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Bag emission measurements on Ultra Low Emission Vehicles require measurement sensitivities in the 1 ppm range for HC and NOx and measurement resolutions well below this to obtain sufficient accuracy and repeatability. Additionally, an analysis of the C2 to C12 components is required. In these emission ranges, adsorption, desorption, diffusion and chemical reaction processes may produce significant effects to the measuring values. Therefore, improvements are necessary to avoid this as far as possible. However, for physical reasons these effects cannot be eliminated completely. For example: Particle filters are not 100% efficient and particles will slowly contaminate the surfaces. Due to physical and chemical processes with some gas components, even stainless steel and Teflon can change their characteristics. Problems resulting from the physical and chemical effects and provisions to minimize the influences to the measuring accuracy and system stability are discussed.