We show, by direct numerical simulation, how space charge effects in an ion source give rise to nonlinearities in quadrupole gas analyzers operating at high pressures. Numerical simulations were performed on a model for a generic electron impact ionizer and the calculations included the effects of both the electron and the ion space charge. By comparing the results of the simulations with previous experimental work we conclude that the most important effects arise in the ion source itself rather than at later stages in the mass analysis process. It was not necessary to invoke ion-molecule scattering mechanisms to reproduce the trends in the experimental data. The results show how competition between the negative and positive space charges can lead to either an increase or a decrease in relative sensitivity as the pressure of gas in the source is raised. Our conclusions are extended to a discussion of nonlinearities in other types of partial pressure analyzer and to extractor gauges measuring total pressure.