Analysis of melamine in milk powder and liquid milk by capillary zone electrophoresis after electrokinetic supercharging

A method was developed to determine melamine in milk powder and liquid milk by capillary zone electrophoresis after electrokinetic supercharging. The effects of buffer acidity and concentration, the type and length of leader and terminator electrolytes, injection time and matrix effects were investigated. The optimum background electrolyte was 50 mM sodium citrate-citric acid (pH 4.5), hydrodynamic injection of 50 mM sodium chloride (24 s, 0.5 psi) as leader, electrokinetic injection of the sample (100 s, 10 kV), hydrodynamic injection of 50 mM tetrabutylammonium bromide (18 s, 0.5 psi) as terminator and then a voltage of 25 kV for separation (+25 kV). Under these conditions, the sensitivity was enhanced 2285 times when compared to a normal hydrodynamic injection with the sample volume <3 % of the capillary volume. The detection limit for melamine with this electrokinetic supercharging (EKS) system is 0.7 ng/mL. The developed method was verified by analysing milk powder and liquid milk samples, and the recovery was 29.8 and 42.1 %, respectively.