Determination of melamine residue in raw milk and dairy products using hydrophilic interaction chromatography-electrospray ionization tandem mass spectrometry

A method for the determination of melamine residue in raw milk and dairy products was developed using hydrophilic interaction chromatography-electrospray ionization tandem mass spectrometry(HILIC-ESI-MS/MS).The melamine residue in the test sample was extracted with 1% trichloroacetic acid-acetonitrile(1∶1,v/v)solution.The homogenate was centrifuged and the supernatant was collected.The extract was cleaned up using a mixed-mode cation exchange(MCX)solid-phase extraction cartridge and then concentrated,and analyzed by HILIC-ESI-MS/MS.The gradient chromatographic separation was performed using a hydrophilic silica column(250mm×4.6 mm,5 μm)with 10 mmol/L ammonium acetate buffer(pH 3.0)and acetonitrile as the mobile phases.Due to its hydrophilic property,melamine was well retained on the column and seperated from other compounds.It effectively reduced matrix effect.A triple quadruple mass spectrometer equipped with an electrospray ionization source was employed for the qualitative and quantitative measurement of melamine.The melamine was quantified using the fragments produced from the protonated melamine ion through multiple reaction monitoring(MRM)in positive ion mode.Two MRM transitions from the protonated melamine ion(m/z 127→85 and m/z 127→68)were monitored.The average recoveries were between 76.3% and 98.7% in the spiked range of 0.5 to 10 mg/kg in raw milk and dairy products,and the relative standard deviations were less than 6.8%.The linear range was from 0.05 to 10.0 mg/L.The limit of quantification(S/N10)was 0.05 mg/kg.The method is highly selective and sensitive for the measurements of melamine and adequate for the analysis of melamine in raw milk and dairy products.