Study of the Degradation by Ozone of Adsorbents and of Hydrocarbons Adsorbed during the Passive Sampling of Air Xu-Llang Cao and C. Nlcholas Hewltt'
Institute of Environmental and Biological Sciences, Lancaster Universlty, Lancaster LA1 4YQ, U.K. The effect of ozone on four adsorbents (Tenax-TA,TenaxGR, Carbotrap, and Chromosorb 106) during the passive sampling of air has been investigated by exposure to ozone at 180 ppb for 1week. Generally no artifact compounds were found to form on Carbotrap or Chromosorb 106. Several new peaks appeared in the chromatograms of Tenax-TA and Tenax-GR, the most significant being benzaldehyde and acetophenone. The effect of ozone on selected hydrocarbons (isoprene, benzene, toluene, pxylene, o-xylene, a-pinene, and @-pinene),adsorbed on Tenax-GR during passive sampling, was also investigated by exposure to ozone a t different concentrations (60,120, and 180 ppb) for 1week. Minor destruction by ozone was only observed for @-pinene(4 % ) at an ozone concentration of 180 ppb. Introduction Passive samplers are devices that are capable of taking samples of gas or vapor pollutants from the atmosphere, at a rate controlled by a physical process such as diffusion, through a static air layer or through permeation of a membrane, but that do not involve the active movement of the air through the sampler. Passive sampling, followed by thermal desorption into a gas chromatograph, has been increasingly used for the sampling and analysis of low concentrations of volatile organic compounds (VOCs) in rural air during recent years (1, 2). The main problem limiting the use of passive samplers for the determination of low concentrations of VOCs in ambient air is the blank level of the whole analytical system, particularly of the adsorbent. Although the blank levels of adsorbents can be minimized by rigorous conditioning procedures, it has been found that their blank signal increases during storage (2). Because of the long sampling periods required by passive methods, some adsorbents may not be suitable in such applications due to the blank buildup problem. A second problem that can affect the blanks of adsorbents is the formation of artifact compounds produced on exposure to reactive trace compounds such as ozone. Some work has been done on the effect of ozone ( 3 , 4 )and other reactive gases (NO, and SO,) (5)on Tenax-GC. The main artifact compounds identified were benzene, toluene, benzaldehyde, phenol, and acetophenone,with the straight-
* Author to whom correspondence should be addressed. 0013-936X/94/092S-0757$04.50/0
0 1994 Amerlcan Chemical Society
chain aliphatic aldehydes (hexanal, heptanal, octanal, and nonanal) also observed after exposure to 110 ppb ozone (3). An additional concern is the degradation of organic compounds adsorbed on the adsorbents during sampling by reaction with ambient ozone and other reactive species. The long passive sampling periods (days or weeks) necessary to collect sufficient analyte for analysis at very low ambient VOC concentrations (ppb or ppt) exacerbates this problem. Roberts et al. (3)investigated the possible destruction of the monoterpene ( C d hydrocarbons by ozone during sampling and found it to be negligible for a-pinene and @-pineneand minor (6 % ) for D-limOnene. In the work of Venema et a1 (6),no oxidation by ozone was observed for ethylbenzene, 1-hydroxyethylbenzene, acetophenone, n-octanol, and octanal adsorbed on TenaxGC during sampling. Bunch and Pellizzari (7)examined the reactions of alkenes with ozone during sampling and found reaction of the analyte species under certain conditions. Pellizzari and Krost (8)observed that styreneds and cyclohexene-dlo were oxidized during sampling when several reactive species (ozone, Clz, NO,, etc.) coexisted. In this work, the effects of ozone during passive sampling on selected hydrocarbons compounds commonly targeted in air-monitoring programs and four different adsorbents, typically used for ambient air monitoring, were investigated by exposure to ozone for 1week at different, realistic, ozone concentrations. Experimental Section
Analytical System. Gas chromatographic measurements were made using a Hewlett-Packard 5890 Series I1 gas chromatograph fitted with a flame ionization detector (FID). The carrier gas was helium, and the makeup gas was nitrogen. The GC capillary column used was an Ultra 2 (cross-linked 5 % phenyl methyl silicone) 25 m X 0.2 mm X 0.33 pm film thickness (Hewlett-Packard). Peak identification was based upon the relative retention times of standard compounds, with independent verification by a second laboratory using a different capillary GC column (Steinbrecher, per communication). Organic vapors were injected into the carrier gas stream by means of a 1-mL gas-tight syringe via a Chrompack thermal desorption cold trap (TCT) injector, interfaced with the GC. The injected vapors were then carried at a flow rate of 30 mL/min Envlron. Sci. Technol., Vol. 28, No. 5, 1994 757
Figure 1. Schematic diagram of the ozone exposure system.
