Single-Drop Extraction versus Solid-Phase Microextraction for the Analysis of VOCs in Water

2 Introduction An important part of any analytical process is sample preparation, which involves isolation and preconcentration of various analytes of interest.1 Liquid–liquid extraction (LLE) is one of the oldest pretreatment procedures and because of its simplicity and low cost is commonly used. However, conventional LLE requires large volumes of high-purity solvents, which are often expensive and/or hazardous. The desire to reduce both time and the volumes of organic solvent required has led to the development of newer techniques, such as solid-phase extraction (SPE)2,3 or solid-phase microextraction (SPME).4–6 SPME, as a universal tool for isolation and preconcentration of pollutants from different matrices, was introduced and developed by Pawliszyn and co-workers.7 It uses a fused-silica fibre coated with a sorbent to extract samples and pass analytes directly into a heated injector for gas chromatography (GC) or with a solvent into a high performance liquid chromatography interface. Coupling with GC is frequently used for the determination of various organic micropollutants in water, allowing extraction without solvents.8 Cantwell and Jeannot9 introduced a micro-LLE method in which a single drop of organic solvent was contained at the end of a PTFE rod. This system required two discrete parts: the first for extraction and the second for injection. Liu and Dasgupta10 described a microdrop liquid extraction system for trace amounts of analytes, which were subsequently identified in situ by UV/vis detection. In 1997 Jeannot and Cantwell11, and He and Lee12 independently introduced a simpler kind of microextraction in which an organic drop hangs from the tip of a GC syringe needle. The latter system has since been applied to speciation investigations.13 In LLE, the distribution of analyte (AN) between the aqueous and the organic phases is described by the distribution coefficient (Kd). Equilibrium can be reached faster by shaking or stirring the aqueous phase containing the analyte, with the organic phase: