Determination of position-specific carbon isotope ratios in propane from hydrocarbon gas mixtures

Position-specific isotope ratios (PSIRs), also termed intramolecular isotope ratios, provide novel information to probe molecular structure, reaction mechanics, and molecular signatures for gas to source correlation. Successful application of this technique to natural gas may provide key insights into gas origin and formation mechanisms, which often cannot be satisfactorily addressed using bulk or compound-specific isotope analyses (CSIA) alone. In this study, we present a method to determine the PSIR of propane from hydrocarbon gas mixtures at natural abundance using a step-wise quantification and compound-specific isotope monitoring approach. First, we purify/enrich propane using a proprietary cryogenic gas processing unit, and then convert it to acetic acid (AA) by both enzyme-catalyzed and chemical reactions. Our method of PSIR analysis makes progress in several regards, including efficient propane separation from low-propane concentration gas mixtures (0.5% v/v) using relatively small sample quantities (below 8 mL propane). This advance in methodology enables more routine analysis and an optimized workflow for isotope analysis with strict quality control. Results obtained from oil-derived natural gas show that the center carbon in propane is more ¹³C-enriched than the terminal carbons by about 19.2‰. Our results are discussed in the context of previous efforts in propane intramolecular analysis and potential future uses of this novel technique to improve understanding of the origin of gases, their formation, and the isotope reversal phenomenon.