through a heated empty Perkin-Elmer stainless steel tube to a coated (CP-Si18 CB, df = 5 pm) fused silica capillary trap (91, which was cooled with liquid nitrogen. After sample concentration, the trap was flash-heated to 220 "C at 15 "C/s for 1min, and the trapped vapors were injected onto the GC column in splitless mode. Spiked and exposed packed Perkin-Elmer diffusion tubes used in the experiments were also analyzed following the above procedures by placing them in the TCT desorption oven. Analytes. The following hydrocarbons were used in this work isoprene, benzene, toluene, p-xylene, o-xylene, a-pinene, and 0-pinene. Their selection was based on their photochemical reactivity and their contributions to the formation of ozone in the troposphere (10-12). Organic vapor mixtures were prepared using the static dilution bottle method (13,14). A known amount of liquid compound was injected into a 1-L flask, which had been purged with nitrogen for 3 min, using a 1-pL wire barrel syringe. The flask and syringe were kept warm by placing them in an oven at 55 f 5 "C in order to prevent wall adsorption. Adsorbents. Four commonly-used adsorbents were chosen for study: Tenax-TA (mesh size 60/80, specific surface area 20 m2/g) was obtained from Chrompack. Carbotrap (mesh size 20/40, specific surface area 100 m2/ g) was obtained from Sulpelco. Tenax-GR (mesh size SO/ 80, specific surface area 20-100 m2/g, obtained from Chrompack) consisted of a Tenax matrix filled with 23% graphitized carbon, but not a simple mixture of the two. Its passive sampling performance has been recently evaluated (2, 15). Chromosorb 106 (mesh size 60/80, specific surface area 800 m2/g) was obtained from Chrompack. Perkin-Elmer stainless steel diffusion tubes (9.0 cm X 0.48 cm i.d.) were packed with 0.2 g of adsorbent (0.16 g for Tenax-TA) and conditioned for at least 16h with helium flow (35 mL/min) at the following temperatures: 300 "C (Tenax-TA),320 "C (Tenax-GR),350 "C (Carbotrap),and 250 "C (Chromosorb 106). Chromosorb 106 has a low maximum operating temperature (250 "C) and so requires 758 Envlron. Scl. Technol., Vol. 28, No. 5, 1994
a much longer conditioning period of at least 48 h. The exposed diffusion tubes were thermally desorbed in the Chrompack TCT unit at the following conditions: 250 "C (5 min) for Tenax-TA, 260 "C (6 min) for Tenax-GR, 280 "C (8 min) for Carbotrap, and 230 "C (10 min) for Chromosorb 106 (16). Vapor Spiking. Packed and conditioned diffusion tubes were placed in the oven of the TCT injector. A known amount of organic vapor mixture (20-30 ng) was then injected into the tube at room temperature (25 "C) via the injection port of the TCT unit with helium flow at 35 mL/min. The flow was maintained for 5 min to distribute the analytes onto the adsorbent. There was no breakthrough, at least for hydrocarbons of Cg and above, under the above spiking conditions. This was confirmed by maintaining the capillary cold trap at -150 "C during the spiking process, with subsequent injection of its contents into the GC. Ozone Exposure Chamber System. The ozone exposure experiments were carried out using an all-Teflon fumigation system designed for the study of reactive air pollutant effect on plants (17). A schematic diagram of this system is shown in Figure 1. Ozone was generated by aTriogen ozone system. Chamber 1was ozone free (mean ozone concentration
