A method of computation to determine the trajectories of large numbers of ions in quadrupole mass spectrometers has been developed. The computer program produces simulated mass scans with over 10(5) ions injected into the quadrupole model at each point on the mass scale. The computation method has been combined with an improved representation of an ion source to predict the behavior of complete quadrupole systems; the results obtained are shown to reproduce most features of experimentally observed mass scans. The calculations show that the shape and height of mass peaks depend strongly on the properties of the ion source. However, ion source properties have a relatively small effect on the resolution of the mass spectrometer and an alternative to the common method of presenting the resolution of a quadrupole is proposed. This alternative better indicates the performance of the instrument as it is less sensitive to effects of the ion source. Results are also presented which indicate that peak splitting, closely matching experimental observations, can be produced by small changes in the exit aperture of the quadrupole.