Normal phase diols sorbents

In liquid chromatography, reversed-phase materials such as Cig and Cg are the most commonly used sorbents (429, 430, 434, 438, 446, 447, 453, 454). Examples of baseline separations with reversed-phase columns of several groups of anabolics including stilbenes, resorcyclic acid lactones, and other, frequently used anabolics have been reported (463-466). In addition to reversed-phase separations normal-phase separations of anabolics using either Hypersil (467) and Brownlee (456) silica or diol-modified silica have been reported. Although not all analytes were completely separated, the latter column could be efficiently used to differentiate between estrogenic and androgenic compounds within a mixture of 15 anabolics and their metabolites (468). 

For normal-phase SPE, cyanopropyl (CN), aminopropyl (NH2), and diol functional groups are chemically bonded to the silica gel. The loading on the cyano, amino, and diol colunms are sufficiently large ( 6-10% as carbon) that they may sometimes be used for reversed-phase applications, especially for the removal of hydrophobic solutes from water or other polar solvents. These hydrophobic solutes would otherwise sorb too strongly to a more hydrophobic C-8 or C-18 sorbent and would be difficult to elute. Straight silica gel also is... 

Finally, sorbents such as aminopropyl, cyanopropyl, and diol can be used for both reversed-phase and normal-phase separations. Many manufacturers supply their sorbents in variety packs that may be used for methods development. Also quality assurance reports are commonly available for the various sorbents, which is a good indication of their reproducibility. Later chapters will show specific applications, such as environmental, drugs and pharmaceuticals, and food and natural products. 

There are three general types of bonded normal-phase sorbents aminopropyl, cyanopropyl, and diol (Fig. 5.3). All three are derivatized as a propyl hydrocarbon, which places them near the surface of the silica. Only the cyanopropyl sorbent is available in either endcapped or nonendcapped forms (Table 5.1). Thus, analytes have the opportunity to interact not only with the bonded phase but also with the underlying silica sorbent. Furthermore, because the hydrocarbon chain is short, only three carbons in length, the... 

Normal-phase sorbents have polar functional groups, e.g. cyano, amino and diol (also included in this category is unmodified silica). The polar nature of these sorbents means that it is more likely that polar compounds will be retained. 

As practiced, normal phase is very much like the adsorption mode (polar stationary-phase surface and nonpolar mobile phase), the only difference being that the normal-phase separation is done on a less-polar surface. This is because the sorbent used in normal-phase separations is bonded with a polar-bonded phase, such as propyl cyano, propyl amino, or organo-diol (from a reagent derived from glycerol). Although select vities are similar to the adsorption mode, the reproducibility is greatly increased due to less sensitivity to moisture in the mobile phase due to a lower silanol content. 

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