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Energy & Fuels 2008, 22, 382–388
Formation of the Secondary Radicals from the Aging of Tobacco Smoke Zofia Maskos and Barry Dellinger* Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803 ReceiVed July 26, 2007. ReVised Manuscript ReceiVed October 23, 2007
The fractional pyrolysis of Bright tobacco was performed in a nitrogen atmosphere over the temperature range of 240-510 °C in a specially constructed, high-temperature flow reactor system. Electron paramagnetic resonance (EPR) spectroscopy was used to analyze the free radicals in the initially produced total particular matter (TPM) and in TPM after exposure to atmospheric, dry- or water-saturated air (argon). When the initially produced TPM was allowed to age, the formation of secondary radicals with an EPR g value of 2.0053 ( 0.0003 was observed. The rate of the formation of these radicals was found to depend dramatically upon the presence of water vapor in the exposure atmosphere. The secondary radicals were nearly undetectable in the water-saturated atmosphere and increased significantly by decreasing the water concentration by partial evacuation or purging with a dry air/argon atmosphere. The EPR spectra and the conditions of the formation of the observed secondary radicals were consistent with many aspects of oxygen-centered, semiquinone radicals formed from hydroxylated aromatic molecules, including hydroquinones and catechols, chemisorbed to metals or other electron-acceptor sites in the TPM. In conjunction with previous studies of primary radicals produced prior to aging of the tobacco, we believe that semiquinone-type radicals are largely produced in aged tobacco smoke and are at much lower concentrations in primary tobacco smoke.
Introduction We have recently reported1–3 that the fractional pyrolysis of Bright tobacco led to the formation at least two types of free radicals: (i) type-I radicals, which displayed five broad lines in the electron paramagnetic resonance (EPR) spectrum with the apparent g value of 2.0064 and were only observed over a pyrolysis temperature range of 240-380 °C, and (ii) type-II radicals, which exhibited a single-line EPR spectrum with g ) 2.0035–2.0040 and ∆Hp-p of 8.5 ( 0.2 G and were only produced in observable concentrations at temperatures g 380 °C. On the basis of the experimental data as well as a comparison with some biological systems in the literature, the spectrum of type-I radicals was suggested to be constrained or immobilized tyrosyl radicals formed from protein components of the cellwall biopolymer of tobacco. Type-II radicals were consistent with surface-associated, carbon-centered radicals, where the unpaired electron is vicinal to an oxygen-containing functional group or a partially delocalized electron associated with the bulk of a phenoxyl-type polymeric matrix.4 Type-II radicals can be found in total particular matter (TPM) as well as in tobacco char.1 * To whom correspondence should be addressed. Telephone: 1-225-5786759. Fax: 1-225-578-0276. E-mail:
[email protected]. (1) Maskos, Z.; Khachatryan, L.; Cueto, R.; Pryor, W. A.; Dellinger, B. Radicals from the fractional pyrolysis of tobacco. Energy Fuels 2005, 19, 791–799. (2) Maskos, Z.; Khachatryan, L.; Dellinger, B. Precursors of radicals in tobacco smoke and the role of particulate matter in forming and stabilizing radicals. Energy Fuels 2005, 19, 2466–2473. (3) Dellinger, B.; Lomnicki, S.; Khachatryan, L.; Maskos, Z.; Hall, R. W.; Adunkpe, J.; McFerrin, C.; Troung, H. Formation and stabilization of persistent free radicals. Proc. Combust. Inst. 2007, 31, 521–528. (4) Maskos, Z.; Khachatryan, L.; Dellinger, B. Formation of the persistent primary radicals from the pyrolysis of tobacco. Energy Fuels 2007, manuscript submitted.
After aging, the concentration of radicals in TPM increased because of the production of secondary radicals with a g factor of 2.0053 ( 0.0003, and their concentration achieved a maximum for TPM samples produced at a pyrolysis temperature of 280 °C. At this temperature, the concentration of these new radicals was 8 × 1015 spins/g of tobacco, which was much higher than the concentration of radicals measured immediately after collection. Our previously published results suggested that hydroquinone/catechol-type species, which were free or chemically bound to the surface of TPM, did not initially produce radicals. Instead, they were converted to semiquinone-type radicals after aging in air.1,2 We have performed additional studies designed to clarify the nature and origin of the secondary radicals and to evaluate the effect of environmental and storage conditions (air versus argon, dry- versus water-saturated air or argon, and light versus dark) on the rate of their formation and stability. Experimental Section Materials. Samples of Bright (flue-cured) tobacco were provided by Philips Morris (Richmond, VA) and used without further treatment. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) (free radical, 98%) and silica gel (Devisil, grade 633, 200–425 mesh) were obtained from Aldrich Chemical Co. Dry air used in our experiments was passed through a reactor with fresh, dehydrated molecular sieves. The EPR tubes were standard X-band Suprasil high-purity quartz tubes [3 and 4 mm inner diameter (i.d.)] that were supplied by the Wilmad/Labglas Co. Fractional Pyrolysis. The samples were pyrolyzed in a commercial Pyroprobe 1000 (CDS Analytical, Inc.), using a continuousflow-reactor system. A description of the apparatus is given elsewhere.1 Samples (80 ( 2 mg) of Bright tobacco diluted with silica in a ratio of 1:1 were placed in a quartz tube (i.d. ) 6 mm) and inserted inside the pyroprobe coil. Both sides of the tube insert were plugged
10.1021/ef700446v CCC: $40.75 2008 American Chemical Society Published on Web 12/11/2007
Secondary Radicals from Aging of Tobacco Smoke
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Figure 2. EPR spectra of the initially produced and aged TPM. The EPR spectrum of the 280 °C fraction of TPM before aging was collected using 50 scans because of the very low radical concentration.
