Future legislation like EURO IV regulation will require massive reduction of particulate emission limits. Besides continuous improvement of the engine combustion process also exhaust aftertreatment systems, like Diesel Particulate Filters (DPF), or Selective Catalytic Reduction (SCR) of Nitrogen Oxides, are under investigation to meet that goal. For the development of the complete powertrain system, which means the engine and the aftertreatment device, with reduced particulate emissions, there is not only demand for high sensitive Smoke and Particulate Measurement, but also for exhaust gas conditioning to enable gas and particulate measurement at the high pressures and temperatures in the exhaust duct when sampling in front of e.g. a particulate trap. Sampling in front of the aftertreatment system as well as behind that device is required when evaluating the catalytic or filter efficiency. Emission test equipment is typically equipped with automatically cleaning functions in order to ensure long time stability of the measuring signal and long service intervals. This is usually performed by purging modes with clean air. On the other hand oxygen within the purge air is influencing the catalytic behaviour of the aftertreatment system which needs to be absolutely avoided.