Chemical Separation of Tungsten and Other Trace Elements for TIMS Isotope Ratio Measurements Using Organic Acids

Abstract

One requirement for isotope ratio measurement results with small measurement uncertainties is that the element of interest is effectively separated from the sample matrix. Efficient chemical separation of W from matrix components, especially Ti, can be challenging, particularly for large test portion masses (> 1 g). We present a new W separation procedure that takes advantage of the distinct complexation behaviour of Ti and W with citrate ligand in a moderately low pH, oxidising solution. This preparation procedure can reduce the Ti/W ratio of large (4–10 g) basaltic (i.e., high‐matrix) test portions by a factor of 105, relative to their original compositions, in a two‐step separation procedure. The procedure additionally provides a separate, well‐purified Mo fraction. We show that optimal separation requires precise selection of reagent concentrations and sample load. The procedure was employed to determine the μ182W composition of BHVO‐2 as −6.7 ± 4.2 (2 standard deviation, 2s). The principles derived from this method may prove useful for chemical separation of other elements used for geochemical and cosmochemical applications given an appropriate selection of organic acid. Future successful applications of this method may reveal that the use of organic acids as procedural reagents is a currently under‐utilised tool for efficient chemical separation protocols.

Combined Lithophile‐Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources

“…Tungsten was separated using the method of Peters et al. (2019) in a three‐step procedure. First, high field strength elements (HFSE: e.g., Ti, Zr, Ta, Nb) were separated from matrix elements using 1M HCl‐0.1M HF on Biorad™ AG50‐X8, 100–200 mesh cation exchange resin, analogous to the procedure used for Hf‐matrix separation.…”

“…When normalizing to normalizing to 186 W/ 184 W = 1.98594, deviations in 183 W/ 184 W from the standard value (0.467143 ± 0.000004) are not observed in sample measurements (average sample µ 183 W = 0.7 ± 5.1, 2σ s.d., n = 20). Two powder batches of BHVO-2 (#0631 and #1369) analyzed over the course of this study (Peters et al, 2019) yielded a mean µ 182 W composition of −6.7 ± 2.4 (2σ s.d., n = 4; see below for a discussion of Baxter (1975) for Mauritius. Abbreviation: CFB, continental flood basalts.…”

Tungsten Isotope Composition of Archean Crustal Reservoirs and Implications for Terrestrial μ182W Evolution

“…Tungsten was separated from bulk rock samples after dissolution following the methods of Peters et al (2019). Total analytical blanks measured by isotope dilution on a Nu Plasma MC‐ICP‐MS were <5.5 ng for isotope composition measurements and <1 ng for W concentration analyses, which corresponds to <1% of the total W in the sample and a negligible blank correction.…”

Improved method for highly precise and accurate ¹⁸²W/¹⁸⁴W isotope measurements by multiple collector inductively coupled plasma mass spectrometry and application for terrestrial samples

“…For example, Touboul and Walker (2012) proposed a procedure consisting of four chromatography steps, and these investigators used their procedure to obtain W isotope data for rocks of various ages. Peters et al (2019) modified Touboul and Walker's procedure and developed a low-blank, four-step separation method that uses cation and anion exchange chromatography with elution not only by HCl, HNO 3 , and HF but also by citric acid; this method is also effective for separating Mo. These investigators then determined precise W isotope ratios by means of negative thermal ionization mass spectrometry (NTIMS).…”