Evaluation of δ13C and δ15N Uncertainties Associated with the Compound-Specific Isotope Analysis of Geoporphyrins

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Abstract
Compound-specific isotope analysis of geoporphyrin, which are derivatives of chloropigments possessed by phototrophs, provide essential records of the biogeochemical cycle of past marine environments. Here, we evaluated uncertainties in carbon and nitrogen isotopic compositions (δ13C and δ15N) associated with high-performance liquid chromatography (HPLC) purifi-cation 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 car-bon 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 δ13C and δ15N values were observed during reversed- and normal phase HPLC isolation of Ni- and VO-porphyrin standards, with the changes in δ13C and δ15N values within ±0.6‰ and ±1.2‰ (2σ), respectively. These values are compara-ble to the instrumental precision of the nano-EA/IRMS system (±1.3‰ for 0.70 µgC and ±1.1‰ for 0.08 µgN, 2σ), con-firming that no substantial artifact in the δ13C and δ15N value would be expected during the reversed- and normal-phase HPLC purification. The sensitivity and precision of our method enable us to determine δ13C and δ15N 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.
Funding Information
  • Japan Society for the Promotion of Science (08J04848, 16H02236, 16J07844)

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