The application of an ultraviolet (UV) light-emitting diode (LED) to on-line sample concentration/fluorescence detection in capillary electrophoresis (CE) is described. The utility of a UV-LED (peak emission wavelength at 380 nm, ∼2 mW) for fluorescence detection was demonstrated by examining both a naturally fluorescent (riboflavin) compound and a nonfluorescent compound (tryptophan), respectively. The detection limit for riboflavin was determined to be 0.2 ppm by the normal MEKC mode, which was improved to 3 -7 ppb when dynamic pH-junction technique was applied. On the other hand, the detection limit of the tryptophan derivative was determined to be 1.5 ppm using the MEKC mode, which was improved to 3 ppb when the sweeping-MEKC mode was applied. In an analysis of an actual sample, the concentrations of riboflavin in beer, and tryptophan in urine and milk samples were determined, respectively.

Experimental ApparatusThe violet-LED (light-emitting diode) light source (∼2 mW), CE set-up and data-acquisition system used were similar to that described previously, 10 and are abbreviated herein. A large-volume sample injection (> 5 µL) with an extremely high theoretical plate number (N > 10 7 ) was achieved when the sweeping-MEKC mode and a coupled-capillary (100 -50 µm i.d.) were simultaneously used in a capillary electrophoresis (CE) separation. A low-cost and compact violet-LED (∼2 mW) was used as the fluorescence excitation source. As a result, the theoretical plate numbers of the detected peaks (two model compounds: naphthalene-2,3-dicarboxaldehyde derivatized-dopamine and -norepinephrine) were 1.0 × 10 7 and 7.4 × 10 6 , respectively. The limits of detection (at S/N = 3) of these were determined to be 2.8 × 10 -10 M (92 ppt) and 2.3 × 10 -10 M (83 ppt), respectively.