What is Solid-Phase Extraction

Solid-phase extraction (SPE) is a method of sample preparation that concen-Irates and purifies analytes from solution by sorption onto a disposable solid-phase cartridge, followed by elution of the analyte with a solvent appropriate for instrumental analysis. The mechanisms of retention include reversed phase, normal phase, and ion exchange. Traditionally, sample preparation consisted of sample dissolution, purification, and extraction that was carried out with liquid-liquid extraction. The disadvantages with liquid-liquid extraction include the use of large volumes of organic solvent, cumbersome glassware, and cost. Furthermore, liquid-liquid extraction often creates emulsions with aqueous samples that are difficult to extract, and liquid-liquid extraction is not easily automated. These difficulties are overcome with solid-phase extraction. Thus, solid-phase extraction was invented in the mid-1970s as an alternative approach to liquid-liquid extraction. 

Initially, SPE was based on the use of polymeric sorbents, such as XAD resins (polymeric adsorbents), which were packed in small disposable columns for use on drug analysis. The early environmental applications consisted of both XAD resins and bonded-phase sorbents, such as C-18 (McDonald and Bouvier, 1995). These precolumns were used for sample trace enrichment prior to liquid chromatography and were often done on-line, which means at the same time as liquid chromatography. However, these first, steel, on-line precolumns quickly were replaced with an off-line column made of plastic in order to be both inexpensive and disposable. Eventually, the term solid-phase extraction was coined for these low-pressure extraction columns (Zief et al., 1982). Thus, solid-phase extraction is an analogous term to liquid-liquid extraction, and in fact, solid-phase extraction might also be called liquid-solid extraction. However, it is the term solid-phase extraction or the acronym SPE that has become the common name for this procedure. 

Solid-phase extraction columns now are typically constructed of polypropylene or polyethylene and filled with 40-pm packing material with different functional groups. A 20-pm polypropylene frit is used to contain from 50 mg to 10 g of packing material. A liquid sample is passed through the column and analytes are concentrated and purified. The sample volume that 

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