US 3835319 A
Mass spectrometer combination and ion source subcombination therefor. The mass spectrometer has a magnetron ion source, a quadrupole ion analyzer and an ion collector with digital counting read-out and this is useful for residual gas analysis, partial pressure analysis and chemical composition determination and is particularly suited for ultra-high vacuum usage yielding high sensitivity and improved signal-to-noise ratio, consistent with low weight and bulk. The ion source is constructed so that photons are not produced at those loci which can be viewed through the ion exit aperture thereof. Thus, the spectrometer collector is free from impingement thereon of light energy and the signal-to-noise ratio of the instrument is improved thereby.
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Description (OCR text may contain errors)
United States Patent .[191 Roehrig et a1.
COLD CATHODE ION SOURCE MASS SPECTROMETER WITH STRAIGHT LINE ARRANGEMENT OF ION SOURCE AND ANALYZER Inventors: Jonathan R. Roehrig; Franklin L.
Torney, both of South Weymouth, Mass.
Assignee: National Research Corporation,
Continuation-impart of Ser. No. 811,139, March 27,
US. Cl 250/290, 250/423, 313/359,
Int. Cl. H0lj 39/34 Field of Search. 250/4l.9 G, 41.9 SB, 41.9 SE, 250/4l.9 DS; 313/7, 63, 230, 231; 324/33 References Cited UNITED STATES PATENTS 8/1962 Redhead 324/33 X 1/1963 Gunther 250/4l.9
11] 3,835,319 [4 1 Sept. 10,1974
3,378,712 4/1968 Lafferty ,313/7 OTHER PUBLICATIONS Primary Examiner-William F. Lindquist Attorney, Agent, or Firm-Kenneth W. Brown; Barry R. Blaker; Lawrence Chaletsky [5 7] ABSTRACT Mass spectrometer combination and ion source subcombination therefor. The mass spectrometer has a magnetron ion source, a quadrupole ion analyzer and an ion collector with digital counting read-out and this is useful for residual gas analysis, partial pressure analysis and chemical composition determination and is particularly suited for ultra-high vacuum usage yielding high sensitivity and improved signal-to-noise ratio, consistent with low weight and bulk. The ion source is constructed so that photons are not produced at those loci which can be viewed through the ion exit aperture thereof. Thus, the spectrometer collector is free from impingement thereon of light energy and the signal-tonoise ratio of the instrument is improved thereby.
9 Claims, 2 Drawing Figures 1 COLD CATHODE ION SOURCE MASS- SPECTROMETER WITH STRAIGHT LINE ARRANGEMENT OF ION SOURCE AND ANALYZER CROSS-REFERENCE Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).
FIELD OF THE INVENTION The present invention relates to mass spectrometer instruments and comprises an improvement in such instrumentation particularly suitable for residual gas analysis in vacuum pumped systems and partial pressure analysis. It may also be used for chemical composition analysis in combination with conventional means for converting a solid or liquid sample to a gas sample and feeding it to the spectrometer.
The invention is especially useful for, and distinctly advantageous in connection with operations under high vacuum mm. Hg. range and below) and specifically comprises an improved mass spectrometer combination and improved ion source subcombination therefor, the spectrometer being of the type which provides a cold cathode ion source in straight line alignment with a non-magnetic analyzer such as one of the quadrupole type.
BACKGROUND OF THE INVENTION Cold cathode ion sources are well known,e.g., US. Pat. Nos. 2,499,289; 2,636,990; 3,075,076; 3,117,224; and 3,287,589. Such sources are essentiallyderived from the Penning Ion Gauge, US. Pat. "No. 2,146,025 and US. Pat. No. 2,197,079. The advantages of cold cathode ion sources over hot cathode sources are summarized in Blum and Torney, Cold Cathode Quadrupole Mass Spectrometer, Review of Scientific Instruments, Vol. 38 No. 10, 1404-1408 (October 1967). This last cited article also sets out recent state of the art in connection with the development of a mass spectrometer comprising a cold cathode magnetron ion source (which source is a modification of the Redhead ion gauge, Pat. No. 3,051,868) coupled with a quadrupole ion analyzer (Pat. No. 2,939,952) in a straight line arrangement. This straight line arrangement and the choice of components thereof minimizes complexity, costs, weight and bulk and on the whole provides good sensitivity and resolution. However, a significant component of noise in the form of light energy is picked up at the multiplier and the multiplier amplifies this photon noise along with the desired signal of ion current. It was discovered that by a specific structural modification of the cold cathode ion source the problem of low signal-to-noise ratio of the instrument could be attenuated.
