Determination of calcium, iron and zinc in milk powder by reaction cell inductively coupled plasma mass spectrometry Original Research Article

An inductively coupled plasma mass spectrometer (ICP-MS) equipped with a dynamic reaction cell™ (DRC) was used for the determination of Ca, Fe and Zn in milk powder samples. The effect of the operating conditions of the DRC system was studied to get the best signal-to-noise (S/N) ratio. The potentially interfering 40Ar+, 40Ar16O+, 40Ca16O+, 48Ca16O+ and 32S16O16O+ at the masses m/z 40, 56 and 64 were reduced in intensity significantly by using CH4 as the reaction cell gas in the DRC while a q-value of 0.7 was used. The limits of detection for 40Ca, 56Fe and 64Zn estimated from the external calibration graphs were 1, 0.01 and 0.001 ng ml−1, respectively, which correspond to 1000, 10 and 1 ng g−1 in the original powder sample. This method was applied to the determination of Ca, Fe and Zn in NIST SRM 1549 non-fat milk powder and two milk powder samples purchased locally. The results for the reference sample agreed satisfactorily with the reference values; the accuracy of the determination was better than 3.8, 18 and 2.2% for Ca, Fe and Zn, respectively. The results for which no reference value was available were also found to be in good agreement between different isotopes. Precision (R.S.D.) between sample replicates was better than 10% for all the determinations.