Determination of the ratio of calcium to phosphorus in foodstuffs by dynamic reaction cell inductively coupled plasma mass spectrometry.

An inductively coupled plasma mass spectrometer (ICP-MS) equipped with a dynamic reaction cell (DRC) was used for the determination of Ca and P in foodstuffs. In this study, two different reaction gases, CH4 and O2, were introduced successively through the different channels to alleviate different interferences in the same analysis run. The effect of the operating conditions of the DRC system was studied to get the best signal-to-noise ratio for each element. The interfering 40Ar+ at m/ z 40 was reduced in intensity by up to five orders of magnitude by using 1.0 mL min(-1) CH4 as reaction cell gas in the DRC. On the other hand, by using O2 as the reaction gas, 31P+ was converted to 31P16O+ that could be detected at m/z 47 where there was less interference. The limits of detection for Ca and P were 0.2 ng mL(-1) and 0.3 ng mL(-1), respectively. This method was used to determine the concentrations of Ca and P and the ratio of Ca to P in NIST SRM 1549 non-fat milk powder and NIST RM 8345 whole milk powder reference materials and an infant milk powder and an infant cereal-rice sample purchased locally. The results for the reference samples agreed satisfactorily with the reference values. The accuracy of the determination was better than 4.1 and 0.9% for Ca and P, respectively. The results for infant milk powder and infant cereal were also found to be in good agreement with the value on the bottle label. Precision (RSD) between sample replicates was better than 4.8% for all the determinations.