Environ. Sci. Technol. 2001, 35, 2717-2720
Degradation of r-Pinene on Tenax during Sample Storage: Effects of Daylight Radiation and Temperature W O L F G A N G S C H R A D E R , * ,† J U T T A G E I G E R , †,‡ DIETER KLOCKOW,# AND ERNST-HEINER KORTE† Institut fu ¨ r Spektrochemie und angewandte Spektroskopie (ISAS), Institutsteil Berlin, Albert-Einstein-Str. 9, 12489 Berlin-Adlershof, Germany, and Institutsteil Dortmund, Bunsen-Kirchhoff-Str. 11, 44139 Dortmund, Germany
The behavior of R-pinene sampled on adsorption cartridges filled with Tenax TA has been investigated in relation to different storage conditions, focusing on daylight radiation and temperature. After sampling, the respective cartridges containing the terpene were placed in sunlight on the windowsill for up to 1 month. Corresponding samples have been wrapped in aluminum foil to prevent the influence of daylight radiation. Additional sample cartridges with R-pinene were stored in the refrigerator at 4 °C and a freezer at -18 °C. All cartridges were analyzed using thermodesorption injection onto a gas chromatograph, and the compounds were detected using either a cryocondensationinterface to a Fourier transform infrared-spectrometer (GC/ FT-IR) or the flame ionization detector (FID). In summary, 12 compounds were detected and identified, from which eight were products that were formed on Tenax through different mechanisms. Two compounds seemed to be formed under the influence of daylight radiation, while the others appear to be mainly autoxidation products. Estimates after 1 month of storage showed recoveries of over 99% for wrapped samples, while for unwrapped cartridges only about 88% of R-pinene was found. A pattern of up to five compounds was found that can be used as an indicator for storage reactions.
1. Introduction Volatile organic compounds (VOC) play an important role in environmental chemistry. The applications range from indoor problems where VOC are suspected to be responsible for the sick-building-syndrome (1) to outdoor studies involving biogenic compounds from plant emissions (2). Biogenic hydrocarbons, mostly isoprene and a variety of terpenes, influence the regional tropospheric chemistry and the formation of atmospheric oxidants. Recent estimates show the amount of biogenic emissions from vegetation (between 825 and 1150 Tg carbon per year) (2, 3) to exceed the atmospheric input from antropogenic sources (between 60 and 140 Tg C/year) (4). * Corresponding author phone: +49(0)208 306 2271; fax: +49(0)208 306 2982; e-mail:
[email protected]. Present address: Max-Planck-Intitut fu¨r Kohlenforschung, Kaiser Wilhelm-Platz 1, 45470 Mu ¨ lheim, Germany. † Institutsteil Berlin. ‡ Present address: LUA-NRW, Wallneyerstrasse 6, 45133 Essen, Germany. # Institutsteil Dortmund. 10.1021/es0002722 CCC: $20.00 Published on Web 05/25/2001
2001 American Chemical Society
The challenge for the analytical approach is to detect trace amounts of compounds in complex matrices with sufficient sensitivity. Therefore, a preconcentration of the VOC on a solid adsorbent has been the method of choice, followed by separation with gas chromatography. In the last 20 years different adsorbents have been used such as charcoal (5), graphitized thermal carbon black (6), carbon molecular sieves (7), siloxanes (8), alumina, zeolite molecular sieves, acrylate polymers (9, 10), and Tenax. Especially Tenax TA, the successor of Tenax GC, is a widely used 2,6-diphenyl-pphenylene oxide polymer which has a higher purity than Tenax GC, produces fewer artifacts, and can be used for the adsorption of compounds with boiling points ranging between 80 and 200 °C (11). Tenax TA is an important adsorbent for the sampling of biogenic emissions, including terpenes (12). We have employed a preconcentration method using sample cartridges filled with Tenax TA and Carbotrap for analyzing emissions from plants (pinus silvestris L and lavender) (13). After thermodesorption the compounds were separated by gas chromatography and detected by cryocondensation-FT-IR (Fourier transform infrared spectroscopy). Most recent studies on the gas-phase reaction of R-pinene with ozone were made by sampling the reaction products exclusively on Tenax TA (14). For these studies it seemed relevant to investigate the behavior of R-pinene on Tenax to find out under which circumstances fragmentation or degradation may occur. While the decomposition of terpenes on Tenax with (12) and without (15) ozone has been thoroughly investigated, the effects of sample storage conditions have not been assessed, although storage conditions may have a strong impact on results from outdoor campaigns where samples are often stored for a long time before being analyzed at the laboratory. To investigate the behavior on Tenax we have studied the influence of daylight radiation and temperature on R-pinene, which is sampled on the adsorbent. In this paper we present the results and identification of the products and suggest a possible mechanism of their formation.
