Johannes Schwieters

High precision calcium isotope ratio measurements using a magnetic sector multiple collector inductively coupled plasma mass spectrometer

Calcium isotope abundances were measured using a Finnigan Neptune magnetic sector multiple collector inductively coupled plasma mass spectrometer capable of resolving all molecular isobaric interferences from 42Ca, 43Ca, 44Ca, 46Ca and 48Ca. Scattering events caused by the intense 40Ar+ ion beam did not contribute to the uncertainty in the baseline of the calcium mass spectrum. Quantitative separation of the calcium from the sample matrix was carried out to ensure that the measurements were independent of the sample type. In addition, thorough desolvation of the aerosol was found to have a significant effect on the stability and sensitivity of the method. The stable mass bias of the instrument enabled normalization of the measured isotope abundance ratios relative to standard reference materials. Delta values including δ44Ca/43Ca, δ44Ca/42Ca and δ48Ca/42Ca were measured with external reproducibilities better than ±0.2‰ n (2s) from 10 ppm solutions of calcium-containing materials, including sea-water and biogenic and non-biogenic marine carbonates.

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

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.

Particle detectors used in isotope ratio mass spectrometry, with applications in geology, environmental science and nuclear forensics

This chapter introduces the reader to mass spectrometry and the instruments used to determine high-precision isotope ratios. These instruments separate ion beams of charged atomic particles with kinetic energies of several keV, by mass-to-charge ratio. Quantitative detection of these ions is a key technology in mass spectrometry. For isotope ratio determination, the main detector types are Faraday cups, the Daly detector, and secondary electron multiplier (SEM) ion counters. For high-precision isotope ratio measurements, arrays of these detectors are arranged to collect several ion beams simultaneously. Examples are given for the application of these detectors in geology, environmental sciences, and nuclear safeguards.