Reducing ion reflections at the quadrupole pre-filter/mass filter boundary: Simulation and experiment.

Rationale: Ion reflections at the quadrupole pre-filter/mass filter boundary lead to the trapping of ions within the pre-filter. The trapped ions lead to space charge effects on the transmission of ions through this region which become more of an issue with the development of high sensitivity mass spectrometers which have bright ions beams.

Methods: Simulations were carried out matching the effective potentials at the pre-filter/mass filter boundary to reduce the field gradients across the boundary by reducing the field radius of the pre-filter. A further rotation of the pre-filter relative to the mass filter was incorporated into the model. Experiments were carried out to reproduce the conditions of the simulations.

Results: Simulations show that matching the effective potentials at the pre-filter/mass filter boundary and rotating the pre-filter by 45° reduced the number of reflected ion trajectories when the mass filter was operated in either the radiofrequency (RF)-only mode or in mass resolving mode. Experiments confirmed the reduced effects of space charge on the transmission of ions through the pre-filter through the measurement of pre-filter transmission profiles with and without the use of an empty step to remove any trapped ions. These results showed significant improvement over the typical pre-filter/mass filter set-up.

Conclusions: Matching the effective potentials at the quadrupole pre-filter/mass filter boundary and rotating the pre-filter result in a pre-filter/mass filter set-up that does not lead to ions that are reflected at the boundary. The modified set-up does not result in the presence of unwanted trapped ions in the quadrupole pre-filter.