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.
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