A residence time chamber and sampling apparatus for use in a system for sampling emission products from an emissions source, for example combustion engines including gasoline, diesel and natural gas engines, for subsequent measurement and analysis of the emission products. The results of the analysis can be used to formulate decisions on changes in engine design strategy, and can be used to determine the effectiveness of aftertreatment systems on the emissions source. The residence time chamber includes a plurality of isoaxial sampling probes, with a plurality of sampling trains connected to the sampling probes to take simultaneous representative emission samples for subsequent analysis. The residence time chamber minimizes many noise factors which can affect the accuracy of the test system, by applying isokinetic sampling and by reducing the interaction of the sampling gas and the particulate matter with the sampling probe inlets and with the various surfaces of the residence time chamber.