Abstract: A mass analyzer in which an ion signal, typically from a plasma, travels through an orifice into a vacuum chamber, and through a focussing system in the vacuum chamber into a mass spectrometer and ion detector. Drift of the detected ion signal, and differences in drift of the detected ion signal for different elements, are greatly reduced by a small shadow stop placed in the vacuum chamber immediately behind the orifice. The shadow stop and plate containing the orifice are both preferably grounded. The focussing system includes a Bessel box lens, and drift is further reduced by insulating the stop in the Bessel box lens from the barrel and biasing the Bessel stop differently from the barrel.