Spatiotemporally resolved LIBS for organic carbon quantification in shale: Decoupling inorganic carbon interference via plasma dynamics analysis

Efficient on-site determination of total organic carbon (TOC) in shale is important for oil and gas resource exploration. However, conventional laser-induced breakdown spectroscopy (LIBS) is hindered by interference from inorganic carbon minerals, which compromises accurate TOC quantification. Here, a spectral-acquisition strategy based on spatiotemporally resolved LIBS was developed to distinguish the contributions of organic and inorganic carbon to C I atomic emission. Measurements of organic-carbon-containing samples (coal and shale) and an inorganic-carbon reference sample (calcium carbonate) showed distinct temporal behaviors of the C I 247.8 nm emission. The C I emission in coal and shale was predominantly concentrated within 0.2-1.4 μs after the laser pulse, whereas that of calcium carbonate was mainly delayed to 1.4-2.6 μs. This temporal separation is interpreted primarily in terms of the delayed availability of neutral carbon atoms from carbonate-derived species and the different expansion dynamics of the plasma plumes. The feasibility of molecular emissions for calibration was also evaluated. The C₂ signal was weak under ambient-air conditions, whereas the onset time of CN emission depended on the total carbon content, preventing the use of a single fixed time window for reliable separation of the two carbon forms. Based on the distinct temporal behavior of C I emission, the detection delay and gate width were optimized. Temporal optimization improved the coefficient of determination (R²) of the univariate shale TOC calibration model from 0.57 to 0.92 and that of the total carbon (TC) model from 0.77 to 0.87. Further optimization of the axial spatial window improved the TOC model to R² = 0.94 with an RMSEC of 0.66 wt% when the 1-2 mm region was selected. This strategy provides a practical approach for resolving contributions from chemically distinct forms of the same element in complex matrices and may be applicable to other multicomponent materials.