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

Silicon isotope ratios (Si-28, Si-29 and Si-30) 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 delta Si-29 and delta Si-30 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 similar to 6 V per mu g g(-1) for Si-28. delta Si-29 and delta Si-30 have been determined for silicon isotope standards IRMM-018 (delta Si-30 = -1.75%), IRMM-018-76 (delta Si-30 = -1.42%), Diatomite (delta Si-30 = 1.34%) and Big Batch (delta Si-30 = -10.52%), for USGS standards BHVO-2 (delta Si-30 = -0.09%) and AGV-2 (delta Si-30 = - 0.01%), and for Aldrich pure silica powder (delta Si-30 = -0.32%). Precision on delta Si-30 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.