Total Gaseous Organic Concentration Using a Flame Ionization Analyzer – USEPA Method 25A

USEPA Method 25A is used to determine total gaseous organic concentration of vapors consisting primarily of alkanes, alkenes, and/or arenes (aromatic hydrocarbons).  The concentration is expressed in terms of propane (or other appropriate organic calibration gas) or in terms of carbon.
 
A gas sample is extracted from the source through a heated sample line and glass fiber filter to a flame ionization detector (FID) analyzer.  Results are reported as volume concentration equivalents of the calibration gas or as carbon equivalents.
 
Direct measurement of an effluent with a FID analyzer is appropriate with prior characterization of the gas stream and knowledge that the detector responds predictably to the organic compounds in the stream.  If present, methane (CH₄) is also measured.  The FID is used under any of the following limited conditions:

• where only one compound is known to exist;
• when the organic compounds consist of only hydrogen and carbon;
• where the relative percentages of the compounds are known or can be determined, and the FID responses to the compounds are known;
• where a consistent mixture of the compounds exists before and after emission control and only the relative concentrations are to be assessed;
• where the FID can be calibrated against mass standards of the compounds emitted.

Another example of the use of a direct FID is as a screening method.  If there is enough information available to provide a rough estimate of the analyzer accuracy, the FID analyzer is used to determine the VOC content of an uncharacterized gas stream.  With a sufficient buffer to account for possible inaccuracies, the direct FID is a useful tool to obtain the desired results without costly exact determination.
 
In situations where a qualitative/quantitative analysis of an effluent stream is desired or required, a gas chromatographic FID system may apply. However, for sources emitting numerous organics, the time and expense of this approach is formidable.