Environ. Sci. Technol. 2008, 42, 7663–7669
Reduction of Biogenic VOC Sampling Losses from Ozone via trans-2-Butene Addition ROBERT R. ARNTS* United States Environmental Protection Agency, National Exposure Research Laboratory, 109 T. W. Alexander Drive, Research Triangle Park, North Carolina 27711.
Received February 25, 2008. Revised manuscript received June 3, 2008. Accepted August 5, 2008.
Thecontinuousadditionoftrans-2-butenetoaircontainingozonereactive volatile and semivolatile organic compounds prior to sampling on Tenax-TA adsorbent was found to be an effective means of removing ozone and reducing analyte losses of ozone reactive biogenic volatile organic compounds (BVOCs). To allow sufficient time for ozone scavenging to occur, the reacting mixture is passed through a heated (40 °C) coil of Sulfinert (Restek Corp., Bellefonte, PA) treated stainless steel tubing. The method was evaluated using a test mixture consisting of terpenes, terpenoid alcohols, and sesquiterpenes at part per trillion (pptv) levels in air in the presence of 100 parts per billion (ppbv) of ozone. The continuous addition of trans-2butene to produce 600 ppm (ppmv) was found to be completely effective in controlling VOC losses on Tenax-TA as long as (1) sufficient time is allowed for the ozone scavenging to occur before the VOCs are adsorbed and (2) analyte enrichment on the adsorbent does not approach the hydroxyl radical scavenging capacity of the trans-2-butene. A manganese dioxide (MnO2) coated copper screen ozone scrubber was also tested and found to be of very limited utility.
Introduction The widespread use of Tenax porous polymer adsorbents to concentrate parts per trillion volume/volume (pptv) concentrations of some monoterpenoids and sesquiterpenoids from ambient air for thermal desorption-gas chromatography is hampered by the presence of cosampled ozone. Numerous studies have demonstrated that significant losses of these compounds can occur through oxidation of adsorbed olefins by ozone passing through the sorbent bed (1-13). Furthermore, ozone can oxidize Tenax adsorbent, creating undesirable, potentially interfering products and alterations to the surface properties of the adsorbent (1, 6, 9). Helmig (14) has reviewed techniques for removal of ozone from sample streams to prevent loss of sampled volatile organic compounds (VOC). These include various reagents, such as sodium thiosulfate, potassium iodide, and sodium carbonate, in the form of packed beds, coated denuders, or glass fiber filters which remove ozone from the sample stream prior to sampling by an adsorbent bed. Manganese dioxide-coated copper screens have been found to be effective in preventing loss of monoterpenes (7, 8, 11, 12) from ozone, but oxygenated terpenes such as linalool, camphor, cis-citral, citronellal, and bornyl acetate are removed by the screens (8, 11). Similarly, * Corresponding author phone: (919)-541-2405; fax: (919)-5410960; e-mail:
[email protected]. 10.1021/es800561j CCC: $40.75
Published on Web 09/19/2008
2008 American Chemical Society
MnO2 screen scrubbers have been shown to remove some sesquiterpenes (11, 13). Contact with any solid surface to remove ozone, carries with it, the potential to also remove VOC analytes by adsorption, reaction, or catalysis. The use of a gas phase ozone scavenger, therefore, may offer a means to remove ozone while avoiding potentially adsorptive solid surfaces. Nitric oxide has previously been investigated to remove ozone but has been found to cause olefin losses (11, 13). For the work described here, trans-2-butene was selected as an ozone scavenger which met the following criteria: (1) an ozonolysis rate constant sufficiently fast, 1.9 × 10-16 cm3molec-1s-1 (15), that it could be added to a sample stream to reduce ozone loss of target analytes to a negligible level within a few seconds, (2) the concentration needed does not create a flammable or explosive mixture (trans-2-butene lower explosion limit is 1.6%), (3) the scavenger is available in sufficient purity (>99%, Sigma-Aldrich Corp., Milwaukee, WI) and the ozonolysis products (formaldehyde, acetaldehyde, methanol, glyoxal, carbon dioxide, carbon monoxide, and methane) created by this addition (16) do not interfere with the target analysis (mono and sesquiterpenes), and (4) the added compound is not efficiently collected by the solid adsorbent (Tenax-TA) sampling method and/or is readily removed by predesorption dry purge (17). The design capacity (flow rate) and size constraints for the system described herein are based on the needs of a relaxed eddy accumulation (REA) system built to measure vertical fluxes of biogenic VOCs above forest canopies (18). A detailed description of this system will be presented elsewhere. In brief, REA methodology requires that a representative range of rising and falling eddies be sampled over a period where quasi-steady state conditions (usually 30-60 min) can be assumed (i.e., day, time, unstable conditions). The accumulated up and downdraft samples are then analyzed to determine the concentration differential of the species of interest. For the specific REA design employed here, canopy air is continuously drawn from near the sonic anemometer, immediately blended with the ozone scavenger, segregated according to origin (up draft, down draft, or quiescent), passed through a heated inert delay (5 s) coil and then accumulated by a pair of adsorbent tubes. The adsorbent tubes are thermally desorbed and analyzed by gas chromatography (with flame ionization and mass spectrometric detection). To achieve desired minimum detectable fluxes (19) on the order of a few micrograms per square meter per hour with typical above canopy concentrations of mono and sesquiterpene between 1 and 500 pptv, flow rates of 1 L min-1 are needed for one hour measurement periods to achieve total sample volumes of about 10 L (under typical conditions where small eddy rejection is employed). A five second delay coil was estimated to be compact enough to fit into the existing REA container. A minimum operating temperature of 40 °C for the delay coil was chosen in order to achieve a constant temperature environment under summer field conditions without condensing water vapor. Reported herein, are the results of testing the trans-2-butene technique along with a comparison with a manganese dioxide screen scrubber which was used in the REA prior to the development of the trans-2-butene technique.
