Baseline resolution of isomers by traveling wave ion mobility mass spectrometry: investigating the effects of polarizable drift gases and ionic charge distribution

• Published in: Journal of mass spectrometry. 2013-09, Vol.48 (9), p.989-997
• Main Authors: Lalli, Priscila M., Corilo, Yuri E., Fasciotti, Maíra, Riccio, Maria Francesca, de Sa, Gilberto F., Daroda, Romeu J., Souza, Gustavo H. M. F., McCullagh, Michael, Bartberger, Michael D., Eberlin, Marcos N., Campuzano, Iain D. G.
• Format: Article
• Language: English
• Subjects: Charge; Charge distribution; Drift; ion mobility; Ionic mobility; isomer separation; Isomers; Mass spectrometry; Nitrous oxides; polarizable drift-gas; Traveling waves
• ISSN: 1076-5174; 1096-9888
• DOI: 10.1002/jms.3245

We have studied the behavior of isomers and analogues by traveling wave ion mobility mass spectrometry (TWIM‐MS) using drift‐gases with varying masses and polarizabilities. Despite the reduced length of the cell (18 cm), a pair of constitutional isomers, N‐butylaniline and para‐butylaniline, with theoretical collision cross‐section values in helium (ΩHe) differing by as little as 1.2 Å2 (1.5%) but possessing contrasting charge distribution, showed baseline peak‐to‐peak resolution (Rp‐p) for their protonated molecules, using carbon dioxide (CO2), nitrous oxide (N2O) and ethene (C2H4) as the TWIM drift‐gas. Near baseline Rp‐p was also obtained in CO2 for a group of protonated haloanilines (para‐chloroaniline, para‐bromoaniline and para‐iodoaniline) which display contrasting masses and theoretical ΩHe, which differ by as much as 15.7 Å2 (19.5%) but similar charge distributions. The deprotonated isomeric pair of trans‐oleic acid and cis‐oleic acid possessing nearly identical theoretical ΩHe and ΩN2 as well as similar charge distributions, remained unresolved. Interestingly, an inversion of drift‐times were observed for the 1,3‐dialkylimidazolium ions when comparing He, N2 and N2O. Using density functional theory as a means of examining the ions electronic structure, and He and N2‐based trajectory method algorithm, we discuss the effect of the long‐range charge induced dipole attractive and short‐range Van der Waals forces involved in the TWIM separation in drift‐gases of differing polarizabilities. We therefore propose that examining the electronic structure of the ions under investigation may potentially indicate whether the use of more polarizable drift‐gases could improve separation and the overall success of TWIM‐MS analysis. Copyright © 2013 John Wiley & Sons, Ltd.