The influence of different acquisition settings and the focus adjustment on Raman spectral maps of pharmaceutical tablets

Abstract Pharmaceutical tablets contain a variety of active substances and excipients which could be monitored by Raman spectroscopy/microscopy. Raman microscopic spectra are affected not only by the sample composition but also by preparation/measurement conditions. The character and variability of surface morphology (of tablet slices), the appropriate levels of focusing and other adjustable settings of the measurement instrumentation represent important parameters to be considered for obtaining relevant and reliable results by Raman microscopic mapping/imaging analysis in the pharmaceutical practice. The aim of the study is to determine the effect of levels of the spotlight, and other instrumental parameters for mapping industrially manufactured pharmaceutical tablets and to find out suitable, preferably robust, parameters for routine analyses. The role of roughnesses produced by commonly used microtome slices of tablets was investigated experimentally by different focusing levels showing relatively weak (statistical) influence on data sets. The spectral quality was evaluated by signal–to–noise ratio. The data sets were processed using two multivariate statistical methods (Principal Component Analysis, Soft Independent Modeling of Class Analogy). Direct Classical Least Squares statistical modeling was optimized to show the distribution of active pharmaceutical ingredient and excipients in Raman maps. The applied different types of data evaluation confirm that the most suitable position regarding the focusing is the middle of tablet's cross-section roughness. However, we have shown that the selection of microscopic measurement modes plays a much more significant role. The selection of proper measurement mode is crucial to describe the actual sample variability and to obtain relevant data for real pharmaceutical samples. The time–consuming requirements of measurement modes need to be considered, as well, to obtain as reasonable data as possible in the given time frame in the applied pharmaceutical analysis.