A method and apparatus for determining chemical properties. The method is based on the sequential measurement of the variation of the oscillation frequency of a single sensing device when different chemically sensitive film materials are deposited on both sides of a resonator such as a quartz crystal microbalance (QCM). Each of the sides of the resonator is sequentially exposed to an analyte while another side is exposed to a blank gas. In this way, the analyte-dependent signal from the resonator is generated only from a single film. Measurements are further made by switching the analyte stream to expose the sensing film on another side of the resonator previously exposed to a blank.