IntroductionVolatile organic compounds (VOCs) are one of the major groups of globalatmospheric pollutants. Large amounts of VOCs enter the troposphere everyyear and an estimated 350–375 million tons yr−1ofmethane and 60–140 million tons yr−1of non-methaneorganic hydrocarbons (NMHCs) were produced from anthropogenic activities (suchas fossil fuel combustion, transport and evaporation, chemical treatment andother domestic and industrial discharges) world-wide.1,2This substantial anthropogenic emission has profound adverse impacts on humanhealth and crop production.3VOCs play a vitalrole in the formation of various secondary pollutants through photochemicalreactions in the presence of sunlight and nitrogen oxides.4,5Furthermore, some VOCs could contribute to the atmospheric ozone depletionand the build-up of persistent pollutions in remote areas.6,7As a consequence, VOCs have attracted significant attention and have beenconsidered as an important environmental issue over the last two decades.A thorough understanding of the formation, distribution, transportation,stability and reactivity of VOCs in the atmosphere requires the availabilityof sensitive and specific analytical methods for measuring these compounds.Sorbent-based or canister-based methods have been successfully developed andfrequently used in the collection of ambient VOCs. A cryogenic preconcentrationprocess is often applied to air samples concentrating the VOC content, priorto chromatographic separation and detection of VOCs by various gas chromatographictechniques. In sorbent-based methods, whole air is pulled through metal orglass tubes that have been pre-packed with appropriate packing materials suchas activated charcoal, Tenax, XAD-2, Carbotrap and Carboseive. Attention mustbe paid on the choice of these sorbent materials because they have variableselectivity towards different types of VOCs. The physical properties and applicabilityof VOCs sorbents have been compared and discussed recently.8In canister-based methods, air is pumped at adjustable flow rates into cleanedand evacuated stainless steel canisters. Canisters of 2–15 L capacityare usually used to hold air samples up to 40 psig and offer a convenientmeans for multiple analyses. However, the inner wall of the canisters mustbe thoroughly cleaned before sampling, otherwise contamination and recoveryproblems might arise.9,10Reviewing the applicability and limitations of sorbent-based and canister-basedmethods,11,12it was decided to adoptthe canister-based sampling method followed by cryogenic preconcentrationand GC-MS detection in our study of 143 ambient VOCs because of the followingconcerns: (1) The time required to arrange transportation of the collectedsamples to the laboratory, tight resources and other factors may lead to aperiod of several days between sampling and analysis. The lead time makessorbent-based methods less favourable as VOCs have been found to be less stableupon storage in sorbents.13(2) This work wasprimarily targetted at monitoring a broad range of ambient VOCs, so canister-basedsampling would have advantages as sorbents are normally selective in, if notlimited to, adsorbing/absorbing certain classes of VOCs. (3) It is difficultto anticipate the possible concentration of analytes under various environmentalconditions as it is not easy to adjust the sampling volume to circumvent thebreakthrough of analytes that is often encountered in sorbent-based methods.Extreme precautions are required for ensuring reproducible and good qualityresults. (4) The generation of artifacts that are frequently found insorbents (e.g., benzene and toluene are the major artifacts reportedon Tenax and Carbotrap)14,15might createadditional technical difficulties in its application.On the other hand, the canister-based technique is thought to be unsuitablefor collecting polar compounds such as aldehydes and terpenes.16Several studies17–20have indicated that the storage stability of VOCs within the canisters isa critical issue of the methodology, although the employment of new typesof canister coated with fused silica might offer increased sample integrity.21Those studies17–20only considered a small selected group of hydrocarbons,halohydrocarbons and simple aromatics, hence comprehensive stability informationon the majority of VOCs was scarce. Furthermore, the stability data for VOCsin the literature were found to be relatively inconsistent. This paper presentsour work on the evaluation of the suitability and investigation of the stabilityof 143 C3–C12VOCs covering a broad range of alkanes,alkenes, halohydrocarbons and aromatic compounds at low ppb levels in pressurizedcanisters. A sampling and analytical protocol, based on USEPA Method TO-14,12was also developed and piloted to measure the concentrationof VOCs inside and outside a semi-confined car park environment. The protocolhas been adopted by the Government Laboratory of the HKSAR as one of the standardmethods in the toxic air pollutant (TAP) monitoring network since early 1999.Results and details of the TAP monitoring work using the above technique willbe reported later.