Evaluation of δC and δN Uncertainties Associated with the Compound-Specific Isotope Analysis of Geoporphyrins.

Compound-specific isotope analyses of geoporphyrins, which are derivatives of chloropigments possessed by phototrophs, provide essential records of the biogeochemical cycle of past aquatic environments. Here, we evaluated uncertainties in carbon and nitrogen isotopic compositions (δC and δN) associated with high-performance liquid chromatography (HPLC) purification and isotopic measurements of geoporphyrins. Evaluation of total blank carbon and nitrogen for the HPLC and our sensitivity-improved elemental analyzer/isotope ratio mass spectrometer (nano-EA/IRMS) analysis confirmed that blank carbon can be corrected and that blank nitrogen is negligible compared to the mass of geoporphyrins required for the isotopic measurement. While geoporphyrins exhibited substantial in-peak carbon and nitrogen isotopic fractionations, no systematic changes in δC and δN values were observed during reversed- and normal-phase HPLC isolation of Ni- and VO-porphyrin standards, with the changes in δC and δN values being within ±0.6‰ and ±1.2‰ (2σ), respectively. These values are comparable to the instrumental precision of the nano-EA/IRMS system (±1.3‰ for 0.70 μgC and ±1.1‰ for 0.08 μgN, 2σ), confirming that no substantial artifact in the δC and δN values would be expected during the reversed- and normal-phase HPLC purification. The sensitivity and precision of our method enable us to determine δC and δN values of both major and minor geoporphyrins found in ancient sediments, which would provide detailed information on the photosynthetic primary producers and the carbon and nitrogen cycles in the past.