Fig. 1 shows the online SP techniques coupled with LC for liquid,
semisolid and solid samples. Since online SP techniques are
generally based on flow analysis approaches, liquid samples are
easier to handle than semisolid and solid samples. Among the
online SP-LC techniques for liquid samples, SPE is the most commonly
used techniques. Besides, microextraction techniques such
as SPME, SBSE and LPME are gaining increasing popularity among
scientists. In addition, dialysis and microdialysis (MD) are efficient
membrane-based techniques for online sampling and cleanup
of liquid samples. As for semisolid and solid samples, the analytes
in the sample matrix are usually extracted into solvents prior
to LC analysis. The most common online extraction methods are
solid–liquid extractions assisted by high temperature and pressure
(ASE), microwave (MAE) or ultrasound sonication (UAE), and
extraction with supercritical fluids (SFE). The extract can be online
injected into a LC system directly or introduced into the online system
for the further treatment and analysis as illustrated in Fig. 1.