Quantification of biogenic carbon in fuel blends through LSC 14C direct measurement and assessment of uncertainty

Abstract

Biogenic carbon in a fuel blend may be quantified by determining its radiocarbon (14C) content, which is most accurately measured using an accelerator mass spectrometer (AMS). An AMS is a highly specialized instrument requiring significant sample preparation and it is not readily available as a tool for rapid, in-house fuel assessment. Direct fuel measurement by liquid scintillation counter (LSC) analysis is an alternative 14C-based technique that requires only limited expertise and can be performed with instrumentation that can be purchased and installed in-house. Although LSC direct measurement is not an accepted standard method for fuel analysis, it has shown promising results. Here, we further describe the use of 14C liquid scintillation for direct-measurement and quantification of fossil fuel-biofuel blend levels and assess uncertainty. Our study includes analysis of four biofuel types (HT processed gasoline, jet fuel, diesel, and commercial bio-diesel) each blended at 0, 1, 2, 3, 5, and 10% biofuel (V/V) by addition of fossil toluene. Additionally, replicate commercial bio-diesel samples blended at 1% biofuel (V/V) were prepared to verify reproducibility. Each sample was counted for repeated 4-hour periods for a total of 24–64 h to quantify uncertainty as a function of counting time. For all samples, fuel count rate and sample counting efficiency were the dominant sources of error but compete and must be optimized. With increasing biofuel content and sample volume, the sample activity increases but at the expense of decreasing counting efficiency. For a 1% fuel blend level and sample size of 5 mL, a target uncertainty of 0.5% biogenic carbon (C/C, 95% confidence interval) absolute precision is achieved in less than 8 hours counting, and the required counting time decreases to 4 h with a sample size of 10 mL.