Ionic Liquids Containing the Tris(pentafluoroethyl)trifluorophosphate Anion: a New Class of Highly Selective and Ultra Hydrophobic Solvents for the Extraction of Polycyclic Aromatic Hydrocarbons Using Single Drop Microextraction

Abstract

A new class of ionic liquids (ILs) containing the tris(pentafluoroethyl)trifluorophosphate (FAP) anion are paired with imidazolium, phosphonium, and pyrrolidinium cations and used as extraction solvents in direct immersion single drop microextraction (SDME) studies coupled to high performance liquid chromatography (HPLC). The selectivity and sensitivity of the extraction method can be tuned and manipulated by varying the cationic component of the IL, thereby providing larger enrichment factors for a variety of analytes, including polycyclic aromatic hydrocarbons (PAHs). Compared to other hydrophobic ILs containing the hexafluorophosphate (PF₆⁻) and bis[(trifluoromethyl)sulfonyl]imide (NTf₂⁻) anions, FAP-based ILs are significantly more hydrophobic and hydrolytically stable permitting them to be used in the sampling of large volumes of aqueous solutions without dissolution or loss of the IL. The highest enrichment factors were obtained with the trihexyl(tetradecyl)phosphonium FAP ([PH₃T] [FAP]) IL for compounds with high molecular weight and fused rings while high enrichment factors were obtained for smaller, more polar molecules using the 1-hexyl-3-methylimidazolium FAP ([HMIM] [FAP]) IL. The detection limits for nine studied PAHs ranged from 0.1 to 0.6, 0.03 to 0.4, 0.04 to 0.7 and 0.1 to 1.2 μg L⁻¹ for [HMIM] [FAP], [PH₃T] [FAP], 1-butyl-1-methylpyrrolidinium FAP ([BMPL] [FAP]), and 1-(6-amino-hexyl)-1-methylpyrrolidinium FAP ([HNH₂MPL] [FAP]), respectively. The reproducibility of the extraction method at 20 °C using the FAP-based ILs was in the range of 1.5−9.4%. Three real water samples including tap water, creek water, and river water were analyzed and yielded recoveries ranging from 79−114%.