Diols sorbents

As another example, desonide and parabens in cream and ointment formulations were cleaned up by SPE by first testing mixtures of hexane-chloroform with silica, diol, and aminopropyl sorbents [36]. The solvent combination of 20% chloroform in hexane was found to be the optimum for dissolving the ointment base and yielding high recoveries of the analytes. The silica and aminopropyl sorbents were found to give nearly identical quantitative results, whereas the diol sorbent gave lower recoveries. Table 2.4 outlines the solvent considerations needed to elute retained compounds on silica and C,8 sorbents. 

Figure 9.5. Sorption of atrazine to diol sorbent via hydrogen bonding.

To date, typical SPE materials are based on silica gel or highly cross-linked styrene-divinylbenzene (PS-DVB). The former is functionalised with distinct chemical groups to yield various sorbents with non-polar or polar characteristics. Non-polar materials are modified with alkyl groups of different chain length (C18, C8, C2), while polar sorbents have cyano-, amino-, or diol-bonded groups. Ion-exchange phases have either anionic or cationic functional groups. 

Besides the above differentiation, restricted-access media can be further subdivided on the basis of the topochemistry of the bonded phase. Packings with a uniform surface topochemistry show a homogenous ligand coverage, whereas packings with a dual topochemistry show a different chemical modification of the pore internal surface and the particle external surface (114). Restricted-access media of the former type are divided into mixed-mode and mixed-function phases, bonded-micellar phases, biomatrix, binary-layered phases, shielded hydrophobic phases, and polymer-coated mixed-function phases. Restricted-access media of the latter type include the Pinkerton s internal surface reversed-phase, Haginaka s internal surface reversed-phase diol, alkyl-diol silica, Kimata s restricted-access media, dual-zone phase, tris-modified Styrosorb, Svec s restricted-access media, diphil sorbents, Ultrabiosep phases. Bio Trap phases, and semipermeable surface phases. 

Solid-phase extraction seems to be more suitable for multiresidue cleanup. This procedure has become the method of choice for isolation and/or cleanup of -lactam antibiotics from biological matrices, because it requires low solvent consumption, it is generally less time-consuming and labor-intensive, and offers a variety of alternative approaches that allow better extraction of the more hydrophilic -lactam antibiotics such as ampicillin. It is usually performed using reversed-phase Ci8 (69-71, 80-83, 90, 92-94, 99, 107, 112-116, 121-125) or Cg (103), anion-exchange (95, 124), and polar diol (95) or alumina (101, 113) sorbents. In recent applications, some workers demonstrated the potential of online solid-phase extraction in the determination of monobasic penicillins in bovine muscle tissues using a reversed-phase Cis 5 m cartridge and an automated... 

Cleanup of macrolides and lincosamides from coextracted material can also be accomplished with solid-phase extraction columns. Nonpolar sorbents such as XAD-2 resin (148) or reversed-phase sorbents (133, 134, 137, 141, 142) are usually employed in solid-phase extraction. In the latter case, ion-pairing with pentanesulfonic acid can also be applied for enhancing retention onto the hydro-phobic Ci8 material (154). However, these sorbents are not always effective for efficient cleanup of liver and kidney extracts. The basic character of macrolides and lincosamides suggests that cation-exchange sorbents such as aromatic-sulfonic acid (145,147), or polar sorbents such as silica (144,152,153), aminopropyl (139), or diol (149-151), can be powerful alternative approaches for isolation and/or cleanup of these compounds. 

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). 

The first step in using SPE is to condition the sorbent with an appropriate solvent. This prewetting increases the capacity of the bonded surfaces by opening up the hydrocarbon chain of the bonded-phase sorbents [31]. For nonploar sorbents, such as C,8) and for the ion exchangers, one column volume of methanol followed by one column volume of distilled water is required. Excessive washing with water will reduce analyte recovery [32]. Polar sorbents such as diol, cyano, amino, and silica should be rinsed with one column volume of a nonpolar solvent such as methylene chloride. Aternate cleanup methods may have to be developed if the analyte is sensi-... 

A vast range of materials have been bonded to silica such as phenyl, cyano, nitro, amino, and diol functionalities. The variety of silica-based sorbents are listed in the United States Pharmacopeia (USP) [23] where it is noted that of the 33 column packings, 19 are silica based. The synthesis and characteristics of over 80 nonconventional bonded silicas have been reviewed [24]. In spite of the number of chromatographic packings, 50% of the chromatographers develop methods using a C18 sorbent [1]. Sorbents... 

Antibiotics represent a chemically quite heterogenous category of compounds, for which the Cl 8 phases appear sorbents of the first choice. A group of these compounds (tetracyclines) was successfully separated by open tubular capillary electrochromatography using Cl8 modified capillaries. The results reported to be obtained in this system were better than those obtained with either monolithic columns or open diol columns [123] (Fig. 10.26). 

These sorbents possess, as functional groups, cyano, amino, and diol residues, bonded by short-chain hydrophobic spacers to the silica matrix. With respect to polarity, hydrophilic-modilied silicas range between nonmodilied silica and the nonpolar alkyl- or aryl-bonded phases ... 

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. 

Another example of modeling the structure of this type of CSP is presented by Francotte and Wolf [47]. They prepared benzoylcellulose beads, in a pure polymeric form as a sorbent, for the chromatographic resolution of racemic compounds like benzylic alcohols and acetates of aliphatic alcohols and diols. Their experimental results implicated multiple interaction sites to be involved in the complexation. Rationalizing the interaction mechanism required a more systematic investigation of the factors influencing separations and, to address the structural features of the cellulose tribenzoate, they carried out molecular modeling with molecular mechanics. The key question being addressed is to what extent is the polysaccharide backbone exposed to small molecules when sterically encumbered benzoates are attached ... 

The comparison of LiChrolut EN (40—120 Xm particles) performance in SPE trace enrichment of herbicides listed in Table 14.2 with that of a restricted-access material (P AM, 25 im), alkyl-diol-silica (25 Xm), reversed phase C-18 (10 Xm), monofiinctional C-18 (40—70 Xm), and, finally, styr-ene-DVB Empore extraction disks revealed a decisive superiority of the hypercrosslinked sorbent in the retention of very polar DIA and DEA [247]. However, the strong retention power of LiChrolut EN causes significant band broadening when analysis is performed in on-line... 

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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