Shift reagents in ion mobility spectrometry: the effect of the number of interaction sites, size and interaction energies on the mobilities of valinol and ethanolamine.

Abstract

Overlapping peaks interfere in ion mobility spectrometry (IMS), but they are separated introducing mobility shift reagents (SR) in the buffer gas forming adducts with different collision cross-sections (size). IMS separations using SR depend on the ion mobility shifts which are governed by adduct's size and interaction energies (stabilities). Mobility shifts of valinol and ethanolamine ions were measured by electrospray-ionization ion mobility-mass spectrometry (MS). Methyl-chloro propionate (M) was used as SR; 2-butanol (B) and nitrobenzene (N) were used for comparison. Density functional theory was used for calculations. B produced the smallest mobility shifts because of its small size. M and N have two strong interaction sites (oxygen atoms) and similar molecular mass, and they should produce similar shifts. For both ethanolamine and valinol ions, stabilities were larger for N adducts than those of M. With ethanolamine, M produced a 68% shift, large compared to that using N, 61%, because M has a third weak interaction site on the chlorine atom and, therefore, M has more interaction possibilities than N. This third site overrode the oxygen atoms' interaction energy that favored the adduction of ethanolamine with N over that with M. On the contrary, with valinol mobility shifts were larger with N than with M (21 vs 18%) because interaction energy favored even more adduction of valinol with N than with M; that is, the interaction energy difference between adducts of valinol with M and N was larger than that between those adducts with ethanolamine, and the third M interaction could not override this larger difference. Mobility shifts were explained based on the number of SR's interaction sites, size of ions and SR, and SR-ion interaction energies. This is the first time that the number of interaction sites is used to explain mobility shifts in SR-assisted IMS. Copyright © 2016 John Wiley & Sons, Ltd.