Degradation of r-Pinene on Tenaxduring Sample Storage: Effects ofDaylight Radiation and TemperatureW 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 , † , ‡

D I E T E R K L O C K O W , # A N DE R N S T - H E I N E R K O R T E †

Institut fur 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 cartridgesfilled with Tenax TA has been investigated in relationto different storage conditions, focusing on daylight radiationand temperature. After sampling, the respective cartridgescontaining the terpene were placed in sunlight on thewindowsill for up to 1 month. Corresponding samples havebeen wrapped in aluminum foil to prevent the influenceof daylight radiation. Additional sample cartridges withR-pinene were stored in the refrigerator at 4 °C and a freezerat -18 °C. All cartridges were analyzed using thermode-sorption injection onto a gas chromatograph, and thecompounds were detected using either a cryocondensation-interface to a Fourier transform infrared-spectrometer (GC/FT-IR) or the flame ionization detector (FID). In summary,12 compounds were detected and identified, from whicheight were products that were formed on Tenax throughdifferent mechanisms. Two compounds seemed to be formedunder the influence of daylight radiation, while theothers appear to be mainly autoxidation products. Estimatesafter 1 month of storage showed recoveries of over 99%for wrapped samples, while for unwrapped cartridges onlyabout 88% of R-pinene was found. A pattern of up tofive compounds was found that can be used as an indicatorfor storage reactions.

1. IntroductionVolatile organic compounds (VOC) play an important rolein environmental chemistry. The applications range fromindoor problems where VOC are suspected to be responsiblefor the sick-building-syndrome (1) to outdoor studies in-volving biogenic compounds from plant emissions (2).

Biogenic hydrocarbons, mostly isoprene and a variety ofterpenes, influence the regional tropospheric chemistry andthe formation of atmospheric oxidants. Recent estimatesshow the amount of biogenic emissions from vegetation(between 825 and 1150 Tg carbon per year) (2, 3) to exceedthe atmospheric input from antropogenic sources (between60 and 140 Tg C/year) (4).

The challenge for the analytical approach is to detect traceamounts of compounds in complex matrices with sufficientsensitivity. Therefore, a preconcentration of the VOC on asolid adsorbent has been the method of choice, followed byseparation with gas chromatography. In the last 20 yearsdifferent adsorbents have been used such as charcoal (5),graphitized thermal carbon black (6), carbon molecular sieves(7), siloxanes (8), alumina, zeolite molecular sieves, acrylatepolymers (9, 10), and Tenax. Especially Tenax TA, thesuccessor of Tenax GC, is a widely used 2,6-diphenyl-p-phenylene oxide polymer which has a higher purity thanTenax GC, produces fewer artifacts, and can be used for theadsorption of compounds with boiling points rangingbetween 80 and 200 °C (11).

Tenax TA is an important adsorbent for the sampling ofbiogenic emissions, including terpenes (12). We have em-ployed a preconcentration method using sample cartridgesfilled with Tenax TA and Carbotrap for analyzing emissionsfrom plants (pinus silvestris L and lavender) (13). Afterthermodesorption the compounds were separated by gaschromatography and detected by cryocondensation-FT-IR(Fourier transform infrared spectroscopy). Most recentstudies on the gas-phase reaction of R-pinene with ozonewere made by sampling the reaction products exclusively onTenax TA (14). For these studies it seemed relevant toinvestigate the behavior of R-pinene on Tenax to find outunder which circumstances fragmentation or degradationmay 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 beenassessed, although storage conditions may have a strongimpact on results from outdoor campaigns where samplesare often stored for a long time before being analyzed at thelaboratory. To investigate the behavior on Tenax we havestudied the influence of daylight radiation and temperatureon R-pinene, which is sampled on the adsorbent. In thispaper we present the results and identification of the productsand suggest a possible mechanism of their formation.

2. Experimental Section2.1. Materials. The chemicals were purchased from thefollowing vendors: hexane (Lichrosolv grade), acetone, andmethanol (analytical reagent grade), from Merck (Darmstadt,Germany); R-pinene, trans-pinocarveole, myrtenal, ver-benone, R-pinene oxide, dimethyldichlorosilane, and TenaxTA (mesh 60/80), from Sigma/Aldrich (Deisenhofen, Ger-many).

