How Reliable Are Radiocarbon Laboratories? A Report on the Fourth International Radiocarbon Inter-Comparison (FIRI) (1998-2001). (Method)

Article excerpt

Rationale

The most recent radiocarbon inter-comparison exercise (FIRI), completed in 2001, was also the most extensive so far, with 85 laboratories participating. The study was designed firstly to assess the comparability of the results from different laboratories and then to quantify the extent and possible causes of any inter-laboratory variation. Radiocarbon dating is universally employed as a dating tool in archaeology, but there is an inevitable diversity of experimental approaches within radiocarbon dating facilities and in this situation the issue of comparability of results amongst laboratories becomes paramount. In keeping with the principles of analytical science, radiocarbon laboratories have always been conscious of the importance of accuracy and precision for their reported results i.e. the ethos of analytical quality control (QC) which in turn is the foundation for the wider concept of quality assurance (QA). The care and effort given to establishing and maintaining primary standards and reference materials exemplify this concern for good quality management within the radiocarbon community.

As early as 1989, Long and Kalin (1990) stressed that it was incumbent on individual radiocarbon laboratories to engage in a formal programme of quality assurance (QA) while Polach (1989) noted that the opportunity for internal checking by individual laboratories in routine [sup.14]C measurement was hampered by a lack of suitable quality control (QC) and reference materials. The work reported here describes ongoing international efforts by means of a laboratory inter-comparison to assure users of laboratory quality and comparability of measurements and to provide suitable quality control and reference materials. This work builds on the previous laboratory inter-comparisons that have taken place over the last 20 years (ISG, 1982; Scott et al, 1991; Rozanski et al, 1992; Gulliksen & Scott, 1995).

A substantial effort has been made by the [sup.14]C community to develop and apply both internal and external QA procedures. FIRI provides a part of these procedures in the form of an independent check of laboratory performance. However, it only provides a spot check of operational performance at the time it was carried out and does not measure consistent performance over a period of time and so should not form the basis of a `league table of laboratory performance'. This is why the FIRI results are published without laboratory attribution.

Objectives

The specific objectives of the Fourth International Radiocarbon Inter-comparison (FIRI) (Bryant et al, 2000) were to provide:

* An unambiguous demonstration of the degree of consistency or otherwise among the results obtained, on a routine basis, from different laboratories. This information is crucial for both laboratories and procurers (researchers and funding agencies).

* Quantification of the extent of, and identification of, the possible causes of, any interlaboratory variation.

* Direct assessment of the comparability of liquid scintillation counting (LSC), gas proportional counting (GPC) and accelerator mass spectrometry (AMS) techniques.

* Creation of suitable, well-characterized quality control and reference materials.

* Assurance of trace-ability of the measurements and provision of an independent check on laboratory performance.

These objectives are directly related to analytical quality control by focusing on experimental accuracy, precision and reproducibility as indices for the assessment and inter-comparison of laboratory performance. Evidence of an acceptable level of analytical quality control is the essential precursor for overall quality assurance. The basic methodology employed in the inter-comparison was to invite laboratories to date a series of reference samples so as to compare their performances and the performance of different radiocarbon methods.

Selection, preparation and testing of control samples

The selection and preparation of large samples of homogeneous [sup. …