Long-lived cosmogenic nuclides such as 36Cl and 129I have found increasing applications for tracing and dating of fluids, as, for example, in subduction zones, gas hydrate occurrences and geothermal systems. Limiting the range of applications is the amount of sample necessary for isotope determinations using AMS, especially for 129I applications. The typical sample mass used for isotope determinations is around 1 mg for nuclides such as 36Cl and 129I. Because the required sample mass puts restrictions on the type of measurement possible, it would be highly desirable if the minimum mass for reliable measurements could be lowered. We report here results on the potential influence of sample mass on isotope ratio and accuracy on AMS determinations. AMS targets (AgI) were prepared from four samples and then subdivided into individual targets with masses decreasing from 1.5 to 0.1 mg, i.e. containing between 0.75 and 0.05 mg of iodine. The measured 129I ratios of the source materials varied between 1500 and 20x10-15. While currents of 127I were reasonably constant for samples with AgI masses above 0.3 mg, a clear decrease in currents was observed for targets below that mass. Isotope ratios were, however, not considerably influenced by the decrease in mass, although it affected the level of accuracy of the results. The results indicate that samples as small as 0.1 mg of AgI (i.e. 0.05 mg of I) can produce reliable 129I/I ratios, even for samples with ratios as low as in the blank material (129I/I = 20x10-15) used for these tests. Based on these results, a minimum of 5000 atoms is sufficient to make a successful AMS determination, a level considerably lower than for any other method used in mass spectrometry.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)