Selected Overtone Mobility Spectrometry

A new means of acquiring overtone mobility spectrometry (OMS) data sets that allows distributions of ions for a prescribed overtone number is described. In this approach, the drift fields applied to specific OMS drift regions are varied to make it possible to select different ions from a specific overtone that is resonant over a range of applied frequencies. This is accomplished by applying different fields for fixed ratios of time while scanning the applied frequency. The ability to eliminate peaks from all but a single overtone region overcomes a significant limitation associated with OMS analysis of unknowns, especially in mixtures. Specifically, a priori knowledge via selection of the overtone used to separate ions makes it possible to directly determine ion mobilities for unknown species and collision cross sections (assuming that the ion charge state is known). We refer to this selection method of operation as selected overtone mobility spectrometry (SOMS). A simple theoretical description of the SOMS approach is provided. Simulations are carried out and discussed in order to illustrate the advantages and disadvantages of SOMS compared with traditional OMS. Finally, the SOMS method (and its distinction from OMS) is demonstrated experimentally by examining a mixture of peptides generated by enzymatic digestion of the equine cytochrome c with trypsin.