Abstract: A new and simple method was developed for the accurate determination of rhodamine B in ketchup, tomato sauce and chili powder samples by micellar electrokinetic capillary chromatography (MEKC) with laser-induced fluorescence (LIF) detection. The separation was performed on an uncoated fused silica capillary 75 μm × 30 cm total length (effective length: 20 cm) with a separation buffer solution consisting of 30 mmol/L sodium tetraborate, 20 mmol/L sodium dodecyl sulfonate (SDS), 20 mmol/L sodium deoxycholate (SD) and 0.2 g/L polyethyleneglycol 35000. A solution containing 10 mmol/L SDS was used for sample extraction. The analysis could be completed within 10 min. A voltage of 8 kV was used for separation. The injection time and pressure were 5 s and 3.448 kPa, respectively. The excitation and emission wavelengths were 488 nm and 520 nm, respectively. The corrected peak area and the concentration of rhodamine B displayed a good linear relationship with a correlation coefficient (r) of 0.999 9 in the ranges of 0.02–1.28 mg/L. The limit of detection (LOD; RSN = 3) and limit of quantitation (LOQ; RSN = 10) were 5 and 15 μg/L, respectively. The intra-day and inter-day precisions of the method were both no less than 2%. The average recoveries of rhodamine B at three spiked concentrations (0.04, 0.16 and 0.64 mg/L) were in the ranges of 100.9%–105.8%. When the MEKC method was used to determine the contents of rhodamine B in 3 ketchup, 3 tomato sauce and 22 chili powder samples, the results were all lower than those measured by ultra-high performance liquid chromatography-fluorescence detection (UPLC-FLR). The possible reasons for the difference were analyzed and discussed.

Key words: micellar electrokinetic chromatography, laser-induced fluorescence, rhodamine B, sauce, ketchup, chili powder