Structurally Informative Isotopic Shifts in Ion Mobility Spectra for Heavier Species

The isotopic molecular envelopes due to stable isotopes for most elements were a staple of mass spectrometry since its origins, often leveraged to identify and quantify compounds. However, all isomers share one MS envelope. As the molecular motion in media also depends on the isotopic composition, separations such as liquid chromatography (LC) and ion mobility spectrometry (IMS) must also feature isotopic envelopes. These were largely not observed because of limited resolution, except for the (structurally uninformative) shifts in LC upon H/D exchange. We recently found the isotopic shifts in FAIMS for small haloanilines (similar to 130-170 Da) to hinge on the halogen position, opening a novel route to isomer characterization. Here, we extend the capability to heavier species: dibromoanilines (DBAs, similar to 250 Da) and tribromoanilines (TBAs, similar to 330 Da). The C-13 shifts for DBAs and TBAs vary across isomers, some changing sign. While Br-81 shifts are less specific, the 2-D C-13/Br-81 shifts unequivocally differentiate all isomers. The trends for DBAs track those for dichloroanilines, with the C-13 shift order preserved for most isomers. The peak broadening due to merged isotopomers is also isomer-specific. The absolute shifts for TBAs are smaller than those for lighter haloanilines, but differentiate isomers as well because of compressed uncertainties. These results showcase the feasibility of broadly distinguishing isomers in the more topical similar to 200-300 Da range using the isotopic shifts in IMS spectra.