• A Suggested Quality Assurance Protocol for Radiocarbon Dating Laboratories

      Long, Austin; Kalin, Robert M. (Department of Geosciences, The University of Arizona, 1990-01-01)
      The current intercomparison of data from 14C laboratories reveals significant variability among liquid scintillation laboratories, suggesting that identical samples submitted to different laboratories may yield values that differ by much more than expected on a purely statistical basis. Erroneous dates (recently corrected) by a well-established 14C laboratory give rise to further concern for quality 14C data. Thus, it is incumbent on each laboratory to develop and implement a quality assurance and control (QA/QC) program in order to ensure accuracy of results and to alert lab personnel to problems. Samples of pure materials (eg, benzene, cellulose) distributed by national or international standardizing groups are valuable, but are not representative of typical samples routinely run in most labs. Inevitably, 14C personnel take special care with intercomparison samples and data that "outsiders" will be scrutinizing and comparing. Here, we reiterate Stuiver and Pearson's (1986) concept of laboratory error multiplier (K-value) and make the case for internally-generated QA/QC programs. We recommend that an ongoing, internal, self-test QA/QC protocol, to be designed and approved at the next 14C conference, is the most practical and effective method of assuring quality of 14C laboratory data. Each laboratory would then be responsible for determining its error multiplier factor by performing analyses on one or more homogeneous batches of wood chips, cellulose or calcite. Laboratories would update these data as they see fit and make this information available in a standard format to all who use their data.
    • An Overview of Some Interlaboratory Studies

      Scott, E. Marian; Baxter, M. S.; Aitchison, T. C.; Harkness, D. D.; Cook, G. T. (Department of Geosciences, The University of Arizona, 1990-01-01)
      Many 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.
    • High-Precision Intercomparison at IsoTrace

      Beukens, Roelf P. (Department of Geosciences, The University of Arizona, 1990-01-01)
      I conducted a high-precision comparison at the 0.2% to 0.3% level with samples supplied by the radiocarbon laboratory of the Quaternary Research Center at the University of Washington (QRC). Four samples with ages ranging from modern to > 50,000 BP were dated in a blind test. The absence of cosmic-radiation background in AMS dating is a major advantage for dating samples > 35,000 BP. The reliability of AMS dates > 35,000 BP depends entirely on understanding the contamination processes. By comparing results with laboratories capable of sample enrichment, such as QRC, it is possible to identify and estimate the intrinsic 14C in the background samples as well as the contamination introduced by sample preparation.
    • International Collaborative Study: Structuring and Sample Preparation

      Cook, G. T.; Harkness, D. D.; Miller, B. F.; Scott, E. Marian; Baxter, M. S.; Aitchison, T. C. (Department of Geosciences, The University of Arizona, 1990-01-01)
      The 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 Problems Using Acetylene as Counting Gas

      Geyh, Mebus A. (Department of Geosciences, The University of Arizona, 1990-01-01)
      An investigation of inconsistent Hannover results in the International Collaborative Study (ICS) led to the conclusion that the main reason was contamination of the acetylene used as counting gas with recent and/or fossil carbon by the lithium used for its preparation. Despite the high level of purity of the lithium guaranteed by the producer and storage under argon in cans, different charges were partly covered with contemporary lithium carbonate and fossil oil sometimes was used to preserve the metal. Thorough cleaning of the surface of the lithium rods decreased the contamination but did not remove it entirely, which is evidenced in the wider scatter of the counting rates of various background gases than that of radiocarbon-free tank acetylene. As a result of the high risk of contamination with fossil and/or recent carbon from the acetylene counting gas, the high price of lithium, and the time-consuming preparation, the Hannover 14C Laboratory will use carbon dioxide instead of acetylene as counting gas in the future.
    • Radiocarbon Dating Reproducibility at the Museo de la Plata Radiocarbon Laboratory

      Figini, Anibal J.; Huarte, Roberto; Carbonari, Jorge (Department of Geosciences, The University of Arizona, 1990-01-01)
      We 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 Sources

      Pazdur, Mieczysław F.; Awsiuk, Romuald; Goslar, Tomasz; Pazdur, Anna (Department of Geosciences, The University of Arizona, 1990-01-01)
      Results 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.