Modelling some rod set imperfections of a quadrupole mass filter.

The problem of modeling the mass peak shape of a quadrupole mass filter (QMF) with round rods is considered. A number of factors leading to the degradation of the mass peak shape are studied, namely, displacement of the electrodes with respect to their original position, changes in the diameter of the electrodes, and asymmetry of the supply potentials. Decomposition of the rod set field on multipole fields allows to obtain an analytical representation of the ion motion equations. Simulations have shown that a parallel shift of one or two electrodes leads to a shift of the peak along the mass number axis and dips at the peak apex. Unbalance of supply voltages does not significantly distort the peak shape, but it shifts the peak along the mass axis and creates a jump of the quadrupole offset potential proportional to the relative unbalance of X and Y rods potentials.