Use of CFCs as internal standards for measurements of non-methane VOCs sampled onto solid adsorbent cartridges

The separation and analysis of the many volatile organic compounds (VOCs) present in the atmosphere is a challenging task. Measurements of the vertical profiles of VOCs are necessary for the evaluation of atmospheric models of tropospheric chemistry, including the formation of ozone, an EPA priority pollutant. Accurate measurements of VOC profiles using adsorbent cartridges for sample collection are difficult due to an interference from water vapor and difficulty in knowing the exact volume of air sampled. In this research, the water vapor interference was thoroughly explored, and methods were developed for quantitative sampling and analysis under highly humid conditions. Also, the use of CFCs, which are now well mixed in the atmosphere, as an internal standard for accurate determination of sample volume was evaluated. Several field experiments covering a wide range of atmospheric conditions allowed the newly developed methodology to be tested.; A multi-bed solid adsorbent (AirToxic™, Supelco) was chosen for its wide sampling range (C₃–C₁₂). These solid adsorbent cartridges underwent extensive testing under humid sampling conditions to determine the minimum mass of water that interfered with analysis by GC/FID/ECD and to evaluate various methods to eliminate this interference. Heating the solid adsorbent cartridges to 10°C above ambient during sampling was found to prevent the adsorption of water vapor that otherwise would have interfered with sample analysis. Cartridges sampled at humidity levels below 60% RH, did not appreciably adsorb water vapor, and therefore diluting a 90% RH sample stream with dry, scrubbed air prior to sampling also was successful in eliminating the water interference.; Long lived chlorinated compounds banned by the Montreal Protocol in 1996 (CFC-11, CFC12, CFC-113, CCl₄, and CH₃CCl₃) were studied for their ability to predict sample volumes from ground based and vertical profile platforms. CFC-12 predicted sample volumes accurately up to its breakthrough volume of approximately 6 STP L. CFC-113 was very stable on spatial and temporal scales and performed well as an internal standard compound for sample volumes of up to at least 9 STP L. Use of other CFCs as internal standards resulted in poor precision and/or accuracy.