Abstract: An apparatus and method for providing mass-selected particles. A mass filter adapted to receive a flux of particles at its entry emits a mass-selected subset of the received flux of particles at its exit, and an electrode configuration provides electric fields for deflecting at least some of the mass-selected subset of particles thus emitted across a substantially open spatial region through substantially 180 degrees of arc. The deflected particles can be collected, converted, fragmented, or detected. The apparatus and method are suitable for use with a multipole (e.g., quadrupole) mass filter, and the electrode configuration can easily be retrofitted to existing installed multipole mass spectrometers as well as adapted to newly manufactured designs.