Determination of the δ¹³C of dissolved inorganic carbon in water; RSIL lab code 1710

The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 1710 is to present a method to determine the δ¹³C of dissolved inorganic carbon (DIC) of water. The DIC of water is precipitated using ammoniacal strontium chloride (SrCl₂) solution to form strontium carbonate (SrCO₃). The δ¹³C is analyzed by reacting SrCO₃ with 100-percent phosphoric acid (H₃PO₄) to liberate carbon quantitatively as carbon dioxide (CO₂), which is collected, purified by vacuum sublimation, and analyzed by dual inlet isotope-ratio mass spectrometry (DI-IRMS). The DI-IRMS is a DuPont double-focusing mass spectrometer. One ion beam passes through a slit in a forward collector and is collected in the rear collector. The other measurable ion beams are collected in the front collector. By changing the ion-accelerating voltage under computer control, the instrument is capable of measuring mass/charge (m/z) 45 or 46 in the rear collector and m/z 44 and 46 or 44 and 45, respectively, in the front collector. The ion beams from these m/z values are as follows: m/z 44 = CO₂ = ¹²C¹⁶O¹⁶O, m/z 45 = CO₂ = ¹³C¹⁶O¹⁶O primarily, and m/z 46 = CO₂ = ¹²C¹⁶O¹⁸O primarily. The data acquisition and control software calculates δ¹³C values.