Analysis of Nitrate in Liquid Milk and Milk Powder

Solid Phase Extraction-Ion Chromatography and the national standard chemical method (cadmium column reduction method) were used for the determination of nitrate in liquid milk and milk powder sold in Shijiazhuang. The Ion Chromatography method was achieved by adding acetic acid to precipitate protein in the prepared samples. After being centrifuged, the top clear solution was loaded to the DIONEX OnGuard RP solid phase extraction column to remove fat and other hydrophobic compounds. The AS11-HC anion chromatographic column was used for analysis. All samples were analyzed by the cadmium column reduction method for comparison. The average contents of nitrate in liquid milk were 10.05 mg/kg and 9.95 mg/kg, respectively. The average contents of nitrate in the milk powder were 25.74 mg/kg and 25.66 mg/kg, respectively. T test analysis it showed revealed that no significant difference was observed (t=0.07, P>0.05). The results support the accuracy of ion chromatography and the convenience of the procedure, which is in concordance with literature. The method of Ion Chromatography was easy to operate. According to the requirements on nitrate in milk powder in China, the evaluated results were within the prescribed limits. Meanwhile, the unqualified ratio of nitrate in liquid milk was about 14%.