OCR Text |
Show procedures are left to the judgement of the user. Recently, EPA has developed a new draft method for VOC sampling in bags that is more specifically directed at products of incomplete combustion (PICs), some of which are also HAPs. Method 0040 (draft) provides specific sampling, QAJQC, and analytical procedures [10]. The sampling train is illustrated in Figure 2a. The sample is extracted using a heated probe and filter, then cooled in a condenser/impinger assembly to remove moisture before being drawn into an evacuated Tedlar® bag. The method specifies rigorous sampling system QC procedures (leak checks, bake-out between runs, etc.) designed to avoid contamination from run to run. The samples (bag and condensate) are analyzed using GCIMS with a cryogenic trap to achieve very low detection limits, on the order of 0.5 ppb for most analytes. Other GC detectors are allowed in the method if the specificity afforded by the MS detector is not needed. Recent field experience with this method at EER in sampling from a gas fired process heater simulator has shown that contamination of quality control field blank samples below method detection limits can be achieved and analytical sensitivity is excellent. The method is nominally applicable for VOCs with bOiling points from below -15°C up to perhaps as high as 120°C. The gas sample is not extracted isokinetically; however, the sampling rate can be adjusted proportionally to stack flow rate based on velocity measurements using a pitot tube when significant variations occur. Method 0040 is not suitable for determining VOCs that are very soluble in water that may be contained in aerosol mists because sampling is not performed isokinetically. Based on recent, albeit limited, field experience thus far, this method appears to be the best choice at present for characterization of lighter VOCs from refinery emission sources. Heavier VOCs may be difficult to completely recover from bags. Therefore, emissions of these compounds should be characterized using a sorbent concentration method. EPA Method 0030 [12], known commonly as the volatile organic sampling train (VaST), employs two condensers and sorbent trap sets in series (Figure 2b). The overall principle is to strip the organics from the sample onto the sorbent. After sampling, the organics are thermally desorbed from the sorbent into a carrier gas that is then analyzed by GCIMS. The sorbent used in the first trap is Tenax®-GC resin, and the second trap is packed with Tenax®-GC followed by activated charcoal. The condensate is usually discarded by may be analyzed. This train works well for VOCs with boiling points below 100°C. The main concerns are breakthrough of lighter VOCs, especially those compounds with boiling points less than 30°C or present at concentrations above about 1 ppm, and ability to desorb heavier VOCs from the sorbent. It is not appropriate for polar, water soluble or reactive compounds. Other potential problems include contamination of samples during field or laboratory handling from solvents such as methylene chloride or refrigerants used in air conditioning systems, and outgassing of benzene from materials used to construct the heated probe has been reported as a potential source of contamination during sampling. 5 |