Rapid determination of calcium, magnesium, sodium and potassium in milk by flame atomic spectrometry after microwave oven digestion
A rapid method for determining Ca, Mg, Na and K in skim, semi-skim and whole milk was studied. These elements can be accurately determined in milk by atomic absorption (Ca and Mg) and atomic emission (Na and K) spectrometry after digestion of the sample with HNO3 in domestic and analytical microwave ovens in sealed poly(tetrafluoroethylene) reactors. Microwave-oven mineralization allowed accurate determination as tested against certified milks. The results were also in good agreement with those obtained after precipitation with trichloroacetic acid in certified and real liquid milk samples. Low recoveries for Ca in whole milk were found, owing to matrix effects, and the method of standard additions was necessary to avoid them. Mean recoveries determined for one of the milk reference materials studied were 99.3, 100, 100.5 and 100.3% for Ca, Mg, Na and K, respectively. The method gave relative standard deviations of 1–2% for the four elements. The limits of detection were 63, 15, 18 and 72 µg l–1 for Ca, Mg, Na and K, respectively. The method is very practical for routine laboratory analyses of relatively large numbers of samples.