When operated as a mass sensor, the QCM-D is often used to study molecular adsorption/desorption and binding kinetics to various types of surfaces. In contrast to optical techniques such as surface plasmon resonance (SPR) spectroscopy, ellipsometry, or dual polarisation interferometry, the QCM determines the mass of the adsorbed film including trapped solvent. Comparison of the "acoustic thickness" as determined with the QCM and the "optical thickness" as determined by any of the optical techniques therefore allows to estimate the degree of swelling of the film in the ambient liquid. The difference in dry and wet mass measured by QCM-D and MP-SPR is more significant in highly hydrated layers as can be seen in.