INTRODUCTION

1. LABORATORI DEL CENTRE DE MEDI AMBIENT (E.T.S. D’ENGINYERIA INDUSTRIAL BARCELONA) Table 1. - Instrumental settings and operating conditions. TD Desorption temp.: 300 ºC Desorption time: 10 min Transfer line: 200 ºC Cold trap sorbent Tenax TA + Carbotrap Cold trap low: -30 ºC Cold trap high: 300 ºC Desorption flow rate: He (50 ml/min) Inlet split: 4 ml/min Outlet split: 7 ml/min Split ratio: 12 % GC Capillary column: DB-624 (60 m x 0.25 mm x 1.4 µm) Temperature program: 40 ºC (1 min), 6 ºC/min until 230 ºC (5min) Carrier gas: He (19.1 psi) MS Interface: 250 ºC Ionization source: 200 ºC Ionization mode: Electron impact Electron energy: 70 eV Mass range 20 - 300 amu COMPARATIVE OF THE ADSORPTION PERFORMANCE OF A MULTI-SORBENT BED (CARBOTRAP, CARBOPACK X, CARBOXEN 569) AND A TENAX TA ADSORBENT TUBE FOR THE ANALYSIS OF VOCs E.Gallego1, F. X. Roca1, F. Perales1, X. Guardino2 and M. G. Rosell2 1 Laboratori del Centre de Medi Ambient. Universitat Politècnica de Catalunya (LCMA-UPC). Avda. Diagonal, 647. E 08028 Barcelona 2 Centro Nacional de Condiciones de Trabajo. INSHT. Dulcet 2-10. E 08034 Barcelona. INTRODUCTION Ambient air is a very complex mixture of compounds, and has a very variable composition and concentration of pollutants. Hence, a good choice of sorbent or a good combination of different sorbents may allow the determination of a wide range of target compounds in air samples (Ribes et al., 2007; Barro et al., 2009), as well as achieve high breakthrough volumes (Demeestere et al. 2007). The selective characteristics of the sorbent chosen would determine the removing of the target compounds from the air matrix (Dąbrowski, 2001). On the other hand, a choice of a proper sorbent for the range of the studied target compounds would eliminate problems derived from breakthrough values (Dewulf and Van Langenhove, 1999). The capacity of a sorbent to retain specific compounds is usually evaluated by measuring the breakthrough volume of a concrete compound on the sorbent (Baya & Siskos 1996). To maximize sampling efficiency, the maximum volume of air that can be sampled without loss of adsorbent must be known (Harper 1993, Dettmer & Engewald 2003). Tenax TA has been determined to be a not suitable adsorbent for very volatile organic compounds (VVOCs, 0<boiling point<50-100ºC (WHO, 1989)). However, Tenax TA continues being one of the most widely used adsorbents for the preconcentration of VOCs (Sunesson, 2007). In addition to that, generally, a single adsorbent cannot be appropriate for the majority of compounds present in ambient air. Hence, a combination of several adsorbents may result in better performances. Two types of adsorbent tubes, one containing a mixture of three adsorbents (Carbotrap, Carbopack X, Carboxen 569) (Ribes et al., 2007; Gallego et al., 2008) and another containing Tenax TA were compared to evaluate their usefulness as active adsorbents of ambient air VOCs, including VVOCs. Breakthrough comparative Breakthrough values for the different volumes sampled are shown in Table 3. Typical VOCs recommended breakthrough value is <5% (U.S. EPA, 1999). For the concentrations obtained, Tenax TA present high breakthrough values for mainly all compounds and sampling volumes studied. On the other hand, multi-sorbent bed tubes do not exhibit important breakthrough values for these compounds, except the VVOCs ethanol (for all sampled volumes), and acetone, dichloromethane and isopropanol (for sampling volumes over 40 litres). Tenax TA has a surface area of 20-35 m2 g-1, whereas Carbotrap, Carbopack X and Carboxen 569 have surface areas of 95-100 m2 g-1, 240-250 m2 g-1 and 387-485 m2 g-1, respectively. Total surface areas are approximately of 6 and 70 m2 for Tenax TA and multi-sorbent bed tubes, respectively. Therefore, multi-sorbent bed tubes have approximately 12 times more surface area than Tenax TA tubes. Table 2. Average ± standard deviation and median values for indoor air concentrations (µg m-3) for multi-sorbent bed and Tenax TA tubes. Compounds are listed by elution order. 