Department of Earth System Science

We have developed techniques for accurately and precisely measuring samples containing less than a few hundred micrograms of carbon, using a compact AMS system (NEC 0.5 MV 1.5SDH-1). Detailed discussions of the sample preparation, measurement setup, data analysis and background corrections for a variety of standard samples ranging from 0.002 to 1 mgC are reported. Multiple aliquots of small amounts of CO₂ were reduced to graphite with H₂ over pre-baked iron powder catalyst. A reduction reaction temperature of 450 °C was adopted for graphite samples below 0.05 mgC, rather than the usual 550 °C used on samples of 0.1–1 mgC. In our regular reactors (∼3.1 cm³), this reduction in temperature improved the graphite yield from ∼60 to 90–100% for samples ranging from 0.006–0.02 mgC. The combination of lower reaction temperature with a reduced reactor volume (∼1.6 cm³) gave yields as high as 100% on graphite samples <0.006 mgC. High performance measurements on ultra-small samples are possible also due to a modified NEC MC-SNIC ion-source that generates C⁻ currents of 1 μA per μg of carbon for samples in the 0.002 to 0.010 mgC range, combined with on-line measurement of ¹²C and ¹³C (AMS δ ¹³C) to correct machine-induced isotopic fractionation. Source efficiencies are in excess of 10%, which enables 4–5% of the radiocarbon atoms in 0.005–0.010 mgC samples to be measured. Examination of the background samples revealed two components: (a) 0.2–1 μg of modern carbon and (b) 0.1–0.5 μg of dead carbon. The latter component can be ignored when measuring unknown samples paired to small standards of precisely identical size (matching size normalizing standard method). Otherwise, one must make corrections for both background components. Ultra-small samples from 0.002 to 0.01 mgC can be measured with accuracy and precision of a few percent, based on scatter in results for multiple aliquots of a primary standard and deviations of secondary standards from their known values.