Browsing Radiocarbon, Volume 32, Number 3 (1990) by Subjects
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An Overview of Some Interlaboratory StudiesMany interlaboratory studies have been made in the 14C community at irregular intervals over the past ten years. At times, the results from these studies have been contentious, mostly because of the lack of consistency in their findings. The importance of regular exercises has become particularly acute due to the large number of operating laboratories and the diversity of their methodologies. Hence, we briefly review the studies that have been made in the 1980s, focusing on those in which our laboratories participated. These include the 14C Interlaboratory Comparison in the UK (Otlet et al 1980), the International Comparison (ISG 1982, 1983) and the first two parts of the current International Collaborative Program (Scott et al 1989a, b). The development of each study, its findings and shortcomings, are highlighted in order to assess the concordance of the conclusions.
International Collaborative Study: Structuring and Sample PreparationThe success of any intercomparison exercise depends largely on participation and cooperation of a sufficient number of laboratories and the selection of a suitable suite of samples. Unless the latter is satisfactorily devised, the former cannot be guaranteed. The hierarchical nature of this study has necessarily resulted in a far more comprehensive set of sample types than has previously been employed. The exercise was structured to satisfy the following criteria: 1) to enable the participating laboratories to assess the experimental precision and accuracy of the component stages of the dating process; 2) samples should be typical of those routinely dated by the laboratories. This takes on a particular significance in Stage 1 where they should resemble as closely as possible the counting medium; 3) an objective statistical analysis of the results at each component stage of the study.
Radiocarbon Dating Reproducibility at the Museo de la Plata Radiocarbon LaboratoryWe discuss here the variability, for our laboratory, in counting for radiocarbon dating of replicate measurements of background and secondary modern standard, duplicate measurements of samples provided by the International Collaborative Study, and replicate measurements of the dilution of the 14C-labeled benzene standard. The variability in the measurements of the International Collaborative Study samples suggest the existence of systematic bias.
Systematic Biases in Results of the International Collaborative Study and Their Probable SourcesResults of the International Collaborative Study show an unexpectedly large scatter of individual dates as well as systematic biases. Very high values of linear correlation coefficients are observed for all results of Stage 2 and for benzene samples of Stage 1. We observed moderate correlations for carbonate samples and the lowest for natural samples of wood and peat of Stage 3. The correlation is practically negligible among results obtained in different stages. The probable reasons for such effects are seen in medium-term changes in the calibration of the counting systems.