Monitoring VOCs in atmospheric air II. Sample collection and preparation
Introduction
With the progress of civilization, whole ranges of chemical compounds are emitted into the atmosphere. Volatile organic compounds (VOCs) play a particular role, from the point of view of both atmospheric air quality and chemistry of the atmosphere. VOCs are discharged into the atmosphere from many sources, both controlled and uncontrolled. Because of their substantial high vapor pressure they are dominant compounds in the atmosphere. VOCs can be transported to areas very far from their sources of emission, thereby magnifying the risk to which human populations are exposed. VOCs in the environment are therefore not just sources of problems related to human health. They also play a significant part in the formation of tropospheric ozone and of the precursors of urban aerosols as a result of photochemical reactions in the atmosphere [1], [2]. It is therefore quite obvious that, if we wish to care for the environment and to assess the degree of endemic exposure of a population to VOCs in ambient air, monitoring concentrations of these pollutants is of the utmost importance.
Methods for monitoring VOCs in atmospheric air include those based on instruments operating in real time, as well as the widely applied indirect, ex situ techniques. Methodologies utilizing indirect, ex situ techniques require an additional step of sample collection, which might involve collecting the gas in appropriate containers (isolation) or collecting the analytes from the air (isolation with simultaneous preconcentration) [3].
The sample-collection step plays a crucial role in the analytical process, as it determines the representativeness of the sample (i.e. whether it reflects the real state of the air being monitored). Where assessment of the quality of atmospheric air is concerned, the choice of sample-collection technique determines the kind of analytical information that can be provided. Usually, there are very low concentrations of VOCs in atmospheric air and, taking into account their diverse chemical compositions, the number of analytical procedures that can be used for monitoring or determining these analytes is relatively large [3], [4].
The legitimacy and the utility of ex situ techniques in environmental analysis were endorsed by the standard methods of the US Environmental Protection Agency (EPA), introduced in the 1990s, which set out exact procedures for determining VOCs in ambient air, including:
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TO-12, for non-methane hydrocarbons in ambient air;
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TO-14 and TO-15, for volatile organics [e.g., vinyl chloride, benzene, methylene chloride, and perchloroethylene (PCE)], in which air is sampled by isolation in vacuum canisters and Tedlar bags.
There are also official procedures (e.g., ASTM, EPA, NIOSH, CEN and ISO protocols) based on passive sampling of pollutants from atmospheric air, including:
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EN 13528-1, general requirements for quantifying gases and vapors;
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EN 13528-2, specific requirements and test methods;
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EN 13528-3, the guide to passive sampler selection, use and maintenance; and,
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EPA TO-1/-2, combined procedure for VOCs (e.g., vinyl chloride, benzene, methylene chloride, and PCE) [5].
This Part II of the article reviews the information in the literature on the analytical methods used to monitor VOCs in atmospheric air. It follows on from Part I that discussed automated monitoring instruments for measuring VOC levels in atmospheric air.
Section snippets
Analytical techniques requiring sampling
Unlike the automated on-line systems that measure pollution levels in situ, ex situ systems require additional sampling and sample-preparation steps. Because the components of atmospheric air need to be determined at different concentrations, including trace levels, available analytical methods are not always sufficiently sensitive for a reliable final determination to be carried out directly on the sample. A preliminary enrichment stage thus has to be dovetailed into the procedure so that
Miniaturization of instrumentation
Miniaturization of instrumentation to simplify the analytical process, facilitate transport and minimize energy consumption, while not jeopardizing the usefulness of the apparatus, presents a challenge to analysts. Passive dosimetry is useful for determining VOCs primarily because of the small size of the samplers. Solid-phase dynamic extraction (SPDE) and the inside-needle capillary-adsorption trap (INCAT) are interesting miniaturizations of denuder techniques for isolation and preliminary
Analytical information obtained
The employment of different analytical techniques yields different types of analytical information concerning analyte levels in atmospheric air. The basic types of such information are:
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instantaneous concentrations, used above all to assess the quality of air and the degree of exposure of an employee at the workplace; and,
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mean, time-weighted concentrations (24 h, mean monthly, or mean annual).
In both cases, analytical information can be obtained from continuous or periodic monitoring [3]. Table 5
Conclusions and future trends
VOCs are a major concern of many scientists worldwide mainly because of the growing awareness of the impact of VOCs on both human health and the global environment. VOCs contribute to major environmental problems (e.g., global warming, stratospheric ozone depletion, photochemical ozone formation and odor nuisance). VOCs and their degradation products may also be important in the epidemiology of respiratory disorders and cancer. Knowledge of the occurrence, the fate and the behavior of VOCs in
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C22 n-alkanes and 2–4 ring PAHs were chosen as surrogates to evaluate the performance of this method. Regression coefficients of external calibration curves were greater than 0.93 and 0.96 for all individual n-alkanes and PAHs, respectively. Average relative standard deviation (RSD) values among replicate samples spiked at 3 ng for each individual standard were 9% ± 5%. The detection limits of this method for individual n-alkanes and PAHs were 3.1–16.2 ng/m3 and 1.0–2.7 ng/m3, respectively. Atmospheric IVOCs were continuously monitored from September 28 to 30 and October 22 to November 9 in 2018, in an urban area of Shanghai. Besides targeted n-alkanes and PAHs, unspeciated complex mixtures (UCM) of IVOCs as well as total-IVOCs concentrations in the atmosphere were also determined. Measured concentrations and compositions of gaseous IVOCs in the atmosphere in this study were comparable to other similar studies.