Determination of sodium, potassium, calcium and magnesium content in milk products by flame atomic absorption spectrometry (FAAS): A joint ISO/IDF collaborative study

A flame atomic absorption spectrometry method to determine sodium, potassium, calcium and magnesium content in milk products using dry ashing or wet digestion was evaluated in an international collaborative trial. The study involved 18 participants from six countries. The method was tested on a total of eight milk products: whey protein concentrate, whole milk powder, two processed cheeses, whey powder, acid casein, freeze-dried cheese, and skim milk powder. A control solution and a certified reference material were included in the study. Materials were presented to the participants of the study as blind duplicates, and the participants were asked to perform only one test on each test sample. Except for acid casein, repeatability relative standard deviation (RSDr) ranges for Na, K, Ca, and Mg were 1.7–6.4%, 1.8–10.0%, 0.9–5.5% and 1.1–4.7%, respectively, while for reproducibility relative standard deviations (RSDR) the ranges were 3.7–9.2%, 2.7–8.9%, 2.1–7.9% and 1.3–7.4%, respectively. Again with the exception of acid casein, the precision of the method was within the range predicted by the Horwitz equation and the Horrat(R) values (i.e., the ratio of the reproducibility relative standard deviation calculated from the data to the predicted RSDR calculated from the Horwitz formula) were <2.0.