US 3641339 A
A small magnetic field is applied to the ion beam from the mass spectrometer ion source to deflect a substantial proportion of the relatively light carrier gas (e.g., helium) ions in the beam from impinging on the beam monitor electrode of the mass spectrometer, while leaving the relatively heavy sample (e.g., hydrocarbon) ions substantially undeflected. This increases the detection sensitivity and alleviates the need for ion source retuning between recording and standby conditions.
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Description (OCR text may contain errors)
Umted States Patent 1 3,641,339 McCormick 1 Feb. 8, 1972  GAS CHROMATOGRAPHY-MASS  References Cited SPE TR METRY C 0 UNITED STATES PATENTS  yf r g gg'z Basmgsmke 3,429,105 2/1969 Llewellyn et a1 ..73/23.1 x P 3,475,604 10/1969 Noda et a1 ..250/41.9 ME  Assignee: United Kingdom Atomic Energy Authority,
London, England Primary Examiner-James W. Lawrence Assistant Examiner-A. L. Birch  June 1969 Attorney-Larson, Taylor and Hinds  Appl. No.: 835,706
ABSTRACT  Foreign Appicafion P i it D m A small magnetic field is applied to the ion beam from the mass spectrometer ion source to deflect a substantial propor- July 5, 1968 Great Britain ..32,335/68 on of the relatively light carrier gas (egl helium) ions in the beam from impinging on the beam monitor electrode of the  US. Cl. ..250/41.9 G, 73/23.l, 250/419 S, mass Spectrometer, while leaving the relatively heavy Sample 250/419 ME (e.g., hydrocarbon) ions substantially undeflected. This in-  Int. Cl. .1101] 39/34 creases the detection Sensitivity and alleviates he need for ion  Search "250/41 4 source retuning between recording and standby conditions.
7 Claims, 2 Drawing Figures PATENTEDFEB @1912 3.6% ,339
GAS CHROMATOGRAPHY-MASS SPECTROMETRY BACKGROUND OF THE INVENTION This invention relates to combined gas chromatographymass spectrometry. This technique is described, for example, in a review paper by Leemans and McCloskey in J. Am. Oil Chem. Soc., Vol. 44, No. I, pp. 11-17(1967).
In combined gas chromatography-mass spectrometry the samples are carried from the chromatographic column to the mass spectrometer ion source in a carrier gas, usually helium, and the total ion current monitor of the mass spectrometer is used as the gas chromatograph detector. The limit of sensitivity is usually determined by the fluctuations of the carrier gas pressure in the mass spectrometer ion source. These are caused by variations in the gas flow throughthe chromatographic column and in the pumping speed of the mass spectrometer, and may be such as to produce fluctuations in the total ion current of amplitude equivalent to that due to emergence of a peak from the chromatograph. This effect can be avoided by reducing the energy of the ionizing electron beam in the spectrometer ion source to e.v. Helium is not ionized at this energy (ionization potential 24.6 e.v.) and variations of the helium pressure thus have no etfect on the mass spectrometer total ion current. However at this lower-than-normal electron energy, ionization cross sections are greatly reduced, and this situation is aggravated by the fact that a lower ionizing beam current of electrons has also to be used to ensure filament stabilization at the reduced voltage. The total result of this can be a reduction of at least fiftyfold in the mass spectrometer sensitivity. Further, mass spectra for various reasons are normally recorded at an energy of the electron beam of 50 to 70 e.v. If the spectrometer is run at 20 e.v. during standby conditions, i.e., until a gas chromatograph peak is detected by the beam monitor electrode, and a mass scan is then required, there must be some means of switching to, say, 70 e.v. at the start of the mass scan and back to 20 e.v. at the end of it. Ion source tuning for maximum sensitivity is generally different for each of these settings, and thus it is impossible to have the maximum sensitivity for the two conditions simultaneously.
The above-described difficulties exist even where means are provided between the column and the ion source for removing carrier gas from the gas stream, since such means inevitably leave some carrier gas in the stream.
