USEPA Method 25

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Determination of Total Gaseous Nonmethane Organic Emissions as Carbon – USEPA Method 25

USEPA Method 25 is used for the determination of volatile organic compounds (VOC) (measured as total gaseous nonmethane organics (TGNMO) and reported as carbon) in stationary source emissions.

An emission sample is withdrawn from the stack at a constant rate through a heated filter and a chilled condensate trap by means of an evacuated sample tank. After sampling is completed, the TGNMO are determined by independently analyzing the condensate trap and sample tank fractions and combining the analytical results. The organic content of the condensate trap fraction is determined by oxidizing the NMO to carbon dioxide (CO₂) and quantitatively collecting the effluent in an evacuated vessel; then a portion of the CO₂ is reduced to CH4 and measured by an FID. The organic content of the sample tank fraction is measured by injecting a portion of the sample into a gas chromatographic column to separate the NMO from carbon monoxide (CO), CO₂, and CH₄; the NMO are oxidized to CO₂, reduced to CH₄, and measured by an FID. In this manner, the variable response of the FID associated with different types of organics is eliminated.

When CO₂ and water vapor are present together in the stack, they can produce a positive bias in the sample. The magnitude of the bias depends on the concentrations of CO₂ and water vapor. As a guideline, if the product of the CO₂ concentration, expressed as volume percent, times the water vapor concentration does not exceed 100, the bias is considered insignificant.

Collection of organic particulate matter in the condensate trap produces a positive bias. A filter is included in the sampling equipment to minimize this bias.

This method is not applicable for the determination of organic particulate matter and is not the only method that applies to the measurement of VOC. Costs, logistics, and other practicalities of source testing may make other test methods more desirable for measuring VOC contents of certain effluent streams. Proper judgment is required in determining the most applicable VOC test method.

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Last Update: October 25, 2006