No abstract

Hung a r y GE-TUNGSRAM Br6dy Laboratory, Budapest, HungaryThe experimental verification of predictions of a new simulation model of quadrupole mass filters is presented. Computer calculations show that the transmission of the prefilter-QMS system is periodical as a function of the prefilter length. The periodicity of the transmission is independent of the mass number of ions of equal velocity. Experimental proof of the simulation has been produced for low mass numbers and a more difficult procedure is required for higher masses (mlz > 100). This is realized with a closed ion source and tandem quadrupole mass spectrometer arrangement. Experimental results prove the mass-independent behaviour predicted by the model.A computer program has been constructed to simulate the motion of ions in the field of quadrupole mass analyzers.' The primary objective was to find a mathematical model which could help to optimize the prefilters used with quadrupole mass filters.* The model predicts periodicity of the transmission as a function of prefilter length, and the present paper gives an account of the experimental proof of the calculations. Further improvement of the model is expected. It has been successfully used to calculate the resolution and transmission of the quadrupole mass analyser (QMS) as well as to optimize the length of the prefilter coupled to the analyzer .2 The program can calculate the average maximum displacement of ions from the axis of the quadrupole rod system: where lp,, is the length of prefilter (PF); rape is the radius of the entrance aperture of the prefilter; x,,, yo are the initial displacements of the ion at the entrance aperture; 6 is the initial phase of the radio frequency; N is a normalization constant; r is the position vector of the ion in the conventional Cartesian coordinate system attached to the mass filter geometry according to Fig. 1.The average maximum displacement function (Dr ) is closely related to the focusing properties of the QMS analyzer and it characterizes the transmission of the quadrupole mass filter: the higher the average maximum displacement, the smaller the probability of free ion transmission through the analyzer. The periodicity of the D, function is determined by the ratio of the axial ion velocity (u,) and radio frequency (f); its amplitude depends on the mass and on f and ro. Calculations revealed that D, , and consequently the transmission of the prefiltered QMS, is a periodic function of the length of the prefilter.* This behaviour was experimentally proved for helium ions earlier,3 and experiments have been extended to higher masses in this work. controlled masses at the entrance of the prefilter/mass filter assembly (Fig. 2). Fragment ions of hexachlorobutadiene were produced in a closed ion source, and the first analyzer (QMS 1) was set to a certain mass number. Eight masses were chosen in the range of 12-260Da. Analyzers 1 and 2 and the prefilter were mounted in the same axis in a cylindrical housing. Prefilters of different lengths were manufactured and the f...

No abstract