A laser processing machine for processing workpieces includes a beam guide containing a gas atmosphere, and also includes an apparatus for investigating the gas atmosphere in the beam guide for impurities. The investigation apparatus makes use of the photoacoustic effect. The measuring apparatus has a measuring chamber and at least one measuring head, where the beam guide acts as the measuring chamber. As a measuring chamber, the beam guide contains the gas atmosphere that is to be investigated and also a modulated laser beam modulated. The measuring head(s) are integrated into the beam guide and are used to detect the photoacoustic effect in the measuring chamber.