USEPA Method 25 Non-Methane Organic Compounds

Measurement of total non-methane organics in ambient air and source samples permits a rapid assessment of potential release of contributors to photochemical smog. All possible volatile organics are separated from methane, carbon dioxide, and carbon monoxide. Then the group is oxidized to carbon dioxide and subsequently reduced to methane, to become an effective count of the total carbon atoms in the sample. Native methane, carbon dioxide and carbon monoxide are not included in this grouping, as they are not significantly active in photochemical reactions. Conversion of all organics to carbon dioxide and then to methane to yield the composite peak for non-methane non-ethane compounds makes the measurement an effective carbon counter. Propane can then be used to generate a response factor that then is applied to all organics.<\p>

The protocol was originally developed by South Coast Air Quality Management District (South Coast AQMD), Diamond Bar, CA, nearly 50 years ago and has been adopted by the US EPA as EPA Method 25. It has undergone several revisions since, and is now updated as South Coast AQMD Method 25.3. This method now provides a better separation of carbon dioxide from the other organics, especially for low levels of nonmethane non-ethane organics and very high levels of carbon dioxide. However, the specified column set does not achieve enough separation from carbon dioxide for ethane, ethene and ethyne, and these three must be allowed to elute on the tail of the carbon dioxide in the foreflush mode. Their concentrations are mathematically added to the nonmethane, non-ethane organic compounds, measured from the backflush peak. Since the detector responses for these three hydrocarbons are equal, concentrations for them can be calibrated against any one of them and the response factor can be applied to the others.<\p>

Non-Methane Organic Carbon (NMOC) Analyzer

The Non-Methane Organic Carbon (NMOC) Analyzer is a gas chromatograph configured for analyzing gaseous samples for total organic carbon content, detected as methane. The system is designed to determine concentrations of methane, carbon dioxide, carbon monoxide, ethane/ethene, and all organics from propane and heavier as a single peak. Additional hardware is configured to measure fixed gases (including oxygen, nitrogen, hydrogen, carbon monoxide, and methane). The methodology employed in this analyzer meets the requirements of both EPA Method 25 and South Coast Air Quality Management District (SCAQMD) Method 25.3.

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Brochure for the Non-Methane Organic Carbon (NMOC) Analyzer