Hydrocarbons and Hydrocarbon-Based Pollutants
Non-methane hydrocarbons of both biogenic and anthropogenic origines exert a consid erable impact on global warming and air pollution. As the concentration of non-methane hydrocarbons in the atmosphere is low, many preconcentration methods have been devel oped to decrease the detection limit of the quantitative gas chromatographic analysis. The results obtained in the development of trace enrichment methods for the determination of volatile organic compounds in air have been earlier reviewed (Camel and Claude, 1995). On-line membrane extraction-microtrap followed by GC analysis was proposed for the continous monitoring of volatile organic compounds in air (Mitra et a l , 1996). The majority of preconcentration methods uses either cryogenic sample traps (Bertman et a l , 1993; Greenberg et a l , 1994) or various solid-phase adsorbents such as graphitized carbon (Bianchi and Varney, 1993), Tenax species, etc. (Helmig and Vierling, 1995), purge-andtrap method (Lucke et a l , 1993) or solid-phase microextraction (SPE) (Mangani and Cenciarini, 1995). It was found that the use of multiple bed sorbents considerably the capacity of trapping and enhances recovery (Seko and Onda, 1997). Besides the commer cial cryoconcentration units the application of laboratory-made instruments has also been reported using a two-step cryogenic procedure and a GC-FTD systems with temperature programming (Lay et a l , 1993). A cryofocusing inlet with reversed-phase flow sample collection has also been developed for the enhancement of the sensitivity of GC deter mination of volatile organic compounds (Klemp et a l , 1993). The combination of cryoconcentration and adsorption enrichment using a multistage solid-adsorbent unit con siderably decreased the detection limit (Helmid and Greenberg, 1994). The adsorption behavior of airborne hydrocarbons and oxygenated (Comes et a l , 1993; Comes et a l , 1998) compounds as well as smelly volatile pollutants on Tenax GC was studied in detail (Comes et a l , 1996). It was established that the adsorption process can be modeled with a Langmuir isotherm. The equations allowed the calculation of the breakthrough volumes of pollutants at infinite dilution and the maximum analyte concentrations. The correspond-
Table 2.1. Sampling conditions defined by the Langmuir constants.