Stable X-islands of quadrupole mass filter

Quadrupole mass filters are normally operated as narrow band pass filters by appropriate choices of rf and DC voltages corresponding to Mathieu a and q values near the apex of the first stability region. We add an auxiliary quadrupole excitation potential to the main drive voltage. As a result, stability islands appear on the (a,q) plane. The method of the islands mapping on the (a,q) plane is discussed in detail. The DC electric field's effect responsible for removing shadowing islands has been studied using a combination of analytic theory and computer modeling. We call a narrow stability region elongated along an iso-beta x line an X-island. Two such stability islands were found where operation is possible without mass spectra interference when the DC potential is used. Those islands are formed by quadrupole potentials with relative excitation frequencies nu=beta,nu=1 +/-beta and nu=2 +/-beta, where beta << 1. Many other stability X-islands are presented in detail and illustrated by their transmission contours. This data is necessary for experimental testing of the separation mode at those islands. The study demonstrates how to use X-islands to achieve relatively high resolution 4000-12,000 at the transmission of about 28-15%, respectively.