On the Separation of Enantiomers by Drift Tube Ion Mobility Spectrometry

Racemic mixtures of twelve common α-amino acids and three chiral drugs were tested for the separation of their enantiomers by drift tube ion mobility spectrometry (IMS)-quadrupole mass spectrometry (QMS) by introducing chiral selectors into the buffer gas of the IMS instrument. (R)-α-(trifluoromethyl) benzyl alcohol, (L)-ethyl lactate, methyl (S)-2-chloropropionate, and the R and S enantiomers of 2-butanol and 1-phenyl ethanol were evaluated as chiral selectors. Experimental conditions were varied during the tests, including buffer gas temperature, concentration and type of chiral selectors, analyte concentration, electrospray (ESI) voltage, ESI solvent pH, and buffer gas flow rate. The individual enantiomers yielded the same drift times and the racemic mixtures could not be separated as opposed to a previous report (Dwivedi et al. Anal. Chem. 2006, 78, 8200). Energy calculations of the chiral selector –ion interactions showed that these separations are unlikely using 2-butanol as a chiral selector but they might be theoretically feasible depending on the chiral selector nature and the type of enantiomers. Several plausible explanations for not succeeding were analyzed. A critical review of previously claimed enantiomer separations by drift-tube IMS-MS is presented and recommendations for potential enantiomer separations by IMS are proposed.