COMPUTER-SIMULATION OF THE QUADRUPOLE MASS FILTER

A computer program, called MMFCODE, has been developed to analyze nume rically the dynamics of ion beams in a Quadrupole Mass Filter (QMF). T he motion of the ions can be known by solving the three-dimensional eq uation of motion. A harmonic expansion of the electric field is introd uced to get a reasonable computing time. For the incident beam, more t han 10(4) ions are generated by a Monte Carlo method. After the mass s can line was determined with the condition that Delta M = const. at tw o different mass numbers (M), the mass spectrum and the acceptance can be simulated. By using MMFCODE, the simulation of a real dimension ca n be done without neglecting the fringing field or using the model fie ld. In this paper, a model of a QMF with an RF-only pre-filter, called the delayed DC ramp [W. Brubaker, U.S. patent 3,371,204 (1968)], was analyzed by using MMFCODE. By our simulation, the transmittance was os cillated with respect to the pre-filter length in the case of a low ma ss number as reported by C. Trajber et al. [Rapid Commun. Mass Spectro m. 6 (1992) 459], but was not oscillated in the case of a high mass nu mber. This is probably because the distribution of the axial velocitie s is narrow in the case of a low mass number but broad in the case of a high mass number. This discrepancy is caused by the influence of the fringing field at the pre-filter entrance.