Fractionation of Oxygen Isotopes by Thermal Ionization Mass Spectrometry Inferred from Simultaneous Measurement of 17O/16O and 18O/16O Ratios and Implications for the 182Hf–182W Systematics

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• 作者：Anne Trinquier
• 刊名：Analytical Chemistry
• 出版年：2016
• 出版时间：June 7, 2016
• 年：2016
• 卷：88
• 期：11
• 页码：5600-5604
• 全文大小：259K
• 年卷期：0
• ISSN：1520-6882

文摘

Accurate ¹⁸²Hf–¹⁸²W chronology of early planetary differentiation relies on highly precise and accurate tungsten isotope measurements. WO₃^– analysis by negative thermal ionization mass spectrometry requires W¹⁷O¹⁶O₂^–, W¹⁷O₂¹⁶O^–, W¹⁸O¹⁶O₂^–, W¹⁷O₃^–, W¹⁷O¹⁸O¹⁶O^–, and W¹⁸O₂¹⁶O^– isotopologue interference corrections on W¹⁶O₃^– species (Harper et al. Geochim. Cosmochim. Acta 1996, 60, 1131; Quitté et al. Geostandard. Newslett. 2002, 26, 149; Trinquier et al. Anal. Chem. 2016, 88, 1542; Touboul et al. Nature 2015, 520, 530; Touboul et al. Int. J. Mass Spectrom. 2012, 309, 109). In addition, low ion beam intensity counting statistics combined with Faraday cup detection noise limit the precision on the determination of ¹⁸O/¹⁶O and ¹⁷O/¹⁶O relative abundances. Mass dependent variability of ¹⁸O/¹⁶O over the course of an analysis and between different analyses calls for oxide interference correction on a per integration basis, based on the in-run monitoring of the ¹⁸O/¹⁶O ratio (Harper et al. Geochim. Cosmochim. Acta 1996, 60, 1131; Quitté et al. Geostandard. Newslett. 2002, 26, 149; Trinquier et al. Anal. Chem. 2016, 88, 1542). Yet, the ¹⁷O/¹⁶O variation is normally not being monitored and, instead, inferred from the measured ¹⁸O/¹⁶O variation, assuming a δ¹⁷O−δ¹⁸O Terrestrial Fractionation Line (Trinquier et al. Anal. Chem. 2016, 88, 1542). The purpose of the present study is to verify the validity of this assumption. Using high resistivity amplifiers, ²³⁸U¹⁷O₂ and ²³⁸U¹⁸O₂ ion beams down to 1.6 fA have been monitored simultaneously with ^235,238U¹⁶O₂ species in a uranium certified reference material. This leads to a characterization of O isotope fractionation by thermal ionization mass spectrometry in variable loading and running conditions (additive-to-sample ratio, PO₂ pressure, presence of ionized metal and oxide species). Proper determination of O isotope composition based on the simultaneous analysis of the ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios could prevent tens of ppm bias or more on the ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W ratios.