Samples for radiocarbon dating received at the lab are immediately subjected to substantial quality control measures starting with verifying that the submittal documentation matches the actual samples received. Redundant crosschecks are performed by multiple persons throughout the log-in, cataloging, pretreatment, chemical conversion, AMS loading and detection, data reduction, calculation, and reporting.
Every sample analyzed is viewed by at least seven persons prior to reporting results through different stages of the analysis. Each person cross-checks the work and labeling of the previous person. Each stage of the analysis has required quality control measures to ensure proper and accurate handling and analysis of the sample.
Calculation and reporting of results are performed using an in-house software with built-in multiple cross-checks for each result. A broad system of cross-checks comprises 300 pages of quality control commitment outlined in Beta Analytic’s ISO-17025 quality assurance documentation.
Daily monitoring of instrumentation and chemical purity is also performed in addition to extensive computer cross-checks of statistical analyses and final age calculations.
Sequential injection of Carbon-13 and Carbon-12 provides for measurement of Carbon-13/Carbon-12 ratio within the accelerator mass spectrometer (AMS), necessary for accurate total fractionation correction without relying upon relative measures and assumptions (something some older machines are limited to). Sequential injection of Carbon-14, Carbon-13, and Carbon-12 allow for calculation of age both using Carbon-14/Carbon-12 and Carbon-14/Carbon-13 ratios. Simultaneous accumulation of the Carbon-14/Carbon-12, Carbon-14/Carbon-13, and Carbon-13/ Carbon-12 ratios ensure ongoing quality control during the detection; the calculation of each provides three different measures to ensure isotope pathway is stable during the analysis.
In addition to measuring the Carbon-13/Carbon-12 within the AMS (correction for total fractionation to derive the most accurate conventional radiocarbon age/pMC), the sample d13C ratio is measured in an isotope ratio mass spectrometer (IRMS). Beta Analytic has access to four in-house Thermo-Finnegan Delta IRMS to ensure redundancy and constant throughput. These same IRMS provide sample d15N on bones and organic materials, d18O on carbonates, and dD/d18O on water.
Multiple parameters are tested for each sample before accepting the data. This includes, but is not limited to, cathode current, extractor current, cesium focus current, high-energy Carbon-14/Carbon-12, high-energy Carbon-14/Carbon-13 ratio, low-energy Carbon-13/ Carbon-12 ratio, low-energy Carbon-12 current, low-energy Carbon-13/ Carbon-12 current, and gated Carbon-14 counts. Once accepted, ratios between and within the Oxalic Acid modern standard are utilized to calculate a fraction of modern value for the unknown.
Why does quality assurance matter in radiocarbon dating?
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Beta Analytic clients receive Quality Assurance reports with their results. The QA report provides the results of reference materials used to validate radiocarbon analyses prior to reporting. Known value reference materials were analyzed quasi-simultaneously with the unknowns. Results are reported as expected values vs measured values. Reported values are calculated relative to NIST SRM-4990C and corrected for isotopic fractionation. Results are reported using the direct analytical measure percent modern carbon (pMC) with one relative standard deviation.