Tuning preparation conditions towards optimized separation performance of thermally polymerized organo-silica monolithic columns in capillary liquid chromatography

Abstract

Tuning of preparation conditions, such as variations in the amount of a porogen, concentration of an aqueous acid catalyst, and adjustment in polymerization temperature and time, towards optimized chromatographic performance of thermally polymerized monolithic capillaries prepared from 3-(methacryloyloxy)propyltrimethoxysilane has been carried out. Performance of capillary columns in reversed-phase liquid chromatography was assessed utilizing various sets of solutes. Results describing hydrophobicity, steric selectivity, and extent of hydrogen bonding enabled comparison of performance of hybrid monolithic columns prepared under thermal (TSG) and photopolymerized (PSG) conditions. Reduced amounts of porogen in the polymerization mixture, and prolonged reaction times were necessary for the preparation of monolithic columns with enhanced retention and column efficiency that reached to 111,000 plates/m for alkylbenzenes with shorter alkyl chains. Both increased concentration of catalyst and higher temperature resulted in faster polymerization but inevitably in insufficient time for pore formation. Thermally polymerized monoliths produced surfaces, which were slightly more hydrophobic (a methylene selectivity of 1.28±0.002 TSG vs 1.20±0.002 PSG), with reduced number of residual silanols (a caffeine/phenol selectivity of 0.13±0.001 TSG vs 0.17±0.003 PSG). However, steric selectivity of 1.70±0.01 was the same for both types of columns. The batch-to-batch repeatability was better using thermal initiation compared to monolithic columns prepared under photopolymerized conditions. RSD for retention factor of benzene was 3.7% for TSG capillaries (n=42) vs. 6.6% for PSG capillaries (n=18). A similar trend was observed for columns prepared within the same batch.

Advances in the development and applications of organic–silica hybrid monoliths

“…They are faster, can be stopped at selected times and monoliths can be formed selectively in certain segments of the capillary, specifically those which have the polymerization mixture directly exposed to light. However, we have observed better batch‐to‐batch repeatability using thermally polymerized conditions (RSD of retention factor of benzene of 3.7%, n = 42, TSG versus 6.6%, n = 18, PSG) .…”

“…The effect of reaction conditions on the monolith morphology, mechanical strength, and separation performance in CEC , capillary LC , and electrochromatographic separations on a chip was evaluated. In the original work by Dulay et al.…”

Use of monolithic supports for high‐throughput protein and peptide separation in proteomics

“…In order to improve shortcomings of both organic and silica based monoliths different strategies were employed. Solutions of sometime similar challenges led to approaches that merged best of silica and organic monoliths resulting in a new hybrid monolith with improved properties . A method that combines silica based cage‐like polyhedral oligomeric silsesquioxane incorporated within organic matrix was explored .…”

Current trends in the development of porous polymer monoliths for the separation of small molecules

“…On the other hand, silica‐based monoliths provide higher solvent resistance, but they lack pH stability and facile surface modification. Organic‐silica hybrid monolithic columns combine advantages of both and are recently receiving significant attention in the analysis of small molecules .…”

“…In a follow‐up study, the same group prepared organo‐silica monolithic columns by thermally initiated polymerization reaction of the same monomer . The concentration of porogen, polymerization time, temperature, and concentration of aqueous catalyst were optimized and excellent column efficiency exceeding 110 000 plates/m was achieved.…”