Identification of Isomers by Multidimensional Isotopic Shifts in High-Field Ion Mobility Spectra

Nearly all molecules incorporate elements with stable isotopes. The resulting isotopologue envelopes in mass spectra tell the exact stoichiometry but nothing about the geometry. Chromatography and electrophoresis at high resolution also can distinguish isotopologues, again without revealing structural information. In high-definition differential ion mobility (FAIMS) spectra, these envelopes universally split in a structure-specific manner, providing a new general approach to isomer delineation. Here, we show that the peak shifts from instances of the same isotope are equal and can be averaged into characteristic elemental shifts, namely ¹³C and ³⁷Cl for dichloroanilines (DCA). Matrices of these shifts, including the gas composition dimension, are unique to the structure. Hence, all six DCA isomers (with four making two unresolved pairs) are readily delineated in the ¹³C/³⁷Cl maps with He/CO₂ buffer gases. Mixtures of coeluting isomers are also distinguished from pure components.