect to both one reference compound peak of a higher position on the mass spectrum than the second sample peak and another reference compound peak of a lower position on the mass spectrum than said one reference peak to locate the position of the second sample peak with respect to a second segment of the spectrum established by said one and said another reference peaks whereby to identify said second sample peak with respect to its position of said second segment thereby minimizing the effects of deviations in the results of the scan prooducing said 13 second sample peak of said one and said another reference peaks from an ideal scan; and, said evaluation of each of said first and second sample peaks being made according to assumed scan conditions established by said first and second and said one and said another reference peaks respectively whereby to compensate for variations from an ideal scan due to hysteresis and the like and whereby to produce mass analysis of a sample without need for a visible spectrum.

9. The method of claim 8 wherein said second reference peak is the same as said one reference peak.

10. The method of claim 8 wherein the evaluation of the sample peaks with respect to the reference peaks is by establishing the ion masses of the sample peaks in accordance with the mass/time relationship determined from the time position and known ion masses of the reference peaks.

11. The method of claim 10 wherein the true time position of each sample peak is identified by determining the centroid of the peak.

12. The method of claim 8 wherein the reference compound is perfiuorokerosene.

13. The method of claim 8 wherein the first and second reference compound peaks are from 10 to 14 mass numbers apart.

14. The method of mass measuring a sample with a mass spectrometer including means to ionize substances, an analyzer, and collector means comprising:

a. simultaneously ionizing the sample and a reference compound;

d. evaluating the outputs and in so doing evaluating a first peak of the sample with respect to a pair of reference compound peaks of different positions on the mass spectrum to locate the position of the first sample peak with respect to a segment of the spectrum established by said reference peaks and with respect to one of the reference peaks whereby to identify said first sample peak with respect to its position relative to said one peak, said segment thereby minimizing the effects of deviations in the results of the scan producing said first sample peak and said first and second reference peaks from an ideal scan; and,

e. said evaluation of said first peak of the sample being made according to assumed scan conditions established by said pair of reference compound peaks whereby to compensate for magnetic variations from an ideal scan due to hysteresis and the like and whereby to produce a mass analysis of a sample without need for a visible spectrum.

15. The method of claim 14 wherein the pair of reference peaks are from 10 to 14 mass numbers apart.

16. The method of claim 14 wherein the reference compound is perfluorokerosene.

17. The method of claim 14 wherein the two reference compound peaks occur within the mass spectrum one on either side of the sample peak in question.

18. The method of claim 14 wherein the analyzer region includes a magnetic analyzer and the scanning is accomplished by varying the magnetic field of the analyzer.

19. The method of claim 14 wherein the evaluation of the sample peak with respect to the reference peaks is by establishing the ion masses of the sample peak in accordance with the mass/time relationship determined from the time position and known ion masses of the reference peaks.

20. The method of claim 19 wherein the true time position of each sample peak is identified by determining the centroid of the peak.

21. The method of mass measuring a sample with a mass spectrometer including means to ionize substances, an analyzer and collector means comprising:

a. simultaneously ionizing the sample and a reference compound;

b. concurrently passing ions from both the sample and the reference compound 'through the analyzer to the collector means while scanning the mass spectrum to collect data as to the mass spectrum of the sample and the reference compound with the collector means;

0. producing electrical outputs with said collector means representative of the data collected;

d. digitizing signals derived from said collection means outputs;

e. evaluating the digitized signals with a computer and in so doing evaluating a first peak of the sample with respect to both a first reference compound peak of a higher position in the mass spectrum than the sample peak and a second reference compound peak of a lower position on the mass spectrum than said first reference peak to locate the position of the first sample peak with respect to a segment of the spectrum established by said first and second reference peaks thereby identifying the mass of said first sample peak from its position on said segment with respect to said first and second reference peaks thereby minimizing the effects of the deviations in the results of the scan producing said first sample peak and said first and second reference peaks from an ideal scan; evaluating the digitized signals with a computer and in so doing evaluating a second peak of the sample with respect to both one reference compound peak of a higher position on the mass spectrum than the second sample peak and another reference compound peak of a lower position on the mass spectrum than said one reference peak to locate the position of the second sample peak with respect to a second segment of the spectrum established by said one and said another reference peaks whereby to identify said second sample peak with respect to its position on said second segment thereby minimizing the effects of deviations in the results of the scan producing said second sample peak of said one and said another reference peaks from an ideal scan; and,

g. said evaluation of each of said first and second sample peaks being made according to assumed scan conditions established by said first and second and said one and said another reference peaks respectively to compensate for variations from an ideal scan due to hysteresis and the like and accordance with the mass/time relationship determined from the time position and known ion masses of the reference peaks.

24. The method of claim 21 wherein the true time position of each sample peak is identified by determining the centroid of the peak.

Claims

1. The method of mass measuring a sample with a mass spectrometer including means to ionize substances, an analyzer, and collector means comprising: a. simultaneously ionizing the sample and a reference compound; b. concurrently passing ions from both the sample and the reference compound through the analyzer to the collector means while scanning the mass spectrum to collect data as to the mass spectrum of the sample and the reference compound with the collector means; c. producing electrical outputs with said collector means representative of the data collected; d. digitizing signals derived from said collection means outputs; e. evaluating the digitized signals with a computer and in so doing evaluating a first peak of the sample with respect to a pair of reference compound peaks of different positions on the mass spectrum to locate the position of the first sample peak with respect to a segment of the spectrum established by said reference peaks and with respect to one of the reference peaks whereby to identify said first sample peak with respect to its position relative to said one peak, said segment thereby minimizing the effects of deviations in the results of the scan producing said first sample peak and said first and second reference peaks from an ideal scan; f. said evaluation of said first peak of the sample being made according to assumed scan conditions established by said pair of reference compound peaks to compensate for variations from an ideal scan due to hysteresis and the like and whereby to produce a mass analysis of a sample without need for a visible spectrum.