Experimental Details Estimation of Conditions to Limit VOC Loss. For purposes of modeling ozone losses, R-terpinene, which has the highest reported rate constant with ozone (about twice as fast as β-caryophyllene) was selected. To estimate the sensitivity of VOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. A) Predicted gas phase decay of 100 ppbv ozone in the presence of 100, 200, and 600 ppmv trans-2-butene at 40 °C over a 5 s interval. (B) Predicted gas phase and on-adsorbent loss of 30 pptv (initial) r-terpinene in the presence of 100 ppbv (initial) ozone at 100, 200, and 600 ppmv trans-2-butene at 40 °C. Left panel shows r-terpinene loss over 5 s period assuming ozone decay rate predicted in A. Right panel shows predicted loss of r-terpinene as a function of sample volume on adsorbent assuming accumulation of r-terpinene remaining at 5 s of gas phase reaction reacted with ozone residual (not accumulated). Gas phase rate constants are assumed in lieu of unavailability of kinetic data describing reaction kinetics of adsorbed r-terpinene with gas phase ozone. Rate constant for r-terpinene was scaled to 40 °C using increase rate in trans-2-butene over same temperature range predicted from Arrhenius parameters for trans-2-butene-ozone rate. R-terpinene loss to initial trans-2-butene concentrations after 5 s of reaction, a simple kinetic model was constructed. Assuming conditions where the rate of destruction of ozone is controlled by the trans-2-butene, the residual ozone (using 0.1 s time steps) at five seconds is computed along with the remaining R-terpinene. Figure 1A shows the computed ozone decay for 100, 200, and 600 ppmv of initial trans-2-butene and an initial ozone concentration of 100 ppbv. Figure 1B illustrates the accompanying gas phase loss of R-terpinene at 100, 200, and 600 ppm of initial trans-2-butene (shown to the left of the ordinate). Clearly, in the presence of 600 ppm of trans-2-butene, R-terpinene is still present at more than 97% of the intial concentration. Even with 200 ppm of trans2-butene, better than 93% of the initial concentration is preserved. After 5 s of scavenging ozone, the residual ozone levels in both cases are below 1 ppbv. If this mixture were then passed into a whole air sample container such as a Summa can or a polymer bag, very little additional loss due to ozone would occur. However, if the mixture is passed through an adsorbent bed which retains the VOCs while passing the residual ozone, the rate of VOC loss will increase 7664
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as the VOC accumulates on the adsorbent bed. The magnitude of this loss is governed by the residual ozone entering the bed. To roughly model this using the gas phase rate constants and typical sampling conditions for the measurements which follow this section (sampling into an adsorbent bed of about 0.23 cm3 at 0.179 L per minute), it is assumed that the VOC is continuously added to the bed (at a rate governed by the flow rate) and that the integrated VOC is continuously fumigated by the residual ozone. To perform this on a spreadsheet, the following steps were executed: (1) for the initial step the loss of the BVOC is computed using the steady state values (defined here as HC5 and OZ5) after 5 s of reaction using conditions after 5 s of scavenging, (2) the remaining concentration is incrementally increased by adding the amount of material defined by HC5 × flow rate × time step, (3) the updated BVOC concentration is reacted again with the OZ5 to calculate the remaining VOC and steps 2 and 3 are repeated until the desired adsorbent sampling volume is attained. A parallel set of calculations without ozone reaction, are computed to compare with the remaining HC to calculate % recovery. The results are shown in the right panel of Figure 1B. These estimates indicate that if trans2-butene is added to the sample stream to achieve 600 ppmv, residual ozone levels are low enough that on-adsorbent reactions can be reduced to negligible levels. Alternatively, the model also predicts that if the time delay were increased this can offset the use of lower standing concentrations of trans-2-butene. The model was tested with various initial concentrations of VOC ranging from 0.1 to 1000 pptv and found to produce the same results. The predicted increasing loss of reactive VOC with increasing sample volume was observed by Roberts et al. (1) where increasing losses of limonene on Tenax-GC was observed with increasing sample volume of air containing 40 ppb of ozone. Limitations of this model are (1) instantaneous mixing is assumed, (2) the kinetics of a gas phase ozone molecule colliding with an adsorbed VOC may not be accurately described by assuming gas kinetics, and (3) the model does not attempt to account for concentration gradients within the adsorbent bed, i.e., deposition of the least volatile compounds at the front of the bed with more volatile species penetrating further into the bed. Test Atmosphere Generation System. A schematic of the apparatus employed is presented in the Supporting Information (SI) (Figure S1). In brief, the system is composed of four parts: (1) a diffusion tube based VOC generator which provides test compounds in dry nitrogen directly to the analytical system and for (2) dilution with clean humidified air or clean