Accurate Procedure for Determining the Calibration Curve of High-Temperature Molten Salt Systems via Raman Spectroscopy

Abstract

The measurement of the intensity of a Raman spectral peak presents one particular difficulty which does not appear in the measurement of absorption spectra of any kind. It is possible in absorption measurements to establish the absorption of the investigated substance by a direct comparison with the unimpaired beam which emerges from the same light source at any wavelength, continuously in two-beam instruments. A direct comparison is not possible for various reasons in case of the usually very weak intensity of the Raman emission, so the determination of a calibration curve in Raman scattering spectroscopy is difficult. The conventional method for obtaining a calibration curve of a Raman-active species is by the addition of an internal standard or by the use of solvent Raman signals as an internal standard. The former method is not suitable for molten salt systems at high temperature. For a molten salt system having no Raman signals other than Rayleigh scattering, the latter method is not applicable either. Here, we want to give a simple yet accurate method of determining calibration curves in such systems. The essence of the method is the use of the shoulder of the Rayleigh line as the normalization point, i.e., the internal standard. We illustrate here the basic principle of the method via the determination of a calibration curve for MgCl2 in a four-component composite melt at 720°C. However, the general method can be extended to determine calibration curves for other molten salt systems.