Assessment of matrix effects associated with Fe isotope analysis using 266 nm femtosecond and 193 nm nanosecond laser ablation multi-collector inductively coupled plasma mass spectrometry

Xin Yuan Zheng, Brian L. Beard, Clark M. Johnson

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This study evaluated matrix effects during Fe isotope analysis using a 266 nm femtosecond (fs) laser and a 193 nm nanosecond (ns) laser coupled to a multi-collector ICP-MS. During 150 second spot analysis on pyrite, Fe isotope fractionation was not observed for fs-laser ablation (fs-LA), but was evident for ns-LA. The observed downhole Fe isotope fractionation during ns-LA is caused by multiple processes comprising ablation, transport, and ionization in the ICP. Contrary to the common perception of "matrix-free" analysis, matrix effects clearly exist during fs-LA analysis; small deviations of up to ∼0.2‰ in the measured 56Fe/54Fe ratios from the true value of magnetite grains with ≥∼8 wt% impurities were resolved using a nearly pure magnetite as the bracketing standard. Moreover, inaccurate and imprecise 56Fe/54Fe results were obtained when magnetite and pyrrhotite was measured against a non-matrix-matched standard (pyrite or Fe metal). The observed matrix effects during fs-LA cannot be explained by formation of a large heat-affected zone during ablation, but result from the influence of different chemical compositions of samples and standards on space-charge effects in the ICP-MS. Such matrix effects can be largely suppressed by water addition during analysis at a price of reduced sensitivity, so that precise and accurate Fe isotope analysis to a ∼0.1‰ level can be routinely achieved under "wet" conditions without matrix-matching between sample and standard. These results may reconcile dramatically different precisions previously reported for Fe isotope analysis by fs-lasers, and also highlight fs-LA-MC-ICP-MS as an appealing option for in situ Fe isotope analysis on samples with complex matrices and high-symmetry minerals, both of which encounter significant analytical difficulties using secondary ion mass spectrometry (SIMS). For ns-LA, in addition to similar composition-related matrix effects experienced by fs-LA in the ICP, matrix effects also originate from ablation-related processes that produce sample particles with matrix-dependent size distributions and, often, larger aerodynamic sizes, resulting in highly inaccurate 56Fe/54Fe results during non-matrix-matched analysis under "dry" conditions. The collective matrix effects during Fe isotope analysis by ns-LA cannot be fully suppressed by water addition, therefore, matrix matching is required for accurate Fe isotope analysis by ns-LA.

Original languageEnglish (US)
Pages (from-to)68-83
Number of pages16
JournalJournal of Analytical Atomic Spectrometry
Issue number1
StatePublished - Jan 2018
Externally publishedYes

Bibliographical note

Funding Information:
This work was supported by NSF grant EAR-1347056 and NASA grant NNA13AA94A. We thank Prof. Ingo Horn for providing details on the LA-MC-ICP-MS setup in his lab, and Prof. Martin Whitehouse for providing details on the spatial resolution of Fe isotope analysis by SIMS in his lab, and Miss Thiruchelvi Reddy for help with data collection, and two anonymous reviewers for their helpful comments.

Publisher Copyright:
© 2017 The Royal Society of Chemistry.


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