Raman spectroscopy and multivariate analysis for quantifying uranium(VI) under uranyl nitrate...

A fiber-optic Raman spectroscopy approach was developed for solution-phase, in situ quantification of U(VI) under uranyl nitrate crystallization-relevant conditions, where temperature fluctuations and chemically complex matrices can compromise calibration performance. A multivariate regression framework is presented that combines Raman spectroscopy, design of experiments, and chemometrics to predict U(VI) concentration in HNO₃ in pre- and post-crystallization regimes. A D-optimal design was used to efficiently select calibration concentrations spanning the relevant composition space. Principal component analysis of Raman spectra acquired over a range of concentrations and temperatures revealed systematic trends, including a temperature-dependent redshift in the ν₁ symmetric U(VI) stretch at high U(VI) concentrations near the solubility limit. Partial least squares regression (PLSR) and support vector regression (SVR) models were developed and evaluated from 20 to 60 °C in the presence of surrogate fission products (Cs, Sr, Ru, Nd, Re, Zr, and Mo) and varying HNO₃ levels. SVR outperformed PLSR for U(VI) and HNO₃ quantification by delivering strong accuracy despite temperature- and matrix-induced spectral variation. These results highlight the utility of Raman spectroscopy coupled with multivariate calibration for solution-phase quantification under uranyl nitrate crystallization-relevant conditions and provide a basis for future monitoring applications in used nuclear fuel recycling and other crystallization-focused process analytics settings.