Headspace Liquid Microextraction

2 Headspace sampling for gas chromatographic (GC) analysis has many advantages, the most important of which is the elimination of many interferences from the sample matrix. In addition to the standard methods for headspace sampling, the technique of solid-phase microextraction (SPME) has found wide acceptance because it is simple and inexpensive. More recently, single-drop microextraction (SDME) has been evaluated as an alternative to SPME. In this straightforward technique, a microdrop of solvent is suspended from the tip of a conventional microsyringe and is then immersed in a sample solution in which it is immiscible or suspended in the headspace above the sample. The original application used an 8 μL drop of n-octane in an aqueous sample,1 and only a fraction of this drop was analysed subsequently by GC. Later, a smaller drop was used (1 or 2 μL) and all of it was injected,2 a technique that was replicated by a second laboratory.3 To date, there are more than a dozen publications 4–13 that describe the use of the SDME method, which has also been called liquid-phase microextraction, solvent microextraction, liquid–liquid microextraction or for headspace sampling, headspace solvent microextraction or headspace liquid-phase microextraction. SDME also has been included in three literature reviews.14-16 Headspace SDME is similar to traditional headspace sampling in that volatiles are sampled from the vapours above the sample, thus avoiding interferences from the sample matrix. A variety of methods and specialized equipment is available for this purpose. An alternative technique, headspace SPME, has the further advantage of improved detectivity, which results from the use of a coated fibre chosen for its selectivity for the analyte. In headspace SDME, the fibre used in SPME is replaced by a liquid microdrop that can also be chosen for its selectivity. The first papers appeared in 2001,17,18 and only five others have followed.19–23 The range of reported analyses includes alcohols, chlorobenzenes, trihalomethanes, and BTEX (benzene, toluene, ethylbenzene, and xylenes). The extracting solvents that have been used for SDME are listed in Table 1. A wide range of solvents and polarities is possible. For headspace sampling, the boiling point of the solvent should be high to avoid significant evaporation during sampling. Because that requirement is often necessary for residual solvents analysis as performed in the pharmaceutical industry, headspace SDME is a natural application. Reference 24 provides further information for those not familiar with residual solvents analysis. Methods for the analysis of residual solvents in pharmaceutical products are presented in the United States Pharmacopeia (USP)25 and include direct liquid injection as well as headspace methods. Recent publications can be consulted to compare the older methods Headspace Liquid Microextraction