An atmospheric pressure (AP) interface for a spectrometer includes wall for separating an ionization chamber from a reduced-pressure region of the spectrometer, an ion inlet defining an ion path from the ionization chamber to the reduced-pressure region, and a passage defining a gas path from the ionization chamber to a gas outlet external to the reduced-pressure region. The passage may have a greater gas conductance than the ion inlet such that most gas into the passage and not the ion inlet. The interface device is configured for applying a static electric field effective for focusing ions in the ionization chamber preferentially into the ion inlet.