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EVALUATION OF SOLID ADSORBENTS FOR THE COLLECTION AND ANALYSES OF AMBIENT BIOGENIC VOLATILE ORGANICS
Citation:
Arnts, R R. EVALUATION OF SOLID ADSORBENTS FOR THE COLLECTION AND ANALYSES OF AMBIENT BIOGENIC VOLATILE ORGANICS. Presented at American Geophysical Union, San Francisco, CA, December 13-17, 1999.
Description:
Micrometeorological flux measurements of biogenic volatile organic compounds (BVOCs) usually require that large volumes of air be collected (whole air samples) or focused during the sampling process (cryogenic trapping or gas-solid partitioning on adsorbents) in order to achieve sufficient sensitivity with conventional chromatographic detectors. The technique of relaxed eddy accumulation has emerged as a powerful tool for measuring vertical fluxes of BVOCs. Although relaxed eddy accumulation (REA) theory can be applied using different hardware configurations, the use of solid adsorbents is particularly attractive in meeting the demands of the high speed up and downdraft switching while not subjecting the sample stream to passage through a pump. Tenax-TA (a porous polymer) and Carbotrap (a graphitized carbon black) have become popular to use as solid adsorbents because of their low water retention and high affinity for organics. In order to evaluate their suitability for use in our REA system, we performed intensive laboratory testing of a seven candidate adsorbents (three graphitized carbon blacks, one activated carbon, a porous polymer and two experimental silicone deactivated porous silicas). A challenge mixture of 20 compounds was used to evaluate optimum desorption temperatures, determine breakthrough volumes and recovery using two tubes of each adsorbent connected in series. It was found that at low loadings the recovery coefficients, i. e. b-pinene, linalool (ratio of analyte recovered to analyte loaded) of the carbon adsorbents, although high with the virgin material, decreased dramatically with subsequent reuse. At higher loadings, recovery coefficients of these compounds increased and were reproducible with reuse of the adsorbent. The low recoveries of these analytes at low loadings were accompanied by the appearance of dehydration products and isomerizations. It was also observed that less of the total carbon was recovered at low loadings compared to the higher loadings even when unknown and artifact peaks are included. Interestingly, total recoveries were poorest with the lowest surface area carbons despite breakthrough not occurring. These results are consistent with known activated carbon behavior. The single activated carbon tested (Anasorb 747, SKC Inc.) behaved similarly to the graphitized carbons. However, it collects large quantities of water making it difficult to use without appropriate water management. The porous polymer (Tenax-TA) was much less reactive towards the analyte mix, displaying good recoveries across the range loaded. Slightly lower recoveries of b-pinene and cis3-hexen-l-of were observed at lower loadings with TenaxTA. The silicone deactivated porous silicas demonstrated excellent recoveries of the alcohols without causing the dehydrations observed with the graphitized carbons.
This work has been funded wholly or in part by the United States Environmental Protection Agency (EPA). It has been subjected to Agency review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.