Anal. Chem. 1995, 67, 1437-1441
Determination of the Isotopic Compositions of Samarium and Gadolinium by Thermal Ionization Mass Spectrometry H. Hidaka* and M. Ebihara Department of Chemisty, Tokyo Metropolitan University, Minami-Osawa 1- 1, Hachioji, Tokyo 192-03, Japan
M. Shima Faculty of Science and Engineering, National Science Museum, Hyakunin-cho 3-23- 1, Shinjuku, Tokyo 169, Japan
Because of the large neutron capture cross sections for 149Sm,155Gd,and 15'Gd, accurate and sensitive isotopic analyses of samarium and gadolinium can be used to investigate natural neutron irradiation exposure. In this paper, chemical separation procedures and mass spectrometric methods for the precise determination of samarium and gadolinium isotopic compositions are described. Isotopic ratios other than 144Sm/152Sm, 152Gd/ 160Gd,and lMGd/lGOGdforbothsamariumandgadoli.nium in commercial reagents, d c i a l l y neutron-irradiated reagents, and geological reference rocks are quantiiied within 0.001%precision at the 95%coddence level. The present method can detect effects caused by timeintegrated neutron flux of 1.3 x 1014neutrons/cm2 in the environmental samples. Isotopic deviations of 149Sm,IsoSm, Is5Gd, 156Gd,Is7Gd, and '%Gd in g e e and cosmochemical materials can be useful indicators to monitor time-integrated neutron flux in environment. Because the nuclides 149Sm,Is5Gd,and Is7Gdpossess extraordinary large neutron capture cross sections, the relative isotopic abundances can be altered by intensive neutron irradiation. The net effect is to enrich the lS0Sm,156Gd,and '%Gd. Thermal ionization mass spectrometry is known to be the best method currently available to determine accurate isotopic compositions and to detect small deviations of isotopic ratios. There is a significant possibility that Sm and Gd isotopic variations in extraterrestrial materials will be present due to cosmic ray-induced neutrons. Following the Apollo missions in the early 1970s, isotopic studies of Sm and Gd in lunar soils and some meteorites were attempted to estimate the integrated neutron flux in The analytical precision at that time was 0.1-0.05% (2a mean of standard error), which was sufficient for detection of the isotopic shifts derived from neutron irradiation at the level of 3 x 1015n e u t r o n s / ~ m ~ .Two ~ - ~ decades later, instrumental developments in mass spectrometry and technical improvements in separation chemistry now enable isotopic abundances to be determined more precisely than before. High-precision isotopic abundances have since been determined (1) Eugster, 0.;Tera, F.; Burnett, D. S.; Wasserburg, G. J.]. Geophp. Res. 1970, 75, 2753-2768. (2) Lugmair, G.W.; Marti, K Eadh Planet. Sci. Lett. 1971,13, 32-42. (3) Russ, G. P.; Burnett, D. S.; Lingenfelter, R E.; Wasserburg, G. J. Earth Planet. SC~.Lett. 1971,13, 53-60.
0003-2700/95/0367-1437$9.00/0 0 1995 American Chemical Society
for some elements by use of new and improved technique^.^-'^ For example, it is now possible for Sr, Ce, and Nd isotopic ratios to be measured with a precision
