Applying Two-Dimensional Correlation Spectroscopy and Principal Component Analysis to Understand How Temperature Affects the Neptunium(V) Absorption Spectrum

Abstract

The visible-near infrared (Vis-NIR) electronic absorption spectrum of neptunium(V) (NpO2+) comprises numerous f-f electronic transitions with mostly undocumented temperature dependencies. The effect of temperature on the absorption spectrum of the pentavalent neptunyl dioxocation (NpO2+) is an important factor to consider with spectrophotometric applications but has often been overlooked. Optical Vis-NIR absorption spectra (400–1700 nm) of Np(V) (0.017–0.89 M) in 1 M nitric acid were evaluated with varying temperatures (T = 10–80 °C). The intensity, position, and overall shape of the bands were sensitive to interactions with the solvent and coordination environment. Numerous temperature-induced isosbestic points were identified resulting from dynamic, overlapping peak shifts. Spectral variations were characterized using principal component analysis (PCA) and 2D correlation spectroscopy (COS). 2D-COS revealed that the absorption band near 1095 nm likely consists of two bands centered near 1087 and 1096 nm, which cannot be explained by current computational methods. 2D-COS analysis also provided an unambiguous assignment of unresolved peaks in the visible region for comparison with computational predictions. PCA was used to identify nonlinearity in the spectral response at elevated Np(V) concentrations ≥ 0.5 M. This unique experimental data and interpretation will foster a deeper understanding of the absorption spectra for complex actinyl ions.

Quantifying neptunium oxidation states in nitric acid through spectroelectrochemistry and chemometrics

“…Strong prediction performance is achieved when RMSEP is less than or equal to 5%, satisfactory performance when RMSEP is between 5% and 10%, and indicative performance when RMSEP is between 10% and 15%. (Sadergaski and Morgan, 2022) The deviation (i.e., uncertainty) in predicted Y-values (i.e., concentrations) for each individual sample was estimated as a function of the global model error, sample leverage, and residual X-variance. (Vries and Ter Braak, 1995) Standard error of prediction (SEP) and bias were also used to evaluate prediction performance.…”

“…Many studies have reported characteristic electronic spectra of Np oxidation states ranging from Np(III) to Np(VI). (Sjoblom and Hindman, 1951;Ryan, 1960;Friedman and Toth, 1980;Ban et al, 2014;Chatterjee et al, 2017;Sadergaski and Morgan, 2022). However, similar redox potentials, differing chemical and redox behavior, the ability to react with certain complexing ligands, and differing stabilities in acid of various concentrations all contribute to the complex redox chemistry of Np and the difficulty in acquiring electronic spectra of single oxidation states.…”

“…Determining the relative proportion of Np oxidation state(s) and total Np concentration by spectrophotometry is challenging because of its complex redox chemistry (adopting oxidation states from +3 to +6), coordination chemistry, interionic associations, and dependence of absorption bands on temperature and solution conditions. (Chatterjee et al, 2017;Sadergaski and Morgan, 2022). Quantifying multiple Np oxidation states is also particularly challenging owing to numerous overlapping peaks.…”

Evaluating Optical Techniques to Characterize Solid State Samples for the Pu-238 Supply Program

“…Spectrophotometry is an important component of the 238 Pu Supply Program for measuring Np and Pu concentrations, oxidation state(s), and monitoring separations. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Spectrophotometry, or absorption spectroscopy, is a technique that quantifies the amount of light absorbed by a sample as a function of wavelength. Ultraviolet (UV)-visible (Vis)-near-infrared (NIR) spectrometers collect spectra at relatively rapid intervals (e.g., 10-1,000 ms) and contain a wealth of information.…”

Partial Least Squares, Experimental Design, and Near-Infrared Spectrophotometry for the Remote Quantification of Nitric Acid Concentration and Temperature

“…Fully integrated spectroscopic monitoring examples in the nuclear field are sparse in part owing to the harsh and restrictive environments often needed to deploy such technologies (e.g., hot cells) [6,7,9]. Two important variables, including (1) temperature and (2) resources (i.e., number of samples), are often overlooked in many laboratory-scale proof of principle studies [10][11][12]. These variables must be accounted for when such technologies are implemented in harsh and restrictive environments such as radiochemical hot cells or caves.…”