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This paper introduces a new resonant mass sensor that is based on a lateral extensional mode (LEM) piezoelectric resonator, and has a minimum detectable mass (MDM) of 10-15g in air at room temperature. The resonator with size of about 200×50×1 μm3has a quality factor (Q) of > 1,400 at 60MHz, and as small as 0.1ppm shift of its resonant frequency can be detected. The 0.1ppm detection capability corresponds to a mass uncertainty of only about 4.6fg. We have experimentally demonstrated a minimum detectable frequency shift of about ~ 1.6ppm due to an absorption of isopropanol vapor (73 fg) on the sidewalls of the parylene-coated LEM piezoelectric resonator.