Optimization of sensitivity.

Abstract

The injection volumes involved in CE are very small (in the order of 1–10 nL). In addition, the area of capillary employed for on-column detection may be only 50 × 200 µm. Both these factors influence the detector sensitivity to a large extent. CE is less sensitive when directly compared to HPLC with typical injections of 10–50 µL and 1-cm detector cells. The difference may be up to an order of magnitude () when comparing at the same UV wavelength. Several strategies may be employed (Table 1) to maximize the sensitivity in CE. These include use of low UV wavelengths, increased capillary bore, and optimized sampling procedure. Table 1 Summary of Approaches Available for Increasing Sensitivity Action to improved sensivity Drawback(s) to consider Employ low-UV wavelength Increased background noise—determine wavelength to give optimum signal-to-noise ratio Increase capillary bore Increased current, reduced EOF giving possible alteration in selectivity Increased injection time Reduction in separation efficiency; excessive time will result in run failure Appropriate use of electrokinetic injection Sampling bias for more mobile ions, sample matrix effects on injection amount Increased electrolyte strength Increased current and associated noise Optimize electrolyte composition Effects on selectivity and current Decrease operating voltage Increase in analysis time Decrease temperature Increase in analysis time Capillary modifications Reduction in separation efficiency and resolution, increased cost Sample derivatization Additional sampling handling Indirect detection Extra method development considerations Wide-bore capillaries EOF profile disturbed, adjustments to rinse and injection times, siphoning effects more pronounced Increased detector slit width Reduction in separation efficiency and resolution