Evaluation of Method 25 Nonmethane Organic Analyzer Design

Under contract to the U.S. Environmental Protection Agency, Research Triangle Institute has been conducting research to improve the precision, accuracy and limit of detection attainable with the EPA Method 25 nonmethane organic (NMO) analyzer. In Method 25, volatile organic carbon (VOC) samples are collected by drawing gas from an emitting source through a dry ice cooled sample trap and into an evacuated collection tank. The hydrocarbon concentration emitted from the source is determined on a per-carbon basis by catalytically converting the trap and tank sample fractions to CO₂ and quantitating the amount of CO₂ produced using the NMO analyzer. A reduction catalyst evaluation led to the selection of an NMO analyzer reduction catalyst which operates at a moderate temperature and displays no appreciable effect on peak shape. A gas chromatographlc column system which provides better permanent gas separation and hydrocarbon quantitation was also selected for use in the NMO analyzer.