Low-Field Differential Ion Mobility Spectrometry of Dipole-Aligned Macromolecules

Ion mobility spectrometry (IMS) with mass spectrometry has grown into a powerful approach to simplify complex mixtures, disentangle isomers, and elucidate their geometries. Two established branches are linear IMS based on the absolute mobility K at moderate normalized electric field E/N and field asymmetric waveform IMS (FAIMS) relying on the evolution of K at high E/N causing strong ion heating. Here, we introduce low-field differential IMS (LODIMS), where the field is too weak for significant heating but suffices to lock the permanent macromolecular ion dipoles, producing novel separations based solely on their alignment. The method is demonstrated for a prototypical large protein-albumin. Its oligomers start separating at fields of just 1 kV/cm (4 Td), or ∼5% of those typical for FAIMS. Negligible ion heating at such fields allows preserving fragile species, in particular the noncovalent complexes up to pentamers (332 kDa) destroyed in FAIMS and not detected without it. The separation parameter (compensation field, E_C) in this regime scales with the field linearly versus cubically in FAIMS. The dipole moments obtained from threshold fields for alignment and directional cross sections estimated from the slope of said linear E_C dependence appear reasonable.