ON THE USE OF PREFILTERS IN QUADRUPOLE MASS SPECTROMETERS

Computer simulation, based on real, circular-rod quadrupole geometry, is used for studying the effect of RF-only pre-filters on the performance of the quadrupole mass analyser. Although they were originally introduced to avoid undesirable fringing field effects, pre-filters have proven to be useful even if these effects are not taken into account. It is also shown, through the example of the He ion, that the fringe fields do not really play a significant role in the case of fast ions. Consequently, mass spectrometers supplied with pre-filters produce better performance than the analyser itself not only for higher masses, where the fringing field effects are considerable, but also for lower ones where they can be neglected. The intensity-pre-filter length curve has a definite maximum. The optimum pre-filter length belonging to this maximum value is found to be about 12% that of the main analyser used and yields nearly 100% higher transmission at a better resolution. It is shown that pre-filters of optimum size remarkably enlarge the effective inlet aperture of the main filter.