Reliability of 129I/I ratios produced from small sample masses

We report here results of a study on the influence of sample mass on isotope ratios and accuracy of 129I/I determinations in the AMS system at PRIME Lab, Purdue U. Iodine from four samples, two coal-bed methane brines, one surface water sample and a blank, was extracted and precipitated as AgI, following established methods. The resulting samples were subdivided into four sequences of targets, containing between 1.5 and 0.1 mg of AgI. Beam currents for samples with masses above 0.3 mg did not show a dependence on mass, but fell off strongly for smaller masses. The resulting isotope ratios were within the instrumental error limits and did not vary with sample mass, but accuracy decreased for samples with masses below 0.3 mg. The results demonstrate that the presence of 5000 129I atoms in the target is sufficient for making a successful AMS determination, a level considerably lower than for other methods used in mass spectrometry. The ability of producing reliable 129I/I ratios from targets as small as 0.1 mg of AgI enlarges considerably the range of applications possible for this isotopic system.