The effect of large, changing concentrations of electrolytes on the behavior of the OH stretching band of water have been investigated with the aim of developing methods for compensating for spectral interferences when solute NH bands are made the basis for mixture analyses. With the use of urea and ammonium salts as analytes, it was found that changing electrolyte concentrations affect the shape of the water band but do not appreciably affect the shapes of either the ammonium ion or urea Raman lines. Chlorides, nitrates, and mixtures of these were used as electrolytes. The identity of the anion had a significant effect on the shape of the OH band. Two methods of compensation were used. One involved factor analyzing the spectra of a set of solutions that contained chlorides and nitrates that are Raman inactive in the vicinity of the OH stretching band. The principal abstract factors were used in place of a water reference for a least-squares mixture analysis. The other method was application of partial least-squares. In addition to urea and ammonium ion, the concentration of KCl and the ionic strength of the system can be determined in the partial least-squares approach with limits of detection better than 0.1 M.
Hsiaoling Wang, Chao Wang, Charles K. Mann, and Thomas J. Vickers, "Effect of the Water Spectrum on the Determination of Dissolved Ammonia, Urea, and Chloride Concentrations by Raman Spectroscopy," Appl. Spectrosc. 52, 240-249 (1998)