Determination of Metal Ions and Melamine in Milk by Capillary Electrophoresis-Capacitively Coupled Contactless Conductivity Detection
A method combining capillary electrophoresis (CE) and capacitively coupled contactless conductivity detection (C4D) was developed for the determination of melamine and metal ions,namely,potassium, calcium and sodium in milk.The milk samples (original samples were employed for metal ion determination and the fortified ones were used for melamine assaying) were first extracted with trichloroacetic acid and aceto-nitrile,and then the extracts were centrifuged.For metal ions,the supernatants were diluted with deionized water and filtrated prior to CE analysis.The supernatants from the melamine-fortified samples were further treated with solid phase extraction before CE assay.CE separation was carried out in a 50 μm × 50 cm capillary; the excitation frequency was set to 180 kHz and the excitation voltage was 8 V.With 20 mmol/L lactic acid as background electrolyte,the detection limits of K+,Ca2+ ,Na+ were in the range of 0.009-0.040 μg/g and those of melanine were in the range fo 0.10-0.20 μg/g for the standard solution and milk sample, respectively.The experiments suggest that the method is cost-effective and suitable for real sample analysis.