δ13C correction to AMS data: Values derived from AMS vs IRMS values

Abstract 14 C data from AMS measurements need to be corrected for isotopic fractionation using the δ 13 C values. Whether to use the δ 13 C values obtained from the AMS stable isotope measurements as they reflect any additional isotopic fractionation or to use Isotope Ratio Mass Spectrometry (IRMS) δ 13 C values due to their high precision and accuracy is the question that finds supporting arguments on either side. In this article, we present our observations by examining the data from the two AMS units at CAIS: a 500 kV CAMS and a 250 kV SSAMS. Offline δ 13 C values resulted in the fraction modern values of standards that are in excellent agreement with the consensus values for both machines. In the case of SSAMS, beyond a certain value, the high energy 12 C beam current is found suppressed and skewed the δ 13 C. In general, the 13 C + / 12 C + ratio on SSAMS is more susceptible to beam current variation than it is on CAMS. But 14 C + / 13 C + ratios are nearly independent of beam current variations on both machines. Our analysis shows that online δ 13 C works well for CAMS, but results are not satisfactory for SSAMS. But we have found that using pre-acceleration 12 C − instead of post-acceleration 12 C + for stable isotope ratio calculation substantially improves the online δ 13 C for SSAMS.