What is a piezoelectric sorption detector?

The resonance frequency of a piezo-electric "tuning fork" is influenced by the mass of adsorbed molecules.

I know this kind of instruments as piezo thickness monitors when evaporating metals in vacuum equipment. They can be very sensitive.

Edit: there is a long article on Wikipedia. I did not read it all but it explains how this works. https://en.wikipedia.org/wiki/Quartz_crystal_microbalance

Edit 2: Such a piezoelectric resonator can be functionalized for sorption with a coating as described in 1964 by King in his article "Piezoelectric sorption detector"