Homogeneity evaluation of Chinese sugar carbon (CSC) standard for AMS 14C measurement

Abstract

The Chinese national radiocarbon standard Chinese Sugar Carbon (CSC), or often called the “Chinese Sucrose Charcoal,” was investigated for its homogeneity and its suitability as a radiocarbon standard for Accelerator Mass Spectrometer (AMS). We randomly chose ten new CSC bottles and two opened bottles and measured their carbon isotope compositions. δ13C results obtained by Isotope Ratio Mass Spectrometer (IRMS) showed relatively large variation: from −15.9 to −22.2‰ with an average of −19.0±1.2‰ (n=49), indicating CSC grains in the mg range are not isotopically homogeneous. However, 14C results of CSC, expressed as Fraction Modern (FM), are well constrained to 2.4‰ after fractionation correction to −25‰. Measurements from all 12 bottles gave an average FM value of 1.3533±0.0034 (n=55). This value is about 7‰ higher that the FM value of 1.346±0.002, converted from the originally reported R value of 1.362±0.002 (n=14, with δ13C of −19.32, n=9) [1,2]. The R value of 1.362 is the ratio of 14C activity of CSC to 95% of OXI, not an FM value. The online simultaneous AMS 13C/12C measurement that allows for correction of all isotopic fractionation, including manufacture, graphitization and machine induced fractionation, is the key for obtaining consistent 14C measurements on this standard. Without further homogenization, CSC may not be a good choice of standard for an AMS that does not have the capability of measuring online AMS δ13C. Further study is needed to see if mixing/grinding can reduce the inhomogeneity. Re-measurement is suggested with more laboratories involved in order to get a better defined consensus value.