The use of normal Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) of cationic-coated, silver substrates to detect nitrate and sulfate ions in aqueous environments is examined. For normal Raman spectroscopy using near-infrared excitation, a linear concentration response was observed with detection limits of 260 and 440 ppm for nitrate and sulfate, respectively. Detection limits in the low parts-per-million concentration range for these anions are achieved by using cationic-coated, silver SERS substrates. Adsorption of the anions on the cationic-coated SERS substrates is described by a Frumkin isotherm.
P. A. Mosier-Boss and S. H. Lieberman, "Detection of Nitrate and Sulfate Anions by Normal Raman Spectroscopy and SERS of Cationic-Coated, Silver Substrates," Appl. Spectrosc. 54, 1126-1135 (2000)