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S A Wren

Publications and source records attributed to S A Wren.

7 recordsLinked to original sources
PubMed2001

Characterisation of electroosmotic flow in capillary electrochromatography columns.

Currently available capillary electrochromatography (CEC) instrumentation using UV-Vis detection dictates the use of duplex columns. Due to discontinuities (electric field strength and conductivity) that arise at the boundary between the packed and open sections in these columns, the determination of the electroosmotic flow (EOF) is complicated. Thiourea has been found to be an accurate EOF marker under the conditions employed in this study. By injecting this compound onto a fully packed column and comparing the obtained mobilities with those calculated from measured zeta potential values a value for tortuosity has been obtained. The use of laser Doppler velocimetry (LDV) for the measurement of zeta potential has been found to be the most direct and rapid method of characterising silica support materials in terms of electroosmotic mobility. The open section in duplex CEC columns has been shown to influence the actual column flow-rate. The EOF measured using duplex columns of varying packed and open section lengths have been compared with those obtained for a fully packed column.

Chromatography, Micellar Electrokinetic Capillary↗
PubMed2001

Influence of the unpacked section on the chromatographic performance of duplex strong anion-exchange columns in capillary electrochromatography.

This work describes initial investigations of strong anion-exchange (SAX) packing materials for capillary electrochromatography (CEC). The use of SAX phases in CEC is theoretically appealing for the analysis of negatively charged species. The reversed direction of the electroosmotic flow (EOF) generated by SAX phases (in comparison to reversed phases and strong cation-exchange phases) means that negative species can migrate with the EOF, not against it, hence the analysis times, of such species should be decreased and efficiencies improved. Duplex CEC columns (the standard for instruments using UV detection) consist of a packed and an unpacked section. Using common reversed-phase packing materials the direction of the EOF in both sections is co-linear, however when normal fused-silica capillaries are packed with SAX material the direction of the EOF in the two sections oppose one another. It has been shown, using conventional duplex CEC columns and fully packed CEC-MS columns that the opposing direction of EOF causes a massive degradation in column performance. Consequentially, it is demonstrated that if the EOF in the open section of the duplex SAX column can be controlled via pH or capillary derivatisation then good, reproducible CEC can be performed on anionic species using SAX packed CEC columns.

Anion Exchange Resins↗
PubMed1996

Self-consistent framework for standardising mobilities in free solution capillary electrophoresis: applications to oligoglycines and oligoalanines.

A theoretical analysis of deviations from ideality in ionic transport is presented to correct mobilities, mu, measured in free solution capillary electrophoresis (CE) to mobility at infinite dilution, mu degree (limiting mobility). Non-ideality is treated at the same level of approximation as in equilibrium, using a correction factor for the sum of the analyte and counter-ion radius originally suggested by Robinson and Stokes (Electrolyte Solutions, 1961). Unlike previous corrections using Debye-Hückel-Onsager theory, which are strictly applicable only at very low ionic strengths, this treatment is expected to be valid for univalent ions migrating in a uni-univalent background electrolyte for ionic strengths up to 0.075 mol kg-1, a range typical of CE experiments. The analysis is applied to the determination of mu degree in acidic and basic buffers for oligoalanines and oligoglycines with degree of polymerisation 2 to 6. Limiting mobilities for the fully protonated and deprotonated peptides are found to be numerically equal but opposite in sign, consistent with a change in charge from +1 to -1. In all uni-univalent buffers studied (borate, citrate, low pH lithium phosphate and sodium phosphate) mu degree values established using data over a range of pH and ionic strength are found to be identical and in excellent agreement with previous values from isotachophoresis. Values of mu degree in high pH sodium phosphate buffer are systematically 0.2.10(-8) m2 V-1 s-1 higher than those in other buffers; this may be attributed to limitations of the model for a buffer with 1+:2- and 1+:3- ions. This self-consistent framework for standardising mobilities in free solution CE is expected to be widely applicable to univalent analytes migrating in a 1:1 background electrolyte.

Chemical Phenomena↗
PubMed1995

Chiral separation in capillary electrophoresis.

A mathematical model which explains separation in chiral capillary electrophoresis is presented. The model is analysed to give a more general solution than previous work and is supported by further experimental work.

Atenolol↗
PubMed1994

Molecular size/shape effects in the separation of the monosubstituted alkyl pyridines using capillary electrophoresis.

A series of monosubstituted alkyl pyridines have been used to investigate the presence of molecular size/shape effects in capillary electrophoresis and the applicability of various descriptors in discriminating not only between derivatives of differing chain length but also between positional isomers. Although van de Waals' radii and Offord's parameters are extremely good descriptors in predicting the relative mobilities of the derivatives, a further descriptor involving length and/or diameter of the molecule is necessary for discrimination between the positional isomers.

Alkylation↗
PubMed1993

Theoretical aspects of chiral separation in capillary electrophoresis. III. Application to beta-blockers.

Enantiomers of the beta-blockers propranolol, atenolol, metoprolol, and oxprenolol have been separated by the addition of methyl-beta-cyclodextrin (MeBCD) to the operating buffer. There is an optimum concentration of MeBCD for separation for each of the beta-blockers, the magnitude of which could be ranked by the use of a mathematical model and log P data--an indication of the hydrophobicity of the molecule.

Adrenergic beta-Antagonists↗