Reverse-phase sorbents

Table 3.42 lists the main factors influencing optimisation of SPE. When considering a specific extraction problem, many different aspects influence column selection, including nature of the analytes and of the sample matrix degree of purity required nature of major contaminants in the sample and final analytical procedure. Reversed-phase sorbents have nonpolar functional groups and preferentially retain nonpolar compounds. Thus, for a nonpolar analyte, to remove polar interferences using a polar sorbent phase, the sample... 

The choice of solvent directly influences the retention of the analyte on the sorbent and its subsequent elution, whereas the solvent polarity determines the solvent strength (or ability to elute the analyte from the sorbent in a smaller volume than a weaker solvent). Dean [272] gave solvent strengths for normal- and reversed-phase sorbents. The elution solvent should be one in which the analytes are soluble and should ideally be compatible with the final analysis technique. For example, for HPLC analysis, a solvent similar to the mobile phase is a good choice of elution solvent. For the elution step it is also important to consider the volume of the solvent. A minimum volume of elution solvent (typically 250 xL per 100 mg of sorbent) allows maximum concentration of the analytes. 

Schroder, H.E (2003). Determination of fluorinated surfactants and their metabolites in sewage sludge samples by liquid chromatography with mass spectrometry and tandem mass spectrometry after pressurized liquid extraction and separation on fluorine-modified reversed-phase sorbents. J. Chromatogr. A 1020(1), 131-151. 

Modified silica with a C18 reversed-phase sorbent has historically been the most popular packing material, owing to its greater capacity compared to other bonded silicas, such as the C8 or CN types [22]. Applications of C18 sorbents include the isolation of hydrophobic species from aqueous solutions. The mechanism of interaction with such sorbents depends on van der Waals forces, and secondary interactions such as hydrogen bonding and dipole-dipole interactions. Nevertheless, the main drawbacks of such sorbents are their limited breakthrough volumes for polar analytes, and their narrow pH stability range. For these reasons, reversed-phase polymeric sorbents are also used frequently in environmental applications for the trace enrichment of soluble molecules that are not isolated by reversed-phase sorbents such as C18. 

Hydrophilic-Lipophilic Balance sorbent (a reversed-phase sorbent). 

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. 

Tire preferred type of reversed-phase sorbents is Cjg bonded silica (Table 29.4). Using this reversed-phase sorbent, ion-pair separation of lincomycin (154), spiramycin (138), and tylosin (145) residues has also been reported through use of octanesulfonate, heptanesulfonate, and tetrabutylammonium pairing ions, respectively. Phenyl-bonded silica or polymeric stationary phases have also been described for the separation of tilmicosin (133) and lincomycin (146) residues, respectively. 

Cleanup and concentration of both polar and nonpolar anticoccidial drugs from coextracted substances have mainly been accomplished with polar sorbents, such as silica (379, 391, 396, 398, 399, 402, 407, 410, 411), alumina (227, 257, 403, 404, 406, 412), or Florisil (388), since they provide high recovery of the analytes. However, in many cases, cation exchange sorbents (409), reversed-phase sorbents (405), or combinations of silica with reversed-phase sorbents (395, 400), alumina with Sephadex LH-20 (406), or alumina with Sephadex LH-20 and silica (397, 401) sorbents have been reported to be a powerful approach for the isolation and/or cleanup of these compounds. It is significant to note that cleanup systems using alumina are not suitable for isolation of lasalocid residues since they are irreversibly bound to the sorbent. 

Boonjob W et al (2010) Online hyphenation of multimodal microsolid phase extraction involving renewable molecularly imprinted and reversed-phase sorbents to liquid chromatography for automatic multiresidue assays. Anal Chem 82(7) 3052-3060... 

Dissolved organic sulfur was determined in aqueous solutions after isolation by solid-phase extraction on macroporous resins and reversed-phase sorbents.156 The sulfur in the extracts was determined by pyrohydrogenolysis of the extract in a heated quartz tube (1100°C) in a hydrogen atmosphere followed by flame photometric detection. 

Relative elution solvent strength (or eluotropic strength) is depicted in solvent polarity charts (Figure 2.39). The relative elution strength for a solvent on a polar, normal-phase sorbent such as silica or alumina increases in reverse order to that measured on a nonpolar, reversed-phase sorbent. Ac-... 

