an ion source for a mass spectrometer is disclosed comprising a lens and mirror arrangement which focuses a laser beam onto the upper surface of a target substrate . the lens has an effective focal length ≦ 300 mm . the laser beam is directed onto the target substrate at an angle θ with respect to the perpendicular to the target substrate , wherein θ ≦ 3 °. one or more ion guides receive ions released from the target substrate and onwardly transmit the ions along an ion path which substantially bypasses the lens and mirror .