Radiocarbon analysis of biogenic carbon in liquid fuels using accelerator mass spectrometry

Determination of biogenic carbon in liquid fuels is essential not only for verifying compliance with renewable energy regulations and supply-chain integrity but also for understanding anthropogenic perturbations of the radiocarbon reservoir. Radiocarbon (¹⁴C) analysis provides a definitive means to distinguish modern, biogenic carbon from fossil-derived hydrocarbons, serving as a direct tracer for environmental and regulatory applications. We present a validated workflow for ¹⁴C determination in liquid fuels using sealed-tube MnO₂ combustion coupled with accelerator mass spectrometry (AMS). Liquid aliquots of 2 μL were introduced into glass capillaries, combusted in borosilicate tubes at 550 °C with an excess of MnO₂, and the evolved CO₂ was cryogenically purified using a -30 °C water trap and a liquid-nitrogen trap before graphitisation by catalytic H₂ reduction for AMS measurements. Calibration was performed using isopropanol-ethanol mixtures spanning 0-100% biogenic carbon. OxII (NIST SRM 4990C) was used as the primary standard, and a Constant Contamination Model (CCM) was applied to correct for processing blanks. The method achieved generally acceptable CO₂ recoveries (typically >90%, with lower values for several volatile commercial samples) and excellent linearity across the full bio-fraction range. Analysis of commercial E5/E10 petrol and B7 diesel fuels showed biogenic volume fractions (V_bio) ranging from 7.0 to 9.5 vol% in petrol and from 6.0 to 7.0 vol% in diesel, consistent with the corresponding legislative requirements. The presented approach is robust, cost-efficient, and readily applicable not only for routine biofuel verification but also for environmental ¹⁴CO₂ source apportionment and assessing the impacts of fossil-fuel emissions on atmospheric radiocarbon levels, providing a direct link between fuel composition and regional carbon-cycle dynamics.