Quantum Cascade Laser Infrared Spectroscopy for Online Monitoring of Hydroxylamine Nitrate

Hindawi International Journal of Analytical Chemistry Volume 2018, Article ID 7896903, 9 pages https://doi.org/10.1155/2018/7896903 Research Article Quantum Cascade Laser Infrared Spectroscopy for Online Monitoring of Hydroxylamine Nitrate Marissa E. Morales-Rodriguez,1,2 Joanna McFarlane 1 2 ,1 and Michelle K. Kidder1 Oak Ridge National Laboratory, USA The Bredesen Center at the University of Tennessee Knoxville, USA Correspondence should be addressed to Joanna McFarlane; Received 27 March 2018; Revised 26 July 2018; Accepted 29 August 2018; Published 23 September 2018 Academic Editor: Charles L. Wilkins Copyright © 2018 Marissa E. Morales-Rodriguez et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We describe a new approach for high sensitivity and real-time online measurements to monitor the kinetics in the processing of nuclear materials and other chemical reactions. Mid infrared (Mid-IR) quantum cascade laser (QCL) high-resolution spectroscopy was used for rapid and continuous sampling of nitrates in aqueous and organic reactive systems, using pattern recognition analysis and high sensitivity to detect and identify chemical species. In this standoff or off-set method, the collection of a sample for analysis is not required. To perform the analysis, a flow cell was used for in situ sampling of a liquid slipstream. A prototype was designed based on attenuated total reflection (ATR) coupled with the QCL beam to detect and identify chemical changes and be deployed in hostile environments, either radiological or chemical. The limit of detection (LOD) and the limit of quantification (LOQ) at 3𝜎 for hydroxylamine nitrate ranged from 0.3 to 3 and from 3.5 to 10 g⋅L−1 , respectively, for the nitrate system at three peaks with wavelengths between 3.8 and 9.8 𝜇m. 1. Introduction The monitoring of chemical processing in hazardous or extreme conditions challenges methods that rely on sampling followed by offline analysis. Continuous processes with reactive species are particularly difficult to control and would benefit from active online monitoring of reagents or products or both. Nuclear isotope separations depend on careful control of redox chemistry, using reactive species such as hydroxylamine nitrate, HAN, to change the oxidation state of actinides dissolved in aqueous solution. Hence, we describe a spectroscopic method that could be used to monitor HAN reactions in real time. Because its flexibility, the method could be applied to any aqueous species with absorption in the midinfrared. Vibrational IR spectroscopy is a tool that offers the selectivity required for identifying molecular species as IR absorptions are characteristic and specific to molecular groups. Vibrational spectra can be interpreted to give thermal energies of IR-active compounds, allowing these to be included in chemical kinetic and dynamical models. Traditionally, IR transmittance is not utilized to characterize aqueous solutions because of the absorption of H2 O, but advances in Mid-IR FTIR and the incorporation of the attenuated total reflection accessory (ATR) make it usable for aqueous solution chemistry [1–3]. The same principle of minimizing matrix absorption by using ATR was employed here. As an IR source, we used a set of four quantum cascade lasers (QCL). Potential advantages of the QCL system over a broadband source such as that used in an FTIR include portability because an evacuated light path is not required, spectral resolution based on the laser linewidth, and enhanced sensitivity through high peak power of the excitation laser [4–7]. A recent study by Pengel and colleagues [8] demonstrated the feasibility of using QCLs to monitor chemicals in solution in a static system and Alcaráz and colleagues used an external cavity QCL for measurements of proteins in the mid-IR [9]. In the work described here, the goal was to demonstrate the capability of a QCL-ATR compact and off-set system to continuously monitor (with samples taken every minute) an aqueous phase reaction in a nuclear application. 2 International Journal of Analytical Chemistry The utility of QCL standoff detection of molecules has been demonstrated in the solid and gas phase at ORNL, e.g., methane in field experiments and in the detection of explosive dust collected on solid surfaces [10–16]. However, many chemical processes occur in solution phase, and involve different molecules with distinguishing functional groups. In aqueous solution, there are two issues that need to be addressed, the high background and spectral selectivity. Hence, Raman is usually the method of choice for vibrational spectroscopy as it avoids background absorption from H2 O. Because of selection rules, Raman is generally much less sensitive than IR absorption, unless methods such as surface enhanced Raman are used [17, 18]. For instance, Raman has been used to monitor the degradation of anion-exchange resins used for the separation of plutonium isotopes in highly acidic conditions, e.g., Buscher et al. [19]. Van Staden and colleagues cite a detection limit for both nitrate and nitrate as 500 mg/L [20]. Resonance Raman has been used to study nitrate and nitrite in wastewater treatment processes, with detection limits of 7 𝜇g [21]. This method depends on far UV excitation; however, this method becomes unfeasible for use in applications involving high concentrations of nitrate because self-absorption becomes problematic at concentrations above 3.5 mM. The QCL-ATR system was used to monitor and assay nitrate-nitrite chemistry representative of the process for plutonium-238 production for NASA deep space missions. The chemical processing of neptunium-237 targets after irradiation involves several steps to (a) separate fission products, (b) separate the neptunium and plutonium, and (c) make purification and polishing. This process achieves separation of neptunium and plutonium through redox chemistry and selective liquid-liquid extraction from nitric acid solution (where the target is dissolved) in a tributyl phosphate (TBP)-organic mixture. Recovery of the plutonium from the organic phase needs introduction of hydroxylamine nitrate (NH3 OH+ ⋅NO3 − ) or HAN that is used to reduce Pu(IV) to Pu(III). Hydroxylamine, or HA, is classified as a self-reactive substance [22]. The autocatalytic reaction scheme that takes place in nitric acid solution is given in Reaction (1), showing the conversion of nitric to nitrous acid [23, 24], and the decomposition of NH3 OH+ , Reaction (2). 2HNO3 + NH2 OH 󳨀→ 3HNO2 + H2 O (1) HNO2 + NH3 OH+ 󳨀→ N2 O + 2H2 O + H+ (2) As Reactions (1) and (2) progress consuming NH3 OH+ , in strong nitric acid the amount of HNO2 can increase causing an uncontrolled reaction that can affect the recovery of the plutonium. Hence, it is important to be able to monito (...truncated)