BLANK ASSESSMENT FOR ULTRA-SMALL RADIOCARBON SAMPLES: CHEMICAL EXTRACTION AND SEPARATION VERSUS AMS

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 (14)C-dead carbon contamination (MC and DC, respectively) can be assessed by measuring (14)C-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 (14)C 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 (14)C 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.