Oxyanion impurities in reagent grade chemicals may be detected and measured by Raman spectrophotometry. During recent studies of NaNO3, samples were prepared from a bottle of certified reagent grade chemical. A weak band at 981 cm−1 was detected in the Raman spectrum when high sensitivity was employed—conditions that cause the 1051 cm−1 line of NO3− to be much off scale (Fig. 1). The origin of the 981 cm−1 band was traced to SO42− impurity, listed by the manufacturer as 0.002 wt %. Phosphate ion also contributes intensity in this spectral region, but its molar intensity is considerably less than that of the symmetric stretch of sulfate, and interference in this case was not considered significant.
D. E. Irish and J. D. Riddell, "The Use of Raman Spectroscopy for the Quantitative Analysis of Impurities: Sulfate in NaNO3," Appl. Spectrosc. 28, 481-482 (1974)