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%).