High-precision Fe Isotope Analysis on MC-ICPMS Using a Fe-57-Fe-58 Double Spike Technique

Herein we report procedures based on multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) for high-precision Fe isotopic analysis using a Fe-57-Fe-58 double spike technique. Iron purification was achieved using AG1-X8 in HCl media following previously or newly established procedures. In the new procedure, smaller columns with 4 mm diameter were used, containing 0.4 mL AG1-X8, thus greatly reducing the operation time and the amount of acid and resin consumed compared to the previously established method using 1 mL resin. Potential trace Ni interference on Fe-58 was suppressed by increasing the total Fe ion intensity to >= 120 V. Measurements of GSB Fe solutions doped with mono-elements demonstrated that a mass bias correction by the Fe-57-Fe-58 double spike was robust if Ca/Fe <= 1.0, Al/Fe <= 1.0, Cu/Fe <= 1.0, Co/Fe <= 0.1, Ni/Fe <= 10(-4), and Cr/Fe = 10(-4). Monitoring of pure Fe standard solutions, viz. IRMM-014 and NIST3126a, and geological reference materials, viz. JP-1, BHVO-2, W-2a, GSP-2, and COQ-1, over nine months yielded delta Fe-56 (relative to IRMM-014) values of 0.003 +/- 0.013 parts per thousand (2 SD, N = 20), 0.368 +/- 0.011 parts per thousand (2 SD, N = 30), 0.019 +/- 0.018 parts per thousand (2 SD, N = 15), 0.109 +/- 0.017 parts per thousand (2 SD, N = 30), 0.049 +/- 0.018 parts per thousand (2 SD, N = 17), 0.155 +/- 0.018 parts per thousand (2 SD, N = 14), and -0.066 +/- 0.022 parts per thousand (2 SD, N = 20), respectively, consistent with the recommended values within quoted errors. Based on repeated analyses of the standards, the long-term precision of our double spike method is better than 0.02 parts per thousand for d56Fe on average, proving its ability to distinguish small isotope fractionation among high-temperature samples.