Using this method, it is possible to measure the concentration of solute in the liquid phase not just in the neat liquid but throughout phase changes such as during crystallisation. In liquid systems, the coupling to photons to the vibrational density of state (VDOS) forms the so-called microscopical peak, which is responsible for an increasing absorption coefficient with frequency observed in the terahertz region. In crystalline systems, the VDOS is depleted and instead spectral peaks are formed at discrete frequencies, thereby decreasing the baseline. Hence, terahertz spectroscopy can probe liquid relaxation dynamics as well as vibrational transitions in crystals simultaneously, represented by the baseline and the peaks on a spectrum, respectively. It is demonstrated at the example of the crystallisation of the model system magnesium sulfate heptahydrate.