A sample fluid (14) is pumped through a first cavity (38) associated with a first piezoelectric resonator (20.1) and pumped through a second cavity (40) associated with a second piezoelectric resonator (20.2). An electrode (26) of the first piezoelectric resonator (20.1) exposed to the sample fluid (14) in the first cavity (38) is coated with an adsorption layer (34.1) that provides for adsorbing a substance (12) to be detected in the sample fluid (14). The adsorbed substance (12) changes the resonant frequency of the first piezoelectric resonator (20.1) relative to that of the second piezoelectric resonator (20.2), wherein a change in the frequency difference therebetween relative to an initial frequency difference is responsive to and provides a measure of the mass of adsorbed substance (12). The adsorption layer (34.1) of the first piezoelectric resonator (20.1) is automatically refreshed when a change in the frequency difference crosses a threshold (ΔFEOR).