2.2. Instrumental. For GC/Cryocondensation-FT-IR mea-surements 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 gaschromatograph series 8060 (Mainz, Germany). The wholesystem has already been described elsewhere (13, 14). TheGC was equipped with a laboratory-built thermodesorptioninjector connected to a DB-5 column (J+W Scientific, Fisons;60 m × 0.25 mm i.d., film thickness ) 0.25 µm) and theoriginal liquid injector connected to a BPX-5 column (SGEDeutschland, Weiterstadt; 50 m × 0.22 mm i.d., film thickness) 0.25 µm). Both columns are connected to a pneumaticallyactuated 4-way-valve (Valco, GAT Analysentechnik, Berlin),located inside the GC oven, which allows to use the flameionization detector (FID) as well as the TRACER as detectiondevices for both columns.

Spectra were obtained on-the-move by co-adding fourscans with a resolution of 8 cm-1, which gives a timeresolution of 1 spectrum every 0.8 s.

* Corresponding author phone: +49(0)208 306 2271; fax: +49(0)208306 2982; e-mail: wschrader@mpi-muelheim.mpg.de. Present ad-dress: Max-Planck-Intitut fur Kohlenforschung, Kaiser Wilhelm-Platz1, 45470 Mulheim, Germany.

† Institutsteil Berlin.‡ Present address: LUA-NRW, Wallneyerstrasse 6, 45133 Essen,

Germany.# Institutsteil Dortmund.

Environ. Sci. Technol. 2001, 35, 2717-2720

10.1021/es0002722 CCC: $20.00 2001 American Chemical Society VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2717Published on Web 05/25/2001

For the analysis the compounds were thermally desorbedfrom the Tenax cartridges at 270 °C and cryotrapped at -110°C within a deactivated glass-lined tube (15). Afterward, theanalytes were rapidly vaporized and injected onto the columnby heating the cryotrap to 250 °C. The temperature of thecolumn was held at 40 °C for 3 min and afterward increasedwith a rate of 2 °C/min to 120 °C. Thereafter, the columntemperature was increased with a rate of 5 °C/min up to250 °C.

GC/FID experiments were done accordingly by justswitching the separation column to the FID detector.

2.3. Experimental Conditions. The sampling routinecorresponds to the one used for our gas-phase studies ofR-pinene with ozone (14). The samples were taken usingself-made quartz tubes of 12.5 cm length, each filled with 60mg of Tenax TA which was prepared as described byHoffmann (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 witha concentration of 6 ppmv and sampled for 1 min on a Tenaxcartridge with a sample flow of 25 L/h. After sampling bothends of the cartridge were closed with Swagelok connectorsusing graphite ferrules. The sample tubes were then exposedto daylight radiation for 1 day, 1 week, and 1 month by placingthem in the sunlight on the windowsill. Correspondingcartridges were wrapped in aluminum foil and placed nextto the cartridges that were not wrapped for the same periodof time.

Additional cartridges were sampled, wrapped in alumi-num 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 throughthe infrared spectra obtained with the TRACER-system. Foran estimate of quantities the detector was switched to theFID, and calibration functions were obtained using availablestandard compounds. Reference samples were measureddirectly without any storage with both detectors.

3. Results and DiscussionThe effect of daylight radiation on R-pinene containingTENAX cartridges is illustrated in Figures 1-3. These figuresshow the functional group (FG) chromatograms of the CH-stretching vibrations between 3000 and 2750 cm-1.

The results of GC/cryocondensation-FT-IR data can bedisplayed in either a Gram-Schmidt chromatogram, whichshows an overview about all compounds that have anyabsorption along the scan range of the spectra or in so-called functional group (FG) chromatograms. These FGchromatograms show only signals from a predefined ab-sorption region, i.e., of a specific functional group and aresimilar to mass traces in mass spectrometry.

