Carbon Stable Isotope Analysis Method of Marine Dissolved Organic Carbon with Laser Absorption Spectroscopy

The stable carbon isotope (δ¹³C_DOC) of dissolved organic carbon (DOC) is one of the most informative tracers for marine carbon cycling studies. Yet its determination is critically constrained by reliance on isotope ratio mass spectrometry (IRMS). Conventional IRMS-based methods require large sample volumes and labor-intensive pretreatment but offer low measurement throughput, significantly limiting their application in field research and high spatiotemporal resolution studies. To resolve these issues, we developed a field-deployable δ¹³C_DOC analysis method and established an IRMS-independent device by coupling a self-developed ultrahigh-sensitivity mid-infrared tunable diode laser absorption spectroscopy (MIR-TDLAS) isotope spectrometer with a high-temperature catalytic oxidation (HTC) module, enabling a continuous "injection-oxidation-analysis" workflow on a single platform. Integrated with hardware enhancements and a physical-model-based spectral processing algorithm, the device achieved an approximately 100-fold improvement in sensitivity for both ¹²CO₂ and ¹³CO₂, yielding detection limits of 3.98 ppbv and 70 pptv, respectively. This performance enabled quantification of isotope signals from ppmv-level CO₂ without any CO₂ trapping or enrichment. Only 300 μL of seawater was required to determine δ¹³C_DOC within approximately 6 min, providing a δ¹³C precision of ∼1.0‰. A measurement deviation of less than 0.336‰ was achieved using an empirical correction model developed to correct concentration-dependent isotopic bias. By demonstrating its rapidness, precision, and portability, this laser absorption spectroscopy-based method establishes a novel approach for high-throughput and field-deployable DOC carbon isotope analysis.