A mass filter package for atmospheric sampling in the stratosphere has been designed and constructed so that ambient positive ions can be detected in one mode of operation (PI) and ambient negative ions in a second mode of operation (NI). The range of the quadrupole mass filter is 1-150 amu. Any one of the 150 mass positions can be selected for a fixed peak operation, or the total range can be covered in a continuous sweep at the rate of 300 amu/s. Filtered ions are individually counted. The atmosphere is sampled through a valved 0.03-cm-diam orifice which has a conductance of about 9 x (10(-3)) l/s and width-to-thickness ratio greater than 10. Ambient temperature, ambient pressure, atmospheric electrical conductivity, and cosmic ray ionization detectors are also part of the package. An on-board clock and logic circuitry control the gathering of data. However, the mass filter operations can also be preferentially controlled via telemetry. Data are concurrently recorded with an on-board digital tape recorder and telemetered to earth for real-time data analysis. The entire package has been designed and constructed to minimize contamination of the atmosphere being sampled. Preliminary results from a recent flight show evidence for the presence of hydrated protons in the stratosphere.