A method for direct measurement of ion mobilities using a travelling wave ion guide

The relatively recent introduction of a commercial ion mobility-mass spectrometry instrument has provided access to ion mobility for many investigators for the first time. The mechanism of mobility separation in this instrument differs from classical drift tubes in that travelling voltage waves are used, which has necessitated calibration of the device when collision cross-section (CCS) data are required. Here we present a study into a mode of operation of the travelling wave mobility separator which enables direct determination of mobility and hence CCS values. It is shown that for fixed travelling wave pulse amplitudes and velocities, a minimum mobility is required for an ion species to fully surf on a single wave through the mobility device. For a species with the minimum mobility, the trajectory on the travelling wave front is unique and reproducible. SIMION modelling shows the electric potential profile of the travelling wave to be approximately Gaussian and this is used to establish the field experienced by an ion species on the wave front, ultimately allowing determination of mobility values. With the travelling wave mobility device operating with Helium at 2.5 mbar and equivalent wave velocities of 200-300 m/s, wave amplitudes between 7.9 and 16.2 V were required to establish the mobilities of various singly and multiply charged species ranging in CCS from 147 to 3815 A(2). We show that using this method, CCS values can be obtained which are within 5% of the results obtained using drift tubes. (C) 2009 Elsevier B.V. All rights reserved.