Octadecyl-coated particles

The power-law exponent /x is around 4.0-5.0. A similar result is obtained for the yield stress in shear cr, except the exponent is smaller, coated silica particles. Such power laws have been derived from theories that model the gel as a network of interconnected fractal clusters (BuscaU et al. 1988 Shih et al. 1990 Potanin et al. 1995). 

Several different analytical and ultra-micropreparative CEC approaches have been described for such peptide separations. For example, open tubular (OT-CEC) methods have been used 290-294 with etched fused silicas to increase the surface area with diols or octadecyl chains then bonded to the surface.1 With such OT-CEC systems, the peptide-ligand interactions of, for example, angiotensin I-III increased with increasing hydrophobicity of the bonded phase on the capillary wall. Porous layer open tubular (PLOT) capillaries coated with anionic polymers 295 or poly(aspartic acid) 296 have also been employed 297 to separate basic peptides on the inner wall of fused silica capillaries of 20 pm i.d. When the same eluent conditions were employed, superior performance was observed for these PLOT capillaries compared to the corresponding capillary zone electrophoresis (HP-CZE) separation. Peptide mixtures can be analyzed 298-300 with OT-CEC systems based on octyl-bonded fused silica capillaries that have been coated with (3-aminopropyl)trimethoxysilane (APS), as well as with pressurized CEC (pCEC) packed with particles of similar surface chemistry, to decrease the electrostatic interactions between the solute and the surface, coupled to a mass spectrometer (MS). In the pressurized flow version of electrochromatography, a pLC pump is also employed (Figure 26) to facilitate liquid flow, reduce bubble formation, and to fine-tune the selectivity of the separation of the peptide mixture. 

Precoated plates are also available for reversed-phase liquid-liquid partition thin-layer chromatography. Here the silica gel has been treated with an octadecyl silylating reagent thus coating the particles with a non-polar chemically-bonded thin film. The solvent employed is more polar than the film and chromatographic development results from partition between these two phases. 

Stationary phases for modern, reversed-phase liquid chromatography typically consist of an organic phase chemically bound to silica or other materials. Particles are usually 3, 5, or 10 p,m in diameter, but sizes may range up to 50 p,m for preparative columns. Small particles thinly coated with organic phase allow fast mass transfer and, hence, rapid transfer of compounds between the stationary and mobile phases. Column polarity depends on the polarity of the bound functional groups, which range from relatively nonpolar octadecyl silane to very polar nitrile groups. 

The most common R group is the octadecyl group (C-18), which leaves the silica particles coated with hydrocarbon chains. The silica particles are very small (—40 p dia. = 4 x 10 cm dia.) and very uniform in size. With very small, uniform particles, a molecule in the solvent can rapidly diffuse to the surface of the packing and undergo partitioning between the station-... 

Separation on reversed-phase columns with an aqueous mobile phase. This rapid and inexpensive system is used where only separation of mono- and disaccharides is required without further separation within either group. The nonpolar column packing is silica particles coated with octadecyl (C-18) groups. Separation occurs in order of decreasing polarity and, therefore, for bulk sweeteners, in order of molecular weight. 

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