Solid‐phase extraction (SPE) using small cartridges filled with sorbents of a small particle size has rapidly established itself as an important sample clean‐up technique since the 1980s. It has prospered at the expense of traditional liquid–liquid extraction (LLE) which is considered labor‐intensive and frequently plagued by problems, such as emulsion formation and use of large volumes of hazardous solvents. A remarkable characteristic of SPE is its easy adaptation to on‐line mode by column switching techniques; switching can be affected manually or by automated controllers.
The same analyte/sorbent interactions that are exploited in liquid chromatography (LC) are of use in SPE, but particle sizes employed are generally greater. Chemically bonded silica (SI), usually with C
18
and C
8
organic groups, is by far the most commonly used material. Cross‐linked polystyrene and other polymeric resins and mixed‐mode SPE sorbents are also widely used. Recently, different sorbents have been introduced in order to improve the selectivity such as molecularly imprinted polymers (MIPs) and immunoaffinity sorbents or to improve the extraction capacity through the increment of the surface‐to‐volume ratio (i.e. hypercross‐linked resins). Advances in material science in general and in nanomaterials in particular mark a trend in this field. Also, ionic liquids (ILs), carbon‐based materials, sol‐gel‐based compounds, and metal‐organic frameworks (MOFs), among others, are being introduced.
Current trends in SPE are aimed toward the development of miniaturized systems which facilitate its automation and increase its precision, throughput, and cost‐effectiveness. Its compatibility with the goals of sustainable chemistry is remarkable. Examples of such trends are the new formats of SPE such as solid‐phase microextraction (SPME) including in‐tube solid‐phase microextraction (IT‐SPME) and tip‐based microextraction, between other formats.
There are different approaches to automation for both off‐line and on‐line SPE, which involve different levels of apparatus cost. SPE is generally easier and cheaper than LLE for achieving this last goal. The precision obtained is similar for both techniques, while the recoveries obtained by SPE are generally better than those obtained by LLE.
Enrichment, derivatization, and fractionation can also be carried out by use of SPE sorbents as supports.