humidified air containing ozone, along with (3) trans-2-butene immediately prior to introduction to delivery to (4) a sampling manifold. The diffusion tube system consists of four 1 L PFA (perflouroalkoxy) Teflon (Savillex Inc., Minnetonka, MN) jars with TFE (tetraflourinated ethylene) Teflon ported lids (with external clamps) that each contain TFE Teflon carousels of eight diffusion tubes containing 1 mL glass vials with fused silica or glass capillaries. Capillary sizing was estimated using guidance from Altshuller and Cohen (20). Sources and purities of each compound are summarized in SI Table S1. Two of the jars were held at constant temperature in a water bath at 10 °C (terpenes and C5 to C9 compounds) and the other two jars were held at 30 °C (C11-C16 alkanes, oxygenated terpenes, and sesquiterpenes) in an oven. Flow restrictors were used to maintain sufficient pressure (about 6 cm H2O) in the diffusion tube containers so that downstream sampling switching actions did not cause pressure fluctuations in flow rates through R1 and R2. Clean dry air was supplied by a zero air generator (model 737, Aadco, Cleaves, OH). Humidification was effected by passage through a 1 m section of microporous TFE Teflon tube (Goretex, Newark, DE) submerged in a jacket of
deionized water. Ozone could be added by passing the humidified air through a quartz tube illuminated by a mercury vapor lamp. An adjustable lamp shroud was used to regulate the amount of ozone added (range 0 to 170 ppbv at 8 L min-1). The 10% trans-2-butene in nitrogen was prepared from 99% trans-2-butene (Sigma-Aldrich) and clean dry nitrogen. This was performed by evacuation of a Scott aluminum cylinder and controlled pressurization with trans-2-butene followed by clean nitrogen. Diluted aliquots in a Teflon bag were measured by direct cryogenic trapping and gas chromatography and found to be within 10% of 600 ppmv. Because 10% trans-2-butene was found to swell Kel-F and Kalrez polymers in mass flow controllers, flow control was achieved by pressure regulation and capillary restriction (see SI Figure S1 for details). A PFA Teflon Tee just prior to adding the air stream to the BVOC stream was used to continuously add trans-2-butene. The BVOC stream in nitrogen (200 cm3 min-1) was introduced in the center of the air stream (8.8 L min-1) and was immediately directed to a narrower passage containing a twisted filament of PFA Teflon (to induce turbulent mixing) before delivery to the larger diameter manifold (at atmospheric pressure). For later tests a high speed motor driven mixer (set at 4000 rpm) was added immediately after the twisted filament (see SI Figure S1). The gas mixture then passed into a FEP (fluorinated ethylene propylene) Teflon manifold having eight sampling ports and an end vent to a fume hood. All gas flows with the exception of the trans-2-butene were mass flow controlled using MKS Instruments transducers. Mass flow transducers were calibrated against a BIOS DC-1 Dry Cal certified (BIOS International, Butler, NJ). All samples were collected at 24 ( 1 °C and 35 ( 4% relative humidity. BVOC Sampling and Analysis. See SI Figure S2 for a schematic and details of the analytical system. In brief, samples were collected by drawing air samples (0.179 L min-1) through Sulfinert passivated stainless steel tubes packed with 400 mg of Tenax-TA adsorbent. Tubes were analyzed by thermally desorbing (at 220 °C) the BVOCs, cryogenically focusing them and analyzing by capillary gas chromatography with flame ionization (for quantitation) and mass spectrometry (identification). The flame ionization detector was calibrated using an isooctane and 2,5-dimethylhexane standard in nitrogen (Scott Gases). This transfer standard was periodically recalibrated against another gas chromatograph calibrated with a NIST-certified propane in nitrogen standard. The output of the undiluted BVOC mixture in dry nitrogen exiting the diffusion tube manifold is directly cryotrapped and analyzed by the same analytical system used to assay the adsorbent tubes. Comparison of directly measured hydrocarbon generator output with computed diluted concentrations compared well with adsorbent tube measurements of samples drawn directly from the manifold. Table 1 summarizes the average concentrations of the diluted test mix. Ozone Measurements. Two instruments were used for this effort: (A) a TECO UV photometric ozone analyzer (Series 49, Thermo Electron Corp., Franklin, MA) and a Bendix ethylene chemiluminescene model 8002 ozone analyzer (Bendix Corp., Lewisburg, WV). The photometric instrument was calibrated against the NIST certified UV absorption photometer at Research Triangle Park, NC which in turn was used to calibrate the Bendix instrument. The chemiluminescence instrument was used to measure manifold and residual ozone level downstream of both the coil and the MnO2 scrubber. The TECO instrument was not used directly since it requires 2 L min-1 of sample. The Bendix instrument has a limit of detection 1 ppbv of ozone (S/N ) 3). Sulfinert Five Second Delay Coil. Before the efficacy of this approach could be evaluated, a suitable means of providing a sufficient time for the ozone scavenging to occur
TABLE 1. Test Mixture Concentrations of Biogenic VOCs and Reported Rate Constants with Ozone
compound
concentration pptv, s.d.
