US3641340A

US3641340A - Multichannel readout mass spectrometer

Info

Publication number: US3641340A
Application number: US859866A
Authority: US
Inventors: Willem J Van Der Grinten; George Jernakoff
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1969-09-22
Filing date: 1969-09-22
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08

Abstract

A multichannel readout mass spectrometer is described wherein the amplitude of the control voltage governing the passage of an ionized stream of a gas mixture through a quadrupole mass filter is employed to supervise the selection of readout channels individually recording various constituents of the ionized stream. For each readout channel, the control voltage is compared with a first preselected voltage to activate logic circuitry initiating recordation of a constituent of selected mass upon the readout channel and readout of the constituent is continued until a signal is generated from a second control voltage comparator circuit. Switching circuitry also is provided for recording the mass spectra of all constituents on a common channel simultaneously with a signal indicative of the recording interval for any one of the individual readout channels. The recording interval then is correlated with a single constituent forming the mass spectra whereupon readout of only the single constituent can be switched to the individual readout channel.

Description

United States Patent Van Der Grinten et al.

[ 1 Feb. 8, 1972 [54] MULTICHANNEL READOUT MASS SPECTROMETER [72] Inventors: Willem J. Van Der Grinten, Scotia; George Jernakoff, Loudonville, both of N.Y.

Kronenberger ..250/41.9 R

OTHER PUBLICATIONS Continuous Observation of Several Mass Peaks with Quadrupole Mass Spectrometer, by Arthur et al. Review of Sci. lnstr. V37, N. 3, pp. 794-5,June 1966.

A Programmable Magnetic Field Mass Spectrom with On- Line Data Processing, by Wasserberg et al. Review of Sci. Instr. V. 40, N. 2, Feb. 1969.

hul 240 Primary Examiner-Walter Stolwein Assistant Examiner-D. C. Nelms Atmrney-Richard R. Brainard, Paul A. Frank, John J. Kissane, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman ABSTRACT A multichannel readout mass spectrometer is described wherein the amplitude of the control voltage governing the passage of an ionized stream of a gas mixture through a quadrupole mass filter is employed to supervise the selection I of readout channels individually recording various constituents of the ionized stream. For each readout channel, the control voltage is compared with a first preselected voltage to activate logic circuitry initiating recordation of a constituent of selected mass upon the readout channel and readout of the constituent is continued until a signal is generated from a second control voltage comparator circuit. Switching circuitry also is provided for recording the mass spectra of all constituents on a common channel simultaneously with a signal indicative of the recording interval for any one of the individual readout channels. The recording interval then is correlated with a single constituent forming the mass spectra whereupon readout of only the single constituent can be switched to the individual readout channel.

4 Claims, 5 Drawing Figures RHMP PARA P10!!! GEN ERR 70R READOUT emu/v54 M5 SELECT/0N CIRCUIT CIRCUIT CHANNEL SELECT/01V CIRCUIT PATENTEUFEB a ma SHEET 3 0F 3 TIME M G 0R NQL TIME [7) ve n tors: I Willem J van der Gr-v'n ten,

George L/Qf'fld/(Ofi by Q Ava/0 he/r' tormey MULTICIIANNEL READOUT MASS SPECTROMETER This invention relates to a multichannel readout mass spectrometer and in particular, to a mass spectrometer wherein the electrical signal controlling the selective passage of diverse constituents of an ionized stream of a low pressure gas mixture is utilized to supervise readout of various constituents of the stream on distinct channels.

Mass spectrometers characteristically monitor gaseous streams containing a plurality of constituents by the visual display of a mass spectrum portraying the quantity of each constituent as a function of time. During continuous monitoring, the mass spectrum is repeated at regular cycles impeding identification and analysis of diverse critical constituents within the mass range of the spectrometer. To alleviate the difficulty of identifying peaks in repeated mass spectra, DC control voltages having one or more stepped amplitude levels have been employed to record on a single chart the peaks of only selected constituents of the gaseous stream. While a stepped DC control voltage somewhat eases interpretation of repeated mass spectra, only a single mass peak is obtained for each amplitude step and some peak identification problems still occur during continuous cycling of a plurality of mass peaks on a single chart utilizing a multistepped control voltage. To alleviate the analysis problem associated with repeated mass spectra of a plurality of constituents, it heretofore has been suggested that the mass spectra of various constituents be recorded on diverse charts.

