The use of single capillaries (25 and 50 pm inner diameter (ID)) and coupled capillaries of different diameters (100-50 and 75-25 mum ID) based on micellar electrokinetic chromatography (MEKC) and sweeping-MEKC modes is compared and reported. Naphthalene-2,3-dicarboxaldehyde (NDA)-derivatized dopamine was selected as the model compound by examining the fluorescence intensity when a violet (410 +/- 7 nm, similar to2 mW) light-emitting-diode (LED) was used as the light source. When a single capillary (50 mum ID) was used, the detection limit for NDA-derivatized dopamine was determined to be 2.0 x 10(-7) M (Signal-to-nose ratio S/N = 3) based on the MEKC mode. This was improved to 4.0 x 10(-9) m when the sweeping-MEKC mode was applied. In addition, this can be further improved to 1.0 x 10(-9) m and 5.6 x 10(-10) M when 100-50 and 75-25 mum ID coupled capillaries are used. The use of the coupled capillary is also helpful for improving the separation efficiency. Based on the sweeping MEKC mode, the number of theoretical plates (M for the detected peaks were determined to be 6.3 +/- 2.7 x 10(5) by means of a single capillary (50 mum ID). This can be improved to 9.4 +/- 3.6 x 10(5) and 9.4 +/- 0.9 x 10(6) when the 100-50 and 75-25 mum ID coupled capillaries were applied.