OBJECTS OF THE INVENTION It is the object of the present invention to providean improvement in cold cathode ion sources and mass spectrometers utilizing such ion sources to enhance their utility for the above stated applications by improving the signal-to-noise ratio of the spectrometer consistent with retained high sensitivity and resolution,
low weight and bulk, simplicity and economy of the instrument as a whole.
GENERAL DESCRIPTION :OF THE INVENTION The mass spectrometer of the invention utilizes an essentially straight line alignment of an ion source, nonmagnetic analyzer and electron-multiplier collector.
Noise is reduced by a particular cold cathode ion source construction which mitigates against reception by the collector of photon noise generated in the ion source. This desirable feature of operations is obtained by the use of an axially extending cathode rod stub, hollowed at its free terminus and which rod, when viewed through the ion exit aperture of the source, presents a substantially photon-free view.-
SPECIFIC DESCRIPTION OF THE INVENTION The invention is now specifically described with reference to the accompanying drawings wherein FIGS. 1 and 2 are schematic sectional views of the apparatus of the invention, or critical portions thereof, showing two embodiments of the apparatus.
Referring now to FIG. 1, there is shown a mass spectrometer l with a cylindrical body having an inlet opening 2 at one end surrounded by an inlet flange 3 (alternatively the flange opening could be in the side of the cylindrical body).
There are three major components of the spectrometer a cold cathode ion source 10, a quadrupole ion analyzer 20 and electron multiplier/ion collector 30. Associated with the spectrometer is an output reading and/or recording means 40.
The ion source 10 represents an important modification of the Blum and Torney ion source described in the authors hereinbeforecited article. It comprises an open-ended cylindrical anode 11 with essentially plate form discharge cathodes 12 and 13 located at the respective ends thereof. Said discharge cathodes l2 and 13 may also comprise auxiliary cathodes or polished cathode margins 12A and 13A whose function and precise structure are more fully described in Pat. No. 3,051,868. Downstream cathode 13 bears a centrally located ion discharge exit aperture 14 through which ions are extracted into analyzer 20. Upstream cathode 12 is provided with a cathode stub 15 located at the center thereof and extending at least partially into the space defined by cylindrical anode 11. A critical feature of said cathode stub 15 resides in the provision of a hollowed-out free terminus 16 thereon, the function of which will be discussed in more detail hereinafter. This hollowed-out feature of the terminus 16 may be provided such as by employing a tubular construction of the entire stub cathode 15 (as shown in FIG. 1) or, alter-natively, as shown in FIG. 2 by employing a solid cathode stub which is inwardly dished at the free terminus thereof. A magnetic field produced by an externally or internally arranged magnet 19 threads the ion source structure as indicated by the arrow H.
The quadrupole analyzer 20 comprises four rods 21 and may also desirably comprise an entrance aperture 17 and an exit aperture 18. The spacing of the quadrupole analyzer 20 from ion source -10 and electron multiplier/collector 30 is enlarged in the drawing for purposes of illustration.
The electron multiplier/collector 30 comprises an electron multiplier comprising an ion accelerating screen grid 31 and a plurality of dynodes 32, and a collector 39.
The secondary electron current at collector 39 is read out by bracketed means 40 which comprises a counting preamplifier 41, pulse amplifier 42 and a scaler 43. Alternatively, the read-out means can comprise an electrometer.
The use of an electron multiplier improves the sensitivity (and range) and signal-to-noise ratio of the instrument as a whole by amplifying the ion current output of the analyzer through several stages of secondary electron emission. However, the gain stability of commercial multipliers is ordinarily inadequate to provide reproducible data unless the gain is constantly reviewed or recalibrated. The latter step is ineffective in working at the lowest levels of ion current (correspond-- put for digital data acquisition. The'only drawback of counting technique is the cost and complexity of the circuitry involved. However, on the whole, it is normally a desirable feature in the instrument.
