Evaluation of δ13C and δ15N 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 (delta C-13 and (delta N-15) 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 delta C-13 and delta N-15 values were observed during reversed- and normal-phase HPLC isolation of Ni- and VO-porphyrin standards, with the changes in delta C-13 and delta N-15 values being within +/- 0.6 parts per thousand and +/- 1.2 parts per thousand (2 sigma), respectively. These values are comparable to the instrumental precision of the nano-EA/IRMS system (+/- 1.3 parts per thousand for 0.70 mu gC and +/- 1.1 parts per thousand for 0.08 mu gN, 2 sigma), confirming that no substantial artifact in the delta C-13 and delta N-15 values would be expected during the reversed and normal-phase HPLC purification. The sensitivity and precision of our method enable us to determine delta C-13 and delta N-15 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.