Evaluation of the spectral information acquired by an in-line high-speed near-infrared reflection sensor in a tableting environment

Near-infrared (NIR) spectroscopy is widely used in process analytical technology during manufacturing of tablets. Therefore, the aim of the present study was to investigate an NIR sensor capable of in-line monitoring of tablets during manufacturing regarding the influence of the tablet temperature on the spectral data and the relevance of the effective penetration depth of the NIR radiation. The influence of the temperature was assessed by subjecting two tablet formulations to a temperature program while simultaneously recording NIR spectra. Whereas chemometric methods, such as differentiation techniques and variable selection merely suppressed the temperature influence, a novel correction method based on the linearity between temperature and NIR spectra showed promising results in reducing this influence, while maintaining the relevant spectral features. To determine the effective NIR penetration depth of the tablets, bilayer tablets with a bottom layer consisting of polytetrafluoroethylene and a top layer with variable thickness consisting of a common immediate-release tablet formulation were used. The effective NIR penetration depth profiles showed a wavelength dependence and strongly correlated with the reflection spectra of the top layers. In conclusion, the temperature dependence of the spectral data and the effective NIR penetration depth are useful for the design of NIR calibration models.