Raman Spectroscopy Coupled with Chemometric Analysis for Speciation and Quantitative Analysis of Aqueous Phosphoric Acid Systems

Abstract

Complex chemical systems that exhibit varied and matrix-dependent speciation are notoriously difficult to monitor and characterize online and in real-time. Optical spectroscopy is an ideal tool for in situ characterization of chemical species that can enable quantification as well as species identification. Chemometric modeling, a multivariate method, has been successfully paired with optical spectroscopy to enable measurement of analyte concentrations even in complex solutions where univariate methods such as Beer’s law analysis fail. Here, Raman spectroscopy is used to quantify the concentration of phosphoric acid and its three deprotonated forms during a titration. In this system, univariate approaches would be difficult to apply due to multiple species being present simultaneously within the solution as the pH is varied. Locally weighted regression (LWR) modeling was used to determine phosphate concentration from spectral signature. LWR results, in tandem with multivariate curve resolution modeling, provide a direct measurement of the concentration of each phosphate species using only the Raman signal. Furthermore, results are presented within the context of fundamental solution chemistry, including Pitzer equations, to compensate for activity coefficients and nonidealities associated with high ionic strength systems.

Leveraging Multiple Raman Excitation Wavelength Systems for Process Monitoring of Nuclear Waste Streams

“…Monitoring approaches based on optical spectroscopy are uniquely suited to providing detailed chemical composition information on a process batch or stream. − Optical approaches can provide a range of chemical information, including identification and quantification of elemental and molecular species. ,− Optical approaches are also generally mature and can be flexibly integrated into a variety of process types. ,, As an example, optical spectroscopy probes can be plumbed into hazardous environments and are robust enough to withstand radiation dose, corrosive media, high temperatures, and wide ranges of pressures. ,− Most beneficially, for several optical approaches such as Raman, instrumentation and control equipment can be located in remote, safe locations with only fiber optics traversing the space between detectors and probes.…”

“…The Raman probe itself contains only optics and no electronics, which enables it to maintain a long functional lifetime in radiation and chemically harsh environments . Raman spectroscopy can also be paired with multivariate analysis to enable real-time analysis of highly complex data. ,,, …”

“…9,13−16 Optical approaches are also generally mature and can be flexibly integrated into a variety of process types. 13,17,18 As an example, optical spectroscopy probes can be plumbed into hazardous environments and are robust enough to withstand radiation dose, corrosive media, high temperatures, and wide ranges of pressures. 7,17−19 Most beneficially, for several optical approaches such as Raman, instrumentation and control equipment can be located in remote, safe locations with only fiber optics traversing the space between detectors and probes.…”

“…19 Raman spectroscopy can also be paired with multivariate analysis to enable real-time analysis of highly complex data. 9,13,14,21 However, a significant gap can be found in the application of Raman approaches to very low-concentration waste streams. This can potentially be addressed by taking a two-pronged approach: (1) optimizing instrument parameters and (2) modifying data analysis approaches.…”

Microfluidic In-Situ Spectrophotometric Approaches to Tackle Actinides Analysis in Multiple Oxidation States

“…3 In addition to U(VI) (uranyl), many Purex relevant compounds are Raman active, including phosphate, nitrate, and water. 3,[18][19][20][21][22] Raman spectroscopy can also provide insight into matrix conditions relevant to spent nuclear fuel reprocessing, such as pH; for instance, the Raman water vibrational band changes in response to acid concentration. 23,24 Raman spectroscopy can be used to detect UV-Vis inactive molecules and polyatomic ions in aqueous and organic phase solutions, but it does not directly detect free, monoatomic ions.…”