Abstract:

Three chromatographic methods, i.e., reversed-phase ion pair chromatography, hydrophilic interaction chromatography and cation exchange chromatography, were established to determine melamine in powered milk. Before hydrophilic interaction chromatographic analysis, samples were extracted with formic acid, precipitated with acetonitrile for the removal of proteins and centrifugated. While for other two methods, the sample pretreatment was based on extraction with aqueous acetic acid solution, precipitation, centrifugation and cleanup on a RP column. The chromatographic separation was performed on a C18 column using a mobile phase composed of a buffer solution containing 10 mmol/L sodium heptanesulfonate and 10 mmolL citric acid and acetonitrile (85:15, V/V) in reversed-phase ion pair chromatographic analysis. A HILIC column and 20 mmol/L ammonium formate/acetonitrile (10:90, V/V) as mobile phase were used for hydrophilic interaction chromatographic analysis. The column and mobile phase employed for cation exchange chromatographic analysis melamine were CS17 column and 6 mmol/L of sulfuric acid. A mobile phase flow rate of 1.0 mL/min was employ for all three chromatographic methods and the detection wavelength was set at 240 nm. The results showed that the calibration curves of melamine derived from the methods were all linear within the range from 0.1 to 25.0 mg/L (r = 1.0000). The recoveries for melamine determined by reversed-phase ion pair chromatography, hydrophilic interaction chromatography and cation exchange chromatography were 95.75%, 99.96% and 93.91%, respectively. Reversed-phase ion pair chromatography had the highest response to melamine, cation exchange chromatography could detect melamine within the shortest time and while hydrophilic interaction chromatography could be used as a supplementary method for the determination of false positive samples.

Key words: powered milk, melamine, reversed-phase ion pair chromatography, hydrophilic interaction chromatography (HILIC), cation exchange chromatography