提出了用离子色谱法测定奶粉中蛋白质氮、钠、钾、钙及镁.用浓硫酸及过氧化氢处理样品使完全炭化并氧化,从而得到一无色透明样品溶液.将此溶液继续加热蒸发至1 mL以下,加入一定量的氢碘酸与溶液中硫酸作用,而产生碘及二氧化硫.此两反应产物在150 ℃温度下加热时从溶液中挥发除去.通过上述处理使溶液的酸度控制在pH 1.5~7之间,使溶液通过Ionpac CS12A色谱柱并采用20 mmol·L-1甲烷磺酸作为淋洗液达到Na+、NH+4、K+、Mg2+及Ca2+的分离.采用电导检测器进行测定.方法的检出限(3S/N)为0.3~5.0 mg·kg-1.对方法的回收率及精密度作试验,测定回收率为95.0%~102.0%;相对标准偏差(n=6)为1.1%~3.7%.

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%.