MULTIFUNCTIONAL ION DETECTOR FOR A TIMS

MULTIFUNCTIONAL ION DETECTOR FOR A THERMAL IONIZATION MASS SPECTROMETER

  1. Pamula, I. Dobocan, N. Gligan, Adriana Benţa

Natioanal Institute for Research and Development of Isotopic and Molecular Technologies

Abstract

A three mode operating ion detector was designed and built for a thermal ionization mass spectrometer. It allows ion detection:

-on a Faraday cage

-on a secondary electron multiplier after the conversion of incident ions in secondary electrons

-on a secondary electron multiplier by direct bombardment of the first dynode by the incoming ions

Introduction

In a thermal ionisation mass spectrometer, the ion beams energy ranges, usually, in between 5 – 10 keV. If, for sensitivity reason, a secondary electron multiplier is used, than the first dynode is subject to a very hard heavy ion bombardment, which, in time, leads to the alteration of its surface. As a result, an amplification loss occurs. To avoid this, an ion-electron conversion outside the multiplier is proposed.

The ion optics of the detection system was numerically modelled,[1] and the system was designed and build up in the Mass Spectrometry Laboratory of the institute.

Figure 1. Numerical modelling of the multifunctional ion detector. Ion and electron trajectories are shown for the ion-electron conversion mode.

The detector was mounted in the analyser chamber of the SMIT-1 thermal ionisation mass spectrometer. An adjustable resolution slit (0 – 1 mm) was placed in front of it. The width of this slit can be adjusted from the exterior via a micrometric screw. Figure 2 shows the adjustable slit mounted in front of the detector.

A throughput assembly was built to supply the necessary potentials to the detector electrodes.The throughputs were realised from tubular ceramic insulators of 70 mm long and 10 mm exterior diameter. Epoxy resins ensured the tightening. The assembly can be outgased at 80° C. The typical vacuum in the mass spectrometer tube is 3,5 x 10-7torr

Performance testing

The starting potential values supplied to the detector electrodes were those determined by numerical modelling (figure 1).

With all the electrodes grounded except the re-focussing electrode maintained at 3500 V, the spectrometer was tuned on the 238U+ ion. The secondary electron multiplier was supplied with 2.1 kV, corresponding to a gain of approximately 104. A stable output signal of 1.524 V (mean value) was observed The standard deviation of the values registered in 60 sec. interval was  = 0,005V. This value represents fluctuations of the output signal caused both by electrical. A 5000 V potential was applied on the conversion and electron re-focussing electrodes, while maintaining unchanged the voltage on the deflection electrode (0.0 V). The output signal becomes zero. Then, the potential of the deflection electrode was gradually risen; watching the electrometer output signal. At 6900 V on the deflection electrode we got a stable output signal of 1.225 V (mean value) The intensity and the standard deviation of the output signal remains in the same range of magnitude as in the case of the direct detection of the ions.

At 3850 V on the deflection electrode we obtain a signal on the Faraday cage. This signal is about 104 times less intense then in the preceding situation

Conclusions

  • The potentials determined by numerical modelling are in very good agreement with those experimentally determined
  • In the ion-electron conversion mode, the detected signal has the same magnitude order as the signal registered in the direct ion detection mode. The ion-electron conversion should be preferred to avoid a hard bombarding of the first dynode of the secondary electron multiplier.
  • The ion-electron conversion doesn’t alter the accuracy of the measurements.
  • The multifunctional detector requires a very limited number of different voltages.
  • The system being non-dependent of the multiplier type, the detector could fit any mass spectrometer functioning with ion energies of several keV.

Reference

  1. David A. Dahl and Anthony D. Appelhans “Ion Optics Through the Eyes of SIMION 6.0”, An Asms Short Course, Portland ASMS Conference, May 11-12, 1996