C18-modified silica

T. Galeano-Diaz, M.I. Acedo-Valenzuela, F. Salinas, Determination of the pesticide Naptalam, at the ppb level, by FIA with fluorimetric detection and on-line preconcentration by solid-phase extraction on C18 modified silica, Anal. Chim. Acta 384 (1999) 185. 

Different procedures based on that chemistry and using FIA [183], SIA [166,184], MSFIA [185,186], and MCFIA [187] have been described. Most of them make use of online sample preparation, including micellar-assisted preconcentration (cloud point extraction (CPE)) or solid-phase extraction (SPE) [185,186,188]. To this end, either a microcolumn packed with Amberlite XAD-4 resin [185] or C18-modified silica material may be used. 

With regard to the resolution of enantiomers, some applications can be found with modified silica gel supports. Thus, a Pirkle-type CSP was used for the separation of 200 mg of a racemic benzodiazepinone [75]. Also tris-(3,5-dimethylphenyl)carba-mate of cellulose coated on silica C18 [91, 92] was applied successfully to the resolution of the enantiomers of 2-phenoxypropionic acid and to oxprenolol, alprenolol, propranolol among other basic drugs. However, the low efficiency of this technique and the relative high price of the CSPs limits its use to the resolution of milligram range of sample. 

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. 

Srinivasan, G, Kyrlidis, A., McNeff, C., and Muller, K., Investigation on conformational order and mobility of diamondbond-C18 and Cl 8-alkyl modified silica gels by Fourier transform infrared and solid-state NMR spectroscopy, J. Chromatogr. A, 1081, 132, 2005. 

Packed columns Porous and non-porous silica Lipophilic modified silica gels, e.g., C18 Mixed mode phases, e.g., C8/SCX ... 

S. Rio Segade and J. F. Tyson, Determination of methylmercury and inorganic mercury in water samples by slurry sampling cold vapor atomic absorption spectrometry in a flow injection system after preconcentration on silica C18 modified, Talanta, 71(4), 2007, 1696-1702. 

Lysozymes, cytochromes c, aspartame Silica capillary was etched and chemically (C18) modified pH 3.7 515 or 580 mm x 20 pm i.d. 220 or 260 mm effective length, open tubular electrochromatography... 

They are synthesised at room temperature from an aqueous mixture of tetraethoxysiloxane, isopropanol, hexadecylamine and ammonium hydroxide. Hydrothermal treatment of the spheres in the mother liquor induces the formation of wider pores of 50A and a BET surface area of 350 m /g. The silica spheres possess surface silanol groups which can be used to attach C8 tind C18 ligands, thus converting the surface from a hydrophilic to a hydrophobic one. Subsequent application of these surface modified silica spheres in fast separation HPLC has proven to be successful. The synthesis has been successfully up-scaled (70g) to yield spheres of sizes 0.5, 1,2, 6 and 20 pm. 

The lifetime of a stationary phase in small molecule purification strongly depends of course on the type of cmde material that has to be purified. In general, the current spherical phases on the market (bare silica, C8, C18, cyano, diol, amino. . . ) based on purified metal-free silica have a very good mechanical as well as a long-term chemical stability if they are used under the appropriate conditions. For example, on production scale, a cyano-modified silica stationary phase could be kept in the same column for about 4 years allowing to purify multiton quantities of product. 

In RP-HPLC, the stationary phase is less polar than the mobile phase and is usually comprised of spherical silica particles (typically, 3-5 pm in diameter). The acidic functionalities on the silica material have been modified by deriv-atisation with alkyl (C2 to C18), phenyl, cyano and amino groups. Typical mobile phases used in RP-HPLC consist of mixtures of aqueous buffers mixed with water-miscible organic solvents, such as methanol and acetonitrile. In addition to modified silica stationary phases, other new developments in RP-HPLC are now available, e.g. porous polymeric, carbon and mixed modal phases. 

C2, Cg, C18, C30, and phenyl-modified silica Cg-modified alumina cellulose triacetate... 

Lambert and DeLeenheer (1992) contributed a review on the TLC analysis of K vitamins. They noted that although HPLC is usually the method of choice for analysis, there is some interest in HPTLC-densitometric analysis of the K vitamins. Their review covers isolation, extraction, and cleanup methods uses of polar inorganic solvents uses of modified silica (i.e., nonpolar bonded phases such as C2, Cg, C18, and phenyl) uses of mobile phases and methods of detection and quantification of the K vitamins. Madden and Stahr (1993) assayed vitamin K in bovine liver by reversed-phase TLC with dichloromethane-methanol (7 3)... 

Ikegami, T., Hara, T., Kimura, H., Kobayashi, H., Hosoya, K., Cabrera, K., Tanaka, N. (2005). Two dimensional reversed-phase liquid chromatography using two monolithic silica C18 columns and different mobile-phase modifiers in the two dimensions. J. Chromatogr. A, Forthcoming. 

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. 

The retention of analyses in RP-HPLC markedly depends on the adsorption of the organic constituent of the mobile phase on the surface of the stationary phase. The excess adsorption isotherms of ACN, THF and methanol were measured on silica support modified with C, C6, C8, C10, C12 and C18 monomeric phase and a model was developed for the description of the retention of solutes from the binary mobile phase. The dependence of the retention factor on the partition coefficient can be described by... 

For the preparation of capillary columns for gas chromatography, the fused silica column wall is deactivated using polysiloxanes, and modified with a suitable stationary phase. Hetem19 discussed the use of polymethylhydrosiloxanes (PMHS) for deactivation and subsequent coating with a polymerized C18-type silane for stationary phase formation. Stationary phases used in packed GC are analogous to HPLC. 

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