Abstract: An assembly for use in a vacuum treatment process, the assembly including a process changer 1. Gas analysis apparatus to sample and analyse the gas composition within the chamber 1 is provided. The gas analysis apparatus includes a measuring apparatus 14 based either on a miniature mass spectrometer or on a miniature plasma source, which is located within an elongate housing 18. Part of the housing 18 is located within the process chamber 1 such that the gas is analysed within the chamber 1.