It is therefore an object of this invention to provide a novel mass spectrometer wherein the mass spectra of different constituents are recorded on diverse channels. I

It is also an object of this invention to provide a novel multichannel readout mass spectrometer wherein the mass spectrum recording range of each channel can be individually varied.

It is a still further object of this invention to provide a multichannel readout mass spectrometer having means for correlating the mass spectrum recording range of each readout channel with various peaks in the mass spectrum.

These and other objects of this invention are achieved by a multichannel readout mass spectrometer having means for ionizing a gaseous stream containing a plurality of constituents, selection means responsive to a control signal having an amplitude varying as a function of time for selectively passing ionized constituents of given mass and means for directing to diverse readout channels the recordation of the measured quantity of selected constituents passed through the selection means. To supervise the recording of various mass spectra on differing readout channels, means are provided for comparing the amplitude of the control signal with a signal of preselected amplitude and means responsive to the comparison means serve to direct recordation of the quantity of a constituent in the gaseous stream to a first readout channel for a preselected period whereupon suitable means are activated to terminate the recording upon the first channel. The mass spectrometer also is characterized by second comparison means for comparing the amplitude of the control signal with a second signal of second predetermined amplitude and means responsive to the second comparison means function to direct the recording of a differing constituent of the gaseous stream to a second readout channel. Desirably the spectrometer also includes means for effecting a readout of a plurality of constituents of the gaseous stream upon a common channel along with means for independently indicating upon the common channel the recording interval of a differing readout channel. The recording interval then can be varied by suitable means to correlate with a selected constituent desired to be recorded thereon.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and adyantages thereof may best be understood by reference to the following description, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of a multichannel readout mass spectrometer in accordance with this invention,

FIG. 2 is a pictorial illustration of various waveforms in the mass spectrometer,

FIG. 3 is an enlarged pictorial illustration of a mass spec trum peak during correlation of a portion of the mass spectrum with a single readout channel,

FIG. 4 is an isometric illustration of an alternate mass selec tor suitable for use with this invention, and

FIG. 5 is a pictorial illustration of a control waveform suitable for utilization with the mass spectrometer of this invention.

A multichannel readout mass spectrometer in accordance with this invention is illustrated in FIG. I and generally comprises a quadrupole mass spectrometer 10 for analyzing a flowing gaseous stream, portrayed by flow line 12. having a plurality of constituents and a readout channel selection circuit l4 controlling the selection of a single readout channel I6A16D of the spectrometer for individually recording the mass of selected constituents of the gaseous stream. In the spectrometer portrayed in FIG. 1, four differing constituents of gaseous stream 12 are recorded on diverse channels l6A-16D under the supervision of four structurally identical readout channel selection circuits l4A-l4D while a fifth readout channel 16E is provided to function as a conventional mass spectrometer readout channel for recording the mass spectra of all constituents within the gaseous stream.

Quadrupole mass spectrometer 10 is conventional in design and includes an electron beam source 18, e.g., a l-milliampere electron beam gun, for ionizing flowing gaseous stream 12 whereupon the ionized stream is accelerated by a potential gradient formed between suitably energized plates 20 to pass axially through a quadrupole mass filter 22 formed by four elongated cylindrical conductors 24 disposed at 90 intervals about the ionized gaseous stream. One end of each conductor disposed on diametrically opposite sides of the ion bean are interconnected to permit electrical energization of the mass filter with the ends of elongated conductors 24A and 24C at the output of the mass filter being joined by lead 26A while the ends of conductors 24B and 24D at the input of the mass filter are interconnected by lead 268.

