A QCM is a quartz crystal between two gold electrodes. Applying an alternating voltage across the piezoelectric crystal induces vibrations. When gas ad-/de-sorbs into/out of the thin film of ZIF attached to the top of the QCM, increasing/decreasing its mass, the frequency of the vibrations of the QCM decreases/increases. Using the Sauerbrey equation, we can convert the change in vibration frequency of the QCM to a change in the mass of the thin film of ZIF due to the ad-/de-sorption of gas.