ozone reaction rate constant, 10-18 cm3 molecule-1s-1 at 298°K
trans-2-butene 2-methyl-3-buten-2-ol cis-3-hexen-1-ol terpin-4-ol R-terpineol linalool camphene β-pinene R-pinene D-limonene R-terpinene R-copaene β-cedrene β-farnesene R-farnesene β-caryophyllene R-humulene
6 × 108 201 ( 47 104 ( 12 170 ( 11 48 ( 4 90.1 ( 4.9 325 ( 39 626 ( 63 526 ( 57 122 ( 9 481 ( 32 45.0 ( 1.9 47.5 ( 2.2 7.70 ( 0.80 9.90 ( 0.27 32.6 ( 1.3 28.1 ( 2.5
190a 8.3b 64c 250d 300e 430c d 0.90a 15a 86.6a 200a 21 100a 160a N/A N/A N/A 11 600a 11 700a
e
a From ref 15; From ref 24
b
From ref 21;
c
From ref 22;
d
From ref 23
(5 s delay). Coils of 6.35 mm o.d. × 5.33 mm ID × 417 cm L stainless steel (type 304) were tightly wrapped around a short section of aluminum pipe (7.62 cm o.d. × 2.38 mm wall × 16.5 cm L). Silicone rubber heaters were applied to the inner surface of the pipe to permit operation above ambient temperatures. A K-type thermocouple and a proportional integral derivative controller model CN9000A (Omega Engineering, Stamford, CT) were used to monitor and control the temperature of the coil. The coil-pipe assembly was wrapped in a ceramic blanket (Kaowool, Thermal Ceramics, McMaster-Carr Supply, Atlanta, GA) to promote constant temperature. A second identical unit was constructed using Sulfinert treated stainless steel tubing (Restek Corp., Bellefonte, PA). According to the patent on this process, the Sulfinert surface is a hydrogenated amorphous silicon surface derivatized with a hydrocarbon (25) to produce an nonpolar surface. Manganese Dioxide-Coated Copper Screen Ozone Scrubber. See the Supporting Information for the schematic (Figure S3). Screens (50 mm o.d.) were obtained from O.B.E. Corp. (Fredericksburg, Texas) and punched to size using an arch punch. The optimum face velocity determined by Calogirou et al. (8) was used as a guide to estimate the number of plies of screen needed. For 1 L/min of sample flow, seven screens with an internal cross sectional face area of 2.85 cm2 were selected. The scrubber uses 0.64 cm spacers between each screen to obtain a more reproducible sample path through the screen stack (stacking screens in direct contact can produce more or less direct path depending on alignment of the mesh openings). Spacers, nozzle, and tubing internal surfaces were all Teflon.
Results and Discussion Analyte Recovery in the Presence of Ozone. To demonstrate the impact of cosampled ozone on measurement accuracy of the test mix with Tenax-TA adsorbent tubes, two consecutive sets of samples were collected directly from the manifold: one with 100 ppbv of ozone and one with the ozone generator switched off. The recovery of each compound was then computed as the ratio of the compound measured with ozone to that measured without ozone. Figures 2 summarizes these results in the first bar. Tabulated results of these and additional compounds are presented in SI Table S2. The alkanes and 2-heptanone were unaffected by the presence VOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. (A) Comparison of monoterpenoids and (B) sesquiterpenes recovered in the presence of 100 ppbv of ozone without ozone mitigation (1st bar), transmission of monoterpenoids through seven plies of MnO2 coated copper screens without ozone present (2nd bar) and monoterpenoids recovered in the presence 100 ppbv ozone with screens (3rd bar). Compounds shown: camphene (CAMPH), β-pinene (b-PIN), r-pinene (a-PIN), D-limonene (LIM), r-terpinene (a-TERP), 2-methyl-3-buten-2-ol (232MB), cis-3-hexen-1-ol (C3HXOL), terpin-4-ol (T4OL), r-terpineol (a-TER), linalool (LIN), r-copaene (a-COP), β-cedrene (b-CED), r-farnesene (a-FAR), β-farnesene (b-FAR), β-caryophyllene (b-CAR) and r-humulene (a-HM). Data shows mean and standard deviation which includes uncertainty of treatment and reference (control). of ozone. Quantitative recovery was observed for the camphene and p-cymene, whereas the olefinic monoterpenes exhibited losses correlated with their ozone rate constants (R-terpinene > D-limonene > R-pinene > β-pinene > p-cymene, camphene). VOC-ozone rate constants are summarized in Table 1. Similarly, severe losses were found for of all the olefinic alcohols (with the exception of 2-methyl3-buten-2-ol which has the slowest ozonolysis rate of the alcohols tested). Of the sesquiterpenes tested only β-cedrene, was modestly recovered (74%) and the rest were poorly recovered within the range of 4-30%. These results are similar to those reported (1-13) elsewhere and clearly demonstrate the need for mitigating the loss of reactive VOCs by cosampled ozone. Analyte Transmission through Manganese Dioxide Scrubber without Ozone. These tests were conducted by concurrently sampling the manifold test mix and downstream of the scrubber with 1.0 L min-1 of total flow. Recovery was calculated as the ratio of the downstream to the upstream concentration. The manganese dioxide scrubber performance is summarized in the second bar of Figure 2 with additional data in SI Table S2. The MnO2-coated screens, with the exception of p-cymene and R-terpinene, efficiently transmitted the simple monoterpene hydrocarbons. The R-terpinene was apparently dehydrogenated to p-cymene 7666
