Caffeine and glucosamine mobility shifts by adduction with 2-butanol depended on interaction energy, charge delocalization, and steric hindrance in ion mobility spectrometry.

Ion mobility spectrometry (IMS) is an analytical technique that separates gas-phase ions drifting under an electric field according to their size to charge ratio. We used electrospray ionization-drift tube IMS coupled to quadrupole mass spectrometry to measure the mobilities of glucosamine (GH ) and caffeine (CH ) ions in pure nitrogen or when the shift reagent (SR) 2-butanol was introduced in the drift gas at 6.9 mmol m . Binding energies of 2-butanol-ion adducts were calculated using Gaussian 09 at the CAMB3LYP/6-311++G(d,p) level of theory. The mobility shifts with the introduction of 2-butanol in the drift gas were -2.4% (GH ) and -1.7% (CH ) and were due to clustering of GH and CH with 2-butanol. The formation of GBH was favored over that of CBH because GBH formed more stable hydrogen bonds (83.3 kJ/mol) than CBH (81.7 kJ/mol) for the reason that the positive charge on CH is less sterically available than on GH and the charge is stabilized by resonance in CH . These results are a confirmation of the arguments used to explain the drift behavior of these ions when ethyl lactate SR was used (Bull Kor Chem Soc 2014, 1023-1028). This study is a step forward to predict IMS separations of overlapping peaks in IMS spectra, simplifying a procedure that is trial and error by now.