Changes in the resonance frequency (Δf) are primarily related to mass uptake or release at the sensor surface. When employed as a mass sensor, the instrument has a sensitivity of about 0.5 ng/cm according to the manufacturer. Changes in the dissipation factor (ΔD) are primarily related to the viscoelasticity (softness). The softness, in turn, often is related to structural changes of the film adhering at the sensor surface.