Spectral Measurements on Undiluted Samples

Spectral analysis depends heavily on optical density (OD). Depending on the quality of the spectrometer, around an OD of 2, spectral shapes become distorted and results unreliable. Real world samples such as paints, lacquers or foodstuffs often lie far outside this range. Measuring them under process conditions, e.g. during a reaction or cooling, is even harder, because any workaround that alters the sample also alters the very behavior being observed.

The invention uses a draining liquid film on an optical wall as a dynamically variable path length. A light beam passes continuously through the film, capturing spectra at many effective thicknesses in a single run; software automatically selects the optimal optical density region. The setup is simple, supports UV–VIS–NIR–IR depending on the spectrometer, and can be temperature- controlled. Film thickness can be determined interferometrically or via a tracer, enabling absolute absorption values (e.g. molar extinction coefficients). Reactive components can be added upstream so that chemical reactions in the film are monitored in real time.