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de Souza, Evelyn da Motta Freres

University of British Columbia

Multi-Collector Inductively Coupled Plasma Mass Spectrometers (MC-ICP-MS) are widely used instruments in Earth sciences to measure isotopic ratios in rock and environmental materials with high sensitivity, accuracy, and precision (i.e., sub-ppm range). However, even when samples undergo careful chemical separation treatment and the instrument is properly calibrated and operated, the measured ratios always differ from their “true” value due to fractionation caused by processes inherent to the technique. Traditional correction methods such as fractionation laws and standard-sample-bracketing (SSB) can mostly account for this instrumental bias. However, other sources of fractionation may arise when even slight modifications are made to the instrumental set-up and sample composition. This thesis investigates the instrumental conditions and sample preparation methods that govern some common sources of fractionation. Chapter 2 explores oxide formation fractionation and its effect on Nd isotopic ratios using different instrumental setups, including wet and dry plasma and different sampler and skimmer cone geometries. We provide a quantitative description of the influence of NdO⁺ formation on mass bias and a qualitative mechanism of the different mass bias contributions to Nd isotopic ratios. Chapter 3 investigates how self-induced matrix effects (SIME) affect isotopic ratios for several isotopic systems (Li, Mg, Fe, Zn, Sr, Nd, Hf and Pb) using pure elemental solutions and varying the concentration of the samples within ± 50 % relative to the bracketing standard. This characterization allowed us to create several descriptive equations describing how SIME affects the isotopic offsets in each ratio. Chapter 4 systematically studies SIME and matrix effects when measuring Nd, Hf and Pb isotopic ratios in a Plasma 3 MC-ICP-MS using pure elemental solutions and geological reference materials of different matrices. We also investigate a range of Pb/Tl ratios to assess Tl correction accuracy for different matrices and compositions when measuring Pb ratios. This study highlights the robustness of measurements performed using the Plasma 3. Finally, we provide an extensive data set characterizing how the instrumental mass bias affects the isotopic ratios measured by MC-ICP-MS. We provide comprehensive analytical guidelines for several isotopic systems when addressing Nd oxide formation, SIME and matrix-related fractionation effects.

Text

2024-01-11

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/87212

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/