In situ Fe isotopic analyses of fourteen reference materials using a synthetic Cr standard for mass bias and isobaric interference corrections by femtosecond LA-MC-ICP-MS

Highly precise and accurate measurements of Fe isotope ratios for fourteen reference materials from the USGS, MPI-DING and CGSG were successfully carried out using a developed analytical technique by fs LA-MC-ICP-MS. A synthetic standard, i.e., Cr-979S, was produced as a calibration standard for instrumental mass bias and isobaric interference corrections. The results for samples with low Cr concentration (Cr-54/Fe-54 < 0.0001), e.g., delta Fe-56 values of BCR-2G (delta Fe-56 = 0.12 +/- 0.10 parts per thousand, 2SD, n = 15), were in excellent agreement with data from solution MC-ICP-MS within the measurement precision without the use of the Cr-979S standard. For the samples with high Cr concentration (Cr-54/Fe-54 > 0.0001), e.g., BHVO-2G, delta Fe-56 values deviated from reference values when using the standard-sample bracketing method alone. Combining the standard-sample bracketing method with Cr-979S standard corrections and standard sample cone + H skimmer cone, the measured Fe isotopic compositions of GOR128-G and GOR132-G with extremely high Cr concentrations (Cr-54/Fe-54 > 0.01) were identical to reference values within errors (delta Fe-56 = -0.01 +/- 0.13 parts per thousand, 2SD, n = 18; delta Fe-56 = 0.00 +/- 0.13 parts per thousand, 2SD, n = 13; respectively). A series of reference materials with varying Cr concentrations, including the USGS reference glasses (BCR-2G, BHVO-2G, BIR-1G, TB-1G, NKT-1G, GSD-1G and GSE-1G), MPI-DING reference glasses (KL2-G, ML3B-G, GOR128-G and GOR132-G) and CGSG reference materials (CGSG-1 and CGSG-4), were used to test our developed method of in situ Fe isotopic measurements using the combined standard-sample bracketing method and Cr-979S standard corrections by fs LA-MC-ICP-MS. All the reference materials yield relatively homogeneous Fe isotopic compositions with delta Fe-56 values ranging from -0.03 parts per thousand to 0.13 parts per thousand and delta Fe-56 reproducibility is below 0.15 parts per thousand (2SD), suggesting that the proposed isobaric interference correction and mass bias correction method is a useful tool for providing high-quality in situ Fe isotopic data of geological samples with both low and high Cr concentrations.

Automated summary

Highly precise Fe isotope ratio measurements were successfully conducted on fourteen reference materials using fs LA-MC-ICP-MS. A synthetic standard Cr-979S was utilized for calibration and corrections, yielding excellent results for samples with low Cr concentration but requiring additional corrections for samples with high Cr concentration. Combining standard-sample bracketing method and Cr-979S standard corrections proved to be effective in obtaining accurate measurements for samples with high Cr content.