Figure 1. Effect of the pyrolysis temperature on the concentration of primary and secondary radicals [(“primary + secondary” radicals) (“primary” radicals)].
with quartz wool to prevent sample loss. Prior to the experiments, samples were flushed with nitrogen for 24 h to remove traces of oxygen. Pyrolysis was induced by rapidly heating the reaction tube at a rate of 100 °C/s. The temperature in the center of the tobacco bed was measured using a retractable chromel-alumel (K-type) thermocouple (o.d. ) 0.25 mm). The temperature gradient along the layer of tobacco in the reaction tube did not exceed (5 °C. Nitrogen containing no more than 5 ppm of oxygen was used as a carrier gas in the pyrolysis experiments. The pyrolyzing gas was introduced at a flow rate of 200–330 cm3/min, which corresponded to a residence time of 0.05 s over the temperature range studied. The flow rate was controlled by a mass flow controller (McMillan Co., model 80D). The sample was heated for a reaction time of 60 s at each temperature, and TPM was collected on the filter. The filter with TPM was flushed for 5 min with the carrier gas, then removed, and replaced with a new filter. The entire experiment was performed with the same bed of tobacco, and the temperature varied from 200 to 510 °C in 30–60 °C increments. The smoke from the heated tobacco was passed through a cellulose filter (Osmonic, Inc.) supported by a conventional glass fiber Cambridge filter. The TPM was collected and weighed. The cellulose filter had an active surface area of 0.5 cm2 and was efficient for particles larger than 0.8 µm in diameter. After the smoke sample was collected, the cellulose filter was cut into 2-mm wide strips, and the strips were inserted directly into an EPR tube. The sample was stored at 77 K until it was analyzed by EPR at room temperature. In some experiments, the condensable products were extracted with methanol, and the extract was analyzed via gas chromatography–mass spectroscopy using a model 6890N/5973 GC-MSD (Agilent Technology). EPR. All EPR spectra were recorded at room temperature on a Bruker EMX-10/2.7 EPR spectrometer (Bruker Instruments, Billerica, MA) with dual cavities, X-band, and microwave frequency (9.72 GHz). The typical parameters were as follows: (i): sweep width, 100 G; power, 2 mW; receiver gain, 3.56 × 104; modulation amplitude, 4 G; and time constant, 1.28 ms. Values of the g factor, ∆Hp-p, and hyperfine coupling constants were calculated using Bruker’s WINEPR program. The concentration of free radicals in the samples was calculated using double integration of the first derivative signal and a comparison with the DPPH standard.
Results Radicals in TPM—Effect of the Temperature. The fractional pyrolysis of Bright tobacco was performed over the temperature range of 240–510 °C. The concentration of radicals in TPM was measured by EPR spectroscopy and is presented in Figure 1. The curve labeled “primary” displays the concentra-
tion of the primary radicals that were produced during the pyrolysis and analyzed immediately after pyrolysis. The curve labeled “primary + secondary” presents the maximum concentration of radicals formed after aging of the TPM in open EPR tubes exposed to room air. A comparison of these two curves reveals the dramatic increase in the concentration of radicals (“secondary radicals”) over the range of pyrolysis temperatures. The total concentration (primary + secondary) of the radicals after exposure to room air exhibited maxima at 280 and 450 °C. For TPM collected at 280 °C, the concentration of radicals after exposure to air increased from 1.2 to 8.1 × 1015 spins/g of tobacco, and this first maximum was mainly due to the formation of the secondary radicals. The maximum observed for TPM collected at 450 °C results from a significant contribution of primary radicals, while the maximum at 280 °C is primarily due to secondary radicals. Figure 2 depicts the EPR spectra of TPM generated during the pyrolysis at 280 and 450 °C, before and after aging. The 280 °C TPM spectrum represents the radicals generated for pyrolysis temperatures of 240-380 °C and exhibits five lines with a g factor of 2.00576 and the total line width of 42 G. After aging, the five-line spectrum was overlaid by a highintensity singlet derived from the secondary radicals with a g value of 2.00525 and ∆Hp-p ) 9.4 G. The spectrum for the 450 °C fraction exhibited a singlet with a g value of 2.00358 and ∆Hp-p ∼ 8.5 G that was typical for almost all fractions collected above 380 °C. After aging, this spectrum was overlaid by the signal derived from secondary radicals, with the resulting spectrum also being a singlet with a g value of 2.00406 and ∆Hp-p of 7.8 G. To compare the nature of these radicals, two TPM samples, (1) 280 °C fraction (five-line spectrum with an apparent g factor of 2.00576) and (2) 450 °C fraction of TPM (single-line spectrum with a g value of 2.00358 and ∆Hp-p of 8.5 G), were subjected to microwave power dependence measurements. The microwave power was varied in the range of 0.32–101 mW. The intensities of the signals, presented as a normalized double integration [DI/N value is the double-integrated (DI) intensity of the EPR spectrum that has been normalized (N) to account for the conversion time, receiver gain, number of data points, and sweep width] of the first-derivative spectra versus the square root of the microwave power, are depicted in Figure 3. The EPR signal for the 280 °C fraction (type-I primary radicals) is almost saturated at a microwave power
