Non-destructive determination of TiO2 concentration in cream formulation using Raman spectroscopy

The concentration of TiO2 in a cream formulation has been non-destructively measured using Raman spectroscopy without further sample pretreatments. The distribution of TiO2 particles in a highly viscous cream may not be homogeneous on a microscopic scale and local aggregation of the particles is possible; therefore, acquisition of Raman spectra capable of representing the whole sample identity rather than the localized chemical information was critically necessary to ensure reliable quantitative analysis. A wide area illumination (WAI) scheme, applying 6-mm diameter laser illumination area (total coverage: 28.3 mm2) on a sample, was used to achieve representative sample presentation and improved accuracy. When Raman spectra were collected for the same sample using a conventional scheme with a much smaller laser spot (∼100 μm), the spectral reproducibility was degraded, as expected. The local aggregation of TiO2 particles was confirmed by analyzing both SEM (scanning electron microscopy) and Raman images of a cream sample. When the WAI scheme was used, the resulting error in the determination of TiO2 concentration was 0.03 wt%, while 0.17 wt% in the case of using the conventional scheme. The proposed Raman scheme would be advantageous over the conventional titration method that requires destructive incineration of the organic cream matrix, as well as a large consumption of chemical reagents.