Selective Monitoring of Oxyanion Mixtures by a Flow System with Raman Detection

Raman spectroscopy is a selective detection system scarcely applied for the flow analysis of solutions with the aim of detecting several compounds at once without a previous separation step. This work explores the potential of a portable Raman system in a flow system for the selective detection of a mixture of seven oxyanions (carbonate, sulphate, nitrate, phosphate, chlorate, perchlorate, and thiosulphate). The specific bands of these compounds (symmetric stretching Raman active vibrations of carbonate at 1068 cm⁻¹, nitrate at 1049 cm⁻¹, thiosulphate at 998 cm⁻¹, phosphate at 989 cm⁻¹, sulphate at 982 cm⁻¹, perchlorate at 935 cm⁻¹, and chlorate at 932 cm⁻¹) enabled their simultaneous detection in mixtures. Although the oxyanions’ limit of detection (LOD) was rather poor (in the millimolar range), this extremely simple system is very useful for the single-measurement detection of most of the oxyanions in mixtures, without requiring a previous separation step. In addition, quantitative determination of the desired oxyanion can be performed by means of the corresponding calibration line. These are important advantages for controlling in-line processes in industries like those manufacturing fertilizers, pharmaceuticals, chemicals, or food, among others.