Fractional number density measurements for a rf plasma "needle" operating at atmospheric pressure have been obtained using a molecular beam mass spectrometer (MBMS) system designed for diagnostics of atmospheric plasmas. The MBMS system comprises three differentially pumped stages and. a mass/energy analyzer and includes an automated beam-to-background measurement facility in the form of a software-controlled chopper mechanism. The automation of the beam modulation allows the neutral components in the plasma to be rapidly and accurately measured using the mass spectrometer by threshold ionization techniques. Data are reported for plasma generated by a needle plasma source operated using a helium/air mixture. In particular, data for the conversion, of atmospheric oxygen and nitrogen into nitric oxide are discussed with reference to its significance for medical applications such as disinfecting wounds and dental cavities and for microsurgery. (c) 2006 American Vacuum Society.