Elimination of Interference from carbon Dioxide when Sampling High-Moisutre/High-CO_2 Gases for VOC Using Method 25

摘要

CO_2 trapped in the water-ice matrix that forms in the cryogenic Method-25 trap during sampling remains in the ice matrix during the first step of the sample recovery process. This first step is back flush of the cryogenic trap with zero air which "purges" it of non-condensable gases. When the back flush, or "cold purge" is complete, the cryogenic trap is removed from dry ice and heated to 200 deg C while the effluent is passed through an oxidation catalyst, then collected in a clean, evacuated tank. CO_2 released in this step is collected along with the oxidized organic compounds and counted as VOC. Current EPA guidelines advise that when the product of percentCO_2 times percentH_2O content in a gas stream exceeds a value of 100, the interference from trapped CO_2 may be significant. A straightforward method of virtually eliminating this interference has been used successfully on samples with (percentCO_2) (percentH_2O) products>500. The procedure consists of placing a clean, dry-ice cooled trap in the M-25 recovery system between the sample trap and the original sample tank. The sample trap is then purged of non-condensable gases until the CO_2 concentration in the purge gas is <50 ppm. The sample trap is then removed from dry ice and heated to 100 deg C. This drives off entrained CO_2 along with a small amount of water and some VOC. All of these gases pass through the clean cryogenic trap that re-traps the VOC and moisture while allowing the CO_2 to pass through to the sample tank. This procedure reduces the CO_2 interference from tens of ppm to a few ppm or less.