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

Published online by Cambridge University Press:  18 July 2016

Guaciara M Santos*
Affiliation:
KCCAMS Facility, Earth System Science Department, B321 Croul Hall, University of California, Irvine, California 92697-3100, USA
John R Southon
Affiliation:
KCCAMS Facility, Earth System Science Department, B321 Croul Hall, University of California, Irvine, California 92697-3100, USA
Nicholas J Drenzek
Affiliation:
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
Lori A Ziolkowski
Affiliation:
KCCAMS Facility, Earth System Science Department, B321 Croul Hall, University of California, Irvine, California 92697-3100, USA
Ellen Druffel
Affiliation:
KCCAMS Facility, Earth System Science Department, B321 Croul Hall, University of California, Irvine, California 92697-3100, USA
Xiaomei Xu
Affiliation:
KCCAMS Facility, Earth System Science Department, B321 Croul Hall, University of California, Irvine, California 92697-3100, USA
Dachun Zhang
Affiliation:
KCCAMS Facility, Earth System Science Department, B321 Croul Hall, University of California, Irvine, California 92697-3100, USA
Susan Trumbore
Affiliation:
KCCAMS Facility, Earth System Science Department, B321 Croul Hall, University of California, Irvine, California 92697-3100, USA Max-Planck Institute for Biogeochemistry, Jena D-07745, Germany
Timothy I Eglinton
Affiliation:
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
Konrad A Hughen
Affiliation:
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
*
Corresponding author. Email: gdossant@uci.edu
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Abstract

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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 μg C, while DC is ∼0.3 ± 0.15 μg 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.

Type
Sample Preparation
Copyright
Copyright © 2010 by the Arizona Board of Regents on behalf of the University of Arizona 

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