Cross-sensitivity plagues gas sensors in Figure : A gravimetric sensor comprised of a thin film of metal-organic framework (MOF) (the recognition element), in this case HKUST-1 , mounted on a quartz crystal microbalance (QCM) (the signal transducer). The QCM measures the (total) adsorbed mass of gas, m, in the thin film of MOF. The QCM-MOF functions as a sensor because changes in the gas composition, x, are expected to cause changes in m.