When is it correct to use the Sauerbrey relation to calculate the mass adsorbed on the quartz crystal?
The Sauerbrey relation describes the linear relation between frequency changes and changes in mass for thin films adsorbing to the sensor surface. It gives a good estimation of mass/ thickness, as long as the dissipation is relatively low. When the dissipation value typically reaches above 1×10-6 per 10 Hz, the film is too soft to function as a fully coupled oscillator- the upper parts, far away from the surface, do not couple to the oscillation of the sensor. This means that the normal relation to calculate the mass directly from the change in frequency, will underestimate the mass. However, by measuring both dissipation and frequency at several harmonics it becomes possible to extract correct thickness estimations even in these cases and also calculate viscoelastic properties using a viscoelastic model incorporated in Q-Sense software.
