Hydrogen Effect on the δ¹³C Value of CO₂Measured by Mass Spectrometry with Electron-Impact Ionization

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In order to investigate whether coexisting H₂S, H₂and N₂affect the δ¹³C value, we measured the δ¹³C and δ¹⁸O values of CO₂ gas mixed in arbitrary ratios with each gas. H₂S and H₂increase the δ¹³C value, while N₂produces no change. On the other hand, the δ¹⁸O values are not affected by the presence of H₂S, H₂or N₂. These results suggest that the hydrogen of H₂S or H₂may convert a part of CO₂⁺to HCO₂⁺in the ionization chamber, which could increase each CO₂mass number
by one. The shifting effect from mass 44 to mass 45 is more pronounced than that from mass 45 to mass 46, since the amount of ⁴⁴CO₂ is much larger than ⁴⁵CO₂or ⁴⁶CO₂. This is likely to be the reason why the δ¹³C value increases with increasing hydrogen/CO₂ ratio, while the δ¹⁸O value shows little change, irrespective of the hydrogen/CO₂ ratio. In order to minimize the coexisting H₂S effect, we have improved the purification method of CO₂ by using a CuCl₂ solution trap to remove H₂S.