New High Resolution Ion Mobility Mass Spectrometer Capable of Measurements of Collision Cross Sections from 150 to 520 K.

Abstract

We present a new variable temperature (VT), high resolution ion mobility (IM) drift tube coupled to a commercial mass spectrometer (MS). Ions are generated in an electrospray ion source with a sampling cone interface and two stacked ring RF guides which transfer ions into the mobility analyzer located prior to a quadrupole time-of-flight mass spectrometer. The drift cell can be operated over a pressure range of 0.5-3 Torr and a temperature range of 150-520 K with applied fields typically between 3 and 14 V cm-1. This makes the instrument suitable for rotationally averaged collision cross section (CCS) measurements at low E/N ratios where ions are near thermal equilibrium with the buffer gas. Fundamental studies of the effective ion temperatures can be performed at high E/N ratios. An RF ion trap/buncher is located at the beginning of the drift region, which modulates the continuous ion beam into spatially narrow packets. Packets of ions then drift in a linear electric field, which is 50.5 cm long, and are separated according to their mobility in an inert buffer gas. Post-drift, an ion funnel focuses the radially spread pulses of ions into the inlet of a commercial MS platform (Micromass QToF2). We present the novel features of this instrument and results from VT-IM-MS experiments on a range of model systems-IMS CCS standards (Agilent ESI Tune Mix), the monomeric protein Ubiquitin (8.6 kDa), and the tetrameric protein complex Concanavalin A (103 kDa). We evaluate the performance of the instrument by comparing ambient DTCCSHe values of model compounds with those found in the literature. Several effects of temperature on collision cross sections and resolution are observed. For small rigid molecules, changes in resolution are consistent with anticipated thermal diffusion effects. Changes in measured DTCCSHe for these rigid systems at different temperatures are attributed primarily to the effect of temperature on the long-range attractive interaction. Similar effects are seen for protein ions at low temperatures, although there is also some evidence for structural transitions. By heating the protein ions, their conformational profiles are significantly altered. Very high temperatures narrow the conformational space presented by both Ubiquitin and Concanavalin; it appears that diverse conformational families are "melted" into more homogeneous populations. Because of this conformational heterogeneity, the apparent IMS resolution obtained for proteins at ambient and reduced temperatures is an order of magnitude lower than the expected diffusion limited resolution (Rmax). This supports a hypothesis that the broad DTCCSHe features frequently observed for proteins do not correspond to interconverting conformers, but rather to high numbers of intrinsically stable structures.