The infrared (IR) spectrum of a sample in a saline solution cannot be retrieved adequately when the spectrum of pure water is subtracted. Some water bands remain in the spectrum. The retrieved spectrum is good only when the spectrum of NaCl in water at the right concentration is subtracted. However, when the concentration of the salt is unknown, it is still possible to obtain a good spectrum of the sample free of water. This is done by subtracting from the original spectrum a fraction of two eigenspectra of water: one for pure water and one for NaCl solvated water. The fraction of the eigenspectra is determined with the 2100 and 3300 cm -1 bands. The effectiveness of the method is illustrated with a solution of sodium monochloroacetate in a saline solution.
Jean-Joseph Max, Michel Trudel, and Camille Chapados, "Subtraction of the Water Spectra from the Infrared Spectrum of Saline Solutions," Appl. Spectrosc. 52, 234-239 (1998)