Solid-phase microextraction (SPME) was introduced as a solvent-free sample preparation technique in 1990. The basic principle of this approach is to use a small amount of the extracting phase (usually less than 1 |L) compared to the sample matrix. Sample volume can be very large, when the investigated system, for example air or lake water, is sampled directly. The extracting phase can be either a high molecular weight polymeric liquid, similar in nature to chromatographic stationary phases, or it can be a solid sorbent, typically of a high porosity to increase the surface area available for adsorption.

To date the most practical geometric configuration of SPME utilizes a small fused silica fibre, usually coated with a thin film of polymeric phase. The fibre is mounted for protection in a syringe-like device (Figure 1A). The analytes are absorbed or adsorbed by the fibre coating (depending on the nature of the coating) until an equilibrium is reached in the system. The amount of an analyte extracted by the coating at equilibrium is determined by the magnitude of the partition coefficient (distribution ratio) of the analyte between the sample matrix and the coating material.

In SPME, analytes typically are not exhaustively extracted from the matrix. However, equilibrium methods are more selective because they take full advantage of the differences in extracting phase/ matrix distribution constants to separate target analytes from interferences. Exhaustive extraction can be achieved in SPME when the distribution constants are large enough. This can be accomplished for most compounds by cooling the fibre coating. This phase extraction applied to the isolation of estrogens from urine. Journal of High Resolution Chromatogra-phy 21: 481-490.

Sellergren B (1999) Polymer- and template-related factors influencing the efficiency in molecularly imprinted solid-phase extractions. Trends Analytical Chemistry 18: 164-174.

Simpson NJK (2000) Solid Phase Extraction: Principles, Strategies, and Applications. New York: Marcel Dekker.

Thurman EH and Mills MS (1998) Solid-Phase Extraction, Principles and Practice. New York: John Wiley.

concept was tested using a piece of microtubing coated on the outside instead of a solid rod and supplying liquid carbon dioxide into the tube to achieve an internally cooled fibre. In exhaustive extraction, selectivity is sacrificed to obtain quantitative transfer to target analytes into the extracting phase. One advantage of this approach is that, in principle, it does not require calibration, since all the analytes of interest are transferred to the extracting phase. On the other hand, the equilibrium approach usually requires calibration through the use of surrogates or standard addition to quantify the analytes and compensate for matrix-to-matrix variations and their effect on distribution constants.

Since equilibrium rather than exhaustive extraction occurs in microextraction methods, SPME is ideal for field monitoring. It is unnecessary to measure the volume of the extracted sample and therefore the SPME device can be exposed directly to the investigated system for quantitation of target analytes. Thin coatings of extracting phase result in fast separations. In addition, extracted analytes are introduced to the analytical instrument inlet system by simply placing the fibre in the desorption unit (Figure 1B and 1C). This convenient, solvent-free sample introduction process facilitates sharp injection bands and rapid separations. These features of SPME result in the integration of the first steps in the analytical process: sampling, sample preparation and introduction of extracted mixture to the analytical instrument. For example, total analysis time in field applications can be as low as a few minutes when portable instrumentation is used.

The equilibrium nature of the technique also facilitates speciation in natural systems since the presence of a minute fibre, which removes small amounts of target analytes, is not likely to disturb the system. Because of the small size, coated fibres can

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