2. Experimental Section 2.1. Materials. The chemicals were purchased from the following vendors: hexane (Lichrosolv grade), acetone, and methanol (analytical reagent grade), from Merck (Darmstadt, Germany); R-pinene, trans-pinocarveole, myrtenal, verbenone, R-pinene oxide, dimethyldichlorosilane, and Tenax TA (mesh 60/80), from Sigma/Aldrich (Deisenhofen, Germany). 2.2. Instrumental. For GC/Cryocondensation-FT-IR measurements a Bio Rad FTS-60-A interferometer has been used, coupled to a GC/IR Interface-unit (TRACER) (Digilab Division, Krefeld, Germany) which is connected to a Fisons gas chromatograph series 8060 (Mainz, Germany). The whole system has already been described elsewhere (13, 14). The GC was equipped with a laboratory-built thermodesorption injector connected to a DB-5 column (J+W Scientific, Fisons; 60 m × 0.25 mm i.d., film thickness ) 0.25 µm) and the original liquid injector connected to a BPX-5 column (SGE Deutschland, Weiterstadt; 50 m × 0.22 mm i.d., film thickness ) 0.25 µm). Both columns are connected to a pneumatically actuated 4-way-valve (Valco, GAT Analysentechnik, Berlin), located inside the GC oven, which allows to use the flame ionization detector (FID) as well as the TRACER as detection devices for both columns. Spectra were obtained on-the-move by co-adding four scans with a resolution of 8 cm-1, which gives a time resolution of 1 spectrum every 0.8 s. VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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For the analysis the compounds were thermally desorbed from the Tenax cartridges at 270 °C and cryotrapped at -110 °C within a deactivated glass-lined tube (15). Afterward, the analytes were rapidly vaporized and injected onto the column by heating the cryotrap to 250 °C. The temperature of the column was held at 40 °C for 3 min and afterward increased with a rate of 2 °C/min to 120 °C. Thereafter, the column temperature was increased with a rate of 5 °C/min up to 250 °C. GC/FID experiments were done accordingly by just switching the separation column to the FID detector. 2.3. Experimental Conditions. The sampling routine corresponds to the one used for our gas-phase studies of R-pinene with ozone (14). The samples were taken using self-made quartz tubes of 12.5 cm length, each filled with 60 mg of Tenax TA which was prepared as described by Hoffmann (16). R-Pinene was emitted from a test gas generator (14, 16, 17) whose temperature was controlled by a thermostat at 30 °C. The analyte was carried in a stream of synthetic air with a concentration of 6 ppmv and sampled for 1 min on a Tenax cartridge with a sample flow of 25 L/h. After sampling both ends of the cartridge were closed with Swagelok connectors using graphite ferrules. The sample tubes were then exposed to daylight radiation for 1 day, 1 week, and 1 month by placing them in the sunlight on the windowsill. Corresponding cartridges were wrapped in aluminum foil and placed next to the cartridges that were not wrapped for the same period of time. Additional cartridges were sampled, wrapped in aluminum foil, and placed for 7 days in either a refrigerator at 4 °C or in a freezer at -18 °C. The identification of the compounds was made through the infrared spectra obtained with the TRACER-system. For an estimate of quantities the detector was switched to the FID, and calibration functions were obtained using available standard compounds. Reference samples were measured directly without any storage with both detectors.
FIGURE 1. FG chromatograms of CH-stretching band (3000-2750 cm-1): Comparison of r-pinene after 1 day storage: lower trace shows sample unexposed to sunlight, upper trace exposed sample. The numbers correlate to the compounds listed in Table 1. (IM1 indicates tricyclen, IM2 camphene, IM3 β-pinene; these are impurities from the standard of r-pinene while IM indicates siloxane artifacts.)