1The line indicates the limit of 100ºC of boiling point of the compounds. Above the line all compounds have a boiling point <100ºC. Below the line, all compounds have a boiling point >100ºC, except trichloroethylene. ‡ Significant differences observed between average concentrations obtained from multi-sorbent bed and Tenax TA tubes (t-test, p<0.05). Normal data. † Significant differences observed between average concentrations obtained from multi-sorbent bed and Tenax TA tubes (U ofMann-Whitney, p<0.05). Not normal data. Table 3. Average ± standard deviation breakthrough values for each sampling volume (% VOC found in the back tube) for multi-sorbent bed and Tenax TA tubes. Compounds are listed by elution order. MATERIALS AND METHODS Sampling Duplicate samples of multi-sorbent bed and Tenax TA tubes of 10, 20, 40, 60 and 90 litres were taken in Barcelona city (Spain) on July 2009. VOCs were dynamically sampled connecting two custom packed glass multi-sorbent cartridge tubes in series (Carbotrap 20/40, 70 mg; Carbopack X 40/60, 100 mg and Carboxen 569 20/45, 90 mg) (Ribes et al., 2007) and two Tenax TA tubes in series (60/80, 200 mg) to an air collector pump sampler specially designed in the LCMA-UPC laboratory. The flow sampling rate was 70 ml min-1. The temperature and relative humidity during the sampling ranged between 28-31ºC and 35-48%, respectively. Desorption and analysis Analysis of VOCs was performed by Automatic Thermal Desorption (ATD) coupled with capillary Gas Chromatography (GC)/ Mass Spectrometry Detector (MSD), using a Perkin Elmer ATD 400 (Perkin Elmer, Boston, Massachusetts, USA) and a Thermo Quest Trace 2000 GC (ThermoQuest, San Jose, California, USA) fitted with a Thermo Quest Trace Finnigan MSD. VOCs standards were prepared in methanol and injected at 30˚C on the tubes under an inert Helium gas flow (100 ml min-1) using a conventional gas chromatograph packed column injector.The instrumental settings and operating conditions are shown in Table 1. Figure 1. Comparison of different compounds concentrations (µg m-3) using multi-sorbent bed tubes (Carbotrap, Carbopack X and Carboxen 569) and Tenax TA tubes. RESULTS AND DISCUSSION Multi-sorbent bed (Carbotrap, Carbopack X, Carboxen 569)-Tenax TA concentrations comparative Differences between multi-sorbent bed and Tenax TA tubes concentrations are shown in Table 2. Average ± SD and median values are exposed for all samples. Significant differences between concentrations are observed for the very volatile compounds and for alcohols and chlorinated compounds (e.g. acetone, isopropanol, carbon disulphide, dichloromethane, chloroform, carbon tetrachloride). On the other hand, mainly all compounds with boiling points higher than 100ºC (except α-pinene, chlorinated and polar compounds) do not show significant differences between the obtained multi-sorbent bed and Tenax TA tubes concentrations (Table 2, Figure 1). The boiling point of 100ºC is often advised as a guidance value below which the adsorption of the compound is not satisfying for Tenax TA (Sunesson, 2007). • REFERENCES • Barro, R., Regueiro, J., Llompart, M., Garcia-Jares, C., 2009. Journal of Chromatography A 1216, 540-566. • Baya, M.P., Siskos, P. A., 1996. The Analyst 121, 303-307. • Dąbrowski, A., 2001. Advances in Colloid and Interface Science 93, 135-224. • Demeestere, K., Dewulf, J., De Witte, B., van Langenhove, H., 2007. Journal of Chromatography A 1153, 130-144. • Dettmer, K.; Engewald, W. Chromatographia Suppl. 2003, 57, S339-S347. • Dewulf, J., Van Langenhove, H., 1999. Journal of Chromatography A 843, 163-177. • Gallego, E., Roca, F.J., Perales, J.F., Guardino, X., 2008. In: Romano, G.C., Conti A.G. (Eds.), Air Quality in the XXI Century, Nova Science Publishers, New York. • Harper, M., 1993. Annals of Occupational Hygiene 37, 65-88. • Ribes et al. 2007. Journal of Chromatography A, 1140, 44-55. • Sunesson, A.-L., 2007. In: Greenwood et al. (Eds.), Comprehensive Analytical Chemistry 48, Elsevier, 57-83. • U.S. EPA, 1999. Method TO-17, Center for Environmental Research Information, U.S. EPA. • WHO, 1989. Indoor air quality: organic pollutants. Euro Reports and Studies Nº 111. Copenhagen.