Herriott-type cavity-enhanced Raman spectroscopy for simultaneous analysis of ten natural gas components

Real-time monitoring of natural gas composition is essential for reservoir quality evaluation, combustion optimization, and safe transportation. Raman spectroscopy can detect multiple gas species simultaneously through molecular fingerprint signatures, yet weak scattering cross-sections limit its sensitivity in gas-phase applications. Here we present a compact Herriott-type cavity-enhanced Raman spectroscopy (CERS) system in which a 532 nm laser undergoes 128 passes within a 382 mm cavity, yielding a geometric beam path of 49.02 m and a calculated power-weighted enhancement factor of 94.7. The system achieves ppm-level limits of detection for all ten target species in natural gas: 2.8 ppm for H₂, 6.0 ppm for CO, 5.6 ppm for CO₂, 48.8 ppm for CH₄, 6.2 ppm for C₂H₆, 4.7 ppm for C₃H₈, 23.2 ppm for nC₄H₁₀, 3.2 ppm for iC₄H₁₀, 16.0 ppm for nC₅H₁₂, and 7.3 ppm for iC₅H₁₂. DFT calculations at the B3LYP/6-311++G(d,p) level using second-order vibrational perturbation theory (VPT2) anharmonic corrections support the vibrational mode assignments, yielding a mean absolute deviation of 12.5 cm^-1. Quantitative analysis of a multi-component mixture yields quantification deviations below 10% for the ten species with RSD below 5%. These results demonstrate the potential of this system for field-deployable natural gas monitoring.