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.