Raman spectroscopic sensors for inorganic salts

Raman sensors are more and more used for various applications. In particular they were recently used for in situ and real time monitoring of inorganic substances in aqueous solution in the sea, in tanks or in vicinity of industrial sites. Among several advantages they provide the identification of many species with a fairly good component resolution and the accurate determination of their concentration. Raman probes are based upon different data treatment of the Raman spectra. Two main methods are emphasized here. In the first method suitable for diluted salts with polyatomic anions, the Raman spectrum exhibits a large and well resolved peak used as the direct signature of a peculiar anion to be detected. Salts with monoatomic anions are studied via their indirect influence on the large OH stretching band of the water. The advantages and limitations of each method are discussed and illustrated with some examples derived from our own experience. The process giving the quantification in the analysis of the species is described, with a particular attention devoted to the data treatment within the calibration procedure. Analysis with lineshape treatment and chemometric methods are both reported and discussed. A comparison with different approaches reported in the literature is provided. Finally, the main devices used for in situ monitoring are briefly described.