Elemental Dependence of Structurally Specific Isotopic Shifts in High-Field Ion Mobility Spectra

Nearly all molecules incorporate at least one element with stable isotopes, yielding ubiquitous isotopologic envelopes in mass spectra. Those envelopes split in differential or field asymmetric waveform ion mobility (FAIMS) spectra depending on the ion geometry, enabling a new general approach to isomer delineation as we demonstrated for chloroanilines. Here, we report that analogous bromoanilines exhibit qualitatively distinct isotopic shifts under identical conditions, some changing signs depending on the gas. This dramatic elemental specificity conveys the breadth and diversity of structural isotopic effect in FAIMS, suggesting unique information-rich patterns for compounds involving various elements and feasibility of enhancing the structural elucidation by atom substitution. We also introduce the capability to make or ensure structural assignments employing major isomer-specific peak broadening due to unresolved isotopomer mixtures.