Calibration of the mobility scale in ion mobility spectrometry: the use of 2,4-lutidine as a chemical standard, the two-standard calibration method and the incorrect use of drift tube temperature for calibration

Abstract

Ion mobility spectrometry (IMS) is an analytical technique that separates ions in the gas phase under the influence of an electric field according to their size to charge ratio. We used electrospray ionization IMS-quadrupole mass spectrometry to study the mobility shifts of 2,4-lutidine with temperature or the introduction of several contaminants in the drift gas. We found the reduced mobility (K₀) of 2,4-lutidine to decrease up to 24% when contaminants were introduced into the drift gas. We also show the significant variation of 2,4-lutidine's K₀ with the drift tube temperature, 8.5% from 100 to 322 °C. These changes in 2,4-lutidine's mobility were due to variations in clustering by changes in temperature or contaminant concentration. This dependence of 2,4-lutidine's K₀ with temperature and contamination in the drift gas makes this chemical standard unsuitable to calibrate the mobility scale. Despite these findings, 2,4-lutidine is still used for this purpose. The shortcomings of the IMS two-standard calibration and the incorrect use of the drift tube temperature for calibration are also discussed. We suggest that accurate reduced mobilities must be determined for small ions only in a highly purified drift gas using calibrants with a well determined K₀ such as di tert-butyl pyridine at high temperatures, where clustering is low, and the drift gas temperature is measured instead of the drift tube temperature.