Determination of melamine residue in liquid milk by capillary electrophoresis with solid-phase extraction

A novel solid-phase extraction-capillary electrophoresis (SPE-CE) method was developed for the determination of melamine residue in liquid milk. The conditions of SPE and CE were investigated and optimized. A 1% trichloroacetic acid plus 2.2% lead acetate solution were used for the extraction of analyte and the removal of protein. A Cleanert PCX SPE cartridges column was used for clean up. The 50 mM sodium dihydrogenphosphate running buffer (pH adjusted to 3.2 with citric acid) was used as a running buffer. The linearity is satisfactory in the range of 0.8-100 μg/mL with a correlation coefficient of 0.9998. Under the optimal conditions, the method limit of detection (LOD) and method limit of quantification were 0.12 mg/kg and 0.37 mg/kg, respectively. The recovery of melamine from different liquid milk samples was in the range of 89.5-98.5% with a relative standard deviation of 1.8-3.5%. The intra- and inter-day assay precision was 2.8% (n = 6) and 4.1% for five days, respectively. The developed method has been applied successfully for the determination of melamine residue in liquid milk samples. The results obtained by the proposed method agree with those obtained by high-performance liquid chromatographic method. The proposed method enables the quantitative determination of melamine residues at levels as low as 0.37 mg/kg in different liquid milk.