Concentration determination of inorganic acids that do not absorb near-infrared (NIR) radiation through recognizing perturbed NIR water bands by them and investigation of accuracy dependency on their acidities

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>When near-infrared (NIR) spectroscopy is employed to determine the concentrations of inorganic acids in etchant solutions, the perturbation of the water bands resulting from the interactions with these acids is the only information available for analysis, since inorganic acids do not directly absorb NIR radiation. The water bands perturbed by dissociated hydronium ions and respective anions of the acids are not highly component-specific; therefore, characterization of their different interactions with water is necessary to understand the results of the multivariate quantitative analysis. For this purpose, the variations in the water bands that occurred with the inclusion of HCl, H<SUB>2</SUB>SO<SUB>4</SUB>, H<SUB>3</SUB>PO<SUB>4</SUB>, and HNO<SUB>3</SUB> were initially examined with the aid of a two-dimensional (2D) correlation analysis. According to this analysis, components with higher acidity, such as HCl, perturbed the water hydrogen bonding network more strongly. Also, the interactions between each dissociated ionic species and water molecules were somewhat dissimilar. This dissimilarity enabled differentiation between the four acids, although the spectral specificity was not high owing to the absence of direct NIR absorption. The accuracy in the determination of concentrations was better for components with lower p<I>K</I> <SUB>a</SUB>s, such as HCl and H<SUB>2</SUB>SO<SUB>4</SUB>, owing to their stronger perturbation of the water bands. When multicomponent samples were measured, the resulting accuracies deteriorated due to the further degradation in component specificity. Acquisition of reproducible spectra and avoidance of model over-fitting are critical for reliable NIR analysis of these types of samples, as the analyte-induced spectral features are broad and rather specificity-insufficient.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Inorganic acids (HCl, H<SUB>2</SUB>SO<SUB>4</SUB>, H<SUB>3</SUB>PO<SUB>4</SUB>, and HNO<SUB>3</SUB>) do not directly absorb NIR radiation. </LI> <LI> Varied NIR water band by dissociated H<SUB>3</SUB>O<SUP>+</SUP> and anion was used to determine concentration. </LI> <LI> 2D correlation analysis revealed that interaction of each acid with water molecule differed. </LI> <LI> Acidity governing strength of acid-water interaction directly influenced on accuracy. </LI> <LI> Multiple PCs were required in PLS modeling since diverse acid-water interactions occurred. </LI> </UL> </P>