Hollow-Fiber Liquid-Phase Microextraction Combined with High-Performance Liquid Chromatography for Analysis of Melamine in Milk Products

A novel liquid-phase microextraction (LPME) technique, based on a hollow fiber (HF), in conjunction with high-performance liquid chromatography, has been developed for analysis of melamine in milk products. Melamine was extracted directly from milk products by use of a hollow-fiber membrane filled with organic solvent. HF-LPME conditions, for example pH, extraction solvent, temperature, stirring rate, and extraction time were optimized. The best extraction efficiency of melamine was achieved under the conditions:pH 9.5, 35 mu L n-octanol as extraction solvent, temperature 55 degrees C, stirring rate 300 rpm, and extraction time 30 min. The HF-LPME technique resulted in a preconcentration ratio of 29-fold. Baseline chromatographic separation of melamine was achieved on a C-18 column with 96:4 (v/v) 0.02 mol L-1 ammonium sulfate-methanol as isocratic mobile phase. The linearity of the method ranged from 1.0 to 100.0 mu g mL(-1), correlation coefficient 0.9994. The limit of detection by use of HF-LPME was 0.021 mu g mL(-1) at a signal-to-noise ratio of 3. The optimized HF-LPME technique was successfully applied to the analysis of melamine in milk products collected from different commodity manufacturing units.