The monopole spectrometer uses an rf electric field between a two‐electrode configuration for analyzing ions of differente/m. One electrode has a right‐angle cross section, the other has a circular cross section whose center is located on the bisector of the right angle. The properties of the spectrometer are derived from an approximate equation for the ion motion. An instrument is described which achieves a sensitivity of 0.3 mA/mm Hg atm/&Dgr;m=190 with good peak shape, using an analyzing field of 27‐cm length.