A simple method for sampling total dissolved carbonate in carbonate-rich natural waters and CO₂ preparation for δ¹³C determination

抄録

A field-oriented, simple technique to determine the stable carbon isotopic ratio of the total dissolved carbonate in carbonate-rich natural water is proposed. The total dissolved carbonate is fixed in situ in a plastic syringe containing saturated Ba(OH)₂ solution. The BaCO₃ precipitate is purified without contact with atmospheric CO₂. The dried precipitate is reacted with concentrated phosphoric acid in a capped, 5 ml glass syringe at 50°C overnight. A majority of the resulting CO₂ is expanded into a vacuum line and purified in a trap before mass spectrometric δ¹³C determination. The effect of coexisting SO₄^2- ion on δ¹³C determination becomes appreciable when the carbonate mole fraction in the precipitate, i.e., BaCO₃/(BaCO₃ + BaSO₄), becomes smaller than 0.3. This technique can be applied for natural waters containing total carbonate waters down to <2.5 mmole/kg with the precision better than ±0.03‰.