UV-vis detection is a semi-universal mode of detection. The spectral information collected when using DAD detection, coupled with the relatively inexpensive nature of these detectors, make them some of the most used detectors in CE.
For UV/vis detection, on-capillary detection is typically performed to ensure maximum efficiency (i.e. minimise detection band broadening); this is done by removal of a small part of the outside polyimide coating to create a detection window. Small axial detection path lengths relative to the width of solute zones entering the detection window are required for high resolution.
The absorbance (A) of an analyte passing through the detection window in on-capillary CE-UV/vis is directly proportional to optical path length (b), the analyte concentration (c) and molar absorptivity (ε) as defined by the Beer-Lambert law [187,226]:
𝐴 = 𝜀𝑏𝑐 (2.29.) Low sensitivity is a well-known limitation of CE and is in fact a consequence of the small path length used in on-capillary detection. To partially overcome the sensitivity limitations of CE, extended path length flow cells such as bubble cells or Z-cells may be used. Furthermore, on-capillary sample concentration techniques, referred to as sample stacking methods, are also used. A few examples of sample stacking methods include field amplified sample stacking (FASS), field amplified sample injection (FASI), isotachophoretic (ITP) sample stacking, high-salt stacking, reversed electrode polarity stacking mode (REPSM), micelle to solvent stacking (MSS), sweeping and reverse migrating micelles. The use of extended path length flow cells and sample stacking methods to enhance sensitivity is beyond the scope of this study and the reader is referred to [187,227-235] for detailed discussions on this subject.
63 2.9. Conclusions
Phenolic compounds are important constituents of herbal teas to which potential health benefits in humans are attributed. Accurate analysis of these compounds is therefore required, but this is often rather challenging. HPLC remains the method of choice for phenolic determination, but requires extensive method optimisation and often re-development for different samples. This is illustrated by the number of HPLC methods reported for rooibos and honeybush phenolic in literature. CE has successfully been used for the analysis of green, oolong and black tea phenolics. The most popular mode for these applications is MEKC. The performance of CE methods for tea analysis varies, but exhaustive method optimisitation is also often required. No CE methods on the analysis of rooibos and honeybush teas have been reported to date. These herbal teas contain unique and diverse phenolic compounds, and alternative separation methods such as CE might prove valuable for their analysis.
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