Dunstan L P, Gramlich J W, Barnes I L, Purdy W C
National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234.
McGill University, Montreal, Quebec, Canada.
J Res Natl Bur Stand (1977). 1980 Jan-Feb;85(1):1-10. doi: 10.6028/jres.085.001.
The accepted atomic weight of thallium has remained at a value of 204.37 ± 0.03 since 1962. At this level of uncertainty, however, the atomic weight becomes a limiting factor to high accuracy analysis. The new mass spectrometric determination of the atomic weight of thallium has been completed. A high precision assay technique was developed so that accurately known quantities of the Tl and Tl separated isotopes could be mixed to produce standards for calibration of the mass spectrometer. This assay technique involved the gravimetric determination of 99.3 percent of the0020thallium as TlCrO. The soluble thallium was then aliquoted and determined by isotope dilution mass spectrometry. Before making up the final solutions from which the assay and calibration samples would be withdrawn, the separated isotopes were purified by solvent extraction and electrodeposition. A tungsten filament surface ionization technique was developed for the determination of precise isotopic abundance measurements for thallium. This technique allowed isotopic analysis of the separated isotopes, calibration standards, and a natural thallium reference standard with precisions of better than 0.1 percent. The Tl/Tl absolute isotopic abundance ratio of the reference sample was found to be 2.38714 ± 0.00101, yielding an atomic weight of 204.38333 + 0.00018.
自1962年以来，铊的公认原子量一直保持在204.37±0.03。然而，在这种不确定度水平下，原子量成为高精度分析的一个限制因素。铊原子量的新质谱测定已完成。开发了一种高精度分析技术，以便将准确已知量的铊及其分离的同位素混合，以制备用于质谱仪校准的标准物质。这种分析技术包括重量法测定99.3%的铊为TlCrO。然后将可溶性铊等分并通过同位素稀释质谱法进行测定。在配制用于取出分析和校准样品的最终溶液之前，通过溶剂萃取和电沉积对分离的同位素进行纯化。开发了一种钨丝表面电离技术，用于精确测定铊的同位素丰度。该技术允许对分离的同位素、校准标准物质和天然铊参考标准物质进行同位素分析，精度优于0.1%。发现参考样品的Tl/Tl绝对同位素丰度比为2.38714±0.00101，得出原子量为204.38333 + 0.00018。
