Ortho-parahydrogen composition: Experimental comparison between Raman spectroscopy and speed of sound measurements

This study presents the first direct experimental comparison of a speed-of-sound-based measurement system and two Raman-based systems for determining hydrogen isomer compositions. All three systems were integrated into the same experimental setup and simultaneously tested with hydrogen samples covering a wide range of ortho-para ratios. Two measurement campaigns were conducted: one using temperature-controlled ortho-para conversion, and another using flow-controlled mixing of normal- and parahydrogen. Across both campaigns, all systems demonstrated good agreement with each other and with the theoretically expected sample compositions, thereby validating their suitability for measuring ortho-parahydrogen composition with low uncertainty within defined operational ranges. Each system's specific strengths and limitations are discussed. The results provide a basis for selecting an appropriate method and instrumentation for given applications, as well as theoretical background relevant to data analysis and uncertainty evaluation. • First direct comparison of Raman and speed of sound systems for o/p-H2 measurements. • Systems show good agreement over the full spin-isomeric composition range. • System- and method-specific strengths and limitations are presented and discussed. • Inherent single-point calibration approach improves comparability of Raman systems.