Testing the limits of micro-scale analyses of Si stable isotopes by femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry with application to rock weathering

Abstract

Abstract. An analytical protocol for accurate in-situ Si stable isotope analysis has been established on a new second-generation custom-built femtosecond laser ablation system. The laser was coupled to a multicollector inductively coupled plasma mass spectrometer (fsLA-MC-ICP-MS). We investigated the influence of laser parameters such as spot size, laser focussing, energy density and repetition rate, and ICP-MS operating conditions such as ICP mass load, spectral and nonspectral matrix effects, signal intensities, and data processing on precision and accuracy of Si isotope ratios. We found that stable and reproducible ICP conditions were obtained by using He as aerosol carrier gas mixed with Ar/H2O before entering the plasma. Precise  29 Si and  30 Si values (better than ±0.23‰, 2SD) can be obtained if the area ablated is at least 50 x 50 µm; or, alternatively, for the analysis of geometric features down to the width of the laser spot (about 20 µm) if an equivalent area is covered. Larger areas can be analysed by rastering the laser beam, whereas small single spot analyses reduce the attainable precision of  30 Si to ca. ±0.6 ‰, 2SD, for <30 µm diameter spots. It was found that focussing the laser beam beneath the sample surface with energy densities between 1 and 3.8 J/cm² yields optimal analytical conditions for all materials investigated here. Using pure quartz (NIST 8546 aka. NBS-28) as measurement standard for calibration (standard-sample-bracketing) did result in accurate and precise data of international reference materials and samples covering a wide range in chemical compositions (Si single crystal IRMM-017, basaltic glasses KL2-G, BHVO-2G and BHVO-2, andesitic glass ML3B-G, rhyolitic glass ATHO-G, diopside glass JER, soda-lime glasses NIST SRM 612 and 610, San Carlos olivine). No composition-dependent matrix effect was discernible within uncertainties of the method. The method was applied to investigate the Si isotope signature of rock weathering at the micro-scale in a corestone sampled from a highly weathered roadcut profile in the tropical Highlands of Sri Lanka. The results show that secondary weathering products accumulated in cracks and grain boundaries are isotopically lighter than their unweathered plagioclase host, consistent with isotopically heavy dissolved Si found in rivers.

Development of routines for simultaneous in situ chemical composition and stable Si isotope ratio analysis by femtosecond laser ablation inductively coupled plasma mass spectrometry

“…[14] Determination of in situ stable silicon isotope ratios was successfully achieved using nanosecond laser ablation. [15] Reducing the pulse length from nanoseconds to femtoseconds enables the quantification using nonmatrix matched calibration materials [10]. This improvement is substantial for metallic and conductive samples but also for stable metal isotope analysis.…”

“…This improvement is substantial for metallic and conductive samples but also for stable metal isotope analysis. [10,13,16,17] Simultaneously determining the chemical composition and isotope ratio from a single ablation can be achieved in four different ways. (i) Measuring isotope ratios and chemical composition on a single instrument (e.g.…”

“…For matrix-independent Si isotope determination, the use of a femtosecond laser ablation system (fs-LA) provides considerable advantages over nanosecond-LA (ns-LA). [10] The use of ns-LA leads to composition-dependent instrumental mass bias. Unknown samples therefore need to be measured against matrix-matched reference materials.…”

“…[28] The laser ablation system used in this study has been described in detail by Schuessler and von Blanckenburg. [10] Briefly, a UV fs-LA system (Spectra Physics Solstice, USA) in combination with a petrographic microscope and a custom-built stage and laser software for automated analysis are used. The fs-pulse generation is based on a Ti:sapphire seed laser, which is amplified using the chirped pulse amplification method, pumped by a Nd:YLF laser.…”

“…Methods for in situ silicon isotope ratio measurements exist, however not in combination with simultaneous chemical analysis. [8][9][10] Extending the capabilities to a more comprehensive elemental composition analysis has a huge potential for new scientific discoveries on micrometre scale samples. In this study we report two different approaches for evaluating in situ Si isotope ratio measurements with simultaneous determination of the chemical composition.…”