Ultraviolet laser-induced fluorescence detection strategies in capillary electrophoresis: determination of naphthalene sulphonates in river water

Various UV-laser-induced fluorescence detection strategies for capillary electrophoresis (CE) are compared, i.e. two UV-laser systems (a pulsed laser providing up to 25 mW of tunable emission, applied at 280, 290 and 325 nm, and a continuous wave (cw) laser providing up to 100 mW of 257 nm emission) and different methods to collect the fluorescence emission signal and to reduce the background. Attention is focused on the determination of amino- and hydroxy-substituted naphthalene sulphonates (NS) in river water; these analytes exhibit native fluorescence upon UV excitation. Optimum results were obtained by applying only a minor portion of the available (average) laser powers, viz. 0.7 mW at 280 nm for the pulsed laser, and 5 mW for the cw laser. For emission collection, the most favourable results were obtained with a mirror-based microscope objective, which facilitates efficient spatial filtering and does not produce impurity fluorescence upon UV-laser irradiation. For standard solutions, the cw laser gave around 20-fold better detection limits (10−9–10−10 M) than the pulsed laser. For river water, excitation of interferences (presumably humic acids, which exhibit native fluorescence) could be much better suppressed if the pulsed laser was used with selective excitation at 280 nm. Therefore, for real-sample analysis the latter combination is to be preferred. The set-up was used for the identification and quantification (at the 1–35 μg l−1 level) of NS in a river Elbe sample.