EFFECT OF A PREFILTER ON THE SENSITIVITY OF A HIGH-RESOLUTION QUADRUPOLE MASS-SPECTROMETER

The effect of a pre-filter on the sensitivity of a high-resolution qua drupole mass spectrometer (QMS) has been experimentally studied. The h igh-resolution QMS uses the condition of a higher stable zone in the M athieu stability diagram where the higher zone is defined as zone II h aving a center of a = 2.83 and q = 3.02 in the a-q plane. The pre-filt er of a short auxiliary quadrupole was added in front of a main quadru pole and only an r.f. voltage was applied. A helium peak intensity (ma ss 4 at 50% peak resolution of 40) increased by a factor of two compar ed with that without the pre-filter, when an r.f. voltage ratio of the pre-filter to the main quadrupole was roughly 0.2. An ion orbit analy sis is used to support the experimental result.