In-situ isotopic ratio analysis of iron using laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

In-situ Fe-54/Fe-56 and Fe-57/Fe-56 ratio measurements for solid samples have been carried out using laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Using a dry plasma condition achieved by laser sample introduction technique, mass spectrometric interferences such as (ArN+)-Ar-54, (ArO+)-Ar-56 or (ArOH+)-Ar-57 could be successfully reduced. Although these argide interference signals were not completely eliminated, the contribution of the remnant signals could be effectively corrected by an "on-peak" baseline subtraction technique. Resultant repeatability or external precision of the measurement over the four month period was almost 0.5-1%, which was insufficient to detect the isotopic variations of Fe in natural samples (typically smaller than 0.3%). We found that the poor repeatability in Fe isotopic ratio measurements was mainly due to instability or poor reproducibility of the high voltage power supply, especially for ion acceleration voltage, of our instrument. The mass discrimination effect on Fe isotopic ratios was strongly correlated with accelerating energy (ion energy) of analyte ions. To overcome this, Fe isotopic ratios of NIST Standard Reference Material (SRM) 665 metallic standard were measured between the sample measurements. This sample-standard bracketing method successfully results in internal precisions of the Fe-54/Fe-56 and Fe-57/Fe-56 ratios of better than 0.1% at the 95% confidence level. The analysis time for one sample was about 50 s, and the total amount of Fe used for an analysis was about 30 ng. The Fe-54/Fe-56 and Fe-57/Fe-56 ratios measured for NIST SRM 661 steel, three Fe-bearing minerals, and six iron meteorites, show no significant variations within the analytical uncertainties achieved in this study. The good agreement between samples suggests that our approach successfully compensates for both the instrumental drift and the background correction. The data presented here demonstrate clearly that the Fe isotopic ratios obtained by the LA-MC-ICP-MS technique can give strong clues to elucidate the Fe isotopic fractionation occurring in biochemical reactions, which may cause a larger isotopic fractionation effect in nature.