This statement applies only to samples analyzed as solid graphite produced at the National Ocean Sciences AMS (NOSAMS) facility. It does not apply to samples analyzed on the gas ion source. Graphite is presently the most common target material.
An AMS radiocarbon measurement determines the ratio of 14C to 12C in an unknown sample relative to the known ratio in concurrently measured standards. The primary standard for 14C measurements is NBS Oxalic Acid I (NIST-SRM-4990). Every group of samples processed includes an appropriate blank, analyzed concurrently. Process blank materials include but are not limited to Carrara marble (IAEA C-1) and Icelandic Doublespar (Third International Radiocarbon Intercomparison F) for inorganic carbon and gas samples; acetanilide (CE Elantech) for organic carbon samples; 14C-free groundwater for dissolved inorganic carbon samples; and glycine (Sigma Aldrich) dissolved in DOC-free water for dissolved organic carbon samples.
Fraction modern (Fm) is the deviation of a sample’s radiocarbon content from that of the modern standard. Modern is defined as 95% of the radiocarbon concentration of NBS Oxalic Acid I normalized to δ13CVPDB = -19‰ (Olsson, 1970). An isotopic correction is made to normalize the sample result to a δ13CVPDB value of -25‰, assuming a quadratic mass fractionation dependency. This correction is made using 13C/12C ratios measured on the AMS system during radiocarbon analysis. These AMS 13C/12C ratios are not reported. Post-analysis stable isotopic corrections are neither necessary nor appropriate for reported 14C results. Reported δ13C values are measured on a split of sample CO2 using a dedicated stable isotope ratio mass spectrometer.
The 14C atoms contained in a sample are directly counted using the AMS method. Accordingly, we calculate an internal statistical error using the total number of 14C counts measured for each target (internal error=1/√n, where n is the number of 14C counts). An external error is calculated from the repeatability of multiple measurements of a given cathode over the course of a run (external error = standard deviation from the mean/√N, where N is the number of determinations). The final reported error is the larger of the internal or external error, propagated with errors from the normalizing standards and blank subtraction.
It should be noted that the reported error is an estimate of the precision (repeatability) of measurement for a single sample. Due to variability in sample homogeneity, sample collection, and sample processing, the variability of replicate samples (reproducibility) is generally greater than the reported error for a single sample. A total measurement error can be estimated by adding in quadrature the reported error with this extra variability, or added variance:
where is the total estimated error, is the reported error, is the measured Fraction Modern, and is the appropriate added variance. At NOSAMS, added variance is determined by pooling differences of measurements of secondary standards from consensus values of those standards. For calendar year 2016, the estimated added variance for samples of the process type OC (Organic Carbon) or HY (Hydrolysis) is 2.6‰ for samples containing > 100 ug C. For other sample types, e.g. sample submitted as gas samples, dissolved inorganic or organic samples, samples with mass < 100 ug C, or reconnaissance gas ion source samples, an estimated added variance has not been determined. For water or dissolved inorganic carbon (DIC) samples, for which no internationally accepted secondary standards exist, we note that analyses of shipboard duplicates, collected on every cruise, demonstrate a pooled standard deviation of 3.0‰. This would indicate that the added variance for these samples is similar to other types of sample measured at NOSAMS. While added variance may give a better estimate of the total error, the best way to determine total experimental error is by replicate sample analyses. If you are working near the limits of AMS precision, or have questions regarding error estimates, please consult with us.
Radiocarbon ages are calculated using the Libby half-life of 5568 years according to the convention outlined by Stuiver and Polach (1977) and Stuiver (1980). We do not report ages with reservoir corrections applied or ages calibrated to calendar year. If a sample collection date is specified on the submittal form, the Δ14C activity normalized to 1950 is also reported, i.e. the activity or Δ14C of the sample is corrected to account for the decay between collection (or death) and the time of measurement.
When publishing results of samples analyzed at the NOSAMS facility, we ask that an accession number (i.e. the reported OS-#####) be listed with the result along with any subsequently made corrections. Published results should acknowledge support from NSF by including the NSF Cooperative Agreement number, OCE-1239667. We encourage you to email references to your publications to email@example.com for inclusion in our web-accessible database of NOSAMS research related publications (http://nosamsresearch.whoi.edu/).
Olsson, I.U., 1970. The use of Oxalic acid as a Standard. In I.U. Olsson, ed., Radiocarbon Variations and Absolute Chronology, Nobel Symposium, 12th Proc., John Wiley & Sons, New York, p. 17.
Stuiver, M. and Polach, H.A., 1977. Discussion: Reporting of 14C data. Radiocarbon, 19:355-363.
Stuiver, M., 1980.Workshop on 14C data reporting. Radiocarbon, 22:964-966.