Energization for the mass filter is provided by a ramp generator 28 which typically may be a relaxation oscillator producing a negative going ramp voltage 30 of, for example, from O to -13 volts to control the mass of the constituents selectively passed by the mass filter. The ramp voltage output from ramp generator 28 is fed to a paraphase amplifier 32 to generate a positive going, e.g., from 0 to 2 kv., ramp voltage on output terminal 34A for energization of cylindrical conductors 24B and 24D while a symmetrical ramp signal from 0 to -2 kv., is generated on output terminal 348 for energization of lead 26A interconnecting cylindrical conductors 24A and 24C. The negative ramp output from ramp generator 28 also is fed as one input to RF modulator 36 along with an RF signal (suitably having a frequency of two megacycles) from RF generator 38 to produce amplitude modulated

RF output signals

40 and 40A each having a Z-megacycle wavetrain within an envelope symmetrically disposed about the zero axis and extending from 0 to 2 kv. Typically the RF

wavetrains forming signals

40 and 40A are phase displaced by l relative to each other with the envelopes containing the signals being in phase. Although

RF signals

40 and 40A are described as being amplitude modulated between 0 to 2 kv., the RF signal can be modulated to any desired voltage level, e..g., from 0-4 kv., provided the ratio between the output signals from paraphrase amplifier 32 and the envelopes of the RF modulated signals is always constant. The RF output signals from modulator 36 then are applied to leads 26B and 26A mass filter 22 in phase with the ramp signals from paraphrase amplifier 32 to produce a crossed DC and RF field within the mass filter.

As the ionized electron stream accelerated by plates 20 enters into the axial interior of mass filter 22, the crossed DC and RF fields of the mass filter interact with the ions passing therethrough the deflect the ions along a spiral trajectory dependent upon the mass of the ions and amplitude of the electrical signals applied to the mass filter. Thus, as the amplitude of the signals from paraphase amplifier 32 and RF modulator 36 linearly increase from O to 2 kv., the mass of the ions passed by mass filter 22 also linearly increases, e.g., from the passage of an ion mass of zero to the passage of an ion mass of 150. At any particular voltage level of ramp generator 28 however, only ions of a preselected mass corresponding to the particular voltage level are passed in a spiral trajectory through the mass filter while ions of a differing mass interact with the combined RF and DC fields of the mass filter to produce an unstable oscillatory trajectory in the ions of differing mass. Those ions having an unstable trajectory are discharged upon cylindrical conductors 24 or metallic sidewall 42 surrounding the mass selector and therefore do not exit the mass filter as charged ions.

In general, the mass of the ions passed through the mass filter varies as a linear function of the control voltage from ramp generator 28 with ions of a given mass only being passed by a single amplitude control voltage, e.g., nitrogen is passed through the mass filter at a control voltage of 2.05 v. while oxygen and carbon dioxide are passed through the mass filter at control voltages of 2.34 and 3.22 volts, respectively. Because ramp voltage 30 is characterized by an amplitude which varies with time, diverse constituents of gaseous stream 12 are passed by differing time intervals with the passage of each constituent being dependent upon ramp signal 30 reaching the amplitude corresponding to the mass of the constituent.

Ions passing through the quadrupole mass filter are collected by a suitable amplifying device, for example, a l4-stage copper-beryllium or silver-magnesium electron multiplier 44. Typically electron multiplier 44 is characterized by an amplification factor of 10 and the output signal from the electron multiplier is fed to a conventional solid-state amplifier 46 to generate a voltage level suitable for driving conventional pen recorders 48A-48E in each or readout channels l6A-16E. To inhibit overloading of amplifier 46 by the readout channels, an operational amplifier 48 is connected as an impedance transformation device between amplifier 46 and readout channels 16, i.e., the output signal from the operational amplifier is fed back through resistor 50 to the negative node of the operational amplifier, to reduce the input impedance of the multiple channels to a value below 1 ohm.

