Novel aspects of utilizing RF-only pre-filters in a quadrupole mass spectrometer are presented. Theoretical considerations converning the optimization of the spectrometer by means of the pre-filter length and the ion entrance velocity are given and are given and are compared with the results of model calculations. Experimental results are shown to illustrate the advantages of the setup constructed according to the proposed optimization method over both the single mass filter and one with a pre-filter of conventional length range. The most effectively focusing pre-filters are shown to have integer multiples of an elementary length. The predictions have only been verified experimentally for helium. Apoproximeately 150% improvement was measured for helium transmission with respect to the single analyser with the suggestede parameters. Since the resolution always inporoves with length at a certain ion velocity, it is preferable to make use of some of the higher local minima of the suggestede optimizing function.