Detailed description of SPME

The top one cm of a SPME-needle is coated with a polymer (thickness between 7 and 100 µm) that acts as a sorbent (for a description of sorbents see Box 11 ). This needle can be inserted through a septum into a vial that contains a liquid or gaseous sample.

The needle is left in the vial until sorption equilibrium is attained (usually between a few minutes and half an hour depending on the thickness of the coating) or for a well defined time span. Finally, the needle is removed and inserted into the injector of a gas chromatograph. Here, the analytes are thermally desorbed and delivered to the GC column with the carrier gas.

Usually the sorption capacity (determined by the product of the partition constant and the volume of the polymer coating) of the SPME needle is not large enough to exhaustively extract a sample. Therefore, the calibration procedure that is needed for a quantitative analysis must also account for the extraction efficiency of the SPME needle.

SPME has the advantage of being a rather simple and quick extraction technique that does not require the use of organic solvents.