Development of an Attenuated Total Reflectance–Ultraviolet–Visible\nProbe for the Online Monitoring of Dark Solutions

Optical\nspectroscopy is a valuable tool for online monitoring of\na variety of processes. Ultraviolet–visible (UV–vis)\nspectroscopy can monitor the concentration of analytes as well as\nidentify the speciation and oxidation state. However, it can be difficult\nor\nimpossible to employ UV–vis-based sensors in chemical systems\nthat are very dark (i.e., have a high optical density), requiring\nexceedingly short path lengths (for transmission approaches) or an\neffective means of backscattering (for reflectance approaches). Examples\nof processes that produce highly absorbing solutions and that would\nbenefit significantly from the diagnostic potential of optical sensors\ninclude used nuclear fuel recycling and molten salt systems with high\nconcentrations of dissolved uranium. Utilizing an attenuated total\nreflectance (ATR) UV–vis approach can overcome these challenges\nand allow for the measurement of solutions orders of magnitude more\nconcentrated than transmission UV–vis. However, determining\nideal sensor specifications for various processes can be time-consuming\nand expensive. Here, we evaluate the ability of a novel ATR–UV–vis\nprobe to measure very concentrated solutions of Co(II) and Ni(II)\nnitrate as well as organic dyes (methylene blue, acid red 1, and crystal\nviolet). This sensor design provides a modular method for exploring\npossible “path lengths” by altering the length of the\nATR fiber that was submerged within solution during spectral measurements.\nMeasurements within the ATR sensor cell were compared to measurements\ngathered by transmission UV–vis of samples within a commercially\navailable 1 cm optical cuvette. The ATR–UV–vis probe\nwas capable of measuring absorbance of solutions with a chromophore\nconcentration 600 times greater than that in the 1 cm cuvette. Advanced\ndata analysis in the form of multivariate curve resolution (MCR) was\nused to analyze the speciation of methylene blue over a large concentration\nrange. The application of this novel ATR–UV–vis probe\nto the investigation of dark solutions is a promising avenue for use\nin online monitoring of nuclear processes.