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 operated in the second stability region with Mathieu parameters (a,q) approximate to (0.02.7.55). Two sources of peak splitting are considered and modeled theoretically: nonlinear resonances caused by field imperfections and ion collection effects caused by the periodic properties of ion motion in the quadrupole field. The conditions for the appearance of the nonlinear resonances and ion collection effects are derived and presented in terms of the beta variables which determine the frequencies of ion oscillation. Comparisons of calculated peak structure to experimental peak shapes show that ion collection effects dominate, at least for the experimental conditions reported here. It is also shown that neither nonlinear resonances nor ion collection effects can distort the peak at high resolution. (C) 1999 American Society for Mass Spectrometry.