Direct quantification of total organic carbon in soils using native CN and C₂ molecular emissions by Laser-Induced Breakdown Spectroscopy (LIBS)

Abstract

The soil carbon cycle plays a central role in global warming, making accurate mapping of Total Organic Carbon (TOC) in soils essential for climate change mitigation. Conventional TOC determination methods are often time-consuming, costly, error-prone, and environmentally unsustainable due to the use of chemical reagents and extensive sample preparation. This study introduces a proof of concept for a chemically grounded LIBS-based approach that enables the direct quantification of TOC in soils by exploiting the emission of native CN and C₂ molecular species. Two Laser-Induced Breakdown Spectroscopy (LIBS) systems, Spark Discharge-assisted LIBS (SD-LIBS) and a handheld LIBS device (hLIBS), were evaluated using soil samples with different textures and TOC levels. Argon purging ensured inert plasma conditions favoring CN and C2 native species in the plasma, while Ar emission lines were used to perform spectral normalization. For SD-LIBS, a Partial Least Squares (PLS) model based on 12 CN and C₂ wavelengths achieved an R = 0.90 in calibration and an R = 0.82 (MAE = 0.33%) in validation. The hLIBS model, combining 12 CN and C₂ emission bands and 8 more correlation-selected wavelengths, yielded R = 0.96 for calibration and R = 0.79 (MAE = 0.38%) for validation. Both systems delivered comparable analytical performance, demonstrating the feasibility of rapid, reagent-free, and in situ TOC quantification in soils. The proposed approach paves the way toward sustainable soil monitoring and carbon management strategies.