Silica gel stationary phases

The addition of CO2 to mobile phases in normal phase chromatography using silica gel stationary phases was used as an adsorption-promoting solvent [56], Tetrahydrofiiran or chloroform with 3.5% ethanol was the organic components in... 

Li, L., Liu, M., Da, S., and Feng, Y. (2004) High Performance Liquid Chromatography of Aromatic Carbonxylic Acids on p-tert-butyl-calix[8]arene-bonded Silica Gel Stationary Phase, Talanta 62, 643-648. 

Kibbey, C. E. and Meyerhoff, M. E. (1993) Preparation and Characterization of Covalently Bound Tetraphenylporphyrin-silica Gel Stationary Phases for Reversed-phase and Anion-exchange Chromatography. Anal. Chem. 65, 2189-2196. 

Although gradient systems have been employed for the HPLC analysis of a wide variety of drug samples, their use does have the major disadvantage that a long equilibration time is required between each determination. This is because the solvent contained in the pores on the surface of the silica gel stationary phase must be allowed to return to equilibrium with the bulk of the mobile phase at the starting composition. 

There are few differences between the separation in gas chromatography [14-16] and the separation in liquid chromatography (LC), because it is assumed that the differential solvation of the diastereomeric compounds during the LC separation does not play a very important role [17], Helmchen et al. [18] explained the separation of diastereomeric amides using LC with a silica gel stationary phase under normal-phase conditions. In order to explain their separation, the authors made some assumptions ... 

Column chromatography is typically the next step after determination of the polarity of the extract components. Nonpolar compounds are usually chromatographed on silica-gel stationary phases. Vacuum column chromatography (VCC) or vacuum flash chromatography can provide a fast and simple first separation of an extract. This can be followed by additional VCC or by HPLC. Both silica-gel and reverse phase packings can be used (29,30). 

Silica-gel stationary phases 100% heptane, 20% ethyl acetate-80% heptane 40% ethyl acetate-60% heptane 60% ethyl acetate-40% heptane 80% ethyl acetate-20% heptane 100% ethyl acetate 25% methanol-75% ethyl acetate 50% metha-nol-50 /o ethyl acetate 100% methanol. For less polar compounds such as many compounds from the red alga Laurencia, a step gradient using 20% steps of dichloromethane in heptane can be used. 

Normal phase chromatography uses a polar stationary phase and a less polar or non-polar mobile phase, e.g. in TLC or HPLC with a silica gel stationary phase and hexane or methanol/chloroform/diethyl ether mobile phase. 

These columns are similar to normal HPLC colunms except that they have an internal diameter of around 1 mm. The packing materials consist of small diameter particles (3-30 /im) packed into stainless steel colunms by normal slurry packing techniques. An examination of the effect of internal diameter on column efficiency using 1 m columns packed with a silica gel stationary phase demonstrated that the lowest plate height values (//) were obtained with an internal diameter of approximately 1.02 mm (Scott and Kucera, 1979 McGuf-... 

Capillary electrochromatography (CEC) is so termed because the separation involves both electrophoretic and chromatographic procedures. The technique involves applying volt es across capillaries filled with silica gel stationary phases. 

Corticosteroid sodium phosphate salts used in parenteral preparations, i.e., eye, and ear drops, have been separated by reversed phase ion pair chromatography on silanized silica gel stationary phase, using aqueous triethylamine-methanol (3 2, v/v, pH s 4.2) as mobile 4tase. The separated spots were assayed by densitometry at a wavelength of 240 nm (73). Sodium phosphate salts of betamethasone, dexamethasone, and prednisolone have been quantitatively determined in the presence of chloramphenicol in eye and ear drops, also methyl and propyl parabens, and phenols. 

Fig. 24.30 (a) Synthetic steps towards a paira-ter/-butyl-calix[4]arene-bonded silica stationary phase [94]. (b) Chemical structure of para-tert-butyl-calix[8]arene-bonded silica gel stationary phase [95]. (c) Isocratic separation of nucleosides cytidine, uridine, guanosine and adenosine on an Aren Si 60 column with 0.02 M NaH2P04 (pH 3.5) as mobile phase (x-axis time in min y-axis absorbance at 254 nm) [93]. (Reprinted from Ref [93])... 

Adsorption chromatography. Separations are usually normal-phase with a silica gel stationary phase and a mobile phase of a nonpolar solvent blended with additions of a more polar solvent to adjust the overall polarity or eluting power, e.g. n-hexane + dichloromethane or di-ethyl ether. The choice of solvent is limited if a UV absorbance detector is to be used. Traces of water in the solvents must be controlled, otherwise solute retention will not be reproducible. Solutes are retained by surface adsorption they compete with solvent molecules for active silanol sites (Si-OH), and are eluted in... 

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