A study on Raman spectroscopic scheme enabling fast and accurate determination of calcite concentration in gypsum

Abstract This study intended to develop a potential Raman spectroscopic measurement scheme able to simply and accurately determine the calcite concentration (impurity) in gypsum. The sample coverage area for measurements was a major concern since it directly influenced on the representation of sample composition. Initially, 16 synthetic gypsum samples containing varying calcite contents (0.01 ∼ 5.1 wt%) were prepared. For Raman spectral collection, both small spot illumination (50 μm in diameter) and wide area illumination (WAI, 6 mm in diameter) schemes were employed. When the microscope-based (small spot) scheme was used for the analysis, the accuracy of calcite concentration determination was not satisfactory since the sample coverage area was too small to average out the local compositional heterogeneity. The accuracy substantially improved by using the WAI-Raman measurement due to better representation of calcite concentrations in the samples. Next, when real process gypsum samples containing other minor impurities such as carbon residues were analyzed, the fluorescence background appeared due to the colored impurities. Nonetheless, the calcite peak clearly presented on top of the fluorescence background and thereby determination of calcite concentration was feasible. Again, the accuracy of WAI-Raman measurement was superior to the small-spot measurement. In overall, the more correct representation of calcite concentrations in gypsum samples by the WAI scheme was responsible for the improved accuracy.