ABSTRACT
The last decade has seen widespread adoption of triple quadrupole-based inductively coupled plasma-tandem mass spectrometry (ICPMS/MS) technique using a collision/reaction cell in combination with a precell bandpass mass analyzer to measure isotopes otherwise masked by spectral interferences. High-precision isotope ratio analysis containing such isotopes would benefit from a similar capability on a multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) platform, but using a quadrupole-based precell mass analyzer for MC-ICPMS/MS has several limitations. To overcome these limitations, we developed a novel precell mass analyzer for MC-ICPMS/MS using sector field technology. The new precell mass analyzer, comprising two Wien filters and a selection aperture, and a hexapole collision/reaction cell were integrated together in a single module and added to the commercially available Thermo Scientific Neptune XT MC-ICPMS to create a prototype MC-ICPMS/MS we named Vienna. Vienna was proven to retain the same performance of the base MC-ICPMS in terms of sensitivity, accuracy, and precision. Using the Vienna mass filter to eliminate Ar-based species, the abundance sensitivity achievable was equivalent to TIMS at mass 237.05, which was used to accurately determine the low 236U/238U isotope ratio of the uranium reference material IRMM184 (certified value, 1.2446 × 10-7). The performance of Vienna was then tested for a variety of geoscience applications that were expected to benefit from MC-ICPMS/MS technique, including Ca, K, Si, and in situ Rb/Sr dating by laser ablation.
Subject(s)
Isotopes , Mass Spectrometry , Spectrum AnalysisABSTRACT
Different approaches for the determination of the 87Sr/86Sr isotope ratio of high-Rb glass are compared in this work to assess the suitability of minimally invasive approaches for applications on medieval stained glass (from the ancient Abbey of Stavelot in Belgium). It was found that pneumatic nebulization multicollector inductively coupled plasma-mass spectrometry (PN-MC-ICP-MS) after acid digestion and chromatographic isolation of the target analyte out of the sample matrix can still be seen as the preferred method for the high-precision isotopic analysis of Sr in glass with high Rb and rare-earth element (REE) concentrations. Alternatively, the use of laser ablation (LA) for sample introduction is a powerful technique for the direct analysis of solid samples. However, both the high Rb/Sr ratios in the samples of interest and the presence of REEs at sufficiently high concentrations lead to a large bias in LA-MC-ICP-MS, which cannot be corrected for, even by operating the MC-ICP-MS instrument at higher mass resolution and/or using mathematical corrections. It was demonstrated that LA tandem-ICP-MS (LA-ICP-MS/MS) using CH3F/He as the reaction gas to overcome spectral overlap in a mass-shift approach (chemical resolution) provides a viable alternative when (quasi) nondestructive analysis is required. This approach relies on the monitoring of Sr+ (m/z = 86, 87, and 88) ions as the corresponding SrF+ reaction product ions (m/z = 105, 106, and 107), thus avoiding the occurrence of spectral interference. Self-evidently, the isotope ratio precision attainable using sequential quadrupole-based ICP-MS instrumentation (0.3% RSD) was found to be significantly worse than that of high-precision MC-ICP-MS (0.03% RSD) with simultaneous detection, although it was still fit for the purpose of current applications. In addition to Sr isotopic analysis, the REE patterns and their potential influence on the Sr isotopic composition were evaluated by LA-ICP-MS.
ABSTRACT
We document the utility for in situ Rb-Sr dating of a one-of-a-kind tribrid mass spectrometer, 'Proteus', coupled to a UV laser ablation system. Proteus combines quadrupole mass-filter, collision cell and sector magnet with a multicollection inductively-coupled plasma mass spectrometer (CC-MC-ICPMS/MS). Compared to commercial, single collector, tribrid inductively-coupled plasma mass spectrometers (CC-ICPMS/MS) Proteus has enhanced ion transmission and offers simultaneous collection of all Sr isotopes using an array of Faraday cups. These features yield improved precision in measured 87Sr/86Sr ratios, for a given mass of Sr analysed, approximately a factor of 25 in comparison to the Thermo Scientific™ iCAP TQ™ operated under similar conditions. Using SF6 as a reaction gas on Proteus, measurements of Rb-doped NIST SRM (standard reference material) 987 solutions, with Rb/Sr ratios from 0.01-100, yield 87Sr/86Sr that are indistinguishable from un-doped NIST SRM 987, demonstrating quantitative 'chemical resolution' of Rb from Sr. We highlight the importance of mass-filtering before the collision cell for laser ablation 87Sr/86Sr analysis, using an in-house feldspar standard and a range of glass reference materials. By transmitting only those ions with mass-to-charge ratios 82-92 u/e into the collision cell, we achieve accurate 87Sr/86Sr measurements without any corrections for atomic or polyatomic isobaric interferences. Without the pre-cell mass-filtering, measured in situ 87Sr/86Sr ratios are inaccurate. Combining in situ measurements of Rb/Sr and radiogenic Sr isotope ratios we obtain mineral isochrons. We utilise a sample from the well-dated Dartmoor granite (285 ± 1 Ma) as a calibrant for our in situ ages and, using the same conditions, produce accurate Rb-Sr isochron ages for samples of the Fish Canyon tuff (28 ± 2 Ma) and Shap granite pluton (397 ± 1 Ma). Analysing the same Dartmoor granite sample using identical laser conditions and number of spot analyses using the Thermo Scientific™ iCAP TQ™ yielded an isochron slope 5× less precise than Proteus. We use an uncertainty model to illustrate the advantage of using Proteus over single collector CC-ICPMS/MS for in situ Rb-Sr dating. The results of this model show that the improvement is most marked for samples that have low Rb/Sr (<10) or are young (<100 Ma). We also report the first example of an in situ, internal Rb-Sr isochron from a single potassium-feldspar grain. Using a sample from the Shap granite, we obtained accurate age and initial 87Sr/86Sr with 95% confidence intervals of ±1.5% and ±0.03% respectively. Such capabilities offer new opportunities in geochronological studies.