Silica gel plates are modified to form a reversed-phase sorbent by being impregnated with liquid paraffin, silicone oil, or fats. Reversed-phase plates of this type are used in the identification of steroid hormones [8,9]. Table 3.2 lists various other substances that have been used for impregnation [10,11]. Sorbents that are impregnated with amminium oxalate, ammonium sulfate, magnesium acetate, sodium acetate, and silver nitrate have one time or the other been commerically available. As an example, impregnated adsorbents have been used for the resolution of sulfa drugs [19]. 

In the above approaches to choosing a TLC system, silica gel has been the sorbent of choice. This is due to the greater separation potential that silica gel has over reversed-phase sorbents. For purity testing, it is advisable to use both silica gel and reversed-phase sorbents. 

Addition of surfactants, such as cetyltrimethylammonium bromide (CTMA), causes silica to mimic separations obtained with reversed-phase sorbents. The impurities of propranolol and pharmaceutical preparations of catecholamines have been chromatographically studied using this surfactant [310,316]. The one apparent advantage of using these surfactants is that the brand-to-brand variations in selectivity commonly seen for bonded phases is avoided [310]. For basic analytes, the addition of either methanol or acetonitrile changes the sorbent selectivity. Commonly, a retention minimum occurs at about 50% organic solvent content with increases in retention at either increased or decreased organic content [312]. 

The authors compared this aqueous silica method with alkyl-bonded procedures by testing 12 different corticosteroids on 8 different silica sorbents and on 6 different reversed-phase sorbents. The variations in selectivity of the 12 corticosteroids among the silica columns were found to be substantially less than those based on the reversed-phase sorbents. Based solely on this comparison, silica-based separations using aqueous mobile phases would be preferred over reversed-phase sorbents for assay of steroids [328]. The arguments on which sorbent is best for the assay of steroids will more than likely continue. 

In addition to the silica-based materials mentioned above, modem polymers are widely used for TTA and QTA sample preparation allowing SPE not impaired by undesirable silanol activities. HLB Oasis (Waters) is the tradename for a hydrophilic-lipophilic balance reversed-phase sorbent enabling lipophilic interaction to benzene moieties and hydrophilic interactions to pyrrolidone groups as present in the macroporous copolymer of poly(divinylbenzene-co-iV-vinylpyrrolidone). Elution of analytes is often performed with solvents containing MeOH or ACN. Applying this adsorbent TA such as atropine and scopolamine were extracted from human viscera [15], human serum [97-99], human urine [12] as well as from rat plasma and brain microdialysate [77], Furthermore, this hydrophilic-lipophilic balance phase was also suitable for extraction of the QTA trospium from human and rat plasma [77, 84] and methyl scopolamie from rat plasma [77] (Table 4). 

Silica gel 60, the most versatile and most frequently used TLC sorbent, was taken as a basis. The mean particle size of this sorbent was optimized simultaneously, the particle size distribution was brought to within as narrow limits as possible (L- ) The sorbent material thus obtained was used t prepare HPTLC pre-coated plates silica gel 60, being followed subsequently by the development of other sorbents for processing into HPTLC pre-coated layers. The materials chosen were largely "reversed phase" sorbents, that is to say, chemically modified silica gels with a non-polar surface (, 8), as well as microcrystalline cellulose ( )""... 

FIGURE I 5 Illustrative examples of the retention dependencies for the polypeptide hormones, /3-endorphin and glucagon, as plots of the experimentally determined In k (proportional to AG°ssoc, ) versus the volume fraction of organic solvent measured with a n-butylsilica reversed-phase sorbent at temperatures from 5° to 8S°C under isocratic elution conditions at a flow rate of I mL/min, encompassing the range of acetonitrile concentrations from 0.25 < ip < 0.30. Data from Ref. 237. 

The attainable enrichment and clean-up in SPE depend primarily on the selectivity and affinity of the sorbent for the selected target analyte or analytes, the sample load capacity for the analytes and the rate of mass transfer to and from the binding sites, the latter affecting the minimum desorption volume and thus the enrichment that can be obtained. Other factors of importance are the reproducibility of the recovery yields and the stability and reusability of the sorbent when online procedures are desired. For hydrophobic analytes satisfactory results are usually obtained using standard reversed phase sorbents. Thus hydrophobised silica (C8, Cl8), styrene-divinylbenzene copolymers (PS-DVB) and graphitised carbon black (GCB) are the conventional sorbent materials used in SPE (Fig. 15.2)... 

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