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E. Gallego, F.J. Roca, J.F. Perales, X. Guardino
Talanta, Volume 85, Issue 1, 15 July 2011
Abstract
A simple comparison is made to evaluate the relative performance of active and passive sampling methods for the analysis of volatile organic compounds (VOCs) in ambient air. The active sampling is done through a multi-sorbent bed tube (Carbotrap, Carbopack X, Carboxen 569) created in our laboratory and the passive sampling through the Radiello® diffusive sampler specified for thermal desorption (filled with Carbograph 4). Daily duplicate samples of multi-sorbent bed tubes were taken during a period of 14 days. During the same period of time, quadruplicate samples of Radiello® tubes were taken during 3 days, 4 days, 7 days and 14 days. The sampling was carried out indoors during the months of February and March 2010 and outdoors during the month of July 2010 in La Canonja (Tarragona, Spain). The analysis was performed by automatic thermal desorption (ATD) coupled with capillary gas chromatography (GC)/mass spectrometry detector (MSD). The analytical performance of the two sampling approaches was evaluated by describing several quality assurance parameters. The results show that the analytical performances of the methodologies studied are quite similar. They display low limits of detection, good precision, accuracy and desorption efficiency, and low levels of breakthrough for multi-sorbent bed tubes. However, the two monitoring methods produced varying air-borne concentration data for most of the studied compounds, and the Radiello®samplers generally gave higher results. Sampling rates (Qk) were determined experimentally, and their values were higher than those supplied by the producer. As the experimental calculation of Qk values is generally carried out by the suppliers in exposure chambers with only the target compounds present in the air samples, as well as in concentrations dissimilar to those found in ambient air, the use of constant settled Qkcan lead to inaccurate results in complex samples.
Additional Information
VOCs are responsible for odorous annoyance of varying degrees of nuisance and represent a threat to human health (irritation of mucous membranes, psychological stress and long-term toxic reactions) and comfort. Hence, sensitive, selective, fast and reliable methodologies are needed to sample and analyse these pollutants in ambient air. The sampling step is a critical part of ambient air analysis. Active and passive sampling approaches are useful to determine low VOC concentrations in indoor and outdoor air. It has to be taken into account that active sampling has been a traditional sampling technique for determining pollutants in air. However, nowadays passive sampling is being an increasingly used technique for ambient air measurements, especially in urban environments. The easy operability and the low cost of passive samplers make them an ideal tool for long-time averaged pollution concentrations studies. Additionally, in the passive Radiello tubes the analyte diffuses radially through all the cylinder of the tube, and due to its diffusive surface, uptake rates are higher than the observed for other diffusion samplers (e.g. axial), making suitable their use for both short and long term samplings.
A comparison between two types of adsorbent tubes and methodologies, an active multi-sorbent bed (Carbotrap, Carbopack X, Carboxen 569) tube developed in our laboratory, and the Radiello diffusive sampler indicated for thermal desorption (filled with Carbograph 4), has been done to evaluate their usefulness in the analysis of VOCs in ambient air. TD-GC/MS has been applied to the sampling tubes, a methodology that had been used in previous studies to identify and determine a wide range of VOCs that cause odour nuisance and affect indoor and outdoor air quality.
It is known that passive samplers, that represent time-averaged concentrations, do not show properly the daily variability of VOC concentrations as the active samplers (e.g. multi-sorbent bed) do (Figures 1). However, as it can be expected, the shorter the sampling period, the better the daily variability is shown in the passive samplers. This aspect has to be taken into account when sampling in places where pollutant concentrations fluctuate in an important way during the day or during the days of the week.
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