SUMMARY OF THE INVENTION According to the present invention in apparatus comprising a gas chromatographic column and a mass spectrometer for analyzing the output from said column of samples mixed with a carrier gas, said mass spectrometer including an ion source to which the output of the column is fed and an ion beam monitor electrode, a magnet is provided for applying a magnetic field to the ion beam from the ion source to deflect a substantial proportion of the relatively light carrier gas ions in the beam from impinging on the beam monitor electrode while leaving the relatively heavy sample ions substantially undeflected.
Said beam monitor electrode may be preceded by an apertured electrode and said magnetic field may be arranged to cause said deflected carrier gas ions to strike said apertured electrode while said undeflected sample ions pass therethrough.
Said beam monitor electrode-may be preceded by an electrostatic analyzer, said magnet being located between the ion source and the electrostatic analyzer.
The beam monitor electrode may be followed by a magnetic analyzer and the magnetic field of said magnet may be aligned approximately normal to:the field of said magnetic analyzer. Said mass spectrometer may comprise beam-aligning electrodes following said ion source, and said magnet may be located adjacent said beamaligning electrodes.
Said mass spectrometer may comprise at least one pair of beam-aligning electrodes following said ion source and substantially parallel to the beam, said magnet preceding said electrodes and being arranged to deflect the carrier ions outside said pair of electrodes.
DESCRIPTION OF THE DRAWINGS To enable the nature of the present invention to be more readily understood, attentionis directed, by way of example, to the accompanying drawings, wherein FIG. 1 is a diagrammatic view of a combined gas chromatography-mass spectrometry apparatus and FIG. 2 illustrates a modification.
DESCRIPTION OF A PREFERRED EMBODIMENT In FIG. 1 a chromatographic column is represented at 1. The arrow 2 represents a flow of helium carrier gas through the column, carrying with it molecules of the samples, e.g., hydrocarbons, to be analyzed. The successive carrier-borne sample peaks enter an ion source 3, where they are subjected to an ionizing electron beam 4. As in the known technique, positive ions are extracted from the source 3, pass through an electrostatic analyzer 5, and thereafter through a magnetic sector 6 (having its field normal to the plane of the drawing) to a beam current collector 7. The presence of a peak of sample ions is detected by a beam monitor 8 connected to a beam monitor electrode 14 having an aperture 15. The detection of these peaks can be used, manually or automatically, to initiate magnetic scans in sector 6 to determine the mass spectra of the successive corresponding samples entering the ion source from the column.
About 10-20 percent of the total beam current of ions approaching electrode 14 is intercepted by electrode 14, and hitherto a large proportion of this current has been due to carrier ions, leading to the difficulties of peak detection described above. In accordance with the present invention a relatively weak magnetic field is provided by a magnet 9 located between the ion source 3 and the electrostatic analyzer S. The field of magnet 9 is in the plane of the drawing and therefore tends to deflect the ion beam from the source in a direction normal to the plane of the drawing. The field of magnet 9 is insufficiently strong to substantially deflect the relatively heavy sample ions, but the relatively light helium carrier ions are deflected, and continue through sector 5 along a path 10 which diverges from the path 11 of the sample ions. (Path 10 is shown deflected in the plane of the drawing for clarity, but is actually deflected in a plane normal to the drawing.)
At the end of sector 5 is an electrode 13 having an aperture 12 which is larger than aperture 15, and which is provided in existing practice to repel secondary electrons emitted from electrode 14. The sample beam 11 passes through aperture 12 and, after part has impinged on electrode l3, continues on to sector 6 as before, but the deflected carrier beam 10 strikes electrode 13 and is stopped thereby. Whether beam 10 is deflected upwards or downwards relative to the plane of the drawing depends on the direction of the field of the magnet 9. In the illustrated embodiment the magnet 9 is a simple permanent bar magnet oriented to produce a field in the direction shown by arrow 16, but this direction may be reversed by reversing the magnet. Nor need the field be parallel to the plane of the drawing; it may, for example, be normal thereto.
