Reverse phase method development separation mechanism

Modern polysaccharide columns are based on cellulose or amylose derivatives coated onto silica. Chiral discrimination and applications have been extensively documented, but the mechanism of chiral discrimination is not yet fully understood. Whereas numerous phases are available within this subset, orthogonality can generally be obtained from a set of three or four columns as a first approach to method development. A typical choice of columns would be to try a set of different amylase (Chiralpak AD and AS) and cellulose (Chiralcel OD or OJ) columns and defer more extensive method development to the subset of samples not separated by these columns. The columns specified are run in the normal-phase mode and, accordingly, mobile phases are typically mixtures of hexane with small amounts of isopropanol or ethanol to control retention. However, selectivity is changed by different polar modifiers. Tailing may be minimized by the addition of 10-50 mM trifluoroacetic acid (TFA) or triethylamine (TEA). Analogue of the columns specified (AD-R, AS-R, OD-R, and OD-J) are available for reversed-phase separation. 

An understanding of simple methods development is crucial to developing effective environmental applications of solid-phase extraction (SPE). The lour mechanisms outlined in Chapter 2 (reversed phase, normal phase, ion exchange, and mixed mode) are sufficient for the majority of SPE applications in environmental analysis. The molecule s structure and the sample matrix are (he main factors that will determine which mechanism of isolation and separation will be the most appropriate. The fundamental approach to selection of sorbents will be a key topic and many examples are given. This chapter will also discuss applications of SPE to environmental matrices. These include water, soil, and air, for a variety of compounds. 

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