Anal. Chem. 2005, 77, 7098-7099
Correspondence
Mass Spectrometric Determination of Ratio with Improved Precision
234
U/238U
Ejaz ur Rehman,*,† Riffat Naheed,† and Shakeel ur Rehman‡
Applied Chemistry Laboratories, PINSTECH, P.O. Nilore, Islamabad, Pakistan, and Department of Physics and Applied Mathematics, PIEAS, P.O. Nilore, Islamabad, Pakistan
A new mass spectrometric method is proposed for measurement of 234U/238U ratio with a single Daly electron multiplier detector using the general peak jump method. The method is based on precise measurement of the 235U/ 238U ratio and 234U/235U ratio, which are used to calculate the 234U/238U ratio using the equation 234U/238U ) 235U/ 238U × 234U/235U. The results show a significant improvement, i.e., more than 35 times better precision in measuring the 234U/238U ratio with this method (σ ) 2.9 × 10-8) as compared to direct measurement of 234U/238U (σ ) 1.1 × 10-6). The method widens the applicability of the single collector system, and it will potentially be helpful to improve the precision in the case of the static multicollector system also. Natural uranium consists mainly of 234U, 235U, and 238U isotopes with their approximate atom percentages, 0.0057, 0.7235, and 99.2708, respectively. Analysis of variation in 234U isotopic composition is a powerful tool in geological, environmental, and nuclear studies.1-3 The ratio of 234U/238U has particularly been successfully applied to find the age of marine sediments, ferromanganese nodules, and corals.4-7 Depending upon the accuracy and precision of 234U/238U ratio analysis, the age ranging from days to ∼106 years, could be determined. Therefore, a number of attempts have been made to improve the precision in thermal ionization mass spectrometric determination of the 234U/238U ratio.4,8 As an example, ionization activators such as silicic aciddilute phosphoric acid mixture have been used to stabilize uranium emissions.9 * To whom correspondence should be addressed. E-mail: erehman@ gmail.com. † PINSTECH. ‡ PIEAS. (1) Sahoo, S. K.; Masooda, A. Anal. Chim. Acta 1998, 370, 215-220. (2) Green, L. W.; Leppinen, J. J.; Elliot, N. L. Anal. Chem. 1988, 60, 34-37. (3) Sahoo, S. K.; Masooda, A. Analyst 1995, 120, 335-339. (4) Edwards, R. L.; Chen, J. H.; Wasserburg, G. J. Earth Planet Sci. Lett. 1986, 81, 175-192. (5) Moore, J. G.; Clague, D. A.; Ludwig, K. R.; Mark, R. K. J. Volcanol. Geotherm. Res. 1990, 42, 273-284. (6) Handerson, G. M.; Burton, K. W. Earth Planet. Sci. Lett. 1999, 170, 169179. (7) Stirling, C. H.; Esat, T. M.; McCulloch, M. T.; Lambeck, K. Earth Planet. Sci. Lett. 1995, 135, 115. (8) Chen, J. H.; Edwards, R. L.; Wasserburg, G. J. Earth Planet. Sci. Lett. 1986, 80, 241-251.
7098 Analytical Chemistry, Vol. 77, No. 21, November 1, 2005
This report describes a new approach for determination of ratio by single Daly electron multiplier detector with more than 35 times precision as compared to the existing method. For all isotopic analyses, 1 µg of our laboratory reference natural uranium sample in nitrate form was loaded on one of the lateral filaments of triple rhenium filament assemblies, supplied by Cathedon (Cambridge, U.K.). The triple rhenium filament assemblies were baked at 4.0 A for central and 2.0 A for lateral filaments for ∼2 h under vacuum (
