A measurement apparatus and method are provided for determining the material composition of a sample. An X-ray fluorescence detector (412) detects fluorescent X-rays coming from said sample under irradiation with incident X-rays. A laser source (301) is adapted to produce a laser beam. Focusing optics (302) focus said laser beam into a focal spot on a surface of said sample. An optical sensor (312) detects optical emissions coming from particles of said sample upon being exposed to said laser beam at said focal spot. A gas administration subsystem (104, 105, 106, 107, 108) is adapted to controllably deliver gas to a space (101) around said focal spot.