Contamination on AMS Sample Targets by Modern Carbon is Inevitable

Abstract

Accelerator mass spectrometry (AMS) measurements of the radiocarbon content in very old samples are often challenging and carry large relative uncertainties due to possible contaminations acquired during the preparation and storage steps. In case of such old samples, the natural surrounding levels of 14C from gases in the atmosphere, which may well be the source of contamination among others, are 2–3 orders of magnitude higher than the samples themselves. Hence, serious efforts are taken during the preparation steps to have the samples pristine until measurements are performed. As samples often have to be temporarily stored until AMS measurements can be performed, storage conditions also become extremely crucial. Here we describe an assessment of this process of contamination in background AMS samples. Samples, both as pressed graphite (on AMS targets) and graphite powder, were stored in various storage conditions (CO2-spiked air) to investigate the extent of contamination. The experiments clearly show that the pressed targets are more vulnerable to contamination than the unpressed graphite. Experiments conducted with enriched CO2-spiked laboratory air also reveal that the contaminating carbon is not only limited to the target surface but also penetrates into the matrix. A combination of measurements on understanding the chemical nature of the graphitization product, combined with long-available knowledge on “adventitious carbon” from the surface science community, brought us to the conclusion that contamination is to a certain extent inevitable. However, it can be minimized, and should be dealt with by sputter-cleaning the samples individually before the actual measurement.

Radiocarbon analysis of stratospheric CO<sub>2</sub> retrieved from AirCore sampling

“…MCC and DCC affect samples differently, depending on the age and mass of the sample (Paul et al, 2016;Brown and Southon, 1997;Santos et al, 2007b;de Rooij et al, 2010). Very small samples such as those in the present work (≤ 10 µgC) are severely affected by both MCC and DCC.…”

Sample Preparation at the Jena 14C Laboratory

“…But to be able to clearly quantify the effects, the storage time in the investigation was extended up to 30 months, much longer than any expected delay. The long-term observations of this work are supplementary to those of Paul et al (2016), which focus on the chemical nature of this contamination.…”

“…The storage background was investigated in this work not because it is a large contribution, but because it is usually not investigated; one exception to this is Paul et al (2016). We sought to understand the effects of several months’ storage, as storage for such extended periods could be necessary under some circumstances, e.g., significant damage to the AMS system followed by urgent measurement requests increasing further the delay of some samples.…”

A Preliminary ion Source Background Study at Lalonde AMS

“…These have been studied before (e.g. Santos et al 2007;Paul et al 2016). Here, a further example is presented to display the release of the different types of non-sample carbon by separate time constants in the 13 C 3 and 12 C 3 currents measured from Fe-only blanks.…”