For comparison, each figure contains two differenttraces: the lower trace has been obtained from a cartridgethat was wrapped in aluminum foil, while the upper tracewas measured from the exposed sample tube withoutaluminum foil. The exposition time from Figure 1-3 wasincreased from 1 day over 1 week to 1 month, and it isapparent that the number of peaks is increasing and also arethe intensities. While the original sample of R-pinenemeasured directly after the sampling shows just the impuritiestricyclene (IM1), camphene (IM2), and â-pinene (IM3), upto nine different compounds resulting from degradation werefound in the chromatograms from the stored cartridges. Whilein Figure 1 no major artifact can be determined for thewrapped sample, the exposed cartridge already shows anumber of signals. This signals increase with the durationof storage time, as is the same for the unexposed samples,while just in a smaller rate. The identification of the peaks

with their estimated yields is listed in Table 1. The knowndegradation products of Tenaxsbenzaldehyde and aceto-phenon (11)swere observed but are not considered here.

For an estimation of the yields calibration curves forR-pinene, trans-pinocarveol, myrtenal, and verbenone wereobtained from commercially available standards throughdetermination of the FID areas. Other identified productswere not available as standards, and, therefore, the yieldshad to be calculated according to the effective carbon number(ECN) concept from Scanlon and Willis (18), which allowsto quantify hydrocarbons compared to a standard hydro-carbon after detection with FID. This concept was used forthe calculation of the relative response factor (RRF) ofverbenene, since no compound with comparable structurewas measured. The experimental value of trans-pinocarveolwas used in agreement with the concept for the two alcoholstrans-pinene-3-ol and trans-verbenol; for pinocarvone theexperimental value of myrtenal and verbenone, which wereidentical, was taken. These results allow to distinguish twodifferent effects: the influence of daylight radiation on

FIGURE 1. FG chromatograms of CH-stretching band (3000-2750cm-1): Comparison of r-pinene after 1 day storage: lower traceshows sample unexposed to sunlight, upper trace exposed sample.The numbers correlate to the compounds listed in Table 1. (IM1indicates tricyclen, IM2 camphene, IM3 â-pinene; these areimpurities from the standard of r-pinene while IM indicates siloxaneartifacts.)

FIGURE 2. FG chromatograms of CH-stretching band (3000-2750cm-1): Comparison of r-pinene after 1 week storage: lower traceshows sample unexposed to sunlight, upper trace exposed sample.

FIGURE 3. FG chromatograms of CH-stretching band (3000-2750cm-1): Comparison of r-pinene after 1 month storage: lower traceshows sample unexposed to sunlight, upper trace exposed sample.

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R-pinene and the influence of the temperature on thewrapped cartridges. The main compounds formed on thecartridges wrapped in aluminum foil are verbenene, trans-pinene-3-ol, trans-verbenol, and verbenone. For the samplestored for 1 month the sum of the yields of the degradationproducts is ca. 0.75%. For the unwrapped samples alreadyafter 1 day most of the degradation products can be detected.After 1 month 12% of the R-pinene has reacted. The mainproducts are trans-pinocarveol and pinocarvone. To comparethe effect of daylight radiation with the effect of storage inthe dark, in the last column of Table 1 the ratios of therespective yields calculated for the samples after 1 month ofstorage are given. All ratios are greater than one, which meansthat light promotes the formation of all the products. It isobvious that the ratios for trans-pinocarveol and pinocarvoneand to some extent also for myrtenal are higher than theratios of the other compounds.

This degradation of R-pinene may be explained with anautoxidation process. It is possible to propose a radical chainmechanism as has been reported by Moore et al. (19) andby Schenk et al. (20, 21) and also is described by March (22).A slow atmospheric oxidation of C-H bonds to hydroper-oxides, which is called autoxidation, can lead to the observedcompounds. The formation of these compounds is in accordwith an H-atom abstraction in the allylic position of olefinswith an intermediate of the according hydroperoxides.

These can further react to alcohols and ketones. The allylic

radicals R• are resonance stabilized. In Figure 4 the mesomericallylic radicals that can be formed from R-pinene are shown.

