Performance of Micro graphitization versus Gas Ion Source on the EnvironMICADAS at HEKAL Laboratory

Mihály Molnár1, Róbert Janovics1, István Major1, Katalin Hubay1, Botond Buró1, Tamás Varga1, Titanilla Kertész1, László Rinyu1, Anita Molnár2, Gergely Orsovszki2, Mihály Veres2, A.J. Timothy Jull1,3, Virág Gergely1

1INTERACT Centre, Institute for Nuclear Research, Debrecen, Hungary, 2Isotoptech Zrt., Debrecen, Hungary, 3University of Arizona AMS Laboratory, Tucson, United States

keywords: water, DIC, radiocarbon, automated, graphitization

The aim of this study was to compare the ultimate performance of sealed tube graphitization (Zn-method) and gas ion source (GIS) based methods for micro-sample analyses on the EnvironMICADAS AMS of the HEKAL AMS facility (Debrecen, Hungary). To accomplish this purpose both methods, which are routinely used in HEKAL have been tested in the range of 10–100 ug carbon content. To exclude the side effects caused by the possible contamination by the chemical pretreatment and combustion of such small samples, all micro-samples of this tests have been produced by splitting large CO2 gas from combusted samples (containing 1 mg C) and large amount of radiocarbon free fossil CO2 borehole gas without any chemical preparation.
Advantages of the micro-graphitization (Zn-method) relative to gas ion source application are the higher number of total counts and better statistics on the same sample. Memory effect in the ion source is negligible in case of micro-graphite AMS analyses. Full magazines with up to 40 samples could run for tens of hours without any personal assistance in the EnvironMICADAS. Losing of samples due to pure ion current and/or bad target quality is very rare when micro-graphites are measured. Drawbacks of the solid graphite based analyses are the higher and mass dependent blank level which strongly limits the measurable maximum age (C-14 detection limit). The mass dependent blank level requires careful planning of the magazines including a proper number of blanks and normalization samples (Oxa-2) in the same mass range as the actual set of samples.
In case of micro-graphites 1-10 pMC average background level can be reached depending the sample size (10 -100 ug C). The lower C amount the higher blank level, which show exponential rise below 20 ug C sample amount. The available precision for modern samples varies between 0.5% to 2% depending the sample size (10 to 100 ug C). Available ion current shows a linear correlation to the sample size, about 0.1 uA/ ug C on the high energy side. Figure of Merit factor of the micro-graphite analyses shows similar performance compared to the GIS analyses, in cases when the worse detection limit allows the application of graphite-based analyses.
The research was supported by the European Union and the State of Hungary, co-financed by the European Regional Development Fund in the project of GINOP-2.3.4-15-2020-00007 “INTERACT”. This work was carried out in the frame of a János Bolyai Research Scholarship (to Mihály Molnár) of the Hungarian Academy of Sciences.


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Date

Nov 08 - 19 2021