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We present ppt-level detection of dimethyl methylphosphonate (DMMP), a common simulant used in detector calibrations for sarin gas, using a new capacitive micromachined ultrasonic transducer (CMUT) design with a mass sensitivity improved from 130 zg/Hz/μm2 to 48.8 zg/Hz/μm2. A low-noise oscillator using the CMUT as the frequency selective component exhibits an Allan deviation of 0.55 Hz at the presence of air flow. The CMUT resonant sensor was functionalized with a 50-nm thick proprietary polymer layer (Sandia National Laboratory, Albuquerque, NM). Our sensor performance was reliably measured at the MIT Lincoln Laboratory where the accurate delivery of low concentrations of gases was ensured through stringent calibrations. Our sensor response to various concentrations of DMMP in air (10 ppb-1 ppm) showed an excellent volume sensitivity of 30.6 pptv/Hz. Based on the system noise floor, we achieved a mass resolution of 26.9 zg/μm2 and a limit of detection of 16.8 pptv. In addition, the sensor showed good selectivity to DMMP.