Stable isotopes are powerful tools for tracking biogenic carbon in biofuel petroleum coprocessing. High-precision determination of delta C-13 and delta N-15 values in bio-oils and coprocessed biofuels by continuous-flow elemental analyzer coupled to an isotope ratio mass spectrometer is challenging due to the physical and chemical properties of the samples. Standard sample preparation and measurement techniques such as syringe injection are susceptible to fractionation due to evaporation of volatile compounds, incomplete sample introduction due to sample viscosity, or contamination from atmospheric nitrogen and carbon dioxide. In this work, a modified cold-weld capsule sealer combined with an argon gas purge was used to prepare a wide variety of bio-oil and coprocessed biofuel samples. Measurements done in triplicate and quadruplicate and run on an EA-IRMS showed no measurable atmospheric nitrogen contamination and resulted in excellent reproducibility and precision of +/- 0.01% and 0.025%, respectively, for delta C-13. Additionally, this method allows for even high-volatility samples to be stored for long periods of time with no mass loss and analyzed, concurrently, with commonly available solid isotope reference standards, eliminating concerns of fractionation resulting from sample preparation, handling, and introduction. The high precision of this measurement technique makes it well suited for measurement of the often-small isotopic differences seen in blended fuel products.