Selector coverage

Ekborg-Ott, K.H., Wang, X., and Armstrong, D.W., Effect of selector coverage and mobile phase composition on enantiomeric separations with ristocetin A chiral stationary phases, Microchem. J., 62, 26, 1999. 

Bosakova, Z., Cufinovd, E., and Tes ovd, E., Comparison of vancomycin-based stationary phases with different chiral selector coverage for enantioselective separation of selected drugs in high-performance liquid chromatography, J. Chromatogr. A, 1088, 94, 2005. 

In addition to the common use of vancomycin and teicoplanin, the use of other antibiotics as chiral selectors in HPLC were limited. Only a few reports are available in the literature dealing with these antibiotics as HPLC CSPs. Armstrong et al. [42] resolved about 230 racemates on the ristocetin A antibiotic covalently bonded to silica gel. The resolution was carried out using the three modes of mobile phases. The results were complimentary to those obtained on vancomycin and teicoplanin CSPs. In another study, the effect of selector coverage and mobile phase composition on enantiomeric resolution with ristocetin A CSP was carried out by Armstrong et al. [43]. Thiostrepton-based CSP was also used to resolve the enantiomers of thioridazine, 2,2,2-trifluoro-l-... 

Over 100 stationary phases with immobilized chiral selectors are commercially available with further stationary phases described in the literature. Any attempt at a general classification into a reasonable number of groups with similar properties is not easy. Probably the most widely adopted classification scheme was proposed by Wainer, and is loosely based on the chiral recognition mechanism. Table 10.3 [61]. In the following sections, a different approach has been adopted based on the type of chiral selector. Coverage is incomplete, but includes all of the widely used stationary phases. 

First, they compared CSPs 1 and 3 prepared by the two-step solid-phase methodology with their commercially available counterparts (CSPs 2 and 4) obtained by direct reaction of the preformed selector with a silica support. Although no exact data characterizing the surface coverage density for these phases were reported, all of the CSPs separated all four racemates tested equally. These results shown in Table 3-3 subsequently led to the preparation of a series of dipeptide and tripeptide CSPs 5-10 using a similar synthetic approach. Although the majority of these phases exhibited selectivities lower or similar to those of selectors built around a single amino acid (Table 3-3), this study demonstrated that the solid-phase synthesis was a... 

In a typical reaction, the macrocycle is treated with a 2-3 M excess of (3-isocyanatopropyl)triethoxysilane in dry DMF. The derivative is then added to a dry DMF slurry of silica gel ( 2 g of functionalized selector to 4 g of silica gel). The solution is stirred, allowed to react for 20 h at 107°C, and then cooled, filtered, and washed with methanol, 50% aqueous methanol, and methanol again, and finally dried. Surface coverage data are reported only in the case of the commercially available vancomycin CSP (see Table 2.2), immobilized by joining on average three linkers per vancomycin molecule, probably via a 3-isocyanatopropyl-silane [48]. 

Avery recent study [128] deals with the comparison of two commercially available vancomycin-based CSPs with different surface coverage of the chiral selector in the enantioseparation of P-blockers and profens, by RP and POM separation modes. Higher retention and better resolution were obtained on the CSP with higher coverage of vancomycin in both the separation modes. However, in the case of pro fens, higher retention was not always accompanied by an improvement of the enantioselectivity in the RP mode. An accurate study of the influence of the mobile phase composition was also performed in both the separation modes. 

Due to the low molecular weight of the selector, high molar SO coverages can be achieved in the immobilization step, leading to high loadability in (semi)preparative separations. 

These procedures serve a number of purposes in addition to maintaining the files. For example, the selections made in Chemical Abstracts are checked for completeness of coverage both at Mellon Institute and at Gulf Research, and a mimeographed bulletin is prepared for each issue it contains all the abstracts considered by the selectors to be of interest to Gulf Research personnel. This bulletin, prepared by the Information Section, is so arranged that individual abstracts can be clipped from it, pasted on file cards, and added to each man s personal file. The patent abstracts are published monthly in a mimeographed bulletin put out by the Patent Section. Thus, the entire technical personnel of the company is kept abreast of current developments in the field, and each man may build a personal file to suit his particular needs. 

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