High-Precision Tungsten Isotopic Analysis by Multicollection Negative Thermal Ionization Mass Spectrometry Based on Simultaneous Measurement of W and 18O/16O Isotope Ratios for Accurate Fractionation

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• 作者：Anne Trinquier ; Mathieu Touboul ; Richard J. Walker
• 刊名：Analytical Chemistry
• 出版年：2016
• 出版时间：February 2, 2016
• 年：2016
• 卷：88
• 期：3
• 页码：1542-1546
• 全文大小：247K
• ISSN：1520-6882

文摘

Determination of the ¹⁸²W/¹⁸⁴W ratio to a precision of ±5 ppm (2σ) is desirable for constraining the timing of core formation and other early planetary differentiation processes. However, WO₃^– analysis by negative thermal ionization mass spectrometry normally results in a residual correlation between the instrumental-mass-fractionation-corrected ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W ratios that is attributed to mass-dependent variability of O isotopes over the course of an analysis and between different analyses. A second-order correction using the ¹⁸³W/¹⁸⁴W ratio relies on the assumption that this ratio is constant in nature. This may prove invalid, as has already been realized for other isotope systems. The present study utilizes simultaneous monitoring of the ¹⁸O/¹⁶O and W isotope ratios to correct oxide interferences on a per-integration basis and thus avoid the need for a double normalization of W isotopes. After normalization of W isotope ratios to a pair of W isotopes, following the exponential law, no residual W–O isotope correlation is observed. However, there is a nonideal mass bias residual correlation between ¹⁸²W/ⁱW and ¹⁸³W/ⁱW with time. Without double normalization of W isotopes and on the basis of three or four duplicate analyses, the external reproducibility per session of ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W normalized to ¹⁸⁶W/¹⁸³W is 5–6 ppm (2σ, 1–3 μg loads). The combined uncertainty per session is less than 4 ppm for ¹⁸³W/¹⁸⁴W and less than 6 ppm for ¹⁸²W/¹⁸⁴W (2σ_m) for loads between 3000 and 50 ng.