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kccams/uci facility
organic-carbon
c-14
fractionation
contamination
delta-c-14
california
system
Abstract:
The Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS/UCI) has developed protocols for analyzing radiocarbon in samples as small as similar to 0.001 mg of carbon (C). Mass-balance background corrections for modem and C-14-dead carbon contamination (MC and DC, respectively) can be assessed by measuring C-14-free and modern standards, respectively, using the same sample processing techniques that are applied to unknown samples. This approach can be validated by measuring secondary standards of similar size and C-14 composition to the unknown samples. Ordinary sample processing (such as ABA or leaching pretreatment, combustion/graphitization, and handling) introduces MC contamination of similar to 0.6 +/- 0.3 mu g C, while DC is similar to 0.3 +/- 0.15 mu g C. Today, the laboratory routinely analyzes graphite samples as small as 0.015 mg C for external submissions and congruent to 0.001 mg C for internal research activities with a precision of similar to 1% for similar to 0.010 mg C. However, when analyzing ultra-small samples isolated by a series of complex chemical and chromatographic methods (such as individual compounds), integrated procedural blanks may be far larger and more variable than those associated with combustion/graphitization alone. In some instances, the mass ratio of these blanks to the compounds of interest may be so high that the reported C-14 results are meaningless. Thus, the abundance and variability of both MC and DC contamination encountered during ultra-small sample analysis must be carefully and thoroughly evaluated. Four case studies are presented to illustrate how extraction chemistry blanks are determined.
