Capillary electrophoresis with laser-induced fluorescence detection for natively fluorescent analytes

Laser-induced fluorescence (LIF) detection systems for capillary electrophoresis (CE) are nowadays widely available. Many classes of analytes show native fluorescence, but often only upon absorption of short-wavelength UV light. Unfortunately, most CE-LIF systems make use of visible laser lines so that they can only be applied to analytes that have been chemically derivatised with a suitable fluorescent label matching the excitation wavelength. This paper illustrates how the applicability of Ce-LIF can be broadened to include natively fluorescent analytes by employing either UV laser systems or multiphoton-excitation. Several pulsed and continuous wave (CW) lasers were compared in terms of analytical sensitivity and selectivity; in particular the CW 275-nm line from an adapted Ar+ laser proved very suitable. For identification purposes emission spectra were recorded online. The usefulness of such systems in environmental chemistry is shown for amino- and hydroxy-substituted naphthalensulphonates in river water samples. Extra spectral selectivity can be obtained by performing CE-LIF under fluorescence line-narrowing conditions using a cryogenic interface. The introduction of high-repetition rate, femtosecond Ti:sapphire lasers paved the way for two- and three-photon excited LIF detection of a wide array of biologically relevant fluorophores.