In the Array of Micromachined UltraSonic Electrosprays (AMUSE) , the process of droplet formation and droplet charging are decoupled: an RF signal transmitted to a piezoelectric transducer. The ultrasonic wave in the sealed analyte reservoir forms a pressure gradient that emits droplets from the microarray (Figure ). Droplet charging is further induced by applying a DC potential difference between the array and the mass spectrometer entrance.