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FOCUS they are more complex and expensive and require significant calibration and maintenance in the field. Collection methods for vapor-phase organics commonly involve collection on a solid sorbent, such as a polymeric resin (e.g., Tenax GC or XAD), activated carbon, or a carbon molecular sieve. These sorbents can be used for a wide variety of VOCs, and because they have a low affinity for water, their collection efficiency is not strongly dependent on humidity. Inorganic adsorbents (such as silica gel, alumina, and florisil) are not commonly used for collecting VOCs in air, however, because they readily adsorb water, which deactivates their surface sites. Either thermal or solvent desorption is then used to desorb collected organic compounds from the sorbent. Because polymer resins reversibly adsorb organic compounds, thermal desorption followed by GC/MS analysis, which provides positive identification of target organics as well as broad spectrum analysis, is normally used. Activated carbon, on the other hand, chemically binds organic compounds and thus requires solvent desorption for efficient recovery of collected compounds. Solvent desorption usually precludes GC/ MS analysis for low molecular weight organics because the solvent front interferes with the analysis. In these cases, solvents such as carbon disulfide or acetone are used, and GC detectors that are relatively insensitive to the eluting solvent, such as flame ionization, electron-capture, or nitrogenphosphorus detectors, are employed. Overall, the sensitivity of the solvent desorption methods is less than that for thermal desorption because only a fraction of the sample is analyzed. An alternative to sorbent methods involves collecting air samples in stainless steel canisters rather than on a sorbent cartridge. "Essentially," says Sheldon, "air is sucked into the canister in the field. Once the canister is brought back to the lab, about 200 mL of the collected air is cryofocused and analyzed using GC/MS with selected ion monitoring." This method has replaced the Tenax-desorption method for many very volatile compounds. Although real-time analytical methods can provide more information than the passive collection methods, they are not commonly used for organics because of contamination problems at the ambient levels necessary for effective indoor air monitoring. Most existing real-time methods have been developed for inorganic species (e.g., NO2 and SO2), although instruments for monitoring formaldehyde and acrylonitrile are also available. "Although
500 A • ANALYTICAL CHEMISTRY, VOL. 62, NO. 8, APRIL 15, 1990
these instruments have potential," says Sheldon, "they were primarily developed for workplace monitoring, and they just don't quite have the sensitivity necessary for detecting part-pertrillion levels of organics." Formaldehyde Because formaldehyde from insulation and building materials is among the more common indoor pollutants, specialized methods have been developed for determining formaldehyde in air. These methods involve in situ derivatization with 2,4-dinitrophenylhydrazine (DNPH) followed by reversedphase LC. Air samples are collected either on silica gel cartridges coated with acidified D N P H or in impingers containing acidified DNPH-acetonitrile solution. Back in the lab, the cartridges are eluted with acetonitrile while the impinger solutions are brought to volume with acetonitrile; the solutions are then analyzed for formaldehyde by LC. Semivolatiie and nonvolatile organic compounds Semivolatiie and nonvolatile organics include polycyclic aromatic hydrocarbons (PAHs), organochlorine and organophosphorous pesticides, PCBs, and chlorinated dibenzodioxins and furans. These compounds get into the air primarily from cigarette smoke, plasticizers in building materials, and the use of pesticides for insect and rodent control. Common methods for sampling semivolatiie and nonvolatile compounds involve the use of polyurethane foam (PUF) plugs or XAD resin. PUF has a number of advantages over other adsorbents, including low flow restriction, ease of purification and handling, and low cost. XAD, however, has better retention characteristics for the more volatile pesticides and PCBs, allowing lower detection limits for these compounds. PUF also forms mutagenic artifacts- during sampling, reducing its usefulness as a collection medium for bioassay studies. After sample collection, the analytical techniques used for both PUF and XAD are generally the same as for other media. The sorbent is extracted with a suitable solvent and the extractant analyzed using GC/MS or LC with fluorescence or ultraviolet detection for PAHs, chlorinated dioxins, and furans, and GC with electron capture detection for PCBs. Chromatographic cleanup is often needed to achieve the required sensitivity. Biological factors Indoor biological pollution is only beginning to receive the same type of at-
