Developing Reduced Mass-Only Ion Mobility Separations to Unravel Mass Distribution-Based Isotopic Shifts

Recent advancements in high-resolution ion mobility separations have enabled separations of isotopologues and isotopomers, broadly termed as isotopic shifts, based on changes in their mass distributions (i.e., changes in their center of mass and moments of inertia). However, all previous experimental measurements of these isotopic shifts include contributions from both the reduced mass term as well as the mass distribution-based one. Thus, it is of great interest to develop calibration strategies to decouple the two effects and estimate their overall contributions to experimental isotopic shifts. Herein, we have developed the first such procedure to estimate the absolute contributions from reduced mass and mass distribution in isotopic shifts by generating a reduced mass-only ion mobility-based calibration curve. Using this strategy, we were able to determine that for isotopically permethylated carbohydrates that their mass distribution terms were much larger in magnitude than their reduced mass-based ones. Overall, we envision this reduced mass-only calibration methodology can be broadly applied to study any measurable isotopic shift and will enable improved understanding of ion structure in high-resolution ion mobility separations.