IC Determination of Protein Nitrogen and Na,K,Ca, Mg in Milk Powder

Ion chromatography was applied to the determination of protein nitrogen and Na+,K+,Ca2+ and Mg2+ in milk powder.The sample was digested with conc.H2SO4 and H2O2,to char and oxidize the sample completely and to obtain a colorless and transparent solution which was further evaporated to less than 1 mL.Definite amount of HI acid was added to react with H2SO4 of the solution to produce iodine and sulfur dioxide which were expelled from the solution at the heating temperature (150 ℃).The acidity of sample solution was brought to pH 1.5-7 by the above operation.Ionpak CS12A column was used for separation of Na+,NH+4,K+,Mg2+ and Ca2+ in the sample solution,using 20 mmol·L-1 CH4SO4 as eluent.Conductivity detection was adopted in the determination.Detection limits (3S/N) found were in the range of 0.3-5.0 mg·kg-1.Recovery and precision of the method were tested,and the values of average recovery found were in the range from 95.0% to 102.0%,with values of RSD′s (n=6) ranged from 1.1% to 3.7%.