8. The method of mass measuring a sample with a mass spectrometer including means to ionize substances, an analyzer and collector means comprising: a. simultaneously ionizing the sample and a reference compound; b. concurrently passing ions from both the sample and the reference compound through the analyzer to the collector means while scanning the mass spectrum to collect data as to the mass spectrum of the sample and the reference compound with the collector means; c. producing electrical outputs with said collector means representative of the data collected; d. evaluating said outputs and in so doing evaluating a first peak of the sample with respect to both a first reference compound peak of a higher position on the mass spectrum than the sample peak and a second reference compound peak of a lower position on the mass spectrum than said first reference peak to locate the position of the first sample peak with respect to a segment of the spectrum established by said first and second reference peaks thereby identifying said first peak from its position on said segment with respect to said first and second reference peaks thereby minimizing the effects of the deviations in the results of the scan producing said first sample peak and said first and second reference peaks from an ideal scan; e. evaluating said outputs and in so doing evaluating a second peak of the sample with respect to both one reference compound peak of a higher position on the mass spectrum than the second sample peak and another reference compound peak of a lower position on the mass spectrum than said one reference peak to locate the position of the second sample peak with respect to a second segment of the spectrum established by said one and said another reference peaks whereby to identify said second sample peak with respect to its position of said second segment thereby minimizing the effects of deviations in the results of the scan prooducing said second sample peak of said one and said another reference peaks from an ideal scan; and, f. said evaluation of each of said first and second sample peaks being made according to assumed scan conditions established by said first and second and said one and said another reference peaks respectively whereby to compensate for variations from an ideal scan due to hysteresis and the like and whereby to produce mass analysis of a sample without need for a visible spectrum.

12. The method of claim 8 wherein the reference compound is perfluorokerosene.

14. The method of mass measuring a sample with a mass spectrometer including means to ionize substances, an analyzer, and collector means comprising: a. simultaneously ionizing the sample and a reference compound; b. concurrently passing ions from both the sample and the reference compound through the analyzer to the collector means while scanning the mass spectrum to collect data as to the mass spectrum of the sample and the reference compound with the collector means; c. producing electrical outputs with said collector means representative of the data collected; d. evaluating the outputs and in so doing evaluating a first peak of the sample with respect to a pair of reference compound peaks of different positions on the mass spectrum to locate the position of the first sample peak with respect to a segment of the spectrum established by said reference peaks and with respect to one of the reference peaks whereby to identify said first sample peak with respect to its position relative to said one peak, said segment thereby minimizing the effects of deviations in the results of the scan producing said first sample peak and said first and second reference peaks from an ideal scan; and, e. said evaluation of said first peak of the sample being made according to assumed scan conditions established by said pair of reference compound peaks whereby to compensate for magnetic variations from an ideal scan due to hysteresis and the like and whereby to produce a mass analysis of a sample without need for a visible spectrum.

16. The method of claim 14 wherein the reference compound is perfluorokerosene.

21. The method of mass measuring a sample with a mass spectrometer including means to ionize substances, an analyzer and collector means comprising: a. simultaneously ionizing the sample and a reference compound; b. concurrently passing ions from both the sample and the reference compound through the analyzer to the collector means while scanning the mass spectrum to collect data as to the mass spectrum of the sample and the reference compound with the collector means; c. producing electrical outputs with said collector means representative of the data collected; d. digitizing signals derived from said collection means outputs; e. evaluating the digitized signals with a computer and in so doing evaluating a first peak of the sample with respect to both a first reference compound peak of a higher position in the mass spectrum than the sample peak and a second referenCe compound peak of a lower position on the mass spectrum than said first reference peak to locate the position of the first sample peak with respect to a segment of the spectrum established by said first and second reference peaks thereby identifying the mass of said first sample peak from its position on said segment with respect to said first and second reference peaks thereby minimizing the effects of the deviations in the results of the scan producing said first sample peak and said first and second reference peaks from an ideal scan; f. evaluating the digitized signals with a computer and in so doing evaluating a second peak of the sample with respect to both one reference compound peak of a higher position on the mass spectrum than the second sample peak and another reference compound peak of a lower position on the mass spectrum than said one reference peak to locate the position of the second sample peak with respect to a second segment of the spectrum established by said one and said another reference peaks whereby to identify said second sample peak with respect to its position on said second segment thereby minimizing the effects of deviations in the results of the scan producing said second sample peak of said one and said another reference peaks from an ideal scan; and, g. said evaluation of each of said first and second sample peaks being made according to assumed scan conditions established by said first and second and said one and said another reference peaks respectively to compensate for variations from an ideal scan due to hysteresis and the like and whereby to produce mass analysis of a sample without a need for a visible spectrum.

22. The method of claim 21 wherein the mass spectrum is scanned exponentially.

23. The method of claim 21 wherein the evaluation of the sample peaks with respect to the reference peaks is by establishing the ion masses of the sample peaks in accordance with the mass/time relationship determined from the time position and known ion masses of the reference peaks.