Architecture and Dynamics of C₁₈ Bonded Interphases with Small Molecule Spacers

Abstract

The relationship between alkyl phase structure and chromatographic performance is investigated for a series of octadecyl (C(18))-modified silica surfaces with defined spacing of the alkyl surface by a "pre-end-capping" technique. Stationary phases were prepared by a two step process with (1) reaction with less than stochiometric amounts of a small monofunctional silane, followed by (2) solution or surface polymerization with octadecyltrichlorosilane. The results of solid-state and suspension nuclear magnetic resonance (NMR) spectroscopy are correlated with the chromatographic behavior regarding shape selective separations. Two sets of six different stationary phases were prepared by solution and surface polymerization approaches, yielding materials with surface coverages from 2.7 to 5.6 micromol/m(2). (13)C cross-polarization magic angle spinning (CP/MAS) NMR spectra show a predominance of trans conformations for the set of surface polymerized phases with a C(18) coverage greater than 4.5 micromol/m(2). For the solution polymerized phases, no predominance for the trans conformation was observed, even for surface coverages greater than 5.1 micromol/m(2). Proton spectra in suspension indicate the trend that a higher coverage for the surface polymerized materials correlates with a more rigid alkyl chain conformation. The set of solution polymerized stationary phases confirms this tendency but minor deviations are observed for high coverages. These structural abnormalities are confirmed by differences in the (29)Si CP/MAS spectra. Furthermore, the (29)Si CP/MAS spectra indicate a lower amount of cross-linking for the materials with the highest amount of placeholder (spacer). The use of the different spectroscopic and chromatographic methods provides a wealth of information on the surface morphology of the systematically prepared C(18) materials and extends the understanding of surface morphology of alkyl modified silica and its influences of the molecular recognition process in liquid chromatography.

Design of C18 Organic Phases with Multiple Embedded Polar Groups for Ultraversatile Applications with Ultrahigh Selectivity

“…52 29 Si CP/MAS NMR was carried out for the Sil-MEPG-C 18 -1 and Sil-MEPG-C 18 -2 phases (Supporting Information, Figure S7) attributed to a very high degree of cross-linking, even higher than the best C 22 or C 18 bonded phases. 20,21 The results suggested a smaller number of free OH groups on the surface, which leads to fewer silanophilic interactions in HPLC. 20,53 The novel phases were very suitable for the separation of shape-constrained isomers 20−24 and neutral, polar, and basic compounds 29−31 in RPLC and polar compounds 34−38 in HILIC (i.e., three in one) compared to the most commonly used commercial RP, EPG, and HILIC columns.…”

13C SPE MAS measurement of ligand concentration in compressible chromatographic beads

“…Our study also connects to characterization of silica‐based chromatography media with solid‐state NMR, a field that has been active for many years. Besides investigating the molecular dynamics of components immobilized on silica or on polymeric resins and the interactions between bound ligands and analytes in the mobile phase, a few MAS NMR studies concerned the ligand concentration in stationary phases such as chromatography media or resins for solid‐phase organic synthesis . Here, we present for the first time quantitative MAS NMR to measure ligand concentration in a chromatography medium based on a soft polysaccharide hydrogel resin.…”

High resolution magic angle spinning NMR as a tool for unveiling the molecular enantiorecognition of omeprazole by amylose-based chiral phase

“…The use of 1 H HR/MAS NMR reduces the line broadening effects resulting in better resolved spectra. [9][10][11][12][13][14] It is then possible to use these tools for mechanism elucidation. This approach was used in order to investigate the chiral discrimination of omeprazole enantiomers by the tris-3,5-dimethylphenylcarbamate of amylose CSP.…”