Determination of the δ 13 C 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 δ 13 C of dissolved inorganic carbon (DIC) of water. The DIC of water is precipitated using ammoniacal strontium chloride (SrCl 2 ) solution to form strontium carbonate (SrCO 3 ). The δ 13 C is analyzed by reacting SrCO 3 with 100-percent phosphoric acid (H 3 PO 4 ) to liberate carbon quantitatively as carbon dioxide (CO 2 ), 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 = CO 2 = 12 C 16 O 16 O, m/z 45 = CO 2 = 13 C 16 O 16 O primarily, and m/z 46 = CO 2 = 12 C 16 O 18 O primarily. The data acquisition and control software calculates δ 13 C values.