On-line analysis of process gas during the production of ketene

A process for the on-line analysis of a reactive gas mixture in particular of a gas mixture comprising ketene and/or diketene and/or acetic anhydride and/or acetic acid, which gas mixture is situated in a vessel at a first pressure, which comprises expanding a portion of the gas mixture into a sampling line via a probe to a second pressure which is at least ten times lower than the first pressure in order to slow down the reactions of the reactive gas mixture and feeding it to a mass spectrometer for analysis where a further pressure reduction is performed on entry of the gas mixture into the mass spectrometer.