All data were acquired using an Orbitrap Elite mass spectrometer (Thermo Fisher Scientific GmbH, Bremen, Germany) coupled to an intermediate pressure MALDI source (Spectroglyph LLC, WA, USA) as described previously . Unless specified otherwise, the HPF was operated at an amplitude of ~100 VP-P and frequency of 570 kHz, and the LPF operated at an amplitude of ~44 VP-P and frequency of 840 kHz. Except for pressure optimisation experimtns the HPF pressure was controlled to be 4 Torr with the convectron gauge of the Orbitrap Elite maintained at 1.5 Torr.