The value of the magnetic field is not critical and can be adjusted by simple experiment to give suitable operating conditions. Adjustment is facilitated by the use of an electromagnet instead of the permanent magnet illustrated.
Using an AEI MS 902 mass spectrometer, the magnet 9 is conveniently located in line with the pairs of beam-aligning electrodes 17, 18 of the instrument as shown, but this location is not necessarily critical. In this instrument the latter electrodes are oriented normal to the beam, but in a modified embodiment at least one pair of such electrodes 18' is made substantially parallel to the beam (see FIG. 2), and preceded by the magnetic field. The carrier beam 10 can thus be deflected outside the plates while the sample beam 11' is realigned thereby, as shown in FIG. 2. In this modification the carrier beam does not enter the electrostatic sector at all.
Using an AB] MS 902 mass spectrometer, the provision of magnet 9 as described with reference to FIG. 1 has enabled most of the helium ions to be deflected away from the beam monitor without reducing the ion current thereto due to ions of mass greater than 28 mass units. Total ion current records at 50 e.v. ionizing beam energy have been obtained from the monitor with a signal-to-noise ratio as good as those obtained at 20 e.v. Because a constant energy of ionizing beam is used, the need to adjust the tuning between standby and scanning conditions does not arise; the optimum tuning for both conditions exists simultaneously. Contrary to what might be expected, the additional magnetic field does not reduce the sensitivity of the spectrometer nor is there more than a 10 percent reduction in resolving power, a figure which is of no consequence in instruments normally used for combined gas chromatography-mass spectrometry.
A further advantage of the present invention over the high/low electron energy switching system is that helium is no longer essential as a carrier gas. Hydrogen can be used as an alternative. This is even more efficiently removed than helium by the field 9 and the hydrogen ions produced in the mass spectrometer are more easily discriminated against. For some gas chromatographic applications hydrogen is a desirable carrier gas to use and has the further advantage that it is one-fortieth of the price of helium of comparable purity.
Although the described embodiment (FIG. 1) uses Nier- Johnson geometry, this is not an essential feature of the invention and other known forms of mass spectrometer may be used.
1. Apparatus comprising a gas chromatographic column and a mass spectrometer for analyzing the output from said column of samples mixed with a carrier gas, said mass spectrometer including an ion source to which the output of the column is fed and an ion beam monitor electrode, a means including a magnet located between the ion source and the ion beam monitor electrode for applying a transverse magnetic field to the ion beam from the ion source to deflect a substantial proportion of the relatively light carrier gas ions in the beam from impinging on the beam monitor electrode while leaving the relatively heavy sample ions substantially undeflected.
2. Apparatus as claimed in claim 1 wherein said beam monitor electrode is preceded by an apertured electrode and said transverse magnetic field is arranged to cause said deflected carrier gas ions to strike said apertured electrode while said substantially undeflected sample ions pass therethrough.
3. Apparatus as claimed in claim 1 wherein said beam monitor electrode is preceded by an electrostatic analyzer and wherein said magnet is located between the ion source and the electrostatic analyzer.
4. Apparatus as claimed in claim 1 wherein the beam monitor electrode is followed by a magnetic analyzer and wherein the magnetic field of said magnet is aligned approximately normal to the field of said magnetic analyzer.
5. Apparatus as claimed in claim 1 wherein said mass spectrometer comprises beam-aligning electrodes following said ion source, and wherein said magnet is located adjacent said beam-aligning electrodes.
6. Apparatus as claimed in claim 1 wherein said mass spectrometer comprises at least one pair of beam-aligning electrodes following said ion source and substantially parallel to the beam, said magnet preceding said electrodes and being ar ranged to deflect the carrier ions outside said pair of electrodes.
7. Apparatus as claimed in claim 3 wherein an apertured electrode is located between said electrostatic analyzer and said beam monitoring electrode and wherein said transverse magnetic field is arranged to cause said deflected gas ions to strike said apertured electrode while said substantially undeflected sample ions pass therethrough.