A procedural development for the analysis of 56/54Fe and 57/54Fe isotope ratios with new generation IsoProbe MC-ICP-MS

We have developed a procedure for iron isotope analysis using a hexapole collision cell MC-ICP-MS which is capable of Fe isotope ratio analysis using two different extraction modes. Matrix effects were minimised and the signal-to-background ratio was maximised using high-concentration samples (similar to 5 mu g Fe) and introducing 1.8 mL min(-1) Ar and 2 mL min(-1) H-2 into the collision cell to decrease polyatomic interferences. The use of large intensity on the faraday cups considerably decreases the internal error of the ratios and ultimately, improves the external precision of a run. Standard bracketing correction for mass bias was possible when using hard extraction. Mass bias in soft extraction mode seems to show temporal instability that makes the standard bracketing inappropriate. The hexapole rf amplitude was decreased to 50% to further decrease polyatomic interferences and promote the transmission of iron range masses. We routinely measure Fe isotopes with a precision of +/- 0.05 parts per thousand and +/- 0.12 parts per thousand (2 sigma) for delta Fe-56 and delta Fe-57 respectively.