Abstract: A collision cell for observing the emission spectrum of ions. The substantially enclosed collision cell (8) is bounded by a wall (46), the wall having an entrance aperture (47) and an exit aperture (29) through which an ion beam (5) may be passed to traverse the collision cell, and has an observation region (15) therein adjacent to the entrance aperture and a first window (22) in the wall through which radiation generated in the observation region may pass, and, disposed adjacent to the exit aperture and in communication with the observation region, an exit region (16) comprising radiation-trapping means for minimizing the transmission to the first window of radiation generated in the exit region. The collision cell may be incorporated in a conventional mass spectrometer.