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This paper describes a highly sensitive, resonant mass sensor (built on a micromachined diaphragm with a piezoelectric thin film) that can operate in vapor and liquid. In vapor, the sensor can detect a mass change of 10-9 g/cm2 on its surface, comparable with that detectable by a typical quartz crystal microbalance (QCM). In liquid, by using an apparent second harmonic resonance of the resonator, a minimum 10 ppm resonant frequency shift can be detected. This paper presents also a new measurement technique that allows us to detect resonant frequency shift as small as 0.3 ppm when the resonant quality (Q) factor is about 250.