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RESEARCH PRODUCT

Solid-Phase Extraction and Clean-Up Procedures in Pharmaceutical Analysis

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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. It has prospered at the expense of 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 effected 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 greater. Chemically bonded silica (SI), usually with C18 and C8 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 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). A guide to improve sample clean-up procedure can be established. Current trends in SPE are aimed toward the development of miniaturized systems that facilitate its automation and increase its precision, throughput, and cost-effectiveness. Examples of such trends are the new formats of SPE such as tip-based microextraction and solid-phase microextraction with capillary columns (in-tube solid-phase microextraction, IT-SPME). 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. The precision obtained is similar for both techniques, while the recoveries obtained by SPE are generally better than those obtained by LLE. Enrichment and derivatization can also be carried out by use of sorbents as supports.