Quantitative Analysis of Total Organic Carbon in Shale Using Laser-Induced Breakdown Spectroscopy Based on CN and C2 Molecular Emissions

Abstract

This study addresses the critical need for rapid on-site quantification of total organic carbon (TOC) in shale by developing a method based on molecular emission in laser-induced breakdown spectroscopy (LIBS). The CN violet band and C2 Swan band were selected as the target molecular emissions to systematically investigate the influence of various gas environments (air, Ar, He, O2, N2, and CO2) and pressure ranges (2-80 kPa) on their emission intensity and signal-to-noise (SNR) ratio. Experimental results demonstrated that O2-containing environments significantly reduce CN and C2 emission intensity and SNR by depleting these molecules, whereas a He atmosphere at 20 kPa optimized SNR. Additionally, while N2 enhances CN emission, it introduces inorganic carbon interference. Based on this, we inferred the generation mechanisms of CN and C2 in shale plasma in a He environment and established a calibration model for TOC. Univariate calibration models achieved determination coefficients R2 = 0.929 and R2 = 0.940 using individual CN/C2 emissions. A weighted combined model of CN and C2 intensity further enhanced performance R2 = 0.957, as it accounts for the simultaneous formation of CN and C2 radicals from organic carbon in shale plasma. This work demonstrates the potential of LIBS as a rapid and portable method for on-site shale TOC analysis, while shedding light on the complex evolution of organic carbon into CN and C2 radicals in plasma.