Macromolecular Separations by Ion Mobility Focusing in Divergent Asymmetric Electric Fields

Ion mobility spectrometry (IMS) has grown into a powerful approach to simplify mixtures, delineate isomers, and elucidate molecular geometries. The major IMS branches are linear IMS based on the mobility K at moderate normalized electric field E/N and field asymmetric waveform IMS (FAIMS) relying on the increment of K at high E/N causing strong ion heating (ΔK). In low-field differential (LOD) IMS, the dispersion field (E_D) too weak for significant heating changes the mobilities of macroions by locking their dipoles. The FAIMS units with curved and/or differentially heated gaps feature divergent fields that focus or defocus ions, ostensibly based on the waveform polarity and ΔK sign. We now show the focusing to hinge on the superlinearity of the scaling of the compensation field (E_C) versus E_D governed by the whole K(E/N) function. Unlike the basic cubic E_C(E_D) scaling for rotary ions, the one for aligned species is nearly linear but goes above or below that depending on the species and E_D range. Hence, one can fractionate the macromolecular ensembles into two subpopulations with distinct compositions by flipping the waveform polarity in a new "divergent field ion mobility focusing" (DIFIMOF) separation orthogonal to LODIMS, FAIMS, and linear IMS.