Capabilities of inductively coupled plasma mass spectrometry for the measurement of Fe isotope ratios

Abstract

Reliable isotopic analysis of Fe by means of inductively coupled plasma mass spectrometry (ICP-MS) is traditionally hindered by spectral overlap of the analyte signals with those of Ar- and Ca-based molecular ions. The merits of several approaches for overcoming spectral interferences—operation of the ICP under cool plasma conditions, membrane desolvation of the sample aerosol, use of a double-focusing sector field mass spectrometer operated at a higher mass resolution and ion-molecule chemistry in a dynamic reaction cell (DRC)—for solving this particular problem were critically compared and the most successful approaches were subsequently used for the determination of Fe isotope ratios in a human serum reference material. Although beneficial to some extent and/or for some of the Fe nuclides, cool plasma conditions and membrane desolvation were shown to offer no sufficient reduction in the intensity of the interfering ions. With DRC-ICP-MS, all reaction gases tested, NH₃, CO and N₂, showed a similar behaviour. An excellent performance in terms of both freedom from spectral overlap and isotope ratio precision, not exceeding the theoretical precision calculated on the basis of counting statistics and ≤ 0.2% RSD for 5 successive measurements, was established for aqueous standard solutions. In the presence of Ca, however, the detrimental influence of spectral interferences precluded accurate determination of ⁵⁷Fe/⁵⁶Fe at the standard RPq setting. When using a matrix-matched blank in addition to CO as a reaction gas to correct for the remaining overlap, ⁵⁴Fe/⁵⁶Fe could, however, be accurately and precisely measured in human serum. It was established that, with DRC-ICP-MS, the matrix composition affected the mass discrimination to a significant extent, such that the isotopic standard used for mass discrimination also required matrix-matching. Further optimisation of the RPq value resulted in an improved signal-to-background ratio at a mass-to-charge ratio of 57. Despite a marked increase in mass discrimination, ⁵⁴Fe/⁵⁶Fe could still be accurately determined under these conditions (deviation from true value <0.1%) while the result for ⁵⁷Fe/⁵⁶Fe was substantially improved. The total uncertainty on a single determination (5 replicate measurements of 60 s each) of the ⁵⁴Fe/⁵⁶Fe ratio typically amounted to approximately 0.5%. When using sector field ICP-MS operated at a mass resolution of 3000, the analyte signals could be resolved from those of the above-mentioned molecular ions. For aqueous standard solutions, the isotope ratio precision attainable with sector field ICP-MS, typically 0.2–0.4% for ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe and ≥1% for ⁵⁸Fe/⁵⁶Fe, was significantly worse than that obtained with DRC-ICP-MS. The deviation between the experimental values for ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe in human serum and the corresponding true values, however, was <0.05%, while the total uncertainty on the ICP-MS results was approximately 0.5% (5 replicate measurements of 90 s each). Due to the low isotopic abundance of ⁵⁸Fe, the uncertainty for ⁵⁸Fe/⁵⁶Fe deteriorated to ~2.5%. Conclusions concerning the applicability of ICP-MS for isotopic analysis of Fe are presented and some ideas for further research are discussed.