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.

Buffer gas additives (modifiers/shift reagents) in ion mobility spectrometry: Applications, predictions of mobility shifts, and influence of interaction energy and structure

“…The adduction of SRs yields ion clusters with large CCSs, larger than the original analyte ions, and this increases the collisions against the buffer gas producing a larger mobility shift. Strong ion‐SR interaction energies increase cluster average lifetimes with the same results, and several adduction sites increase the probabilities for cluster‐forming collisions …”

“…Only the sites on the SRs are considered because the interaction of the ion is mainly through the charge site because other sites have lower interaction energies with the SRs. Selective mobility shifts in SR‐assisted IMS were first explained based on the number of SR's interaction sites in 2016. In this study, the mobility shifts of a set of ions upon introduction of 2‐butanol (B), methyl‐2‐chloro propionate (M), and nitrobenzene (N) in the buffer gas at 150°C and humidity ~10 ppm were explained based on the number of SR's interaction sites, size, and interaction energies.…”

“…For compounds that sterically hindered the formation of clusters, temperature affected their reduced mobilities only slightly, or did not at all, when SRs were present because there were no clusters to break. These compounds are atenolol and other diamines due to IHBs, and DTBP and TAA ions due to steric hindrance . Relatively large ions, above 300 amu, might show only small variations in mobility from low to high temperatures when small SRs at low concentrations are used in small drift tubes up to 25 cm, atmospheric pressure and ~10 kV.…”

“…In 2014, Campbell et al explained that the main factor in these selective mobility shifts was the ion‐SR interaction energy and the steric hindrance at the charge site instead of the size of SRs . Before 2014, most researchers explained mobility shifts due to the introduction of SRs in the buffer gas based on the increase of the analytes' CCSs upon clustering with these SRs …”

Adduct-ion formation in trapped ion mobility spectrometry as a potential tool for studying molecular structures and conformations

“…The improved resolution was due to mobility shifts in one of the isomeric forms as a result of conformational changes induced by cation adduct formation. Other common approaches to shift the mobility of ions involve the use of mixed carrier gases, shift reagents (SR) and/or changes in temperature of the buffer gas [14][15][16][17].…”