Speciation Analysis of Mercury in Milk Powder using Ion Chromatography-Inductively Coupled Plasma Mass Spectrometry Technique

Sample pre-treatment difficulties arise during the analysis for mercury speciation in milk powder due to the complexity of the sample matrix. The organic mercury combines easily with sulfydryl of proteins in the matrix sample, to form a stable complex. Therefore it is very important to ensure that all forms are extracted completely and do not transform during the pre-treatment process. The method with a lower detection limit was necessary because the concentration of mercury was too low. For this purpose, the method for determination of mercury speciation (inorganic mercury, methylmercury, ethylmercury) in milk was established by ion chromatography inductively coupled plasma mass spectrometry with optimized pre-treatment conditions. Protein, fat, and starch in milk powder were dissociated using a variety of composite enzymes (protease, lipase and amylase). The sample was ultrasonically extracted using mixed solution of L-cysteine, hydrochloric acid with methanol and was further purified by RP solid phase column. Mercury speciation was separated by Agilent Eclipse XDB-C18 column (5 μm, 4.6 mm×150 mm).The mobile phase contained 10 mmol/L ammonium acetate, 0.12% L-cysteine, and 5% (m:m) methanol solution. Three kinds of? mercury speciation were baseline separated within 5 min. The spiked recoveries of inorganic mercury, methylmercury and ethylmercury were obtained in the range of 79.9%-111.2%. The instrument detection limits were 0.5 μg/kg, 0.6 μg/kg and 0.9 μg/kg, respectively. The results obtained from actual? sample testing show that the extraction rate of total mercury can reach more than 70% in low concentration mercury milk powder, which meets the test requirements. The extraction rate can reach more than 80% because the composite enzyme was used in this method to dissociate mercury speciation from the complex matrix during sample pre-treatment processing. Methanol was applied as a sensitizer to improve detection sensitivity. A simple, rapid and reliable method was developed for the determination of mercury speciation in milk powder.