Selection of the readout channel for recording a constituent of a desired mass is achieved by feeding the negative going ramp signal from ramp generator 28 to readout channel selection circuits I4A-14D with operational amplifier 52 connected between the ramp generator and the readout channel selection circuits serving as an impedance transformer to inhibit overloading of the ramp generator by the readout channel selection circuit contains two

operational amplifiers

56 and 58 having positive input nodes at ground potential while the negative going ramp voltage output from operational amplifier 52 is fed through identical resistors 60 to the negative input nodes of

operational amplifiers

56 and 58 tending to switch the operational amplifiers to a node generating a positive going output level. Positive voltages tending to drive the operational amplifiers to the alternate mode of operation, e.g., producing a negative going output level from the operational amplifiers also are applied to the negative nodes of

operational amplifiers

56 and 58 from window potentiometer 62 with wiper arm 64 applying a first positive voltage to operational amplifier 58 through resistor 66 while a second, slightly higher positive voltage, is applied from wiper arm 68 through resistor 70 to the negative node of operational amplifier 56.

Resistors

60 and 66 function as a voltage-dividing circuit for the negative going ramp voltage from operational amplifier 52 and the potentiometer sets positive voltage from potentiometer 62 and, at the initiation of a mass spectrum cycle with negative going ramp voltage 30 at zero volts, the positive voltage from the potentiometer tends to maintain operational amplifier 58 in an operational mode producing a negative going input level therefrom. Similarly, the negative going ramp voltage from operational amplifier 52 is compared with the positive voltage from window potentiometer 62 in a voltage divider network formed by

resistors

60 and 70 and operational amplifier 56 initially is driven by the voltage setting of window potentiometer 62 to produce a negative going output level from the operational amplifier. The output pulse from operational amplifier 56 then is fed through an inverter circuit 72 to a first input terminal of NAND-gate 74 while the output from operational amplifier 58 is applied directly to the other input and the NAND gate. Thus with

operational amplifiers

58 and 60 producing negative going outputs, inversion of the output level from operational amplifier 56 in inverter circuit 72 inhibits the generation of a positive going output from NAND- gate 74.

When readout channel selection circuit l4A desirably produces the display on readout channel e.g., of only a single element, e.g. water, of the conventional mass spectrum display illustrated in FIG. 2A, wiper arm 64 is set along window potentiometer 62 at a location such that the negative going ramp signal, illustrated in FIG. 2B, drives the negative input node of operational amplifier 58 negative at time T,, i.e., the time interval corresponding to the initiation of a water readout from mass spectrometer 10. Operational amplifier 58 then generates a positive going output level which triggers NAND- gate 74 to produce the positive going level therefrom. The output level from NAND-gate 74 is inverted in inverter circuit 76 and applied as a gating signal to transistor 77 driving the transistor to a conductive state. Conduction through transistor 77 energizes coil 78A to close relay 80A permitting the initiation of readout from the mass spectrometer on channel 16A. Readout is continued until time T i.e., the termination of the water readout interval from the mass spectrometer, whereupon the positive potential applied to the operational amplifier 56 from window potentiometer 62 is exceeded by the negative going ramp signal thereby switching operational amplifier 56 to and alternate mode producing a positive output from the operational amplifier. The positive input from operational amplifier then is inverted and applied to NAND gate 74 to ter minate the triggering output signal from the NAND gate. Coil 78A thereupon is deenergized and relay 80A opens inhibiting further recording of the mass spectrum output on channel 16A. Thus the mass spectrum of only a single constituent of the fluidized stream i.e., water, is recorded on channel 16A, as illustrated in FIG. 2C,,, during each cycle of the mass spectrometer and variations in the water concentration of the ionized stream between cycles is easily observed. In a similar manner, readout channel selection circuit 14B compares the negative going ramp signal from operational amplifier 52 with dual potentiometer set positive voltages to activate relay 80B during the interval T -T,, when a filter to permit recording of nitrogen, spectrum, illustrated in FIG. 2C,,, on readout channel 16B. Readout channel selection circuits 14C and 14D also similarly serve to close relays 80C and 80D, respectively, only during the time period when a constituent, e.g., oxygen and carbon dioxide, desired to be recorded, as shown in FIGS. 2C and 2C respectively, is passed through the mass spectrometer by the negative going ramp signal from ramp generator 28.

