Factors affecting chromium isotope measurements using the double-spike method

The double-spike technique is widely used in stable metal isotope measurements to correct for instrumental fractionation, and this method has achieved prominence for chromium (Cr) isotope measurements in the past decade or so. For this method to work, precise calibration of the double spike was thought to be indispensable. However, the effect of carrying out this precise calibration has not been fully evaluated. Moreover, other factors that can affect the accuracy of Cr isotope measurements using the double spike have also not been thoroughly evaluated.

First using theoretical calculation, we assessed whether precise knowledge of the measured mass fractionation of the double spike is necessary. We then developed a double-spike method for measuring the Cr stable isotopic composition using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). We tested the key factors that can affect the accuracy of Cr isotope measurements.

By theoretical calculation we found that the uncorrected instrumental mass fractionation of the double spike will not result in any systematic bias on the measurements of δ⁵³ Cr values in the sample, provided that the double spike, standard and sample are measured on the same instrument and with the same cup configuration. Incorporating this result, with our new method, the presence of Fe and Ti (up to 5% wt of the Cr concentration in the sample) can be fully corrected. The long-term reproducibility of an internal standard was ±0.04‰ (2SD), and the external reproducibility on the δ⁵³ Cr difference between our internal standard and the NIST Standard Reference Material 3112a was -0.07 ± 0.05‰. The δ⁵³ Cr values for a set of USGS geological reference materials including basalt (BHVO-1,2; BIR-1; GSR-3), mica schist (SDC-1), diabase (W-2-a), peridotite (PCC-1) and dunite (DTS-2) were measured, with our results for BHVO-2 and BIR-1 being consistent with previously reported values.

We have shown that precise knowledge of the measured mass fractionation of the double spike is not necessary during spike calibration and thus the calibration procedure is simplified. We can apply our new Cr isotope method to a range of geological rock samples with a wide range of variation in the chemical composition.