Use of ionic liquids as headspace gas chromatography diluents for the analysis of residual solvents in pharmaceuticals

In this study, two ionic liquids (ILs), 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([BMIM][NTf₂]) and trihexyltetradecylphosphonium bis[(trifluoromethyl)sulfonyl]imide ([P₆₆₆₁₄][NTf₂]) were examined as contemporary diluents for residual solvent analysis using static headspace gas chromatography (SHS-GC) coupled with flame ionization detection (FID). ILs are a class of non-molecular solvents featuring negligible vapor pressure and high thermal stabilities. Owing to these favorable properties, ILs have potential to enable superior sensitivity and reduced interference, compared to conventional organic diluents, at high headspace incubation temperatures. By employing the [BMIM][NTf₂] IL as a diluent, a 25-fold improvement in limit of detection (LOD) was observed with respect to traditional HS-GC diluents, such as N-methylpyrrolidone (NMP). The established IL-based method demonstrated LODs ranging from 5.8 parts-per-million (ppm) to 20 ppm of residual solvents in drug substances. The optimization of headspace extraction conditions was performed prior to method validation. An incubation temperature of 140 °C and a 15 min incubation time provided the best sensitivity for the analysis. Under optimized experimental conditions, the mass of residual solvents partitioned in the headspace was higher when using [BMIM][NTf₂] than NMP as a diluent. The analytical performance was demonstrated by determining the repeatability, accuracy, and linearity of the method. Linear ranges of up to two orders of magnitude were obtained for class 3 solvents. Excellent analyte recoveries were obtained in the presence of three different active pharmaceutical ingredients. Owing to its robustness, high throughput, and superior sensitivity, the HS-GC IL-based method can be used as an alternative to existing residual solvent methods.