Modifying a sealed tube zinc reduction method for preparation of AMS graphite targets: Reducing background and attaining high precision

Abstract

The sealed tube zinc reduction method for converting CO 2 to graphite for AMS 14 C measurements was originally developed for rapid production of graphite in biomedical tracer experiments. The method was usually thought to have low precision and a high background. We have modified the zinc reduction method originally outlined in Vogel [J.S. Vogel, Radiocarbon 34 (3) (1992) 344] by carefully controlling the amounts of reagents (zinc, titanium hydride and Co or Fe catalyst) and now routinely obtain a precision of 2-3& and a relatively low background of $50,000 14 C years when analyzing for 14 C at the Keck Carbon Cycle AMS facility at UC Irvine. Fractionation of carbon isotopes does occur during graphitization and depends on the graphitization yield, which can be affected by the amounts of reagents used and other conditions. The d 13 C of our zinc-reduced graphite is usually lighter by 2-3& than the CO 2 from which it is made, but this is corrected for in our system by simultaneous measurement of 13 C/ 12

Soil aggregate fraction-based 14C analysis and its application in the study of soil organic carbon turnover under forests of different ages

“…Sample preparation for 14 C analysis was undertaken using the method of Xu et al [43] in the Laboratory of Quaternary Global Change Group of Peking University. Soil materials containing about 1-1.5 mg carbon were placed into quartz tubes with clean CuO and desulphurization powder.…”

“…After reduction at 550°C for 7-8 h, graphite is formed at the surface of the Fe powder. For the reagents mentioned above, TiH 2 provided the source of H 2 , Fe powder acted as catalyst, and Zn was not only for reducing CO 2 to CO, but most importantly, for recycling the H 2 O back to H 2 for promoting graphitization [43].…”

Drying/rewetting cycles mobilize old C from deep soils from a California annual grassland

“…Carbon dioxide was collected during the first and last rewetting pulses. Gas samples were collected in stainless steel gas cans, converted into graphite targets using a sealed-tube zinc reduction method (Xu et al, 2007), and analyzed for 14 C enrichment at the W.M Keck Carbon Cycle accelerator mass spectrometry facility at the University of California Irvine (Southon et al, 2004). We report radiocarbon results as D 14 C: ( 1) where the 14 C/ 12 C ratio of the sample is corrected for mass dependent isotope fractionation so all data are reported as if they had a common d 13 C signature of À25& (Stuiver and Polach, 1977).…”

“…We also report d 13 C signatures measured by continuous flow isotope ratio mass spectrometery (Thermo/Fisher Delta plus equipped with a Gasbench II interface) on an aliquot of the purified CO 2 we used for graphitization and radiocarbon analysis (Xu et al, 2007). The data are reported in standard notation as the parts per thousand deviation from the 13 C/ 12 C ratio of the PDB standard.…”

“…Values in parentheses are standard deviations. Analytical precision is AE3& for D 14 C and AE0.2& for d 13 C (Xu et al, 2007 There is a large pool of old, deep soil C (Jobbágy and Jackson, 2000). The mechanisms for its stability, however, have not been clear.…”