Determination of benzene, toluene, ethylbenzene and xylenes in indoor air at environmental levels using diffusive samplers in combination with headspace solid-phase microextraction and high-resolution gas chromatography–flame ionization detection
Introduction
In the past several years volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, and o-, m- and p-xylenes (BTEXs) have become of particular interest in the field of indoor air quality. Since people spend on average about 90% of the day indoors, attention is mainly paid to indoor air instead of outdoor air pollution. Indoor air pollution due to BTEXs results from smoking, building or furnishing materials, paints, adhesives, other consumer products and burning processes 1, 2. The main outdoor sources, which also contribute to indoor air pollution via ventilation, are automobile exhaust and industrial emissions.
Exposure to low-level VOC concentrations in indoor air are suspected to contribute to a variety of non-specific symptoms such as headache, eye, nose and skin irritation, which are known under the term “sick building syndrome”. Benzene, moreover, is known to be carcinogenic to humans.
Sampling of BTEX compounds can be performed by active or passive sampling techniques. Passive samplers are well established for indoor and outdoor air measurements at environmental concentrations 1, 2, 3, 4, 5, because they are easy to handle and require cheap and user-friendly equipment. As passive samplers were originally developed for the assessment of occupational exposures, they are, at environmental concentrations, only suitable for long-term sampling periods of approximately 1–4 weeks 2, 3, 4, 5, 6. For short-term sampling periods, active sampling requires expensive equipment and a skilled staff.
An innovative technique for air sampling is solid-phase microexctraction (SPME) developed by Pawliszyn and co-workers 7, 8, 9, 10. Its principle is based on a partition equilibrium of the analytes between the sample itself or the headspace above the sample and a fused-silica fiber coated with a stationary phase. The amount of analytes extracted by the fiber depends on the type of fiber and is proportional to the initial analyte concentration in the matrix, e.g., water or air. After sampling, the fiber is thermally desorbed into the gas chromatography (GC) injector. SPME which combines sampling, enrichment, and sample introduction in one step 7, 8, has already found widespread use in environmental analysis. It has, for example, been used for the determination of VOCs 11, 12, phenols [13], pesticides [14], polycyclic aromatic hydrocarbons [15]and polychlorinated biphenyls 15, 16in different environmental matrices such as drinking water or wastewater. Martos and Pawliszyn [17]used the SPME fibers as air sampling device for VOCs in ambient and industrial air and found excellent agreement with traditional air sampling methods. Other authors 7, 8, 18, 19used grab sampling with stainless steel canisters or glass bulbs in combination with SPME. One of the problems, the dependence of the adsorption rate on humidity and temperature of the air can be overcome by the use of correction factors [17]. But the main drawback of these procedures is the poor storage stability of the exposed SPME fiber, because uncontrolled losses of analytes can occur by evaporation from the fiber [at least with the poly(dimethylsiloxane) (PDMS) phase]. Preliminary studies of time-averaged sampling were performed by retracting the fiber into the needle and diffusion of the analytes through the needle opening to the fiber 7, 17. The first results are promising, but this method is not routinely applicable until now.
This paper presents an improved procedure which is routinely applicable for integrated short-term passive air sampling in indoor air at environmental concentrations. It is based on a combination of conventional passive samplers with headspace solid-phase microextraction (HS-SPME) and high-resolution gas chromatography with flame ionization detection (HRGC–FID).
Section snippets
Reagents
Before use, all materials and chemicals coming into contact with the samples or standards were routinely checked for contamination. Glass vials were heated at 150°C for 24 h and stored under a clean bench equipped with activated charcoal filters (Bleymehl, Jülich, Germany).
All reagents and standards were of analytical-reagent grade, except sodium methanolate, which could only be obtained in synthesis quality. Methanol “purge-and-trap quality” was purchased from Lab-Scan (Dublin, Ireland), “low
Measures to shorten the exposure period
Using the described procedure, passive samplers without preconditioning (cleaning) are suitable for sampling periods of at least 24 h. If the samplers have to be exposed for shorter sampling periods, the samplers must be cleaned prior to use. Preconditioning results in a drastic reduction of the blanks up to a factor of about 80 in case of benzene and toluene. The blanks before and after cleaning are summarized in Table 1 and illustrated in Fig. 1 given as ng/sampler and converted to μg/m3 for
Conclusions
The present work has shown that passive batch samplers in combination with HS-SPME using a Carboxen–PDMS fiber are suitable for short-term measurements of BTEXs in indoor air at environmental concentrations. Xanthation followed by HS-SPME provides an effective tool for preconcentration of BTEXs from the headspace of CS2 extracts. Compared to traditional passive sampling techniques this results in a drastic gain in sensitivity and enables detection limits for BTEXs below 1 μg/m3 for 2-h and 24-h
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