Evaluation of plasma condition, concentration effect, position effect, and nickel-doping method on non-matrix-matched Fe isotopic analysis by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry

Abstract

This study investigated the influences of plasma condition, concentration effect, ablation position and on-line addition of an aqueous Ni standard for non-matrix-matched Fe isotopic analysis by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs LA–MC–ICP–MS). Deviations up to ~0.7‰ in δ56Fe values caused by non-matrix-matched analyses were difficult to eliminate under dry plasma condition even with fs-LA. However, the nebulized water after the ablation cell efficiently reduces or eliminates the matrix-dependent fractionation of Fe isotopes without reducing signal intensity. Furthermore, concentration- and position-dependent fractionation of Fe isotopes was not observed under wet plasma condition. For instrumental mass bias correction, the standard–sample bracketing (SSB) method and SSB with Ni doping (SSB + Ni) were performed in the presence of nebulized water with fs LA–MC–ICP–MS. Results indicate that both the SSB and SSB + Ni methods are sufficient to correct for instrumental mass bias under wet plasma condition, with the SSB + Ni method being more effective in calibrating short-term fluctuations in mass bias with improved external reproducibility of ~0.15‰ (2SD) for δ56Fe values. Compared to SN–MC–ICP–MS, deteriorated precision offered by fs LA–MC–ICP–MS may represent a major limitation for some applications relying upon precise Fe isotope ratio measurements. This approach for non-matrix-matched analysis of Fe isotopes by fs LA–MC–ICP–MS under wet plasma condition combined with the SSB + Ni method allows for more flexibility in tracing detailed geological processes with large Fe isotope fractionation of a wide range of solids. Relatively precise and accurate Fe isotopic compositions of potential reference materials of Balmat pyrite (δ56Fe = −1.32 ± 0.12‰, δ57Fe = −1.91 ± 0.20‰; 2SD, n = 166) and PZH12–24 ilmenite (δ56Fe = −0.38 ± 0.15‰, δ57Fe = −0.57 ± 0.23‰; 2SD, n = 134) were obtained using this analytical protocol, consistent with reference values obtained by solution nebulization (SN)–MC–ICP–MS within analytical uncertainties (Balmat pyrite: δ56Fe = −1.30 ± 0.04‰, δ57Fe = −1.94 ± 0.07‰, 2SD, n = 3; PZH12–24 ilmenite: δ56Fe = −0.38 ± 0.06‰, δ57Fe = −0.55 ± 0.07‰, 2SD, n = 6).