An improved method for the sealed-tube zinc graphitization of microgram carbon samples and 14C AMS measurement

Advances in accelerator mass spectrometry (AMS) and environmental chemistry techniques have increased demand for the natural abundance ¹⁴C analysis of ultra-small (<25 µgC) and compound-specific samples. The sealed-tube zinc (Zn) method is used for the reduction of sample CO₂ to graphite on an iron catalyst. This method provides reliable, low AMS ¹⁴C backgrounds and high measurement precision over a wide range of sample sizes (1000 µgC to <15 µgC). Recently, Rinyu and co-workers demonstrated improved ion beam currents and ¹⁴C backgrounds when performing sealed-tube Zn graphitization by applying a thermal gradient (from room temperature to 550 °C) on 25–100 µgC samples [1]. However, many compound-specific radiocarbon analysis (CSRA) measurements are limited to smaller sample sizes – often <10 µgC per analyte with repeated collection by gas or liquid chromatography. Using a similar thermal gradient setup, we present an improved sealed-tube Zn method for the graphitization of samples ranging from ultra-small mass (2 µgC) and up to 100 µgC in the Keck Carbon Cycle AMS facility (KCCAMS) at the University of California, Irvine. Our improved method produces high AMS currents (he¹²C⁺ ∼1 µA µgC⁻¹), a low extraneous C blank (0.5–0.7 µgC), high measurement precision (2–10% as relative % error) and accuracy (95–97%, as an absolute % difference of measured vs. consensus fraction modern values) for modern 2–5 µgC samples. These improvements have resulted in a sealed-tube Zn method which equals the performance of the ultra-small mass H₂ reduction method for 2–10 µgC samples [2].