Development of an automated monitoring system for OVOC and nitrile compounds in ambient air

Few studies have been conducted on Oxygenated Volatile Organic Compounds (OVOC) because of problems encountered during the sampling/analyzing steps induced by water in sampled air. Consequently, there is a lack of knowledge of their spatial and temporal trends and their origins in ambient air. In this study, an analyzer consisted of a thermal desorber (TD) interfaced with a gas chromatography (GC) and a flame ionization detector (FID) has been developed for online measurements of 18 OVOC in ambient air including 4 alcohols, 6 aldehydes, 3 ketones, 3 ethers, 2 esters and 4 nitriles. The main difficulty was to overcome the humidity effect without loss of compounds. Water amount in the sampled air was reduced by the trap composition (two hydrophobic graphitized carbons: Carbopack B/Carbopack X), the trap temperature (held at 12.5 degrees C), by diluting (50:50) the sample before the preconcentration step and a trap purge. Humidity management allowed the use of a polar Lowox column in order to separate the polar compounds from the hydrocarbon/aromatic matrix. The breakthrough volume was found to 405 ml for ethanol by analysing a standard mixture at a relative humidity of 80%. Detection limits ranging from 10 ppt for ETBE to 90 ppt for ethanol were obtained for 18 compounds. Good repeatabilities were obtained at two levels of concentration (Relative Standard Deviation < 5%). The calibration (ranging from 0.5 to 10 ppb) was set up at three different levels of relative humidity to test the humidity effect on the response coefficients. Results showed that the response coefficients of all compounds were less affected except for those of ethanol and acetonitrile (decrease respectively of 30% and 20%).