STABILIZATION OF ELECTRON CURRENT IN A MASS SPECTROMETER

One of the factors determining the quality of operation of a mass spectrometer is the stability of the ion beam. If the operation of the ion source is based on the principle of ionization of vapors of substances by electron bombardment, then the stability of the ion beam is determined mainly by the stability of the electron current to the electron collector.

Changes in the electron current may be caused by fluctuations of the voltage in the circuit feeding the cathode (by stabilizing the filament heating these fluctuations can be reduced to a minimum) and by the influence of vapors of various samples under investigation on the emission properties of the cathode. This influence may prove to be very considerable. Thus, hydrocarbons increase emission by 20–30%, while some oxygen-containing compounds or compounds rich in halogens may reduce it by a factor of 2–3.

Stabilization of the electron current is usually carried out by automatic regulation of the cathode heating. However, such a method leads to a disturbance of the temperature regime of the ion source, which has a serious effect on its operation \(^{1—3}\). Existing methods of temperature regulation are inertial and cannot fully ensure constancy of the working conditions in the source.

These difficulties can be eliminated if the power supplied to the cathode is kept constant and the electron current is regulated by changing the electric field at the cathode \(^{4}\), as is done in a vacuum triode.

The circuit of an electron-current stabilizer based on this principle is shown in Figure 5. To regulate the potential of the control electrode, located in front of the cathode, this circuit uses the current of the electron collector. When this circuit was used with a 90° Hittl mass spectrometer, it was found that a change in the voltage of the control electrode by 0.1 V causes a change in the current going to the electron collector by 0.6–0.7 μA.

Inclusion of a 10-megohm load resistance in the collector circuit provides an amplification of the voltage applied to the control electrode equal to 60–70. The use of a simple electronic amplifier with a gain close to 6 leads to the result that, when the emission is reduced by 40%, the current going to the electron receiver changes by only 0.1%; i.e., the coefficient of stabilization of the electron current producing ionization is equal to 400.

The use of a stabilizer based on regulation of the electron current instead of the cathode filament current makes it possible to study various classes of compounds with a short stabilization time and with precise regulation of the current to the collector. In this case it is not necessary to solve the serious problems of regulating the temperature of the ion-source source.

In contrast to the results obtained with a cathode-filament-current regulator, the stabilization time for the spectrum of O₂, which affects the emissive capacity of the cathode, as was mentioned earlier, proves, when the electron current is regulated, to be the same as for N₂.

L. L.

REFERENCES

1. R. E. Fox and J. A. Hipple, J. Chem. Phys. 15, 208 (1947).
2. C. E. Berry, J. Chem. Phys. 17, 1164 (1949).
3. D. P. Stevenson, J. Chem. Phys. 17, 101 (1949).
4. E. B. Winn and A. O. Nier, Rev. Sci. Instr. 20, 773 (1949).
5. V. J. Caldecourt, Rev. Sci. Instr. 22, 59 (1951).