COMPARISON OF ULTRAVIOLET-LASER INDUCED AND CONVENTIONAL FLUORESCENCEDETECTION IN CONVENTIONAL-SIZE LIQUID-CHROMATOGRAPHY OF NATIVELY FLUORESCENT ANALYTES

The characteristics of laser-induced fluorescence (LIF) detection and state-of-the-art conventional fluorescence (CF) detection in the UV re gion below 300 nm in conventional-size column liquid chromatography ar e compared. An argon-ion laser system is used to provide four laser li nes, i.e. 257 and 293 nm obtained by frequency-doubling of visible lig ht and the UV lines at 334 and 352 nm. Light intensities inside the de tector cell were measured by means of actinometry. Flicker noise, espe cially important for LIF detection was suppressed by simultaneously mo nitoring the laser light and ratioing sample and reference signals. On -line removal of fluorescent impurities in the eluent by means of a pr ecolumn filled with carbon material resulted in a three-fold reduction of the background signal for LIF detection with 257-nm excitation. It is shown that LIF detection at 257 nm is more suitable than LIF at 29 3, 334 and 352 nm, not only for some pharmaceutical compounds that can only be excited below 300 nm, but also for polyaromatic hydrocarbons that absorb at wavelengths longer than 300 nm. Compared with CF detect ion at optimum excitation and emission settings, the detection limits obtained with the LIF system were improved typically by 5 to 10-fold.