Peak structure with a quadrupole mass filter operated in the third stability region

Peak structure for a quadrupole operated in the third stability region with Mathieu parameters (a, q) approximate to (3, 3) has been studied experimentally and modeled theoretically. It is shown that the structure is due to the imaging properties of the quadrupole field which are caused by the wavelike properties of the ion trajectories. Ions enter the quadrupole through a small inlet aperture on axis. When ions are focused on the center of the exit aperture the transmission is a maximum. Conversely when ions have trajectories that place them near the rods at the exit aperture the transmission is a minimum and a dip appears on a peak. The positions of dips on a peak can be assigned to lines in the stability diagram. These lines follow iso-beta lines, where beta is the parameter that determines the frequencies of ion motion. The positions of the lines are controlled by the number of rf cycles, which ions spend in the quadrupole field. At high resolution (>300) and low ion axial energy (<20 eV) the peak splitting is minimal or absent. (C) 2000 Elsevier Science B.V.