Quadrupole pre-filter/mass filter transmission profiles: Experiment and simulation

Rationale Quadrupole pre-filter transmission profiles are shown to be affected by the presence of trapped ions within the pre-filter of a quadrupole mass spectrometer. Trapped ions are the result of ions reflected at the pre-filter/mass filter boundary and lead to space charge effects that can affect the shape of the pre-filter transmission profile, ion beam stability, peak width and peak intensity. The development of high sensitivity mass spectrometers with bright ion beams leads to greater numbers of trapped ions. Methods Experimental pre-filter transmission profiles have been measured with and without an empty step to clear the pre-filter region of trapped ions showing the extent that space charge has on ion transmission through the pre-filter. Ion trajectory simulations, using a restricted set of initial conditions, have been used to help understand the structure of the pre-filter transmission profile. Results Experiments show that the effects of trapped ions within the pre-filter can be easily seen in the shape of the pre-filter transmission profile, ion beam stability and peak shape. The pre-filter transmission profile shows structure which is related to the secular frequency of the ion of interest which is related to the Mathieu parameter q of the pre-filter. Simulations show that the nodes observed in the experimental transmission profile are spaced by half the secular period of the ion. Conclusions The structure of the pre-filter transmission profile can be observed experimentally when steps are taken to remove trapped ions from within the pre-filter. A better understanding of the factors contributing to the shape of the pre-filter transmission profile from simulations has been demonstrated.