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 system includes a dilution apparatus, a residence time chamber, a plurality of sampling probes within the residence time chamber, and a plurality of sampling trains connected to the sampling probes to take simultaneous representative emission samples for subsequent analysis. The system has particular use in quantifying chemical and toxic trace species from emissions sources. 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.