Although the system of FIG. 1 is completely adequate for scan rates, of for example, one mass spectrum scan per second, when extremely high scan rates, e.g., I00 scans per second are desired, the system of FIG. 1 preferably is completely fabricated with solid state components. To effect this result, a field effect transistor (not shown) gated by the output pulse from NAND-gate 74 can be substituted for mechanical relays 80A-80D thereby permitting controlled readout of a single constituent on each channel.

A fifth readout channel 16E also is provided to display the entire mass spectrum of gaseous stream 12, illustrated in FIG. 2A, upon the mechanical closure of any of switches 82A-82D to energize coil 84 with current from voltage source 86 thereby closing relay 80E disposed between the common readout channel and the output from the mass selector. A second switch 90 in each of readout selection circuits 14A-14D is mechanically ganged with an individual one of switches 82A-82D and, upon closures of switch 82A, the gating signal for transistor 77 is applied to common readout chan nel 16E simultaneously with the mass spectrum. Thus, a negative going blip 92, illustrated in FIG. 2A, is produced on readout channel 16E with each application of a triggering signal to resistor 77 simultaneously with the mass spectra of the various constituents of gaseous stream 12 and the advent of the transistor-triggering signal can be correlated with the initial passage of the desired element through mass filter 22 by an alteration in the setting of wiper arm 64 of window potentiometer 62. Similarly movement of wiper arm 68 along resistive element 94 of window potentiometer 62 control the period of the transistor-triggering signal to regulate the total readout period of channel 16A. The readout intervals of each of channels 168 and 16D also are accurately correlated with various constituents to the gaseous stream to be individually recorded on the channels by the closure of one of switches 82B82D to display the triggering interval of the corresponding readout channel on common channel 16E and the voltage setting of the window potentiometer in the readout channel selection circuit controlling the display interval on the channel is altered by the desired amount. After correlation of the triggering signal with the desired constituents, switches 82A- 82D are opening deactivating coil 84 to permit readout of only the individual constituents upon channels l6A-l6D.

When only a selected portion of the mass spectrum of a single constituent desirably is read on a channel, the pulse width I and location of negative going blip 92 is correlated with he mass spectrum peak of the constituent by a variation in the setting of window potentiometer 62 to produce both discontinuities, illustrated by reference numeral 93 in the enlarged mass spectrum of FIG. 3, at the edges of the blip locations and a slight diminution in the mass spectrum (as illustrated by dotted lines) between discontinuities. Upon switching the mass spectrum readout to the channel corresponding to the negative going blip, the effects of the negative going blip upon the mass spectrum are removed and only the solid portion of the mass spectrum lying between the discontinuities is displayed.

Although this invention has been described utilizing a quadrupole mass spectrometer as the mass filter for selectively passing various constituents of an ionized gaseous stream, other type mass spectrometers, e.g., the monopole mass spectrometer 94 illustrated in FIG. 4 of magnetic deflection type mass spectrometers, also can be employed in the practice of this invention. Monopole mass spectrometer 94 generally comprises a single elongated cylindrical conductor 96 extending longitudinally above a V-shaped conductive plate 97 with the area between the conductor and V-shaped plate serving as the crossed DC and RF field zone for the selective transmission of the ionized gaseous stream. The negative going ramp signal from paraphrase amplifier 32 and the RF modulated signal from RF modulator 36 are applied to the end 96A of cylindrical conductor 96 remote from ionized stream 12 while the positive going ramp output from paraphrase amplifier 32 and the RF signal from modulator 36 are applied to the gas input end of V-shaped plate 97. Upon the application of the RF and DC signals to the monopole mass filter, the ionized stream passing in the region 98 between the elongated conductor and the V-shaped plate is driven in a spiratprojectory with ions of a mass not comparable to the amplitude of the ramp signal having an unstable oscillatory trajectory to inhibit passage through the monopoletilter. "ljhe operationjgf the multichannel mass readout portion of the spectrometer is identical to the operation described with reference to the quadrupole spectrometer of FIG. 1 with the output signal from ramp generator 28 being compared to two potentiometer set voltages in each of a plurality of readout channel selection circuits to record the mass spectra of various constituents on individual readout channels.

