Anal. Chem. 2009, 81, 5054–5063
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 Cong Yao,† William R. Pitner,‡ and Jared L. Anderson*,† Department of Chemistry, The University of Toledo, 2801 W. Bancroft Street, MS 602, Toledo, Ohio 43606, and Merck KGaA, Frankfurter Strasse 250, Darmstadt D-64293, Germany 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-(6amino-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%. * To whom correspondence should be addressed. E-mail: jared.anderson@ utoledo.edu. Phone: (419) 530-1508. Fax: (419) 530-4033. † The University of Toledo. ‡ Merck KGaA.
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Analytical Chemistry, Vol. 81, No. 12, June 15, 2009
In today’s analytical laboratories, the demands to simplify and accelerate sample preparation steps are accompanied by the need to decrease the quantities of organic solvents used and produced. As a traditional sample pretreatment technique, liquid-liquid extraction (LLE) is considered to be time-consuming and tedious as it typically employs large amounts of organic solvents that are often harmful to workers and result in the production of waste.1 Liquid-phase microextraction (LPME) is a sample preparation technique that is the miniaturization of the traditional LLE method.1-4 Single drop microextraction (SDME) is one category of LPME and is performed by exposing a single microdroplet of solvent to the headspace or directly into the matrix of the sample resulting in rapid preconcentration of analytes into the solvent microdroplet.5-8 Compared to solid-phase microextraction (SPME), another widely used sample preparation method, SDME has the advantages of a wide choice of extraction solvents, cheaper devices, simpler operation, and suffers from no analyte carry-over. However, the application of SDME is limited by the relatively small microdroplet volume produced by traditional organic extraction solvents, often because of their low viscosity and surface tension. Therefore, these methods typically exhibit lower sensitivity when combined with high-performance liquid chromatography (HPLC). Ionic liquids (ILs) are a class of non-molecular ionic solvents with low melting points (
