Monopole mass spectrometer characteristics have been analyzed by determining ion trajectories under various operating conditions. The trajectories were obtained by numerical integration of the equations of motion. Peak shapes were derived for ions spending 20 rf cycles in the field, for various entrance/exit slit ratios, with ion injection parallel to the instrument axis. Comparison with earlier results shows that resolutions at half-height can be expressed as (n/p)2, where n is the number of cycles an ion spends within the field and p is constant for a given exit slitwidth. Ion injection at an angle to the instrument axis was also considered. Peaks then showed much less spread on the low mass side. For ions entering the field at a fixed angle, the peaks may exhibit a central dip if the exit slitwidth is too small.