Comparing the structures of the compounds in Table 1with the radicals in Figure 4, the formation of most of thecompounds in the autoxidation process can easily beaccounted for. Reaction via radical Ia can lead to trans-verbenol and verbenone, via Ib to trans-pinene-3-ol; IIa tomyrtenal; and IIb to trans-pinocarveol and pinocarvone. Theformation of verbenene has been explained by dehydrationof an initially formed less stable cis-verbenol.

Taking the stability of free radicals to decrease from tertiaryto secondary to primary, the main products of autoxidationshould be formed via the initial secondary radical Ia and itsmesomeric form Ib. This is the case for the wrapped samples,where verbenene, trans-pinene-3-ol, trans-verbenol, andverbenone are found as the main compounds. A generalincrease of the autoxidation due to daylight radiation hasbeen reported by Schenck, but the strong increase of theproducts trans-pinocarveol and pinocarvone must haveanother cause. In the presence of daylight radiation anadditional process can take place: Oxygen can be promotedto the excited singlet state and then undergo a so-called

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 recoveryis given. Relative response factors (RRF) were calculated in reference to R-pinene. *experimental data.

R1OO• + RH f R• + R1OOH

R• + O2 f ROO•

FIGURE 4. The allylic radicals that can be formed after H-atomabstraction from r-pinene in an autoxidation process.

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photosensitized reaction with R-pinene. Different mecha-nisms for this reaction have been postulated to explain thestereoselective formation of an intermediate allylic hydro-peroxide. The two most favored are a pericyclic mechanism,similar to that of the ene synthesis (23), and the addition ofsinglet oxygen to the double bond, followed by an internalproton transfer. For R-pinene in both cases the productsformed would be trans-pinocarveol and pinocarvone, themain compounds found in this study under the influence ofdaylight radiation.

Another compound detected in minor traces is camp-holene aldehyde. It has been shown that campholenealdehyde can be formed from a primary product R-pineneoxide, due to thermal rearrangement inside the GC injector(14). In some studies R-pinene oxide was also found as anautoxidation product of R-pinene (19-21).

Influence of Temperature. In Figure 5 three differentchromatograms are shown from sample cartridges afterstorage of 1 week in a freezer at -18 °C, a refrigerator at 4°C, and in sunlight on the windowsill. All samples have beenwrapped in aluminum foil to prevent influences of daylight.

From the chromatograms it can be seen that the autoxi-dation is impeded by lower temperature. These results showthat even in the refrigerator autoxidation of R-pinene onTenax occurs, while this effect is significantly reduced if thesample cartridges are stored in the freezer.

These results show that not only the influences of oxidizingagents, like ozone, have to be considered during the samplingof volatile organic compounds but also that effects of daylightradiation and temperature lead to oxidation of R-pinene onTenax. It was shown that the observed pattern of up to fivecompounds, pinocarveol, pinocarvon, verbenon, myrtenal,and verbenol, can be taken as an indicator for storagereactions after sampling of R-pinene on Tenax, especiallyfor samples from field studies that require long sample timesor have been stored for longer times. Estimates indicate adegradation of up to 12% after 1 month of storage. It usuallydoes not help to just wrap the sample cartridges in aluminumfoil. Even basic storage for 1 week in a refrigerator is unsat-isfactory. To avoid degradation of the terpenes collected onsample cartridges they have to be protected from daylightradiation and temperature influences.

The formation of the degradation products may beexplained by a two way mechanism, where most componentsare formed during a radical autoxidation mechanism, whilethe most abundant compounds pinocarveol and pinocarvonare formed through a photosensitized reaction with singletoxygen.

AcknowledgmentsThe financial support by the Senatsverwaltung fur Wissen-schaft, Forschung und Kultur des Landes Berlin and theBundesministerium fur Bildung und Forschung is gratefullyacknowledged.

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Received for review November 17, 2000. Revised manuscriptreceived April 2, 2001. Accepted April 12, 2001.

ES0002722

FIGURE 5. Influence of temperature: FID chromatograms from TENAXcartridges with r-pinene stored for 7 days in a freezer at -18 °C(lower trace), a refrigerator at 4 °C (middle trace), and in thelaboratory at 25 °C (upper trace)

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