Solid-phase-based extraction techniques are based on the partition
of analytes between the stationary phase and liquid matrix,
which can reduce or eliminate the use of solvents and usually
exhibit the higher speed and sensitivity compared with conventional
liquid–liquid extraction techniques. Thus, these techniques,
especially SPE, have become the major online SP techniques for
liquid samples. An ideal solid-phase format for online extraction
should be able to easily employ all kinds of efficient absorbents in a
flow system and facilitate the rapid mass transfer. Fig. 2 shows the
typical formats of online SPE (Fig. 2A and B), SPME (Fig. 2C and D),
MEPS (Fig. 2E) and SBSE (Fig. 2F) techniques.