The circuitry of FIG. 1 also is operable when stepped voltage. as illustrated by waveform 99 of H6. 5 is employed as the mass spectrometer control voltage rather than the ramp signal from ramp generator 28. With a stepped control voltage for the mass spectrometer, the output signals from para phase amplifier 32 are positive and negative going stepped voltages while the amplitude modulated RF output from RF modulator 36 is enclosed within a stepped envelope of similar configuration. The stepped control voltage also is fed to each of readout channel selection circuits l4A-l4l) wherein the 7 window potentiometers ofthe individual selection Circuits are set to produce recordation of an individual readout channel for each amplitude step of the control voltage signal. The operation of the circuitry in all other respects is identical to that described with reference to FIG. I wherein a ramp signal was employed asthe control voltage.

Patent of the United States is:

l. A multichannel readout mass spectrometer comprising means for ionizing a gaseous stream containing a plurality of constituents in unknown degrees of concentration, means for generating a control signal having an amplitude varying as a function of time, selection means responsive to the amplitude of said control signals for selectively passing ionized constituents of a given mass, means for measuring the quantity of ionized constituents passed by said selection means, a plurality of readout channels for recording the measured quantity of ionized constituents passed by said selection means, means for comparing the amplitude of said control signal with a signal of preselected amplitude, means responsive to said comparison means for directing the recording of said constituents to a first one of said readout channels means for terminating recording of said constituents upon said first readout channel, second comparison means for comparing the amplitude of said control signal with a second signal of a second predetermined amplitude, and means responsive to said second comparison means for directing the recording of differing ionized constituents of said gaseous stream to a second readout channel.

2. A multichannel readout mass spectrometer according to claim 1 further including means for effecting readout of said plurality of constituents upon a common channel, means for independently sensing upon said common channel, the recording interval of a differing readout channel, and means for correlating the recording interval of said differing readout channel with a constituent of a given mass derived to be recorded upon said differing readout channel.

3. A multichannel readout mass spectrometer according to claim 1 wherein means selectively passing ionized constituents of said gaseous stream comprise at least two elongated conductors disposed on opposite sides of said ionized stream and means for simultaneously applying DC and RF signals to said elongated conductors to effect a spiral trajectory in the ionized stream passing between said elongated conductors.

4. A multichannel readout mass spectrometer according to claim 3 wherein said control signal is a ramp voltage.

by LetterS

Claims

1. A multichannel readout mass spectrometer comprising means for ionizing a gaseous stream containing a plurality of constituents in unknown degrees of concentration, means for generating a control signal having an amplitude varying as a function of time, selection means responsive to the amplitude of said control signal for selectively passing ionized constituents of a given mass, means for measuring the quantity of ionized constituents passed by said selection means, a plurality of readout channels for recording the measured quantity of ionized constituents passed by said selection means, means for comparing the amplitude of said control signal with a signal of preselected amplitude, means responsive to said comparison means for directing the recording of said constituents to a first one of said readout channels, means for terminating recording of said constituents upon said first readout channel, second comparison means for comparing the amplitude of said control signal with a second signal of a second predetermined amplitude, and means responsive to said second comparison means for directing the recording of differing ionized constituents of said gaseous stream to a second readout channel.

2. A multichannel readout mass spectrometer according to claim 1 further including means for effecting readout of said plurality of constituents upon a common channel, means for independently sensing upon said common channel interval of a differing readout channel, and means for correlating the recording interval of said differing readout channel with a constituent of a given mass desired to be recorded upon said differing readout channel.