FIGURE 2. FG chromatograms of CH-stretching band (3000-2750 cm-1): Comparison of r-pinene after 1 week storage: lower trace shows sample unexposed to sunlight, upper trace exposed sample.
3. Results and Discussion The effect of daylight radiation on R-pinene containing TENAX cartridges is illustrated in Figures 1-3. These figures show the functional group (FG) chromatograms of the CHstretching vibrations between 3000 and 2750 cm-1. The results of GC/cryocondensation-FT-IR data can be displayed in either a Gram-Schmidt chromatogram, which shows an overview about all compounds that have any absorption along the scan range of the spectra or in socalled functional group (FG) chromatograms. These FG chromatograms show only signals from a predefined absorption region, i.e., of a specific functional group and are similar to mass traces in mass spectrometry. For comparison, each figure contains two different traces: the lower trace has been obtained from a cartridge that was wrapped in aluminum foil, while the upper trace was measured from the exposed sample tube without aluminum foil. The exposition time from Figure 1-3 was increased from 1 day over 1 week to 1 month, and it is apparent that the number of peaks is increasing and also are the intensities. While the original sample of R-pinene measured directly after the sampling shows just the impurities tricyclene (IM1), camphene (IM2), and β-pinene (IM3), up to nine different compounds resulting from degradation were found in the chromatograms from the stored cartridges. While in Figure 1 no major artifact can be determined for the wrapped sample, the exposed cartridge already shows a number of signals. This signals increase with the duration of storage time, as is the same for the unexposed samples, while just in a smaller rate. The identification of the peaks 2718
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FIGURE 3. FG chromatograms of CH-stretching band (3000-2750 cm-1): Comparison of r-pinene after 1 month storage: lower trace shows sample unexposed to sunlight, upper trace exposed sample. with their estimated yields is listed in Table 1. The known degradation products of Tenaxsbenzaldehyde and acetophenon (11)swere observed but are not considered here. For an estimation of the yields calibration curves for R-pinene, trans-pinocarveol, myrtenal, and verbenone were obtained from commercially available standards through determination of the FID areas. Other identified products were not available as standards, and, therefore, the yields had to be calculated according to the effective carbon number (ECN) concept from Scanlon and Willis (18), which allows to quantify hydrocarbons compared to a standard hydrocarbon after detection with FID. This concept was used for the calculation of the relative response factor (RRF) of verbenene, since no compound with comparable structure was measured. The experimental value of trans-pinocarveol was used in agreement with the concept for the two alcohols trans-pinene-3-ol and trans-verbenol; for pinocarvone the experimental value of myrtenal and verbenone, which were identical, was taken. These results allow to distinguish two different effects: the influence of daylight radiation on
TABLE 1. Listing of Degradation Products from r-Pinene Storagea
a Yields are determined after 1 month of storage in the dark and in the daylight and are calculated from FID results. For R-pinene the recovery is given. Relative response factors (RRF) were calculated in reference to R-pinene. *experimental data.
R-pinene and the influence of the temperature on the wrapped cartridges. The main compounds formed on the cartridges wrapped in aluminum foil are verbenene, transpinene-3-ol, trans-verbenol, and verbenone. For the sample stored for 1 month the sum of the yields of the degradation products is ca. 0.75%. For the unwrapped samples already after 1 day most of the degradation products can be detected. After 1 month 12% of the R-pinene has reacted. The main products are trans-pinocarveol and pinocarvone. To compare the effect of daylight radiation with the effect of storage in the dark, in the last column of Table 1 the ratios of the respective yields calculated for the samples after 1 month of storage are given. All ratios are greater than one, which means that light promotes the formation of all the products. It is obvious that the ratios for trans-pinocarveol and pinocarvone and to some extent also for myrtenal are higher than the ratios of the other compounds. This degradation of R-pinene may be explained with an autoxidation process. It is possible to propose a radical chain mechanism as has been reported by Moore et al. (19) and by Schenk et al. (20, 21) and also is described by March (22). A slow atmospheric oxidation of C-H bonds to hydroperoxides, which is called autoxidation, can lead to the observed compounds. The formation of these compounds is in accord with an H-atom abstraction in the allylic position of olefins with an intermediate of the according hydroperoxides.
