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PREMATURE OXIDATION DURING ARGON PLASMA CLEANING OF WATER-RICH RADIOCARBON SAMPLES

Published online by Cambridge University Press:  25 January 2022

J Royce Cox ,
Eric Blinman ,
Lukas Wacker Open the ORCID record for Lukas Wacker [Opens in a new window]  and
Marvin W Rowe Open the ORCID record for Marvin W Rowe [Opens in a new window]
J Royce Cox
Affiliation:
Office of Archaeological Studies, Center for New Mexico Archaeology, 7 Old Cochiti Road, Santa Fe, NM87507, USA
Eric Blinman
Affiliation:
Office of Archaeological Studies, Center for New Mexico Archaeology, 7 Old Cochiti Road, Santa Fe, NM87507, USA
Lukas Wacker
Affiliation:
Laboratory of Ion Beam Physics, ETH Zürich, HPK, H29, Otto-Stern-Weg 5, CH-8093 Zürich, Switzerland
Marvin W Rowe*
Affiliation:
Office of Archaeological Studies, Center for New Mexico Archaeology, 7 Old Cochiti Road, Santa Fe, NM87507, USA
*
*Corresponding author. Email: PlasmaC.lab@state.nm.us
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Abstract

Plasma oxidation for 14C sampling utilizes low-pressure (133 Pa), low-energy (<50 W), and low- temperature (<50°C) Ar- and O2-plasmas generating CO2 for AMS dating. O2-plasmas on empty chambers remove organic contamination. When clean, a new specimen is inserted and Ar-plasmas dislodge adsorbed atmospheric CO2 from surfaces. Finally, O2-plasmas oxidize organic materials to CO2 for AMS analysis. During some Ar-plasmas we observed anomalous pressure increases and unexpectedly high CO2. Residual gas analysis detected water, hydrogen and oxygen species with Ar and CO2 indicating water plasmas that produced excited oxygen species that prematurely oxidized specimen organic matter. Evolution of excess CO2 during Ar cleaning compromises the ability to affirm that atmospheric CO2 was removed. Standards, TIRI Belfast Pine and VIRI I Whalebone, were dated to determine whether water-induced oxidation was a confounding influence in dating. TIRI wood was sampled twice, once a water-soaked specimen in an Ar plasma and once with water-vapor-plasma only. The TIRI dates agreed with six earlier dates on usual specimens. A colloidal extract from VIRI I whale bone was also sampled and dated twice using both water–plasma oxidation in an Ar-plasma and in an O2-plasma. Dating agreement suggests that water plasmas do not pose undue risks of contamination.

Keywords

argon plasmasCO2 productionoxygen plasmasradiocarbon datingwater plasmas
Type
Research Article
Information
Radiocarbon , Volume 64 , Issue 1 , February 2022 , pp. 21 - 34
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Copyright
© The Author(s), 2022. Published by Cambridge University Press for the Arizona Board of Regents on behalf of the University of Arizona

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