In working with the combination described above but with a continuous rather than hollow terminused cathode stub 15, major noise problems were discovered at high vacuum operating pressures. Specifically, a pressure and anodevoltage dependent noise was noticeable at theselow pressures and research has shown that the noise is not attributable merely to leakage currents or the like. It was discovered that the cold cathode ion source was the source of this noise. It is surmised that the origin of suchnoise resides in the production of photons within the ion source and the emission therefrom of soft x-rays which, absent the hollow terminused cathode stub of theinvention, are produced at a locus directly receivedby the collector. it was further surmised that the critical portion of the ion source with respect to this problem was the terminus of the cathode stub, since it comprised substantially the only internal ion source structure positioned in line of sight relationship with the collector. This surmise may or may not be correct. But in acting upon it, the cathode stub was modified to the hollow terminus style of the invention with excellent results in attenuating the noise problem, thereby vastly increasing the utility of the resulting overall instrument. 1
As mentioned, the key structural modification involved is the provision of hollow terminus 16 in cylindrical stub 15 as shown in FIG. 1. Apparently, the collector views the hollow interior of the stub terminus 16 which is field free and thus produces few if any photons. The stub 15 outer diameter is preferably equal to or slightly larger than the diameter of ion exit aperture 14. Generally, the length of cathode stub 15 is about one-half the distance from its associated cathode '12 to the cathode 13. The cathode stub is disposed along the central axis 50 of the device. The above dimensions and geometric relationship are desirable for optimum sensitivity and resolution of the ion current signal produced by the source. The reduction of photon noise is achieved by the hollow terminused stub configuration without disturbing these other optimum dimensions.
FIG. 2 shows a second embodiment of the invention in which the cathode stub of an ion source 210 is of generally a solid construction and has an average outer diameter less than that of aperture 14. However, terminus is provided with the critical hollowed feature by dishing thereof. Additionally, flared edge 68 is of equal or preferably somewhat greater diameter than the diameter of aperture 14. The cathodes 12, 13 and the anode 11 are as in the FIG. 1 specie and the auxiliary cathodes or equivalents (such as polished edges of 12A and 13A) of FIG. 1 are not shown.
Testing has shown a substantial reduction of ion source derived photon noise through the use of the improved ion sourceof the present invention without loss of instrument sensitivity and resolution and without the addition of anysignificant weight, bulk, complexity or cost to the instrument.
In experiments with the ion sourcespecies of FIGS. 1 and 2 it was found that the employment of the tubular cathode stub of the FIG. 1 specie yielded a 50-fold reduction in photon noise compared with a mass spectrometer equipped with a cathode stub having a continuous terminus profile in the ion source. That is, the FIG. 1 specie had only one-fiftieth the noise of the prior Blum and Torney device. The FIG. 2 specie, comprising the solid stub cathode having the dished and outwardly flared terminus, effected a further reduction in noise by a factor of 3. Each of the l and 2 species appears to provide easy starting of the ion source at low pressures. Further, high sensitivity is maintained in both the FIGS. 1 and 2 species.
The cold cathode ion source of the present invention can be combined with other known techniques of spectrometer noise reduction (e.g., a slightly offset multi plier as shown in Bennewitz and Wedmeyer, I72 Zeitschrift fur Physik l-l8(l963)), although at a cost of some complexity, cost and loss of sensitivity by a factor of about two. I
Still other variations within the scope of the present invention will be apparent to those skilled in the art. Accordingly, it is intended that the present disclosure shall be construed as illustrative in nature and not as limiting of the invention.
What is claimed is:
l. A cold cathode ion discharge source comprising a cylindrical anode having open ends and a central longitudinal axis; first and second plate form discharge cathodes located at the respective ends of said anode, each said plate form cathode being substantially perpendicular to said axis,
said second cathode having an ion exit aperture therethrough in coaxial alignment with said axis and in open communication with the space defined by said anode,
said first cathode comprising a stub extending into the space defined by the cylindrical anode and in coaxial alignment with the longitudinal axis thereof, said stub having its terminus located short of the ion exit aperture of said second cathode, said terminus being open-ended and hollowed out to a depth sufficient to prevent glow discharge therefrom.
2. The cold cathode ion source of claim 1 wherein the diameter of the terminus of said stub is equal to or greater than the diameter of said ion exit aperture.
3. The cold cathode ion source of claim 1 wherein said stub is of essentially solid construction and is dished inwardly at its terminus.
4. The cold cathode ion source of claim 1 wherein said stub is of tubular construction.
5. The cold cathode ion source of claim 1 wherein the terminus of said stub is flared.
6. The cold cathode ion source of claim 1 where the length of said stub is sufficient to locate the terminus thereof at about mid-way between said first and second cathodes.
7. A mass spectrometer comprising A. a cold cathode ion discharge source comprising a cylindrical anode having open ends and a central longitudinal axis; first and second plate form discharge cathodes located at the respective ends of said anode, each said plate form cathode being substantially perpendicular to said axis, said second cathode having an ion exit aperture therethrough in coaxial alignment with said axis C. ion collector means located downstream from said analyzer means and having an ion collector entry in coaxial alignment with said longitudinal axis of said cylindrical anode.
8. The mass spectrometer of claim 7 wherein the analyzer means of (B) is of the quadrupole type.
9. The mass spectrometer of claim 7 wherein the terminus of said stub is flared.