[M-ZZ42] 地球化学の最前線：新しい挑戦と将来の展望

[MZZ42-P05] Orbitrap質量分析計を用いた多環芳香族炭化水素の同位体分子計測

*Kim Beomsik¹、上野 雄一郎^1,2,3 (1.東京科学大学理学院地球惑星科学系、2.東京科学大学地球生命研究所、3.海洋研究開発機構)

キーワード：オービトラップ質量分析計、多環芳香族炭化水素、同位体分子、多重置換同位体分析、分子内同位体分布計測

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed in the atmosphere, ocean, and soil. In organic geochemistry, PAHs can be extracted from sediments and/or hydropyrolysis of sedimentary organic matter, including highly matured kerogen, which can provide insights into ancient biogeochemical carbon cycling. Isotopic analysis of PAHs has been so far limited to Con-flow analyses using GC-C-IRMS systems. Here, we aim to expand the isotope analysis of PAHs to clump isotope analysis and position specific isotope analysis using an Orbitrap high-mass-resolution mass spectrometer equipped with a dual syringe system. First, for clump isotope analysis, four PAHs (anthracene, phenanthrene, pyrene, and fluoranthene) are prepared and measured their M0 (all ¹²C), M1 (single ¹³C substitution), M2 (double ¹³C substitution), M3 (triple ¹³C substitution) species separated by their precise mass. Their isotope ratios were calculated from intensity of each mass peak: ^13x1R=M1/M0, ^13x2R=M2/M0, and ^13x3R=M3/M0. And δ defined as δ^13xnC = (^13xnR_sample/^13xnR_standard-1)x1000‰. The clumped isotopologue abundance represented by the equilibrium constant of the homogeneous isotope exchange reaction, which can calculate as Δ^13x2C=δ^13x2C-2δ^13x1C and Δ^13x3C=δ^13x3C-3δ^13x1C. The dual syringe analysis of two different commercial samples of the four PAHs gave precision of 0.09~0.47‰ for δ^13x1C, and 0.06~1.27‰ for Δ^13x2C, 0.75–2.62‰ for Δ^13x3C for 17.5 nmol sample in 350 µL. Second, for position specific isotope analysis (PSIA), fragmentation of the PAH was performed using the higher-energy collisional dissociation (HCD) cell before introducing Orbitrap mass analyzer. In the case of phenanthrene (C₁₄H₁₀), C₄ to C₁₄ fragments were identified, and their carbon isotope ratios were determined. The results of HCD-Orbitrap were compared with those analyzed by ¹³C-NMR for calibration. The new high-precision PAH isotopologue analysis using Orbitrap MS may provide additional information on sedimentary organic matter to understanding the past biogeochemical carbon cycling.