SPME can be considered as the miniaturization of SPE, which was introduced by Pawliszyn and co-workers in 1989 [8]. Unlike SPE, SPME relies on quantitative but non-exhaustive transfer of analytes on the basis of equilibration between small portions of adsorbents and large amounts of samples. The major advantages of SPME are the easy miniaturization and automation of devices. The technique integrates sampling, extraction, concentration and injection into one single step, which is simple, efficient and solvent-free. There are two main formats of SPME: fiber SPME and in-tube SPME.
In general, the sampling with fiber SPME (Fig. 2D) is more flexible than that with in-tube SPME (Fig. 2C), since the latter format is only suitable for particle-free samples because it is very easy to block the tube.