Ultra-trace analysis of 41Ca in urine by accelerator mass spectrometry: An inter-laboratory comparison

Abstract

A 41Ca interlaboratory comparison between Lawrence Livermore National Laboratory (LLNL) and the Purdue Rare Isotope Laboratory (PRIME Lab) has been completed. Analysis of the ratios assayed by accelerator mass spectrometry (AMS) shows that there is no statistically significant difference in the ratios. Further, Bayesian analysis shows that the uncertainties reported by both facilities are correct with the possibility of a slight under-estimation by one laboratory. Finally, the chemistry procedures used by the two facilities to produce CaF2 for the cesium sputter ion source are robust and don't yield any significant differences in the final result.

A Bayesian approach to an interlaboratory comparison

“…To use equation (13), we plug in the [ x i , σ Li ],[ y i , σ Pi ] from the 47 samples [4]. These numbers are all dimensionless, and we will use units of 10 −10 for all of them; since the 41 Ca/Ca ratios are so small, it is handy to omit the 10 −10 factor.…”

“…We enlarge on the Bayesian discussion in our previous paper[4], again following the approach presented by Dose [10] (see also [8, 9]). A closely related problem arises when a group of independent laboratories measures the same quantity, and we want to combine the results to determine the best estimate.…”

“…These purposes range from routine quality control to investigating problems arising in challenging new techniques. In a recent paper [4] we discussed such a comparison, in which PRIME Lab (the Purdue Rare Isotope Measurement Laboratory) and LLNL (Lawrence Livermore National Laboratory) performed 41 Ca accelerator mass spectrometry (AMS) [5] measurements on a series of samples. These samples were not special, standard material; rather they were unknowns that had already been chemically prepared and measured at PRIME Lab for other studies [6, 7].…”

“…Thus, the results are representative of the actual quality of the measurements being done at both labs. In our paper [4], the results from the two labs were compared in a variety of ways; one particular method was grounded in a Bayesian approach. In this new paper we focus on the Bayesian approach [8, 9, 10], expanding on our original discussion.…”

“…(The data are presented in [4]. ) PRIME Lab and LLNL separately measured an aliquot of each sample, and reported an R value and an uncertainty for each sample.…”