Optimization of Sensitivity

互联网2014-02-13

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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 �m. 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 �L and 1-cm detector cells. The difference may be up to an order of magnitude (2 ) 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