• Blank Assessment for Ultra-Small Radiocarbon Samples: Chemical Extraction and Separation Versus AMS

      Santos, Guaciara M.; Southon, John R.; Drenzek, Nicholas J.; Ziolkowski, Lori A.; Druffel, Ellen; Xu, Xiaomei; Zhang, Dachun; Trumbore, Susan; Eglinton, Timothy I.; Hughen, Konrad A. (Department of Geosciences, The University of Arizona, 2010-01-01)
      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 ~0.001 mg of carbon (C). Mass-balance background corrections for modern and 14C-dead carbon contamination (MC and DC, respectively) can be assessed by measuring 14C-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 14C composition to the unknown samples. Ordinary sample processing (such as ABA or leaching pretreatment, combustion/graphitization, and handling) introduces MC contamination of ~0.6 +/- 0.3 g C, while DC is ~0.3 +/- 0.15 g C. Today, the laboratory routinely analyzes graphite samples as small as 0.015 mg C for external submissions and =0.001 mg C for internal research activities with a precision of ~1% for ~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 14C 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.
    • Compound-Specific Radiocarbon Analyses of Phospholipid Fatty Acids and n-Alkanes in Ocean Sediments

      Druffel, Ellen R. M.; Zhang, Dachun; Xu, Xiaomei; Ziolkowski, Lori A.; Southon, John R.; Dos Santos, Guaciara M.; Trumbore, Susan E. (Department of Geosciences, The University of Arizona, 2010-01-01)
      We report compound-specific radiocarbon analyses of organic matter in ocean sediments from the northeast Pacific Ocean. Chemical extractions and a preparative capillary gas chromatograph (PCGC) were used to isolate phospholipid fatty acids (PLFA) and n-alkanes from 3 cores collected off the coast of California, USA. Mass of samples for accelerator mass spectrometry (AMS) 14C analysis ranged from 13-100 g C. PLFA extracted from anaerobic sediments in the Santa Barbara Basin (595 m depth) had modern ∆14C values (-20 to +54), indicating bacterial utilization of surface-produced, post-bomb organic matter. Lower ∆14C values were obtained for n-alkanes and PLFA from coast (92 m depth) and continental slope (1866 m) sediments, which reflect sources of old organic matter and bioturbation. We present a brief analysis of the blank carbon introduced to samples during chemical processing and PCGC isolation.
    • Is the Consensus Value of ANU Sucrose (IAEA C-6) Too High?

      Xu, Xiaomei; Khosh, Matthew S.; Druffel-Rodriguez, Kevin C.; Trumbore, Susan E.; Southon, John R. (Department of Geosciences, The University of Arizona, 2010-01-01)
      Primary and secondary standards are essential in radiocarbon analyses for the purpose of reporting and comparing data among laboratories, as well as for internal laboratory data quality control. ANU sucrose is one of the IAEA-certified 14C standards (C-6) with a consensus value of 1.5061 +/- 0.0011 fraction modern (Fm). All of our measurements of ANU sucrose (n = 351) as a secondary standard over the last 7 yr result in an average value of 1.5016 +/- 0.0005 Fm (2- standard error). After applying the same outlier tests used for IAEA reference standards, a weighted average value of 1.5016 +/- 0.0002 Fm (n = 294) was calculated. This value is significantly lower than the IAEA C-6 consensus value (t test with unequal variance; p = 0.023). In contrast, our measurements of other secondary standards over the same time period are in excellent agreement with their respective consensus values. Since ANU is the only secondary standard measured in our lab that does not agree with the consensus values, and we have measured a larger number analyses compared to what went into the definition of the consensus value, we suggest that the consensus value of ANU sucrose might be too high by ~0.0045 +/- 0.0011 Fm. Given that some labs routinely use ANU sucrose as a primary standard, our results suggest that revisiting the consensus value of ANU sucrose may be necessary.