Stacking and analysis of melamine in milk products with acetonitrile-salt stacking technique in capillary electrophoresis.

Melamine was measured in real milk products with capillary electrophoresis (CE) based on acetonitrile-salt stacking (ASS) method. Real milk samples were deproteinized with acetonitrile at a final concentration of 60% (v/v) and then injected hydrodynamically at 50 mBar for 40.0 s. The optimized buffer contains 80.0 mmol/L pH 2.8 phosphates. Melamine could be detected within 20.0 min at +10 kV with a low limit of detection (LOD) of 0.03 µmol/L. Satisfactory reproducibility (inter- and intraday RSD% both for migration time and peak area was lower than 5.0%) and a wide linearity range of 0.05 µmol/L ~ 10.0 µmol/L were achieved. The proposed method was suitable for routine assay of MEL in real milk samples that was subjected to a simple treatment step.

Assay of melamine in milk products with a pH-mediated stacking technique in capillary electrophoresis.

A pH-mediated stacking method in capillary electrophoresis as an assay for low concentrations of melamine in milk products was established. Real samples were treated with acetone and sodium acetate and injected directly after centrifugation and filtration. Several experimental factors, such as buffer pH, buffer concentration, sample matrix, injection/sweeping ratio, sweeping time/voltages, separation voltages, as well as sample pretreatment, which affected stacking and separation, were investigated and optimized. Under the selected condition, a low LOD of 0.01 µmol/L (S/N = 5) and a wide range of linearity of 0.01∼1.0 µmol/L could be easily achieved with a good reproducibility (RSDs < 5.8% for both migration time and peak area) and an acceptable recovery of 94.0∼103.2% (for milk, infant formula, yogurt, and milk products). The proposed method was suitable for routine assay of melamine in real milk samples.