Extraction from liquid matrices
Extraction from liquid matrices is relatively easy: the sample is easily homogenized; diffusion into it is fairly fast (especially when stirred) and the compounds to be extracted are easily accessible.
The conventional technique used for liquid matrices is liquid-liquid extraction (LLE): the liquid extractant is chosen regarding for its affinity for the solute to be extracted and also for its non-miscibility with the liquid sample (a low-viscosity solvent is chosen to favor diffusion of the solute at the liquid-liquid interface).
Solid-liquid extraction (also called solid-phase extraction or SPE) now tends to replace liquid-liquid extraction in laboratories because of its numerous advantages. In this case, the extractant is a solid.