Method 18 Engineering

Measurement of Gaseous Organic Compound Emissions by Gas Chromatography

This method is designed to measure gaseous organics emitted from an industrial source.  Samples are collected using a Tedlar bag, adsorbent tube, or by a direct interface and analyzed by GC with an appropriate detector which is typically an FID (flame ionization detector).  The method should not be used for compounds that  (1) are polymeric (high molecular weight), (2) can polymerize before analysis, or (3) have very low vapor pressures at stack or instrument conditions. 

This method as written when used for compliance samples has very strict procedures concerning sample collection, analysis, and associated quality control requirements.  However, this method is very popular for use with purposes other than compliance testing.  Many of the requirements involving triplicate injections and spiked sample analyses will not be performed at the client's request to lessen the analytical costs when the data are for internal or engineering process purposes. As such, many of the stricter requirements will be discussed in greater detail in Method 18 Compliance.

Tedlar bag samples are collected using a lung system or vacuum box to draw a dry sample into the bag.  In this sampling technique, the sample gas never passes through the sampling pump used to draw the vacuum.  Moisture in the stack gas should be removed before the sample enters the bag.  Tedlar bags are appropriate for non-reactive compounds and compounds that will not adhere to the surface of the bag (not alcohols or amines).  Bags should never be filled more than 50% full if they will be transported to the laboratory for analysis via air courier to allow room for the contents of the bag to expand and not burst during transit. 

Adsorbent tubes are collected using a sampling train consisting of a probe, sampling pump and midget impinger located prior to the adsorbent tube to remove the moisture in the stack gas.  This midget impinger is not needed if the moisture levels are not expected to exceed 2-3%.  The adsorbent tube should be in a vertical position in the sampling train to prevent channeling.  The type of adsorbent tube used for sampling should be selected based upon its performance in collecting the analytes of interest.  There are numerous resources (NIOSH, OSHA, tube manufacturer's) to consult for tube selection.  The method requires a check for breakthrough, if thought to be a problem,  which means the front and back half portions of the adsorbent tube must be analyzed separately.  The breakthrough requirement is that an analyte should not be found on the back half at a level greater than 10% of the total on the tube.

The method does not specify any holding time from collection to analysis, however, it is recommended to analyze the samples as soon as possible especially if collection is in a Tedlar bag.

The compounds listed below comprise a list of analytes that can be determined following the collection and analysis procedures noted in this method.  There are a vast number of compounds that can be determined using this method.  If you are interested in a particular analyte and do not see it listed below, please inquire as to the applicability of determining that analyte using this method.

Method Data

Hold Times, Preservatives, Preps, Collection, Analytical & Documentation
Holding Time:	None specified in method, however, it is recommended to analyzed samples as soon as possible after collection.
Preservatives:	None specified in method, however, it is recommended to keep adsorbent tubes at 4°C.
Required Preps:	Tedlar bag, adsorption tubes
Collection Method:	Sampling train for bags, adsorbent tubes or direct injection following Method 18 procedures.
Analytical Methodology:	GC/FID or other suitable detector
Documentation:	18 Engineering

* The analytes and detection limits listed for each method represent the typical detection limits and analytes reported for that particular method. Keep in mind that analyte lists may vary from laboratory to laboratory. Detection limits may also vary from lab to lab and are dependent upon the sample size, matrix, and any interferences that may be present in the sample.