PRECISE MEASUREMENT OF THE ISOTOPIC COMPOSITION OF CERIUM AND ITS ATOMIC-WEIGHT

A procedure for the determination of the cerium isotopic ratio by ther mal ionization mass spectrometry of the Ce+ ion has been investigated. The intensity of the Ce+ ion emitted from Ce(NO3)(3) was considerably enhanced by the addition of graphite to the filament substrate during loading. Interferences from the impurities of barium and lanthanum ca n be overlooked because the Ba+ peak disappeared at the normal tempera ture for emitting the Ce+ ion and the La-138(+) intensity was too weak (10(-17)A). The interference of Nd-142 With Ce-142 can be calibrated by using a Nd-142/Nd-143 ratio of 2.2274. The Ce-136/Ce-140, Ce-138/Ce -140 and Ce-142/Ce-140 ratios in five chemical reagents with different origins have been measured as 0.002122 +/- 0.000004, 0.002865 +/- 0.0 00005 and 0.125201 +/- 0.000055 (SD) in average value, corresponding t o the percentage abundances of 0.1878(4), 0.2535(5), 88.4808(50) and 1 1.0779(44) for Ce-136, Ce-138, Ce-140 and Ce-142, respectively. Based on these ratios, the atomic weight of cerium was calculated to be 140. 1148(1) (SD), which is more precise than the cerium atomic weight of 1 40.115(4), recommended by IUPAC.