In-situ precise isotopic analysis of tungsten using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) with time resolved data acquisition

Abstract

Measurements of 182W/183W, 184W/183W and 186W/183W ratios in solid geological samples have been carried out using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). A time resolved analysis (TRA) data acquisition technique was used to minimise integration losses in transient signals and also to minimise the risk of lowering the precision and accuracy of the measurements due to integration of low signal intensity data. The resulting precisions of the 182W/183W, 184W/183W and 186W/183W measurements were about 0.015%, 0.031% and 0.039% (2σ) when an optimised integration area and an expansion stabiliser were used. These values are only 3-times worse than those achieved by conventional solution nebulisation measurements. However, the total amount of W used for isotopic analysis using the laser ablation technique was only 20 ng, which is much less (0.1–0.01) than the typical amount required for the solution nebulisation method. Measured W isotopic ratios in a pure W filament and wolframite samples showed excellent agreement with those from chemical reagents: 182W/183W = 1.85147 ± 0.00022, 184W/183W = 2.14169 ± 0.00022 and 186W/183W = 1.98764 ± 0.00021 (2σ) normalised to 186W/184W ≡ 0.92805, obtained by conventional nebulisation. The internal precision using LA-MC-ICP-MS was sufficient to detect the natural variations in the 182W/183W isotopic ratio originating from radiogenic contributions of 182Hf (typically 0.04– 0.2%).