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as evidence by the 700% recovery (matching the carbon deficit from the R-terpinene). A similar process has been reported by Mcgraw et al. (26) where the thermal degradation of R-terpinene caused the formation of p-cymene. Hoffman (8), Wedel et al. (11) and Fick et al. (12) reported good transmission of R-pinene, β-pinene, and limonene through a single ply of MnO2-coated screen. However, Wedel et al. observed only 34 and 33% recovery with sabinene and R-terpinene, respectively, but did not report any rearrangement products. The 2-methyl-3-buten-2-ol was unaffected by passage through the MnO2 screens, whereas the cis-3-hexen-1-ol showed about 13% loss. However, the terpenoid alcohol transmission was poor with terpin-4-ol (49%), linalool (57%) and R-terpineol (10%). It is not clear whether these compounds are being lost to simple adsorption or reaction (possible oxidation or dehydration). The greater than 100% recovery of camphene, β-pinene, R-pinene, and limonene may be due to dehydration of these compounds similar to that this laboratory has observed resulting from desorption of these compounds from Carbotrap adsorbents. Other reports of transmission difficulty of oxygenates through MnO2 screens include cis-3-hexen-1-ol (11), linalool (8), camphor (2), and citral (8). Passage of the test sesquiterpenes through the screens in the absence of ozone was also problematic. Only R-copaene and β-cedrene efficiently passed through the screens in these tests. The two farnesene isomers (tetra-olefins) were below 52% recovery, whereas the two caryophyllene isomers, β-caryophyllene and R-humulene, were recovered at 66 and 59%, respectively. In a similar test, Wedel et al. (11) found 72% transmission of R-cedrene through a single screen. Pollmann et al. (13) reported complete removal of eight sesquiterpenes (including R-copaene and both caryophyllene isomers) using an unmodified ozone instrument ozone scrubber (stack of 25 MnO2 screens). However, it should be noted that their study did not follow the previously cited guidance (8) regarding optimum contact time with the screens. This caused approximately 2 orders of magnitude longer contact with the MnO2 mesh compared to previous studies (approximate exposure ) no. plies ÷ face velocity). Analyte Transmission in the Presence of Ozone with the MnO2 Screen Scrubber. To assess the efficacy of removal of ozone by MnO2, two consecutive sets of samples were collected: one downstream of the MnO2 screen scrubber with 100 ppbv of ozone in the test mixture and one directly from the manifold with the ozone generator switched off. The total recovery is computed from the ratio of the analyte concentration measured with the scrubber to the concentration measured before the scrubber without ozone present. See Figure 2 (third bar). No ozone was detectable downstream of the screen scrubber (below 1 ppbv). The screens present no significant advantage for the low reactive camphene compared to no ozone management. For β-pinene, R-pinene, and limonene the screens are effective in providing quantitative recovery compared to no ozone management. However, due to the interaction of R-terpinene with the screens, no benefit is gained for R-terpinene. These results are similar to other reports using single (7, 11) or multiple ply (8, 12) MnO2 screen scrubbers where improved recoveries were found for β-pinene, R-pinene, and limonene. Given the demonstrated poor performance of the MnO2 screens in transmitting the test alcohols, it is not surprising that there is little to no benefit gained by MnO2 ozone removal. Only cis-3-hexen-1-ol shows a substantial improvement over no ozone management. The effect on 2-methyl3-buten-2-ol is neutral, displaying the same good recovery with or without the MnO2 screens. The linalool and R-terpineol, given their demonstrated removal by the scrubber, are better recovered without the scrubber. There has been
little previous investigation of the efficacy of the screens on alcohols. Wedel et al. (11) reported 83% recovery of cis-3hexen-1-ol without ozone management (100 ppbv), and no recovery on passage through a single MnO2 screen. The MnO2 screens improved the recovery of R-copaene from 21 to 47% and had no appreciable effect on the recovery of β-cedrene. The screens caused a slight increase in the recoveries of β-caryophyllene (6-10%), β-farnesene (19-33%), and R-humulene (4-7%). For R-farnesene, recovery decreased from 30% without the screens to 16% with the screens. These data are consistent with those of Hoffman (7) who reported a wide range (2-100%) of recovery in sesquiterpene measurements when comparing scrubbed vs nonscrubbed air containing 23 ppbv ozone. Wedel et al. (11), also using a single screen and 100 ppbv of ozone, was unable to measure any R-cedrene downstream of the scrubber. Calogirou et al. (8) observed an increase of β-caryophyllene recovery from 4% without the scrubber to 35% recovery with an 8-ply scrubber using 85 ppbv of ozone. Analyte Transmission through Delay Coil. For these tests, VOC sampling was performed concurrently by drawing samples directly from the manifold and downstream of the coil with a total flow through the coil of 1.0 L min-1. Recovery was computed as the ratio of the downstream to the upstream concentrations. The untreated stainless