Ultra small-mass AMS 14C sample preparation and analyses at KCCAMS/UCI Facility

Abstract

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 CO 2 were reduced to graphite with H 2 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 cm 3 ), 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 cm 3 ) 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 À currents of 1 lA per lg of carbon for samples in the 0.002 to 0.010 mgC range, combined with on-line measurement of 12 C and 13 C (AMS d 13 C) 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 lg of modern carbon and (b) 0.1-0.5 lg 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.

Variable effects of labile carbon on the carbon use of different microbial groups in black slate degradation

“…Combustion and graphitization of FAME was performed at the UCI Keck Carbon Cycle AMS facility (Santos et al, 2007a(Santos et al, , 2007b. In the first step, carbon dioxide was produced by combustion at 550°C in evacuated quartz tubes in the presence of CuO and silver wire.…”

Grass Material as a Modern Process Standard for 14C Analysis of n-Alkanes

“…The amount of C ex was estimated by mass balance using the formulae by Santos et al (2007) adding an extra term for "dead carbon correction" to include our 14 C-free C ex derived from PCGC isolation and solvent removal. Our modern component of C ex corresponded to the "modern carbon correction" term in the formulae by Santos et al (2007).…”

“…Our modern component of C ex corresponded to the "modern carbon correction" term in the formulae by Santos et al (2007). We estimated the amount of C ex as the mass of extraneous carbon needed to correct the F 14 C values of the processed standard materials to within 1σ of their bulk F 14 C value.…”

“…This work was supported by the NERC Radiocarbon Facility NRCF010001. (c) Estimated by mass balance using the formulae in Santos et al (2007) based on the deviation in the F 14 C value of each processed sample (corrected for combustion and graphitisation) from the F 14 C value of unprocessed material (bulk F 14 C). This is the mass of extraneous carbon needed to correct the F 14 C values of the processed samples to within 1σ of the bulk F 14 C value.…”

“…(d) Estimated by mass balance using the formulae in Santos et al (2007) based on the deviation in the F 14 C value of the n-alkane fraction (as in Table 2, "uncorrected") from the bulk F 14 C value of the grass material. This is the mass of 14 C-free extraneous carbon needed to correct the F 14 C values of the fractions to within 1σ of the bulk grass F 14 C value.…”