Is the Consensus Value of ANU Sucrose (IAEA C-6) Too High?

Abstract

ABSTRACT. Primary and secondary standards are essential in radiocarbon analyses for the purpose of reporting and comparing data among laboratories, as well as for internal laboratory data quality control. ANU sucrose is one of the IAEA-certified 14 C standards (C-6) with a consensus value of 1.5061 ± 0.0011 fraction modern (Fm). All of our measurements of ANU sucrose (n = 351) as a secondary standard over the last 7 yr result in an average value of 1.5016 ± 0.0005 Fm (2- standard error). After applying the same outlier tests used for IAEA reference standards, a weighted average value of 1.5016 ± 0.0002 Fm (n = 294) was calculated. This value is significantly lower than the IAEA C-6 consensus value (t test with unequal variance; p=0.023). In contrast, our measurements of other secondary standards over the same time period are in excellent agreement with their respective consensus values. Since ANU is the only secondary standard measured in our lab that does not agree with the consensus values, and we have measured a larger number analyses compared to what went into the definition of the consensus value, we suggest that the consensus value of ANU sucrose might be too high by ~0.0045 ± 0.0011 Fm. Given that some labs routinely use ANU sucrose as a primary standard, our results suggest that revisiting the consensus value of ANU sucrose may be necessary.

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“…All of our measurements of ANU sucrose as a secondary standard over the last 3 yr result in an average value of 150.33 ± 0.05 pMC. This value is significantly lower than the IAEA C-6 standard value of 150.61 ± 0.11 (Rozanski et al 1992) but statistically indistinguishable from the consensus value (with Waikato/Hogg data removed) of 150.23 ± 0.13 pMC (calculated from data published in Xu et al 2010: Table 2, p 872).…”

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“…For this reason ANU sucrose is of particular interest, since it is relatively enriched in 14 C relative to both recognized oxalic acid standards. We report results similar to other AMS laboratories [8], which find that repeated measurements of ANU Sucrose deviate significantly from the ''accepted'' value. Fig.…”

“…The 95% confidence intervals indicate there is no evidence of significant changes over time, and the value is probably compatible with the recent consensus value of 1.5017 [8]. Without considering the variation in precision and AMS changeover, a significant time trend (p = 0.02; red) could be errantly reported.…”

“…A change is therefore apparent from early EN-Tandem data, but not from later higher precision EN-Tandem data (dashed region). Both later EN-Tandem data and XCAMS data appear broadly consistent with the proposed new consensus value of 1.5015 [8]. The downward trend on newer, more precise AMS systems, including XCAMS, is of concern and perhaps deserves further investigation.…”

“…The downward trend on newer, more precise AMS systems, including XCAMS, is of concern and perhaps deserves further investigation. Recommended steps in this investigation, and of broader relevance to ensuring stable calibration to primary standards at precisions <0.2%, include running OX-I, OX-II and ANU sucrose as a suite [8], calibration of detector dead-time corrections, and improved assessment of process backgrounds.…”