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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.
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 Reporting of ages and/or activities follows the convention outlined by Stuiver and Polach (1977) and Stuiver (1980).
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.
However, limiting ages or "backgrounds" are also determined by process blanks which correspond to the method used to extract the carbon from the sample.
» NOSAMS General Statement of C from contamination introduced during chemical preparation, collection or handling.
At the present time, for a 1 milligram sample of graphite, this limiting age is about ten half-lives, or 60,000 years, if set only by the sample size.
This correction is performed as follows: $$Fm_ = Fm_ ( Fm_ - Fm_b)\frac$$ Where \(M\) is sample mass, and \(M_b\) and \(Fm_b\) are the mass and Fm of the blank.