steel coil was found to be unsatisfactory in the temperature range of 40-100 °C with poor transmission of the terpenoid alcohols and most of the sesquiterpenes. The Sulfinert passivated coil was found to pass all components of the test mixture at greater than 90% (except for R-terpineol at 89%) at 40, 50, and 60 °C. The pooled results for transmission tests at 40, 50, and 60 °C for the Sulfinert passivated stainless steel coil are presented in the first bar of Figure 3 with tabulated results in SI Table S3. Based on these results the Sulfinert coil was judged to be a suitable conductive path for the reactive mixture. Analyte Transmission in the Presence of Ozone with Trans-2-Butene Addition. To measure the effect of addition of trans-2-butene on analyte recovery, two consecutive sets of measurements were made downstream of the 5 s delay coil: one set with the ozone generator ON at 100 ppbv and the other with generator OFF at 0 ppbv. The ozonated air is blended with the trans-2-butene in a union tee which immediately passes into a second union Tee where the VOC/ N2 stream is introduced into the center of the incoming diluent mix. The total mix then passes through smaller diameter Teflon tubing containing a randomly twisted filament of TFE Teflon (to promote mixing) before discharging to the larger diameter sampling manifold. See the SI Figure S1 for schematic of apparatus. The estimated time from the point of blending the ozone, diluent, trans-2-butene and VOC in nitrogen stream to the inlet of the delay coil is only about 130 ms. Three sets of samples were collected with sampling volumes of 1.8, 3.6, and 5.4 L. The results are presented in Figure 3 and SI Table S3. At the lowest sample volume, 1.8 L, all the monoterpenoids and sesquiterpenes were recovered g98%. However, the R-terpinene, β-caryophyllene, and R-humulene display a decrease in recovery with larger sample volume (R-terpinene: 102 ( 2% f 96 ( 1% f 92 ( 1%; β-caryophyllene: 101 ( 7% f 89 ( 1% f 88 ( 2%; R-humulene 101 ( 12% f 82 ( 1% f 80 ( 2%). In contrast the model of Figure 1B predicted no more than 2% loss for R-terpinene at 600 ppmv of trans-2butene. The deviation of the R-terpinene, R-humulene, and β-caryophyllene from that predicted from the simple gas kinetic modeling prompted further investigation into the cause of these anomalies. Assuming the model is at least qualitatively accurate (i.e., response of ozone residuals to standing concentration of trans-2-butene and sensitivity of analyte recovery from Tenax-TA to residual ozone), the model can be used to test some experimental assumptions. The
FIGURE 3. Sulfinert coil VOC transmission (coil/manifold, no ozone), 1st bar, and effect of 600 ppmv trans-2-butene on VOC recovery (coil with ozone/coil without ozone) in presence of 100 ppbv ozone. Second, third and fourth bars are 1.8, 3.6, and 5.4 L samples using passive twisted PFA Teflon filament to create turbulence after mixing trans-2-butene/ozonized diluent air stream with VOC in nitrogen stream. Last bar shows effect of adding 4000 rpm agitator to gas stream emerging from filament passage. first assumption tested was that the ozone residual exiting the delay coil was higher than predicted due to incomplete mixing of the air/trans-2-butene/VOC gas streams. This could be the case if there was insufficient turbulence prior to entry and within the coil. Although ozone measurements were unable to detect any residual ozone, the model suggests that recoveries are sensitive to residual ozone levels in the subppbv range. Since it was not feasible to make subppb measurements of ozone with the instrumentation available, the model was used to indirectly test whether residual ozone is causing the observed high loss of R-terpinene, R-humulene, and β-caryophyllene. The model predicts that if the initial trans2-butene is raised from 600 to 1200 ppm, the residual ozone VOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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exiting the coil should decline more than a factor of 107. Thus, even if imperfect mixing is present, the residual ozone should be decreased to negligible levels. These results (not shown) were indistinguishable from the runs at 600 ppmv of trans-2-butene. A second test to assess the possible importance of incomplete mixing was performed by adding a high speed paddle, rotating in the sample path immediately downstream of the twisted filament (see bottom inset SI Figure S1). The results for these tests are shown in the last bar of Figure 3. Supplementing the filament mixing with an active high speed agitator did not increase recovery. Therefore, both tests appear to rule out the presence of higher than expected residuals of ozone to explain the deviations from the model. If there is insufficient residual ozone remaining to account for the observed loss with increasing sample volume, the possibility that a product of the ozone - trans-2-butene may be reacting with the adsorbed VOC must be considered. It is well-known that ozone-olefin reactions can produce OH radicals. In the gas phase, any OH produced will be scavenged preferentially by the trans-2-butene present at overwhelming concentration in the gas phase. However, the accumulating nature of the adsorbent