R1OO• + RH f R• + R1OOH R• + O2 f ROO• These can further react to alcohols and ketones. The allylic
FIGURE 4. The allylic radicals that can be formed after H-atom abstraction from r-pinene in an autoxidation process. radicals R• are resonance stabilized. In Figure 4 the mesomeric allylic radicals that can be formed from R-pinene are shown. Comparing the structures of the compounds in Table 1 with the radicals in Figure 4, the formation of most of the compounds in the autoxidation process can easily be accounted for. Reaction via radical Ia can lead to transverbenol and verbenone, via Ib to trans-pinene-3-ol; IIa to myrtenal; and IIb to trans-pinocarveol and pinocarvone. The formation of verbenene has been explained by dehydration of an initially formed less stable cis-verbenol. Taking the stability of free radicals to decrease from tertiary to secondary to primary, the main products of autoxidation should be formed via the initial secondary radical Ia and its mesomeric form Ib. This is the case for the wrapped samples, where verbenene, trans-pinene-3-ol, trans-verbenol, and verbenone are found as the main compounds. A general increase of the autoxidation due to daylight radiation has been reported by Schenck, but the strong increase of the products trans-pinocarveol and pinocarvone must have another cause. In the presence of daylight radiation an additional process can take place: Oxygen can be promoted to the excited singlet state and then undergo a so-called VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The formation of the degradation products may be explained by a two way mechanism, where most components are formed during a radical autoxidation mechanism, while the most abundant compounds pinocarveol and pinocarvon are formed through a photosensitized reaction with singlet oxygen.
Acknowledgments
FIGURE 5. Influence of temperature: FID chromatograms from TENAX cartridges with r-pinene stored for 7 days in a freezer at -18 °C (lower trace), a refrigerator at 4 °C (middle trace), and in the laboratory at 25 °C (upper trace) photosensitized reaction with R-pinene. Different mechanisms for this reaction have been postulated to explain the stereoselective formation of an intermediate allylic hydroperoxide. The two most favored are a pericyclic mechanism, similar to that of the ene synthesis (23), and the addition of singlet oxygen to the double bond, followed by an internal proton transfer. For R-pinene in both cases the products formed would be trans-pinocarveol and pinocarvone, the main compounds found in this study under the influence of daylight radiation. Another compound detected in minor traces is campholene aldehyde. It has been shown that campholene aldehyde can be formed from a primary product R-pinene oxide, due to thermal rearrangement inside the GC injector (14). In some studies R-pinene oxide was also found as an autoxidation product of R-pinene (19-21). Influence of Temperature. In Figure 5 three different chromatograms are shown from sample cartridges after storage of 1 week in a freezer at -18 °C, a refrigerator at 4 °C, and in sunlight on the windowsill. All samples have been wrapped in aluminum foil to prevent influences of daylight. From the chromatograms it can be seen that the autoxidation is impeded by lower temperature. These results show that even in the refrigerator autoxidation of R-pinene on Tenax occurs, while this effect is significantly reduced if the sample cartridges are stored in the freezer. These results show that not only the influences of oxidizing agents, like ozone, have to be considered during the sampling of volatile organic compounds but also that effects of daylight radiation and temperature lead to oxidation of R-pinene on Tenax. It was shown that the observed pattern of up to five compounds, pinocarveol, pinocarvon, verbenon, myrtenal, and verbenol, can be taken as an indicator for storage reactions after sampling of R-pinene on Tenax, especially for samples from field studies that require long sample times or have been stored for longer times. Estimates indicate a degradation of up to 12% after 1 month of storage. It usually does not help to just wrap the sample cartridges in aluminum foil. Even basic storage for 1 week in a refrigerator is unsatisfactory. To avoid degradation of the terpenes collected on sample cartridges they have to be protected from daylight radiation and temperature influences.
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The financial support by the Senatsverwaltung fu ¨ r Wissenschaft, Forschung und Kultur des Landes Berlin and the Bundesministerium fu ¨ r Bildung und Forschung is gratefully acknowledged.
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Received for review November 17, 2000. Revised manuscript received April 2, 2001. Accepted April 12, 2001. ES0002722
