Sorptive sample preparation -- a review

Most sample-enrichment procedures currently available rely on adsorption of the analytes of interest by a suitable adsorbent material. Although good performance can be obtained for many practical problems, in some cases the applicability of adsorptive sample preparation falls short, particularly for the enrichment of polar and/or high-molecular-weight compounds, especially in combination with thermal desorption. Because of the very strong retention of adsorbent materials, undesired effects such as incomplete desorption and artifact formation are observed. Polar solutes are easily adsorbed but readily undergo surface-catalyzed reactions and on desorption yield compounds different than those originally sampled. High-molecular-weight compounds cannot be desorbed because of extremely strong interactions with the adsorbent and their low volatility. To overcome some of these problems sample-preparation techniques based on polydimethylsiloxane sorption have been developed over the past 15 years. In contrast with adsorptive trapping, sorption is based on dissolution of the analytes in a liquid polymeric material. This is a much more inert means of solute retention which overcomes some of the limitations encountered when working with adsorbents. In this contribution, the basic principles of sorption, the different instrumentation used, and applications of the technique will be reviewed. The review covers the sorptive sample-preparation techniques, open-tubular trapping (OTT), solid-phase microextraction (SPME), gum-phase extraction (GPE), equilibrium gum-phase extraction (EGPE), and stir-bar-sorptive extraction (SBSE). Because of the nature of sorptive sample-preparation techniques, which perform particularly well in combination with thermal desorption, this review focuses strongly on gas chromatography as the means of chemical analysis.