A novel method for high-precision tungsten isotope measurements on small-size samples

The application of ¹⁸²Hf - ¹⁸²W system for accurate dating and tracing of planetary events relies on highly accurate and precise tungsten isotope measurements. However, high-precision measurement of W isotopes usually requires large samples, which greatly hampers the study of low-content and/or small-size samples. In this study, a two-column W purification procedure was developed for small-size samples with the whole procedure blank of less than 40 pg and the recovery greater than 97 %. Meanwhile, a high-precision W isotopes measurement method was developed for amounts of W larger than 30 ng using static Faraday cups with 10¹³, 10¹², and 10¹¹ Ω amplifiers by negative thermal ionization mass spectrometry (NTIMS). In particular, we designed a high-precision O isotopic composition measurement method in the form of ReO₄, which greatly improved the precision and accuracy of W isotopes. By analyzing the SRM 3163 calibration solution, the long-term repeatability of the ¹⁸²W/¹⁸⁴W ratio was 0.864868 ± 6 ppm (2 RSD, n = 29, t = 12 months) when the W amount was greater than 30 ng. Measurements on actual samples have shown that these methods are applicable to terrestrial rocks, stony and iron meteorites with sample sizes of 20-400 mg. This new protocol significantly reduces the background of the W separation process and improves precision of W isotope measurement for small-size samples, as well as solves the critical problem of uncertainty in W isotope ratios due to the inability to accurately determine O isotope compositions in previous studies.