The use of single capillaries (25 and 50 μm inner diameter (ID)) and coupled capillaries of different diameters (100-50 and 75-25 μm 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, ∼2 mW) light-emitting-diode (LED) was used as the light source. When a single capillary (50 μm ID) was used, the detection limit for NDA-derivatized dopamine was determined to be 2.0 × 10-7 M (Signal-to-nose ratio S/N = 3) based on the MEKC mode. This was improved to 4.0 × 10-9 M when the sweeping-MEKC mode was applied. In addition, this can be further improved to 1.0 × 10-9 M and 5.6 × 10-10 M when 100-50 and 75-25 μm 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 (N) for the detected peaks were determined to be 6.3 ± 2.7 × 105 by means of a single capillary (50 μm ID). This can be improved to 9.4 ± 3.6 × 105 and 9.4 ± 0.9 × 106 when the 100-50 and 75-25 μm ID coupled capillaries were applied.
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