title: Synthetic strategies towards the improvement of columns for use in high pressure liquid chromatography
Creators: name: Skinley, Kevin
Publisher: name: University of Liverpool Repository
Year: 2013

description: The history of chromatographic theory has always been driven to the development of smaller particles. The use of smaller more uniform particle diameters results in more effective separations and faster optimal mobile phase velocities, therefore producing lower plate heights; consequently the use of sub 2 micron silica particles has delivered a significant impact on the practice of high pressure liquid chromatography. The main issue with sub 2 micron silica particles however, is related to their manufacture. It is difficult to produce such highly specialised materials homogeneously and there are only a handful of methods in the literature. This dissertation outlines work that is intended to improve our technical knowledge of this subject by studying the synthesis (of both non porous and totally porous) spherical silica particles to drive improvements. Further investigations into surface functionalisation as well as column packing are also studied. Several experimental tools have been developed to drive improvements to the manufacture of such materials. For example, the experimental challenges necessitated the development and implementation of the following in this work; Full evaluation of the experimental and technical requirements of the Stöber reaction to produce good quality particles greater than one micron in diameter. 2. Transfer of the key outcomes from non porous particle synthesis to produce totally porous particles via modified Stöber reactions and pseudomorphism, 3. Indentified new types of surfactants that could produce high porosity silica particles, 4. The use of microwave irradiation to dramatically speed up surface functionalisation processes, 5. Investigation of column packing’s via computer simulation. On the basis of this works experimental data, it is concluded that good quality non porous and totally porous spherical silica particles can be produced from the ethanol/water/ammonia hydrolysis system of the Stöber reaction up to two microns in diameter with a narrow particle size distribution. Key experimental procedures that are often overlooked in the literature have been highlighted to facilitate successful reactions. The resultant materials can be used for future column packing’s or as starting materials for the production of superficially porous materials. The use of microwave irradiation was shown to dramatically improve surface functionalisation reaction times whilst incorporating reduced solvent volumes and reduced energy expenditure. Reaction times of five minutes were shown to produce packing’s possessing the level of surface coverage associated with commercially produced phases (3 μmoles/m2 ) which traditionally use disordered heating methods such as reflux with long reaction times. The use of microwave “superheating” or slightly longer reaction times of 20 minutes further increased the overall bonding density. Computer simulation of commercial packing’s showed the influence of large particles within the distribution on bed formation. Larger particles are pushed towards the top of the column chamber by the sedimentation of smaller particles. This effect is completely analogous to the phenomenon of “The Brazil nut effect”. More uniform packing’s would reduce the overall impact. This work improves our understanding of the complex nature of HPLC column phase synthesis as well as provides new tools for future research.