bed will raise the local VOC concentration as sample volume increases to the point where the bimolecular rate expression kOH-VOC(OH)(VOCadsorbed) approaches or exceeds kOH-trans-2-butene(OH)(trans-2-butene). For trans-2-butene, OH yields on the order of 0.57-0.75 have been reported (27-29). To evaluate this the previous modeling exercise was repeated assuming that the OH concentrations after 5 s of ozone reaction with trans-2-butene is equivalent to 0.6 × -∆(O3)/d∆ t, and the VOC loss was calculated from the accumulated VOC and the steady state OH flux into the bed. These calculations do indeed show that VOC sampling losses for a-humulene as a result of increasing importance of OH losses may cause losses on the order of 10% for sample volumes in the range of 3.6-5.4 L for R-terpinene. Lastly, the apparent ozone reactivity of the analytes tested, in some cases, are not consistent with reported rate constants. As can be seen in Table 1, the expected order of reactivity based on gas kinetics is R-terpinene > R-humulene > β-caryophyllene. This order is also true for their reaction with OH (15). However, the observed order is R-humulene > β-caryophyllene > R-terpinene. This may be explained by spatial separation of these species on the Tenax-TA adsorbent bed. Because the sesquiterpenes have a higher affinity for the Tenax-TA (larger retention volumes) than the monoterpene R-terpinene, the sesquiterpenes will tend to displace R-terpinene from the front of the bed. Thus, the ozone will contact more sesquiterpenes before it contacts the R-terpinene deeper within the bed. This may explain the higher losses of the R-humulene and β-caryophyllene relative to the R-terpinene. Because the distribution of adsorbed VOCs on the Tenax bed is governed by the linear velocity through the tube and not the volumetric flow rate, application of these results require that other sampling systems use the same linear velocity (13 cm s-1) to achieve similar ozone scavenging results. Chromatographic theory, however, predicts that higher flow rates would cause the VOCs to be distributed in a wider band in the adsorbent bed. This would cause the local VOC density to decrease and should decrease sensitivity to residual ozone or OH. Evidence that this can occur can be found in the work of Caligirou et al. (8) where they collected samples of equal volume of a terpene mix containing 120 ppbv of ozone. Recoveries of the most ozone reactive terpenes were found to improve with higher flow rates. Although they attributed this effect to sample duration, another explanation is that the higher flow rates will tend to cause the terpenes 7668
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to penetrate deeper into the adsorbent bed thus decreasing the collision frequency of the adsorbed VOCs with cosampled ozone. The trans-2-butene ozone management has been integrated into a relaxed eddy accumulator flux measurement system for biogenic VOCs. In this system, the trans-2-butene is immediately added to a sample air drawn at about 14 L min-1 from the zone near the sonic anemometer. After about a 400 ms transit time to the sample segregation valves, a 1 L min-1 flow is directed to either an up, down or vent channel. Samples directed to the up or down draft channels are first passed into matching sets of Sulfinert coils before being directed to an adsorbent tube for collection. Sample volumes are typically about 10 L. Since the REA sample tubes have a cross sectional area of 1.27 cm2 compared with smaller 0.224 cm2 used for the laboratory experiments described herein, the bed loading is equivalent to that of the tests performed at 1.8 L. Hence, the field measurements should be within the no loss limits established by these tests. With approximately 400 one-hour flux measurements completed, the system consistently measured concentration gradients (mean updraft concentrations-mean downdraft concentrations) necessary to calculate vertical fluxes (emissions) of these compounds. Compounds regularly observed in measurements made over a loblolly pine forest include R-pinene, β-pinene, camphene, myrcene, β-phellandrene, p-cymene, D-limonene, linalool, camphor, verbenone, bornyl acetate, R-bergamotene, β-caryophyllene, δ-cadinene. Data from this study will be presented in another publication. The application of this ozone management for reactive VOC analysis is made practical by the poor retention properties of the Tenax adsorbent for the trans-2-butene relative to the high retention of the monoterpenes and sesquiterpenes. This approach could not easily be applied to whole air sample collection and analysis where cryogenic condensation is used to focus the VOCs. The large amounts of trans-2-butene would cause physical obstruction of the traps which are typically cooled below the -106 °C freezing point of the trans-2-butene.
Acknowledgments The United States Environmental Protection Agency through its Office of Research and Development funded and managed the research described here. It has been subjected to Agency’s administrative review and approval for publication.
Supporting Information Available Additional details, Tables S1-S3. and Figures S1-S3. This material is available free of charge via the Internet at http:// pubs.acs.org.
