Reversed-Phase (RP) and Hydrophilic Interaction (HILIC) Separation Mechanisms for the Assay of Nicotine and E-Cigarette Liquids

Nicotine is a highly used addictive substance that has recently also become available through electronic cigarettes. Here we present a study of nicotine from e-cigarette liquids through reversed-phase (RP) and hydrophilic interaction (HILIC) liquid chromatography. Multiple aqueous mobile-phase additives are considered for the RP mechanism, focusing on chaotropic agents, mobile-phase concentrations and mixing ratios, and column temperature. Sample preparation was conducted by toluene liquid-liquid extraction of e-cigarette liquids diluted with aqueous 25 mM NaHCO₃/Na₂CO₃. Optimal RP results for retention and peak symmetry were obtained using aqueous 0.1% formic acid and 20 mM ammonium hexafluorophosphate with 0.1% formic acid in acetonitrile, using a gradient profile with a C18 column, exploited at 40 °C and a 1.5 mL/min flow rate. A dilute-and-shoot alternative with automated flow reversal after isocratic elution is presented. For HILIC, aqueous 100 mM ammonium formate and 0.1% formic acid in acetonitrile were used as mobile-phase components, using a gradient profile, on a Thermo Scientific™ Acclaim™ Mixed-Mode HILIC-1 column, operated at 25 °C with a 1 mL/min flow rate. UV detection was at 260 nm. Absolute limits of quantitation in the 1 μg/mL range were obtained for all tested alternatives, with 1 μL injection volumes.