Quartz crystal microbalance
A quartz crystal microbalance (QCM), also known as quartz microbalance (QMB) and sometimes also as quartz crystal nanobalance (QCN), measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to oxide growth/decay or film deposition at the surface of the acoustic resonator.
Source: Wikipedia — Quartz crystal microbalance (CC BY-SA 4.0)