Comparison of Raman and mid-infrared spectroscopy for quantification of nitric acid in PUREX-relevant mixtures

During the plutonium uranium reduction extraction (PUREX) process, nitric acid facilitates the extraction of actinides from the aqueous phase into the organic phase by forming neutral, organic soluble complexes with tri-n-butyl phosphate (TBP). The concentration of nitric acid is generally measured by titration; however, titration is a time-consuming method that generates significant volumes of additional waste. Optical spectroscopic techniques can be used to perform fast, automated measurements off-line or on-line, without generating any waste. In this work, the effectiveness of Raman and mid-infrared (MIR) spectroscopy has been compared for the first time as an alternative to titration for the quantification of nitric acid in PUREX-relevant mixtures. Samples of 0 – 12 M nitric acid in the aqueous phase and 0 – 1.10 M nitric acid in the organic phase (TBP/odourless kerosene (OK)-H2O-HNO3 model system) were analysed and partial least squares (PLS) regression models were built to predict nitric acid concentration. MIR spectra required less pre-processing than Raman spectra and more accurate predictions of nitric acid concentration were obtained for MIR spectroscopy than for Raman spectroscopy, with root mean square error of prediction (RMSEP) values of 0.099 M versus 0.148 M obtained for the aqueous phase and root mean square error of cross validation (RMSECV) values of 0.006 M versus 0.013 M obtained for the organic phase. To investigate the ability to predict nitric acid concentration in the presence of uranyl nitrate, samples containing uranium (0 – 100 g/L) and nitric acid (0.15 – 0.64 M) in the organic phase (U-TBP/OK-H2O-HNO3 model system) were analysed by Raman and MIR spectroscopy. The RMSECV was 0.027 M and 0.066 M for MIR and Raman spectroscopy, respectively; these values are higher than those obtained in the absence of uranyl nitrate owing to differences in the experimental approaches employed. Therefore, the results obtained demonstrate that MIR or Raman spectroscopy could be used to measure the concentration of nitric acid in the organic and aqueous phases in the PUREX process

Assessment of novel measurement technologies for process monitoring and use of chemometric advances to facilitate their application

Mid-infrared (MIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy are common laboratory techniques, but are not so widely used in process analysis. The high attenuation of MIR light inhibits the ability to locate delicate instruments away from harsh processes using long lengths of optical fibre, and the large size and cost of high-field NMR spectrometers prevent them from being easily installed in process plants. Recent advances in technology have led to the availability of miniaturised, robust MIR spectrometers and benchtop NMR spectrometers operating at low field.;The performance of a novel, robust MIR spectrometer, designed for use in process environments, was assessed for the quantitative in situ analysis of liquids and was found to be comparable to a laboratory MIR spectrometer. A reaction was then monitored using the novel spectrometer and accurate predictions of concentration could be obtained by multivariate curve resolution. This demonstrates the suitability of the spectrometer for in situ process monitoring. Calibration transfer between the two MIR spectrometers was also performed, demonstrating the ability to build a model in the laboratory for subsequent application to a process.;The instrumental stability of a low-field NMR spectrometer was evaluated, and when multiple samples were analysed, shifting of peaks and deterioration of lineshape occurred over time. To eliminate peak shift, a range of alignment methods were assessed. Alignment was successful for small peak shifts, but less effective when large peak shifts were present. However accurate predictions of concentration could still be obtained by PLS.;Calibration transfer and reference deconvolution were compared as a solution to lineshape deterioration, and the transfer of PLS models between low-field NMR spectra collected under different conditions was demonstrated. Calibration transfer was found to be more effective overall, and produced accurate predictions of concentration. These results demonstrate the suitability of low-field NMR spectroscopy for quantitative analysis.Mid-infrared (MIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy are common laboratory techniques, but are not so widely used in process analysis. The high attenuation of MIR light inhibits the ability to locate delicate instruments away from harsh processes using long lengths of optical fibre, and the large size and cost of high-field NMR spectrometers prevent them from being easily installed in process plants. Recent advances in technology have led to the availability of miniaturised, robust MIR spectrometers and benchtop NMR spectrometers operating at low field.;The performance of a novel, robust MIR spectrometer, designed for use in process environments, was assessed for the quantitative in situ analysis of liquids and was found to be comparable to a laboratory MIR spectrometer. A reaction was then monitored using the novel spectrometer and accurate predictions of concentration could be obtained by multivariate curve resolution. This demonstrates the suitability of the spectrometer for in situ process monitoring. Calibration transfer between the two MIR spectrometers was also performed, demonstrating the ability to build a model in the laboratory for subsequent application to a process.;The instrumental stability of a low-field NMR spectrometer was evaluated, and when multiple samples were analysed, shifting of peaks and deterioration of lineshape occurred over time. To eliminate peak shift, a range of alignment methods were assessed. Alignment was successful for small peak shifts, but less effective when large peak shifts were present. However accurate predictions of concentration could still be obtained by PLS.;Calibration transfer and reference deconvolution were compared as a solution to lineshape deterioration, and the transfer of PLS models between low-field NMR spectra collected under different conditions was demonstrated. Calibration transfer was found to be more effective overall, and produced accurate predictions of concentration. These results demonstrate the suitability of low-field NMR spectroscopy for quantitative analysis

Application of multivariate curve resolution to in situ THz - Raman spectroscopy of amorphous solid dispersions in pharmaceutical products

Off-line techniques such as differential scanning calorimetry (DSC) can be used to characterize solidified extrudates from a hot-melt extrusion (HME) process. However, off-line measurements are not representative of the melt mixture in the HME barrel. THz-Raman spectroscopy can be used to monitor solid dispersions of the melt mixture in situ. To avoid needing off-line reference measurements, calibration-free methods for spectral analysis are advantageous. Multivariate curve resolution (MCR) is a calibration-free method of resolving mixture spectra into pure component contributions using an iterative optimization within a set of constraints. The outputs are pure component spectra and concentration profiles for each component