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(7) Hoffman, T. Adsorptive preconcentration technique including oxidant scavenging for the measurement of reactive natural hydrocarbons in ambient air. Fresenius´ J. Anal. Chem. 1995, 351, 41. (8) Calogirou, A.; Larsen, B. R.; Brussol, C.; Duane, M.; Kotzias, D. Decomposition of terpenes by ozone during sampling on Tenax. Anal. Chem. 1996, 68, 1499. (9) Clausen, P. A.; Wolkoff, P. Degradation products of Tenax TA formed during sampling and thermal desorption analysis: indicators of reactive species indoors. Atmos. Environ. 1997, 31 (5), 715. (10) Caligirou, A.; Duane, M.; Kotzias, D.; Lahaniati, M.; Larsen, B. R. Polyphenylenesulfide, Noxone, an ozone scavenger for the analysis of oxygenated terpenes in air. Atmos. Environ. 1997, 31 (17), 2741. (11) Wedel, A.; Mu ¨ ller, K.-P.; Ratte, M.; Rudolph, J. J. Measurements of volatile organic compounds (VOC) during POPCORN 1994: Applying a new on-line GC-MS-Technique. J. Atmos. Chem. 1998, 31, 73. (12) Fick, J.; Pommer, L., B.; Andersson, B.; Nilsson, C. Ozone removal in the sampling of parts per billion levels of terpenoid compounds: an evaluation of different scrubber materials. Environ. Sci. Technol. 2001, 35, 1458. (13) Pollmann, J.; Ortega, J.; Helmig, D. Analysis of atmospheric sesquiterpenes: sampling losses and mitigation of ozone interferences. Environ. Sci. Technol. 2005, 39, 9620. (14) Helmig, D. Ozone removal techniques in the sampling of atmospheric volatile organic trace gases. Atmos. Environ. 1997, 31, 3635. (15) Atkinson, R. Gas-phase tropospheric chemistry of volatile organic compounds: 1. Alkanes and alkenes. J. Phys. Chem. Ref. Data 1997, 26, 215. (16) Tuazon, E. C.; Aschmann, S. M.; Arey, J.; Atkinson, R. Products of the gas-phase reactions of O3 with a series of methylsubstituted ethenes. Environ. Sci. Technol. 1997, 31, 3004. (17) Maier, I.; Fieber, M. Retention characteristics of volatile compounds on Tenax-TA. J. High Resolut. Chromatogr. Chromatogr. Commun. 1988, 11, 566. (18) Businger, J. A.; Oncley, S. P. Flux measurements with conditional sampling. J. Atmos. Oceanic Technol. 1989, 7, 349.
(19) Businger, J. A.; Delaney, A. C. Chemical sensor resolution required for measuring surface fluxes by three common micrometeorological techniques. J. Atmos. Chem. 1990, 10, 390. (20) Altshuller, A. P.; Cohen, I. R. Application of diffusion cells to the production of known concentrations of gaseous hydrocarbons. Anal. Chem. 1960, 32, 802. (21) Klawatsch-Carrasco, N.; Doussin, J. F.; Carlier, P. Absolute rate constants for the gas-phase ozonolysis of isoprene and methylbutenol. Int. J. Chem. Kinetics 2004, 36, 152. (22) Atkinson, R.; Arey, J.; Aschmann, S. A.; Corchnoy, S. B.; Shu, Y. Rate constants for the gas-phase reactions of cis-3-hexen-1-ol, cis-3-hexenylacetate, trans-2-hexenal, and linalool with OH and NO3 radicals and O3 at 296 ( 2K, and OH radical formation yields from the O3 reactions. Int. J. Chem. Kinet. 1995, 27, 941. (23) Hoffman, T.; Odum, J. R.; Bowman, F.; Collins, D.; Klockow, D.; Flagan, R. C.; Seinfeld, J. H. Formation of organic aerosols from the oxidation of biogenic hydrocarbons. J. Atmos. Chem. 1997, 26, 189. (24) Wells, J. R. Gas-phase chemistry of R-terpineol with ozone and OH radical: rate constants and products. Environ. Sci. Technol. 2005, 39, 6937. (25) Smith, D. A. Surface modification of solid supports through thermal decomposition and functionalization of silanes. United States Patent No. 6,44,326, 2002. (26) Mcgraw, G.; Hemmingway, R. W.; Ingram, Jr. L. L.; Canady, C. S.; Mcgraw, W. B. Thermal degradation of terpenes: camphene, ∆3-carene, limonene, and R-terpinene. Environ. Sci. Technol. 1999, 33, 4029. (27) Atkinson, R.; Aschmann, S. M. Hydroxyl radical production from the gas-phase reactions of ozone with a series of alkenes under atmospheric conditions. Environ. Sci. Technol. 1993, 27, 1357. (28) Fenske, J. D.; Hasson, A. S.; Paulson, S. E.; Kuwata, K. T.; Ho, A.; Houk, K. N. The Pressure dependence of the OH radical yield from ozone-alkene reactions. J. Phys. Chem. A 2000, 104, 7821. (29) McGill, C. D.; Rickard, A. R.; Johnson, D.; Marston, G. ; Product yields in the reaction of ozone with Z-but-2-ene, E-but-2-ene and 2-methylbut-2-ene. Chemsophere 1999, 38, 1205.
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