Analysis of Dissolved Inorganic Carbon (DIC) in Water

The SIF provides ¹³C isotope analysis of DIC of waters using a Thermo LC IsoLink system interfaced to a ThermoFinnigan Delta Plus Advantage IRMS (LC-IRMS; Thermo Scientific, Bremen, Germany). The SIF also provides ¹³C isotope analysis of dissolved inorganic C by trace gas from client-prepared exetainers using a GasBench II system interfaced to a Delta V Plus IRMS (Thermo Scientific, Bremen, Germany).

Analysis of ¹³C-DIC by LC
δ¹³C-DIC is analyzed using a Surveyor HPLC coupled to a ThermoFinnigan Delta Plus Advantage isotope ratio mass spectrometer (Thermo Scientific, Bremen, Germany) through a LC Isolink interface (Brandes 2009). Water samples (2 mL) are subsampled via a six-port switching valve fitted with a 5µL, 10µL, 25µL, or 50µL sampling loop, and injected directly into a liquid carrier stream consisting of degassed, weakly acidified water pumped at 300 µL/min. The sample is then mixed with phosphoric acid (1.7 M, degassed), separately pumped at 50 µL/min and then passes through a heater unit at 80ºC, which forces the equilibrium between CO₂ and H₂CO₃ to gaseous CO₂. The sample then enters the IsoLink separation unit where the CO₂ is transferred from aqueous phase to the helium carrier gas (1 mL/min). Following water removal using a two-stage Nafion™ drier, the CO₂ peaks enter the IRMS through an open split. A reference CO₂ peak is used to calculate provisional delta values of the sample CO₂ peak. Final δ¹³C values are obtained after adjusting the provisional values such that correct δ¹³C values for laboratory standards are obtained. Two laboratory standards are analyzed with every 10 samples. The laboratory standards are lithium carbonate dissolved in degassed deionized water and a deep seawater reference material (both calibrated against NIST 8545).

Analysis of ¹³C-DIC by Trace Gas

Water samples (1-4 mL) are injected into evacuated 12 mL septum capped vials (Exetainers, Labco, High Wycombe, UK) containing 1 mL 85% phosphoric acid, which forces the equilibrium between CO₂ and H₂CO₃ to gaseous CO₂. The evolved CO₂ is purged from vials through a double-needle sampler into a helium carrier stream (20 mL/min). The gas is sampled either: a) for high concentration samples, by a six-port rotary valve (Valco, Houston TX) with a 250 µL loop programmed to switch at the maximum CO₂ concentration in the helium carrier or b) for low concentration samples, the entire CO₂ content is frozen in a trapping loop then released to the GC column. The CO₂ is passed to the IRMS through a Poroplot Q GC column (15m x 0.53mm ID, 25°C, 3 mL/min). A reference CO₂ peak is used to calculate provisional delta values of the sample CO₂ peak. Final δ¹³C values are obtained after adjusting the provisional values such that correct δ¹³C values for laboratory standards are obtained. Two laboratory standards are analyzed with every 10 samples. The laboratory standards are lithium carbonate dissolved in degassed deionized water and a deep seawater reference material (both calibrated against NIST 8545).