Influence of synthetic routes on the conformational order and mobility of C18 and C30 stationary phases.

Silica gels modified with n-alkyl chains (n = 18, 30) are prepared by two different synthetic routes and are examined by variable temperature FTIR and solid-state NMR spectroscopy. HPLC measurements of SRM 869, cis/trans ss-carotene isomers and xanthophylls isomers confirm the dependence of the separation mechanism on the alkyl chain length and the synthetic routes. The determination of the silane functionality and degree of cross-linking of silane ligands on the silica surface is achieved by 29Si CP/MAS NMR measurements. The structural order and mobility of the alkyl chains are investigated by means of variable temperature 13C CP/MAS NMR measurements. Variable temperature FTIR studies are performed where conformational order and flexibility of the alkyl chains in C18 and C30 phases are monitored through conformational sensitive CH2 symmetric, anti-symmetric stretching and wagging modes. In addition, the chromatographic properties of the C18 and C30 phases are determined. The results derived from the FTIR, NMR and HPLC measurements are discussed in the context of the applied synthetic routes and alkyl chain lengths.

Contact-angle, ellipsometric, and spin-diffusion solid-state nuclear magnetic resonance spectroscopic investigations of copolymeric stationary phases immobilized on SiO2 surfaces.

SiO2 surfaces-silica gel particles and silica wafers-were modified by covalently immobilizing three poly(ethylene-co-acrylic acid) copolymers, (-CH2CH2-)x[CH2CH/(CO2H)-]y, with different chain lengths and mass fractions of acrylic acid. 13C solid-state NMR spectroscopy on the modified silica gel particles revealed both mobile gauche and rigid trans aligned alkyl chains in the copolymers. For copolymers attached to silica wafers via a 3-aminopropyltriethoxysilane spacer molecule, ellipsometric measurements revealed a mean value of the layer thickness distribution of 6.5 and 4.3 nm, respectively, for the more acidic and the shorter copolymers with mobile alkyl chains mostly in the gauche conformation. For the longest and least acidic copolymer with more rigid trans ordered alkyl chains, however, a mean phase thickness of 10.6 nm was found. When this copolymer was immobilized via a 3-glycidoxypropyltrimethoxysilane spacer molecule we measured a mean layer thickness of 9.9 nm. A model of the surface morphology of this immobilization strategy was derived using spin-diffusion 13C NMR measurements on the corresponding modified silica. It was thereby proven that the trans and gauche-aligned alkyl chains occur in distinct domains of certain sizes on the silica surface. The surface polarity of all modified silica wafers was also investigated by measurement of contact-angle.

Nuclear magnetic resonance and high-performance liquid chromatographic evaluation of polymer-based stationary phases immobilized on silica.

Three poly(ethylene-co-acrylic) acid copolymers (-CH(2)CH(2)-)(x)[CH(2)CH(CO(2)H)-](y) with different chain lengths and mass fractions of acrylic acid were covalently immobilized as stationary phases on silica via two variants of spacer molecules (3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane). Different mobilities of the alkyl chains in the stationary phases were observed using (13)C solid-state NMR spectroscopy. The stationary phases with more rigid trans-ordered alkyl chains had better selectivity for geometric beta-carotene and xanthophyll isomers (provitamin A derivatives). Also, all the separations of the analytes were affected by polar interactions with the chromatographic sorbent. This was further proved by separating more polar cis/trans retinoic acid isomers (vitamin A derivatives). (13)C high-resolution/magic-angle spinning (HR/MAS) NMR measurements of the chromatographic sorbents suspended in the mobile phase confirmed a dependence of molecular shape recognition ability on alkyl chain conformation.