Molecular Imprints for Solid-phase Extraction

Among the most important Relds in analytical chemistry are medical, food and environmental analysis, where the target analytes are often present at very low concentrations in rather complex matrices. Methods currently used for analysis, such as liquid chromatography, gas chromatography or capillary electrophoresis, must therefore be preceded by a selective isolation and concentration step. Commonly used techniques for the extraction and clean up of analytes from environmental and biological samples are liquid}liquid extraction (LLE) and solid-phase extraction (SPE). Supercritical-Suid extraction (SFE) has also attracted interest, although it has not found widespread application thus far because of the specialized equipment required. The advantages of SPE compared with LLE are that it is faster, more reproducible, cleaner extracts are obtained, emulsion formation is not an issue, solvent consumption is reduced, and smaller sample sizes are required. It may also be cheaper than LLE when the costs for solvent disposal are taken into consideration. Moreover, SPE can be easily incorporated into automated analytical procedures. For a detailed introduction to SPE, the reader is directed to the relevant chapters in the Encyclopedia. Solid phases currently employed for SPE include polystyrene divinylbenzene resins and chemically modiRed silica with either hydrophobic or ionexchange groups. These materials generally yield satisfactory results if the conditions for extraction, washing and elution are carefully determined according to the chemical characteristics of the analytes and the other components in the sample. Nevertheless, due to the non-speciRcity of hydrophobic or ionic interactions, a large number of contaminants are often co-extracted, particularly if the sample matrix is very complex. A solution to this problem can be to use afRnity sorbents, thus taking advantage of the speciRcity of biological recognition. Theoretically, biomolecules like antibodies or receptors have the potential to meet the analytical demands for almost any target analyte since, with the advent of phage display antibody libraries and recombinant antibodies, a suitable recognition element can be found in many cases, even if a natural receptor does not exist. Unfortunately, biomolecules also have a major drawback, which is their poor stability, particularly in the presence of organic solvents. ArtiRcial receptors have therefore been gaining in importance as a possible alternative to natural systems. Molecular imprinting is becoming increasingly recognized as a versatile technique for the preparation of synthetic polymers bearing tailor-made recognition sites. These molecularly imprinted polymers (MIPs) are obtained by copolymerizing functional and cross-linking monomers in the presence of the analyte, which acts as a molecular template. The molecular imprinting technique is reviewed in more detail elsewhere in the Encyclopedia (see AfRnity Separation/Imprint Polymers). Molecularly imprinted polymers combine the advantages of biological antibodies or receptors (very speciRc binding) with those of synthetic polymers (high physical and chemical stability, compatibility with both aqueous and organic solvents). Their use in sample preconcentration and clean up by solidphase extraction is therefore highly attractive.