52. Precise Determination of the lsotopic Composition of Tellurium by Negative Thermal lonization Mass Spectrometry

Sarata Kumar Sahoo, Hidenori Yonehara, Katsumi Kurotaki, Shigekazu Yoneda¹ and Akimasa Masuda² (¹National Science Museum, ²University of Tokyo)

Keywords: tellurium, isotoptc composttton, negattve thermal ionizatton mass spectrometry

Precise determination of the isotoptc composutnon of tellurium is of great interest in various fields such as geochemistry, cosmochemistry and environmental sciences. The relative abundance of Te in natural samples is not well documented. In fact, nature has provided three early solar system chronometers based upon refractory-siderophile (Ru, Pd) and volatile-chalcophile (Te) nuclide yields from ²⁴⁴Pu spontaneous fisston.

Te has eight stable isotopes, ¹²⁰Te (O.O96%) , ¹²²Te (2.6O%), ¹²³Te (O.9O8%), ¹²⁴Te (4.816%), ¹²⁵Te (7.139%), ¹²⁶Te (18.952%), ¹²⁸Te (31.687%) and ¹³⁰Te (33.799%) and it has a relatively high elemental abundance owing to its even atomic number (Z=52) and the relatively high abundance of its r process isotopes. Three of the isotopes are s-only ¹²²Te and ¹²⁴Te being shielded from the r-process by heavy tin isotopes, and ¹²³Te by ¹²³Sb. Te has a pprocess nuclide ¹²⁰Te and two r-only isotopes ¹²⁸Te and ¹³⁰Te having enormously long half-lives of more than TO²⁴y and 10²¹y. ¹²³Te, ¹²⁸Te and ¹³⁰Te isotopes are radioactive.

The development of high precision, high ion-yield, negative thermal tonuzatton mass spectrometry (NTIMS) techniques for analysis at low quantity levels is considered important to produce reliable data in terrestrial and extraterrestrial samples. Isotopic analysis by NTIMS has been developed by K.G. Heumann and his coworkers. For non-metals and refractory metals with higher first ionizatuon potential (>7eV) such as Mo, Te, Sn and W, the precision of TIMS measurements is usually restricted by low ionization efficiencies. Recently NTIMS has been widely used to determine the isotopic composition of refractory metals and the precision of the data has been improved considerably compared to that achieved by conventional PTIMS This technique takes advantage of the much higher ionization efficiencies at lower temperature for negatively charged oxide ions relative to positively charged ions for non-metals and refractory metals, hence it is capable of making high precision isotope measurements.

For the NTIMS a VG Sector 54-30 thermal ionization mass spectrometer equipped with 9 Faraday cup collectors, a Daly-ion counting system, WARP (wide aperture retardation potential) filter and double filament ion source were used (Fig 17). The Pt filament was 0.03 mm thick and 0.75 mm wide. Twenty g Ba as a solution of Ba (OH)₂ were deposited on the ionization filament and dried at a 0.6 A filament current in air. Addition of Ba reduces the electron work function of the Pt filamernt and promoles the production of negative thermal ions. 0.51.0 p g Te as H₂ TeO₃ solution were deposited on the evaporation filament. After loading, the filaments were introduced into the mass spectrometer. Intense inn beams of negatively charged species of Te (Te^-) were observed at 850-95OC under an acceler ating voltage of -6kV (ionization filament current 1.4-1.8 A, evaporation filament current 0.7-0.9 A) A mass spectrum obtained by NTIMS is shown in Fig 18. This method is free from oxygen correction during measurement.

In the past, isotope ratios of Te were usually determined by electron impact ionization or by postttve TIMS. The samples are loaded in the later case by electroplating which is more time consuming and complicated. Since ion currents are low with those techniques, isotope ratios are measured by using a secondary electron multiplier detection system. The electron multiplier method introduces a mass discrimination into the isotopic abundances, since the signal from a light isotope is enhanced with respect to the signal from heavier isotopes because the see ondary electron yield at the first dynode is velocity dependent. This mass discrimination is approxi mately proportional to the square root of the mass ratio. Therefore to overcome this, the most suitable method is to measure the isotopic abundances of Te using Faraday cup mode of detection. This does not eliminate all the sources of mass spectrometer bias, but it does account for the major sources of mass discrimination which are introduced by the electron multiplier detection system. Table 6 summarizes resuits obtained by other researchers who have measured the isotope ratios of Te. Since the present TIMS is equipped with the WARP filter, it is possible to measure the abundance of ¹²⁰Te, which is an improvement over the earlier NTIMS technique. The defer minedratios 120/130 122/130 123/130, 124/130 125/ 130, 126/130 and 128/ 130 are within the preci sion range of 0.002 to 0.008j (relative standard deviation)

Fig.17. Schematic diagram of a double filament thermal ion source.

Fig.18. Mass spectrum of tellurium obtained by NTIMS using a single collector.

Table 6. Comparison of Tellurium Isotopic Composition