Evaluation of an ion mobility detector for supercritical fluid chromatography with solvent-modified carbon dioxide mobile phases

Summary

An ion mobility detector (IMD) was evaluated for open tubular column supercritical fluid chromatography (SFC) when organic solvent-modified supercritical CO₂ was used as mobile phase. It was found that the SFC/IMD interface design in which the SFC capillaray restrictor was directly inserted into the ionization region of the IMD was not acceptable because of low sensitivity that resulted from the effect of the modifier on detector temperature and mechanism of detection. A new interface utilizing a heated nebulizer gas to provide heat to the restrictor and to minimize the formation of ion clusters, and a bent nozzle for enhancing the ionization efficiency of the solute in the IMD ion source are described. Using 5% acetonitrile in CO₂, the minimum detectable quantity (S/N=3) for pyrene was improved from 25.2 ng to 2.1 ng with the new detector design. This compares to a minimum detectable quantity of 0.1 ng when using neat CO₂ as mobile phase. The use of molecular connectivity calculations to predict the drift times of selected analytes is also successfully demonstrated.