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

Abstract

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 highconcentration samples (~ 5μg Fe) and introducing 1.8 mL/min Ar and 2 mL/min 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 ‰ and ± 0.12 ‰ (2σ) for δ 56 Fe and δ 57 Fe respectively.

Abiotic Pyrite Formation Produces a Large Fe Isotope Fractionation

“…Freeze-dried mackinawite was prepared as described above. Fe isotope analysis has been described in detail elsewhere (39). Samples (Fe(III) in HCl) were taken to dryness and re-dissolved in 5% HNO 3 .…”

Experimental determination of the equilibrium Fe isotope fractionation between and FeSm (mackinawite) at 25 and 2°C

“…56/54 Fe and 57/54 Fe isotope ratios were measured on a GV IsoProbe (formerly Micromass) multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). The detailed analytical protocol has been described elsewhere (Guilbaud et al, 2010b). The major challenge for accurate and precise measurement of Fe isotopes is the removal of atomic and polyatomic interferences induced by the Ar plasma.…”

Abundance and Impact of Doubly Charged Polyatomic Argon Interferences in ICPMS Spectra

“…Spectral interferences requiring mass resolving power (m/Δm) of >12000, however, can usually not be accounted for by this, and the sensitivity loss additionally limits the attainable precision and accuracy. In a similar fashion, collision or reaction cells − can be employed in specific cases. Frequently, mathematical correction of the interference signal is carried out to account for the spectral interference by measuring an (noninterfered) isotope of the interfering species and by subtracting the fraction of the interference signal via a known isotope ratio or using spectral deconvolution .…”