An ion source for a mass spectrometer is disclosed comprising a lens 308 and mirror 303 arrangement which focuses a laser beam 302 onto the upper surface of a target substrate 305. The lens 308 has an effective focal length ≤ 300 mm. The laser beam 302 is directed onto the target substrate 305 at an angle θ with respect to the perpendicular to the target substrate 305, wherein θ ≤ 3°. One or more ion guides 311 receive ions released from the target substrate and onwardly transmit the ions along an ion path which substantially bypasses the lens 308 and mirror 303.