The Journal of Research of the National Bureau of Standards reports NBS research and development in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement methodology, and the basic technology underlying standardization. Also included from time to time are survey articles on topics closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete citations to all recent NBS publications in NBS and non NBS media. Issued six times a year. Annual subscription: domestic $17.00; foreign $21.25. Single copy, $3.00 domestic: $3.75 foreign. Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Section B "Mathematical Sciences." NATIONAL BUREAU OF STANDARDS The National Bureau of Standards was established by an act of Congress on March 3, 1901 The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benetu. To this end, the Bureau conducts research and provides (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement I aboratory, the National Engineering Laboratory, and the Institute for Computer Sciences and Technology THE NATIONAL MEASUREMENT LABORATORY provides the national system of physical and chemical and materials measureinent, coordinates the system with measurement systems of other nations and furnishes essential services leading to accurate and uniform physical and chemical measurement throughout the Nation's scientific community, industry. and commerce, conducts materials research leading to improved methods of measurement, standards, and data on the properties of materials needed by industry, commerce, educational institutions, and Government, provides advisory and research services to other Government agencies, develops, produces, and distributes Standard Reference Materials, and provides calibration services The Laboratory consists of the following centers THE NATIONAL ENGINEERING LABORATORY provides technology and technical serVices to the public and private sectors to address national needs and to solve national problems, conducts research in engineering and applied science in support of these elforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service, develops engineering data and measurement capabilities, provides engineering measurement traceability services, develops test methods and proposes engineering standards and code changes, develops and proposes new engineering practices. and develops and improves mechanisms to transfer results of its research to the ultimate user The Laboratory consists of the following centers THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and economy in Government operations in accordance with Public Law 89-306 (40 U..SC_759). relevant Executive Orders, and other directives, carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards guidelines, and managing Federal participation in ADP voluntary standardization activities. provides scientific and technological advisory services and assistance to Federal agencies, and provides the technical foundation for computer related policies of the Federal Government The Institute consists of the following centers Programming Science and Technology - Computer Systems Engineering Headquarters and laboratories at Gaithersburg, MD unless otherwise noted, mailing address Washington DC 20234 Some divisions within the center are located at Boulder, CO 80303 U.S. DEPARTMENT OF COMMERCE Philip M. Klutznick, Secretary NATIONAL BUREAU OF STANDARDS Ernest Ambler, Director Order all publications from the Superintendent of Documents U.S. Government Printing Office, Washington, D.C. 20402 The Secretary of Commerce has determined that the publication of this periodical is necessary in the transaction of the public business required by law of this Department. Use of funds for printing this periodical has been approved by the Director of the Office of Management and Budget through June 30, 1981 Absolute Isotopic Abundance and the Atomic Weight of a Reference Sample of Thallium. L. P. Dunstan, J. W. Gramlich, I. L. Barnes, and W. C. Purdy . . . A Practical Test of the Air Density Equation in Standards Laboratories at Differing Altitude. R. R. Schoonover, R. S. Davis, R. G. Driver, and V. E. Bower .. JOURNAL OF RESEARCH of the National Bureau of Standards Absolute Isotopic Abundance and the Atomic Weight of a Reference Sample of Thallium L. P. Dunstan, J. W. Gramlich, I. L. Barnes, National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234 and W. C. Purdy McGill University, Montreal, Quebec, Canada August 8, 1979 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 203T1 and 205 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 the thallium as Tl2CrO. 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 205TI/203TI 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. Key words: Absolute ratios; atomic weight; isotopic abundance; reference standard; thallium; thallium 1. Introduction Since 1962, the Inorganic Analytical Research Division of he National Bureau of Standards has been conducting a ong term program of absolute isotopic abundance ratios nd atomic weight determinations using the mass spectro metric method. Previous atomic weight determinations inlude silver [1]', chlorine [2], copper [3], bromine [4], hromium [5], magnesium [6], lead [7], boron [8], rubidium 91, rhenium [10], silicon [11], potassium [12], and stronium [13]. The present work extends the study to thallium. The determination of the absolute isotopic abundance. and atomic weight of any element to a high level of accuracy requires the development of highly precise chemical assay gures in brackets indicate literature references at the end of this paper. and mass spectrometric procedures. The mass spectrometers used for the isotopic abundance measurements are calibrated for bias by using synthetic mixes of known isotopic composition, prepared from nearly pure separated isotopes. Extensive research [14] has demonstrated that this bias is due primarily to mass dependent isotopic fractionation and to a lesser degree to non-linearities in the measurement circuit. The measured biases are used to calculate a calibration factor which is then applied to the observed isotopic ratio to yield the absolute isotopic abundance ratio of a reference sample. The atomic weight of the sample can then be calculated by summing the product of the nuclidic masses reported by Wapstra and Bos [15] and the corresponding atom fractions of the individual isotopes. For more general applications it is necessary to establish the limits of variation in nature and high purity commercial samples. |