Uranium Ore Concentrate (UOC, often called yellowcake) is a generic term that describes the initial product resulting from the mining and subsequent milling of uranium ores en route to production of the U-compounds used in the fuel cycle. Depending on the mine, the ore, the chemical process, and the treatment parameters, UOC composition can vary greatly. With the recent advent of handheld spectrometers, we have chosen to investigate whether either commercial off-the-shelf (COTS) handheld devices or laboratory-grade Raman instruments might be able to i) identify UOC materials, and ii) differentiate UOC samples based on chemical composition and thus suggest the mining or milling process. Twenty-eight UOC samples were analyzed via FT-Raman spectroscopy using both 1064 nm and 785 nm excitation wavelengths. These data were also compared with results from a newly developed handheld COTS Raman spectrometer using a technique that lowers the background fluorescence signal. Initial chemometric analysis was able to differentiate UOC samples based on mine location. Additional compositional information was obtained from the samples by performing XRD analysis on a subset of samples. The compositional information was integrated with chemometric analysis of the spectroscopic dataset allowing confirmation that class identification is possible based on compositional differences between the UOC samples, typically involving species such as U<sub>3</sub>O<sub>8</sub>, α-UO<sub>2</sub>(OH)<sub>2</sub>, UO<sub>4</sub>•2H<sub>2</sub>O (metastudtite), K(UO<sub>2</sub>)<sub>2</sub>O<sub>3</sub>, etc. While there are clearly excitation λ sensitivities, especially for dark samples, Raman analysis coupled with chemometric data treatment can nicely differentiate UOC samples based on composition and even mine origin.
The selectivity of an optical sensor can be improved by combining optical detection with electrochemical oxidation or
reduction of the target analyte to change its spectral properties. The changing signal can distinguish the analyte from
interferences with similar spectral properties that would otherwise interfere. The analyte is detected by measuring the
intensity of the electrochemically modulated signal. In one form this spectroelectrochemical sensor consists of an
optically transparent electrode (OTE) coated with a film that preconcentrates the target analyte. The OTE functions as
an optical waveguide for attenuated total reflectance (ATR) spectroscopy, which detects the analyte by absorption.
Sensitivity relies in part on a large change in molar absorptivity between the two oxidation states used for
electrochemical modulation of the optical signal. A critical part of the sensor is the ion selective film. It should
preconcentrate the analyte and exclude some interferences. At the same time the film must not interfere with the
electrochemistry or the optical detection. Therefore, since the debut of the sensor’s concept one major focus of our group
has been developing appropriate films for different analytes. Here we report the development of a series of quaternized
poly(vinylpyridine)-co-styrene (QPVP-co-S) anion exchange films for use in spectroelectrochemical sensors to enable
sensitive detection of target anionic analytes in complex samples. The films were either 10% or 20% styrene and were
prepared with varying degrees of quaternized pyridine groups, up to 70%. Films were characterized with respect to
thickness with spectroscopic ellipsometry, degree of quaternization with FTIR, and electrochemically and
spectroelectrochemically using the anions ferrocyanide and pertechnetate.
Spectroelectrochemistry provides improved selectivity for sensors by electrochemically modulating the optical signal
associated with the analyte. The sensor consists of an optically transparent electrode (OTE) coated with a film that
preconcentrates the target analyte. The OTE functions as an optical waveguide for attenuated total reflectance (ATR)
spectroscopy, which detects the analyte by absorption. Alternatively, the OTE can serve as the excitation light for
fluorescence detection, which is generally more sensitive than absorption. The analyte partitions into the film, undergoes
an electrochemical redox reaction at the OTE surface, and absorbs or emits light in its oxidized or reduced state. The
change in the optical response associated with electrochemical oxidation or reduction at the OTE is used to quantify the
analyte. Absorption sensors for metal ion complexes such as [Fe(CN)<sub>6</sub>]<sup>4-</sup> and [Ru(bpy)<sub>3</sub>]<sup>2+</sup> and fluorescence sensors for [Ru(bpy)3]<sup>2+</sup> and the polycyclic aromatic hydrocarbon 1-hydroxypyrene have been developed. The sensor concept has been extended to binding assays for a protein using avidin–biotin and 17β-estradiol–anti-estradiol antibodies. The sensor has been demonstrated to measure metal complexes in complex samples such as nuclear waste and natural water. This sensor has qualities needed for security and defense applications that require a high level of selectivity and good detection limits for target analytes in complex samples. Quickly monitoring and designating intent of a nuclear program by measuring the Ru/Tc fission product ratio is such an application.