Determination of silicon isotope ratios in silicate materials by high-resolution MC-ICP-MS using a sodium hydroxide sample digestion method

Abstract

Silicon isotope ratios (²⁸Si, ²⁹Si and ³⁰Si) can be measured with high precision by multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS). However, the problematic extraction of silicon from geological materials has been a major disadvantage in previous silicon isotope studies with conventional gas source mass spectrometry, whereas available silicon isotope results obtained by MC-ICP-MS techniques have been mainly restricted to waters and high purity silica. We show here that high yields of silicon (>97%) can be achieved from samples ranging from pure silica to basaltic compositions (45–52 wt.% SiO₂) via a three-step digestion and purification procedure. Silicon isotope measurements, performed with a Finnigan Neptune MC-ICP-MS used in medium-resolution mode (resolving power: 2500), indicate that polyatomic interferences can be resolved and that both δ²⁹Si and δ³⁰Si can be determined with high accuracy and precision on interference-free peak plateaux in the mass spectrum. Instrumental blanks (20–65 mV) were reduced to acceptable values with a Cetac Aridus desolvating device fitted with a sapphire injector in the torch. Sensitivity in medium-resolution mode is in the range of ∼6 V per μg g⁻¹ for ²⁸Si. δ²⁹Si and δ³⁰Si have been determined for silicon isotope standards IRMM-018 (δ³⁰Si = −1.75‰), IRMM-018-76 (δ³⁰Si = −1.42‰), Diatomite (δ³⁰Si = 1.34‰) and Big Batch (δ³⁰Si = −10.52‰), for USGS standards BHVO-2 (δ³⁰Si = −0.09‰) and AGV-2 (δ³⁰Si = −0.01‰), and for Aldrich pure silica powder (δ³⁰Si = −0.32‰). Precision on δ³⁰Si is 0.18–0.41‰ (2 s.d.). Our combined procedure for sample preparation followed by high-resolution MC-ICP-MS analysis facilitates straightforward and safe measurement of silicon isotope ratios in silicate materials.