High-precision iron isotopic measurements in low resolution using collision cell (CC)-MC-ICP-MS

Abstract

Here we present an analytical method for Fe isotopic measurements in low resolution mode, using a Nu Sapphire collision cell-equipped multi-collector inductively coupled plasma mass spectrometer by standard-sample bracketing. High Fe sensitivity was obtained through reducing Ar based interference by over 9 orders of magnitude whilst other analytes remained unaffected. The effects of Fe concentration, Fe beam intensity and HNO₃ molarity mismatch between the standard and sample and the presence of matrix elements have been evaluated. The long-term analyses of JMC Fe and BCR-2 indicate that the obtained isotopic ratios are highly reproducible, with precisions of better than ±0.03‰ for δ⁵⁶Fe (2SD). Such accurate and precise data could be acquired via 3 to 6 repeat measurements consuming 50–100 ng Fe, which has improved by a factor 40 compared to those previously reported on other instruments. Accurate measurements were achieved by closely matching Fe intensities between the sample and standard (a 5% mismatch would create a 0.02‰ offset in the ⁵⁶Fe/⁵⁴Fe ratio). On the other hand, the ratio of the concentration between the matrix elements and Fe, such as C_Na/C_Fe, C_Mg/C_Fe, C_Ca/C_Fe, C_Ti/C_Fe and C_Ni/C_Fe, should be kept under 0.1, C_K/C_Fe under 0.3, C_Al/C_Fe and C_Cu/C_Fe under 1, and C_Co/C_Fe and C_Zn/C_Fe under 2 to avoid any matrix effect. In addition, high precision Fe isotopic data were obtained on twenty-one geological reference materials and were highly consistent with the literature values. Furthermore, we obtained δ⁵⁶Fe of standard chalcopyrite (XTC) for the first time: 0.127 ± 0.027‰ (2SD, n = 3).