Methodological advancements for stable carbon isotope measurement of dissolved inorganic carbon using tunable diode laser absorption spectrometers

Dissolved inorganic carbon (DIC) - including aqueous CO₂, carbonic acid, bicarbonate, and carbonate - is often the largest pool of carbon in aquatic systems. Biogeochemical processes result in exchanges of carbon between the various DIC components and may act to move carbon into or out of the DIC pool. The isotopic composition of carbon is a product of both its source and mass-dependent fractionation as carbon changes form through the processes acting on it. Consequently, measurement of the stable carbon isotope composition of DIC is a valuable tool for understanding biogeochemical processes in aquatic systems. However, differences in isotopic composition are small, and separating source contributions requires precise measurement.

Measurement of DIC can be done by conversion to CO₂ in the presence of a strong acid and quantification of liberated CO₂ by gas analysis. To determine isotopic composition of the liberated CO₂ (δ¹³C) historical methods used isotope ratio mass spectrometry (IRMS). More recently, tunable diode laser absorption spectrometry (TDLAS) based gas analyzers have been adopted for these measurements but have continued to base methodological considerations on those developed for IRMS. While IRMS and TDLAS can both be used to determine δ¹³C, there are fundamental differences in the technology, which should be considered during application. In particular, this has meant δ¹³C - DIC measurements have been unable to take full advantage of TDLAS performance characteristics.  

Here we describe methodological advancements from integration of a TDLAS (LI-7825 carbon isotope analyzer) with a DIC measurement system (LI-5370A), that include changes to the pneumatic and analytical approach used in the DIC system. Pneumatic modifications allow the TDLAS to operate at an independent flow rate from the DIC system and serve to manipulate the residence time for CO₂ along the flow path. We describe use of a non-CO₂ free carrier gas, which allows the DIC measurement to take full advantage of analyzer precision and minimize errors intrinsic to δ¹³C as determined by TDLAS. We present data demonstrating measurement precision over a range of conditions and show that under similar conditions, these methodological changes result in precision exceeding that published previously for TDLAS-DIC measurements.