Near-infrared spectra of aqueous solutions were collected by using evanescent-field fiber-optic sensors. The spectra of two- and four-electrolyte solutions were analyzed with a partial least-squares (PLS) algorithm to quantify the measurements. Our results indicate that the electrolytes can be identified and simultaneously determined by using the PLS calibration model. Electrolytes in opaque aqueous mixtures were also studied, and the results are compared with results from clear aqueous solutions.
Ling Han, Donald Lucas, David Littlejohn, and Steven Kyauk, "NIR Fiber-Optic Method with Multivariate Calibration Analysis for Determination of Inorganic Compounds in Aqueous Solutions," Appl. Spectrosc. 54, 1447-1452 (2000)