Ultra small mass AMS 14C sample preparation and analyses at NIES-TERRA AMS facility

Abstract

We have made substantial improvements to the NEC 5 MV tandem Pelletron AMS system, such as installing a new type of MC-SNICS ionizer, reconstructing beam lines with the development of the graphite preparation method using an online elemental analyzer for small mass samples down to 1 μgC. To obtain an adequately stable and strong beam current intensity for reliable measurement of small mass samples, we examined graphite position depths (0.35–2.0 mm) related to the optimal geometric conditions to enhance the efficiency of the iron catalyst. Our AMS system generated the most stable and intense beam current at a graphite depth of 1.5–2.0 mm and 2 mg of the iron catalyst, with ionization efficiency of 18–22 %. Under these conditions, the current produced with the 1–10 μgC samples showed a more stable beam current (10–20 μA) and a fivefold increase in beam output duration (greater than 30 min compared to 5 min at 1 μgC). Effects of small carbon graphite-induced isotopic fractionation and higher background was corrected by measuring samples relative to standards of similar mass and a blank. The pMC results were obtained for ultra-small samples ranging from 1 to 50 μgC, calculated relative to adjacent HOxII (NIST SRM 4990C) and IAEA-C1 standards of similar amounts for corrected 14C contents, were approximately in agreement with consensus values overall. By using the method developed in this study, we confirmed the 14C content of ultra-small carbon samples of (1-10 μgC) of IAEA-C8 standards can be measured with precision and accuracy of a few percent.