Optical spectroscopic techniques, such as UV-VIS-NIR and Raman, allow for the atline and inline, real-time analysis of multiple parameters for quality control. These optical process sensors have been used for concentration measurements in various applications and are easily adapted to different sampling methods including fiber-optic immersion probes, reflection probes and flow-through cells. In UV-VIS-NIR spectroscopy, the sample is illuminated by a continuous, white light source, and the transmitted or reflected light measured, allowing the determination of the absorbance of the light as a function of wavelength. The absorption depends on the electronic and atomic structure of the absorbing molecule. The Lambert-Beer law relates the absorption of light to the concentration of the absorbing species.
In Raman spectroscopy, the sample is illuminated using a single-wavelength laser source. The energy of the inelastic scatter, Raman scattering, differs from the input wavelength by discrete amounts, which depends on the vibrational frequencies of the sample. The Raman signal directly correlates with the concentration of the measured molecule.