An apparatus and method for removing neutral noise from a quadrupole mass filter ion beam. A mask plate has a lobed aperture centered on a longitudinal axis and positioned between a quadrupole mass filter exit end and an ion detector. The mask plate operates to remove neutral atoms from the ion beam that may interfere with instrument sensitivities. The lobed aperture passes the ion beam with little loss of the ion beam intensity. The invention substantially maintains signal intensity and removes unwanted noise from a mass spectrometer.