Trimethylchlorosilane-derivatized silica

Table 5 lists two lines of commercially available silica-based gel permeation columns. The selection was limited to the Zorbax and LiChrospher silicas because these materials were specifically developed for gel permeation chromatography. Zorbax silica has a 6 pm particle size for optimum efficiency. The pore sizes were chosen such that a linear calibration curve is obtained when coupling columns of different pore sizes. In addition to plain silicas, Zorbax silicas are also available derivatized with trimethylchlorosilane, providing a surface that is less adsorptive for certain organic soluble polymers. Several important water-soluble industrial polymers, such as polyacrylamide, polyacrylic acid, and polyvinyl alcohol, do not require deactivation of the silica surface to obtain ideal size exclusion behavior. LiChrospher silicas are 10 pm in size they vary in pore size from 100 to 4000 A to allow the separation of very large polymers. 

Oxysterols in plasma are usually analyzed by GC-MS. Dzeletovic et al. developed a classical methodology which is still widely adopted today [63]. Oxysterols are found in plasma mostly esterified with fatty acids but also with sulfuric acid and as an acetal with glucuronic acid [64,65]. The protocol developed by Dzeletocvic et al. focused on fatty acids esters, so an initial base hydrolysis step was performed with KOH in ethanol. Following neutralization with phosphoric acid and a Folch-like extraction into CHCI3, oxysterols were separated from other less polar sterols on a silica cartridge. Derivatization was performed with pyridine hexameth-yldisilazane trimethylchlorosilane 3 2 1 (v/v/v) and analysis of the TMS ethers by GC-EI-MS. Nine oxysterols were identified in plasma using this protocol includ-... 

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