Method 18 Compliance
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. The description of the method provided below will outline the requirements to follow the method as written.
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. Moisture content of the source is determined following Method 4. After all of the samples are analyzed in triplicate, one bag is spiked with the analytes of interest at a level corresponding to 40-60% of the average concentration found in the samples. The spiked bag is re-analyzed after an interval which is equal to the duration the original sample was held in the bag from collection to analysis. The recovery of the spiked analytes is calculated which is referred to "R" in the method. The recovery of the spiked compounds must meet 70-130% recovery. All sample data are corrected to reflect their "R" or recovery value.
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 spike recovery study for adsorbent tubes requires a spiked adsorbent tube to be collected simultaneously with each sample run (co-located sampling). It is required to determine a recovery factor per compound, per media, per source. The analytes of interest are spiked onto the tubes at a level of 40-60% of the amount expected on the tubes. The recovery of the spiked analytes is calculated which is referred to as "R" in the method. The recovery of the spiked compounds must meet 70-130% recovery. All sample data are corrected to reflect their "R" or recovery value. 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. Note also that the method requires the contents of the midget impingers, if used, to be analyzed for the analytes of interest.
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. It is also recommended to keep sample adsorbent tubes at 4°C.
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.
(EPA 40CFR Part 60 Appendix A)