Comparison of quadrupole mass filters with hyperbolic and cylindrical rods working in the third stability zone

Abstract

The working point of a quadrupole mass filter is generally set in the first zone of stability. For applications that require higher resolutions other stability zones can provide a suitable alternative and in this article it is investigated the third zone in particular for applications in the low mass range. Ion trajectory simulations for 4He + have been carried out for cylindrical as well as hyperbolic rods and the results have been compared. For cylindrical rods the best mass resolution has been obtained with a ratio between rod radius and field radius of 1.120. For this value further simulations have been performed to determine the dependence of transmission on the injection regions and injection angle in the quadrupole field. The analysis of the transmission regarding low and high mass tails has shown that it is possible to get rid of the low mass tails for cylindrical rods by limiting the injection region, instead the high mass tails, which affect the transmission for both cylindrical and hyperbolic rods, can be reduced by increasing the frequency of the radio frequency applied to the rods. Therefore, although the mass resolution at 50% of the peak height for hyperbolic rods is always greater than for cylindrical ones, with a suitable (reduced) ion injection region, the maximum transmission and the baseline resolution of a filter equipped with cylindrical rods can be made to approach that of a hyperbolic rod filter.