Inherent single-point calibration for quantitative Raman analysis of ortho-para hydrogen composition

The global effort to reduce emissions and decarbonize energy has led to the pursuit of carbon-free energy carriers, with hydrogen emerging as one of the most promising options. However, to enable utilization, storage, and transport, especially for liquified hydrogen, accurate monitoring of the composition and conversion kinetics of hydrogen's isomer states, para- and ortho-hydrogen, is crucial to optimize liquefaction and regasification processes. We present a straightforward and accurate method to calibrate Raman spectroscopy systems for direct hydrogen isomer composition measurements based on the inherent nature of hydrogen's vibrational states. The approach requires only one measured reference data set at a known isomer composition and temperature, calibrated against theoretical calculations of the overall hydrogen isomer composition and relative occupation of molecular states. The calibration can be applied to both rotational and vibrational transitions of equilibrium hydrogen down to cryogenic temperatures (T = 77 K), with maximum absolute deviations in ortho-hydrogen fractions of less than 0.7%. Moreover, a detailed analysis of uncertainty shows that the absolute deviations are within the measurement uncertainty of between (0.55 and 1.1)%. The presented calibration method represents a significant improvement in accuracy and measurement methodology compared to the results obtained from uncorrected spectra or other scaling approaches.