Measurement of Gaseous Organic Compound Emissions by Gas Chromatography – USEPA Method 18

USEPA Method 18 demonstrates how to determine the measurement of gaseous organics emitted from an industrial source.  While designed for ppm level sources, some detectors are quite capable of detecting compounds at ambient levels, e.g., ECD, ELCD, and helium ionization detectors.
 
The major organic components of a gas mixture are separated by gas chromatography (GC) and individually quantified by flame ionization, photoionization, electron capture, or other appropriate detection principles.  The retention times of each separated component are compared with those of known compounds under identical conditions.  Therefore, the analyst confirms the identity and approximate concentrations of the organic emission components beforehand.  With this information, the analyst then prepares or purchases commercially available standard mixtures to calibrate the GC under conditions identical to those of the samples.  The analyst also determines the need for sample dilution to avoid detector saturation, gas stream filtration to eliminate particulate matter, and prevention of moisture condensation.
 
The lower range of this method is determined by the sampling system; adsorbents can be used to concentrate the sample, thus lowering the limit of detection below the 1 part per million (ppm) typically achievable with direct interface or bag sampling.  The upper limit is governed by GC detector saturation or column overloading; the upper range is extended by dilution of sample with an inert gas or by using smaller volume gas sampling loops.  The upper limit is also governed by condensation of higher boiling compounds.
 
The sensitivity limit for a compound is defined as the minimum detectable concentration of that compound, or the concentration that produces a signal-to-noise ratio of three to one.  The minimum detectable concentration is determined during the presurvey calibration for each compound.
 
This method will not determine compounds that:

• are polymeric (high molecular weight);
• can polymerize before analysis;
• have very low vapor pressures at stack or instrument conditions.