Peak splitting with a quadrupole mass filter operated in the second stability region

Peak splitting or structure has been studied for a quadrupole mass filter o perated in the second stability region with Mathieu parameters (a,q) approx imate to (0.02.7.55). Two sources of peak splitting are considered and mode led theoretically: nonlinear resonances caused by field imperfections and i on collection effects caused by the periodic properties of ion motion in th e quadrupole field. The conditions for the appearance of the nonlinear reso nances and ion collection effects are derived and presented in terms of the beta variables which determine the frequencies of ion oscillation. Compari sons of calculated peak structure to experimental peak shapes show that ion collection effects dominate, at least for the experimental conditions repo rted here. It is also shown that neither nonlinear resonances nor ion colle ction effects can distort the peak at high resolution. (C) 1999 American So ciety for Mass Spectrometry.