Gas chromatography (GC) resolution and sensitivity are often limited by matrix effects in "real world" samples that originate outside of a laboratory. GC separation and detection may encounter interference from nonvolatile constituents, side-effects of large sample volumes, and the chemical activity of matrix compounds and derivatization residues. A number of sample preparation techniques increase analyte concentrations and detector response while reducing matrix effects, including classical liquid–liquid extraction, chemical derivatization, and sample preconcentration, as well as headspace, thermal desorption, and large-volume sample injection.
Solid-phase microextraction (SPME) is a relatively new sample extraction technique — first described in the 1990s by Pawliszyn (1,2) — that brings some unique capabilities to bear on the chromatographic analysis of dilute solutions in difficult matrices, both liquid and gaseous. Essentially, SPME consists of two discrete steps: solute absorption from the sample matrix into a thick layer of silicone or related adsorptive material, followed by transfer of the absorbed analytes into a chromatography inlet system by thermal or liquid desorption.
SPME has been applied to both GC and liquid chromatography (LC) separations. It eliminates the need for large-volume sample transfer into a GC column by concentrating analytes into the fiber coating while leaving the bulk of the solvent and nonvolatile residues behind. SPME uses orders of magnitude less solvent and has significant potential to greatly reduce or eliminate solvent consumption and the concomitant issues of used solvent disposal as part of sample preparation.Chromatographers should not confuse SPME with solid-phase extraction (SPE), a related predecessor with similar applications. The principal difference is that SPE is carried out with a relatively large sorptive wafer the size of a small filter paper and requires post-sorption liquid-phase extraction of analytes; SPME is accomplished with a small fiber or tube coated with sorptive material. SPME is applied to both gas-phase and liquid-phase extraction, whereas SPE is limited to extraction from liquid-phase samples. This GC Connections installment primarily discusses SPME using absorptive polymeric coatings, although the principles also apply to adsorptive SPME onto active solid layers.
A related technique, stir-bar sorptive extraction (SBSE) uses a magnetic stir bar coated with a thick layer of absorptive polymer. The stir bar is exposed to sample solution for a time, during which solutes are absorbed into the polymer coating. Subsequently the stir bar is removed, dried, and then thermally desorbed for GC injection, or the absorbed analytes can be back-extracted with a different solvent. SBSE uses a larger volume of absorbent than SPME and, thus, is more efficient at extracting analytes with less absorbent solubility. Therefore, SBSE is generally more sensitive than SPME.
SPME applications cover a broad range that includes flower scents (3), chemical warfare agents (4), pharmaceutical process impurities (5), the determination of organochlorine pesticides in Chinese teas (6), volatile compounds in acidic media (7) and cheese (8), volatile phenols in wine (9), environmental pollutants in water samples (10), chloroanisoles in cork stoppers (11), volatile aliphatic amines in air (12), and phenylurea herbicides in aqueous samples (13).