A quadrupole ICP-MS operated in an He/H2 cell mode was used for the determination of iron in plant samples using isotope dilution inductively coupled plasma mass spectrometry. The accuracy of the method was validated by the analysis of three reference materials namely IAEA-359 (cabbage), NIST SRM 1515 (apple leaves) and NIST RM 8412 (corn stalk), which contain calcium in the range of 0.216 to 1.85%. Dry ashing was shown to be an ideal method that achieved complete equilibration of the spike and analyte isotopes. Under the optimized He/H2 cell mode, both argon-based and calcium-based molecular interferences on isotopes of 56Fe and 57Fe were reduced effectively. The results of analysis of iron in the three plant samples, based on the direct measurement of the isotope ratio of 56Fe/57Fe, were in excellent agreement with the certified/reference values. More importantly, ion-exchange chromatographic separation of calcium matrix from the analyte was not necessary.
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