Novel Linac II electrode geometry for creating an axial field in a multipole ion guide

Tandem instruments, such as triple-quadrupole or quadrupole-time-of-flight mass spectrometers, often use collisional damping ion guides for the purpos e of cooling and focusing the primary ion beam, or as a collision cell for tandem mass spectrometry experiments. A small, axial field to reduce the io n residence time in such devices can give considerable improvements in perf ormance. Reduction of the residence time reduces adduct formation caused by unwanted gas-phase reactions and allows multiple reaction monitoring (MRM) and some other types of scan to be carried out more rapidly without the ri sk of reaction cross-talk. Here we propose a novel electrode arrangement fo r creation of a suitable axial field in a multipole ion guide without signi ficantly reducing the m/z window. As an example, a second set of four elect rodes is added to an existing quadrupole collision cell. The same DC potent ial is applied to all four extra electrodes, which are shaped in the longit udinal direction to create a suitable axial field inside the device. The po tential of these electrodes and the DC bias of the main rods determine the axial field strength. The idea was tested on a modified quadrupole collisio n cell of a MALDI-QqTOF instrument and experimental results are presented h ere, as well as a simplified theory of operation.