The Design, Construction and Evaluation of High Performance UV Detectors for Capillary Separation Techniques

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Thesis for the Degree of Doctor of Philosphy
of the University of London
- in preparation

Abstract

A novel UV absorbance detection system has been developed for use with capillary flow cells. The optical interface comprises fast spherical mirrors providing diffraction limited focussing and near normal incidence of light on the cell. This results in high light throughput (wide dynamic range) combined with low level of stray light (wide linear range). Its modular character allows the new system to be configured into single beam or double beam detector, with fixed or variable wavelength. The specific values of noise level and linear dynamic range depend on the configuration and are the best figures of merit reported to date in UV absorbance detection. The `dynamic reserve' thus obtained allows the use of fast data capture rates in order to preserve the high resolution inherent in capillary separations. The passage of separated bands through the detection window may result in signal modulation with components in the kilohertz region.

A fast data acquisition system has been developed for measurement of true peak profiles. The acquisition and data processing software has been written in LabVIEW graphical environment. The data points are acquired at a flat rate of 3-20 kilohertz with subsequent data bunching, averaging and filtering, in order to provide the desired Nyquist sampling rate with corresponding bandwidth of the signal. The use of smoothing routines and digital filters is discussed. The influence of `time constant' and data acquisition rate on peak shape was measured. The fast temporal response of the detector allowing accurate rendering of high velocity band shapes was demonstrated in comparison with a widely used commercial system. The inherent band broadening was measured in cylindrical, rectangular, bubble and z-shaped cells.

The data acquisition system is based on the open architecture of a modular `Virtual Instrument'. The software offers several advantages over commercially available products, such as high sampling rate, user-selectable digital filters, ease of networking and `on the fly' bandwidth adjustment. In addition it allows for computing platform portability (PC, Mac, Sun, HP). The prototype of the new detection system operates to specification in routine capillary chromatographic applications.

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