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 <i>K</i> at moderate normalized electric field <i>E</i>/<i>N</i> and field asymmetric waveform IMS (FAIMS) relying on the increment of <i>K</i> at high <i>E</i>/<i>N</i> causing strong ion heating (Δ<i>K</i>). In low-field differential (LOD) IMS, the dispersion field (<i>E</i>_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 Δ<i>K</i> sign. We now show the focusing to hinge on the superlinearity of the scaling of the compensation field (<i>E</i>_C) versus <i>E</i>_D governed by the whole <i>K</i>(<i>E</i>/<i>N</i>) function. Unlike the basic cubic <i>E</i>_C(<i>E</i>_D) scaling for rotary ions, the one for aligned species is nearly linear but goes above or below that depending on the species and <i>E</i>_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.