The effect of fringing field on the transmission of the prefiltered quadrupole mass spectrometers

The transmission properties of prefiltered quadrupole mass spectrometers (QMS) are significantly modified by the fringing field between the prefilter and the analyser. This effect depends on the length of the fringing field and parameters of the incoming ions as it has been revealed by the systematic study using a mass selective ion source coupled to a prefiltered QMS. The transmission of the prefiltered QMS has been determined experimentally. It depends on the distance (d(f)) between the prefilter and analyser, however, if is not unambiguously characterised by d(f) expressed, as usually, in terms of the number of RF cycles (nRF) that the ions spend in this fringing field. At a fixed nRF the transmission of the system depends on the energy of ions (E) and d(f). Plotting the relative transmission (Tr: the ratio of the transmission of prefiltered QMS and that of QMS without prefilter) as a function of E and d(f) there are favourable (Tr has local maximum) and not favourable (Tr has local minimum) areas which are alternating periodically as a function of E. The difference of maximum and minimum values of Tr is more pronounced at higher nRF. The transmission is decreasing by increasing d(f). (C) 1998 Elsevier Science Ltd. All rights reserved.