Comparison of quadrupole mass filters equipped with rods of different convexity: An analysis by finite element methods and trajectory simulations

A quadrupole mass filter is generally built by using cylindrical rods whose radius and separation are chosen to approximate the ideal hyperbolic field. Recently, concave cylindrical as well as planar geometries have been proposed to replace ordinary cylindrical rods with the aim to reduce the size and weight of the filter. In this article, the geometry of four configurations (convex, modified convex, planar, and concave) is investigated by means of calculations based on finite element methods which provide an accurate modeling of the geometry. For each configuration the optimized geometry which approximates at best the ideal hyperbolic field is determined. In the literature, results on the convex configuration showed that the optimized geometry does not lead to the best values for mass resolution and transmission, therefore, in this study, other geometries around the optimized ones have been investigated. The transmission has been estimated by performing trajectory simulations for ions injected in the field generated by each studied geometry. The comparison among the different rod configurations shows that convex rods work better than concave and planar ones. Therefore, the proper choice for compact and light mass filters is the modified convex geometry.