Determination of cypromazine and its metabolite melamine in milk by cation-selective exhaustive injection and sweeping-capillary micellar electrokinetic chromatography

In this study, we described a high-sensitive on-line preconcentration method for cypromazine (GYP) and melamine (MEL) analysis using cation-selective exhaustive injection (CSEI) combined with sweeping-MEKC. The optimum conditions of on-line concentration and separation were discussed. The BGE contained 100 mM SDS, 50 mM phosphoric acid (pH = 2.0) and 15% acetonitrile (v/v). The sample was injected at 10 kV for 600 s, separated at -20 kV, and detected at 210 nm. The sensitivity enhancements were 6222 for GYP and 9179 for MEL. The linear dynamic ranges were 0.4-25 ng/mL for GYP (r = 0.9995) and 0.2-12 ng/mL for MEL (r = 0.9991). The LODs (signal-to-noise ratio, 3) were 43.7 and 23.4 pg/mL for GYP and MEL, respectively. The proposed method was applied to analyze GYP and MEL in dairy products pretreated using off-line SPE to minimize the influence of the matrix. The recoveries of GYP and MEL were satisfactory (ca. 74-83%). The experimental results suggest that the CSEI-sweeping-MEKC method is feasible for the application to simultaneously detect trace levels of GYP and its metabolite MEL in real milk samples.