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16Polycyclic Aromatic Hydrocarbons and CombustionJohn Fetzer

16.1Introduction

The polycyclic aromatic hydrocarbons (PAHs) are a class of organic compoundsdefined as those that consist only of carbon and hydrogen atoms in fused aromaticrings [1]. There are several subclasses within this broad category of PAHs. The firstand most commonly utilized class is the number of rings; however, this is not asimple arrangement because other factors create confusion in numbering.

Although other ring sizes may be made in the laboratory, only those with sixcarbons or five carbons are found commonly in combustion products. The PAHs ofonly six-carbon rings are known as �alternant� PAHs, while those with some five-carbon rings are known as �nonalternant� PAHs. The simple PAH isomer pair ofpyrene and fluoranthene illustrate this difference; pyrene is alternant, while fluor-anthene is nonalternant. (Some of these structures are shown in Figure 16.1, whichshows the 16 PAHs targeted as priority pollutants.)

The PAHs are aromatic – that is, their p electrons are not located in only onecarbon–carbon bond (as in an alkene, or colloquially also called an olefin); rather, thep electrons move throughout the carbon skeleton, and are shared among the bonds.This situation, which is known as �resonance,� gives PAHs an increased stability.Alternant PAHs are fully resonant. The five-carbon ring in a nonalternant PAH isnot as effective in electron resonance, so nonalternant PAHs are less stable thanalternant PAHs of similar formula (PAHs with the same number of carbon andhydrogen atoms are known as isomers).Within both classes there are the subdivisionsor ortho-fused and peri-fused. Ortho-fused PAHs have rings attached to each otheronly through one face, sharing only a single carbon–carbon bond, whereas peri-fusedPAHs have ring connections to two ormore faces. By convention, if a PAH structurehas a peri-fused part, it is classified as peri-fused even though it may also have ringsthat are ortho-fused.

Handbook of Combustion Vol.2: Combustion Diagnostics and PollutantsEdited by Maximilian Lackner, Franz Winter, and Avinash K. AgarwalCopyright � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32449-1

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This degree of condensation leads to the most confusing aspect of countingrings as a classification of PAHs. Pyrene, C16H10, obviously has four rings, but so dothe isomers of C18H12, naphthacene, benzo[c]phenanthrene, chrysene, and benz[a]anthracene. The confusion grows even greater for higher numbers of ringswhere thestructures are ortho-fused, peri-fused, and some are mixtures of the two.

Isomerism is a key principle in the study of PAHs. The arrangement of ringsinfluences the arrangement of the p electrons and this, in turn, defines a diverserange of individual PAH properties, from coloration to impact on health. Why this isso will be described in the next section.

PAHs have literally been found throughout every part of the planet Earth, from thedepths of the oceans to the high reaches of the atmosphere. They even have beenfound in meteorites, comets, extraterrestrial atmospheres, and in interstellar space.

Napthalene Acenaphthene Acenaphthylene Fluence

Phenanthene Anthracene Fluoranthene Pyrene

Benzo[b]fluoranthene

Diebenz[ab]anthraceneBenzo[a]pyreneBenzo[b]fluoranthene

Blenz[a]anthracene Chrysene

Benzo[ghl]perylene Indeno[1,2,3,ed]pyrene

Figure 16.1 The 16 polycyclic aromatic hydrocarbons on the US EPA priority-pollutant list.

426j 16 Polycyclic Aromatic Hydrocarbons and Combustion

16.2Properties of PAHs

The arrangement of p electrons in a PAH structure determines its UV andfluorescence spectra. PAH spectra are characteristic in containing many spectralfeatures; this is in contrast to most other types of molecules, which often have onlyone broad, featureless UV spectrum and do not fluoresce. PAH spectra are also veryintense, so that small amounts can be readily seen. By having differing arrangementsofp electrons, isomers have very differentUVandfluorescence spectra; these spectracan, therefore, be easily used to identify specific PAHs.

16.3Analytical Approaches for PAHs in Combustion Processes and Products

The complexity ofmost combustion-related samples and the diversity of PAHs limitswhich types of instrumentation and methodologies can be used for PAH analyses.Without some separation, few approaches can provide any details of composition,except on rare occasions for a few specific PAHs [2].

The separations used for PAHs can be divided into two categories: (i) fractiona-tions and cleanups used for sample preparation; and (ii) chromatographic methodsused for the analysis of individual compounds.

Sample preparation involves dividing the original sample into parts. Each parteither is an enriched fractionwith thePAHs inhigher concentrations, or the fractionsare depleted of PAHs and enriched in other types of compound that would make thePAH analysis difficult. Fractionations include adsorption chromatography on silicagel, alumina, and activated carbon. In addition to a PAH-containing fraction, thereoften is a less-polar fraction that contains the aliphatic hydrocarbons and nonaro-matic sulfur compounds and a more-polar fraction (or fractions) of the manynitrogen- and oxygen-containing molecules.

Normal-phase liquid chromatography is a more precise approach than adsorptionchromatography, and can be used to fractionate PAHsby the ring number. It relies onthe interaction between thep electrons of the PAHs and the bonded phase. Commonbonded phases for this include nitro-, amino-, phenyl-, and cyano- (nitrile) phases.The retention is increased according to the number of p bonds, so that even amongfour-ring PAHs, pyrene will elute before chrysene or benz[a]anthracene.

For smaller PAHs, gas chromatography (GC) can be used because the compoundsare volatile. However, for PAHs greater than five rings the volatility is decreased andso liquid chromatography becomes the preferred option.

Gas chromatography is most often used with mass spectrometry (MS) detection.However, the combination of GC-MS is adequate for smaller PAHs for anotherreason. The separating power of GC is sufficient generally to separate the isomers oflower molecular weight; however, as the molecular weight increases the GC sepa-rating power becomes a limitation, because commercial GC columns have littleselectivity for PAH isomers. Yet, the MS cannot differentiate between them, and will

16.3 Analytical Approaches for PAHs in Combustion Processes and Products j427

provide only the molecular weight of each peak. Historically, GC-MS has been themost widely used technique, mainly because of the attention focused on smallerPAHs with regards to environmental regulations.

For PAHs of six ormore rings,MS detection is not useful other than to provide themolecular weight of each individual GC peak. The molecular weight can easily betranslated in a chemical formula; for example 276means a PAHof C22H10.However,this is not useful in itself because a set of isomers will all have the same molecularweight, and this is a dramatic limitation as the ring number increases. For example,in the case of PAHs of molecular weight 302, C24H14, there are 34 possible PAHisomers; clearly, MS identification would not be of much help in this situation!

In one PAH class (known as the ace, pronounced �ay-sa� or cyclopenta class), themolecules have a peripheral five-member ring containing a double bond, and areoften formed through the addition of C2H2 to the PAH structure. This additionalring is not involved in p-electron resonance, and often these PAHs do not showfluorescence, unlike most other classes of PAHs. One commonly regulated PAH,which appears as an example on the US Environmental Protection Agency list ofpriority pollutants, is acenaphthylene; other lists include cyclopenta[cd]pyrene.

In order to overcome this limitation, alternative approaches are used that rely onhigh-performance liquid chromatography (HPLC) with UV detection. For the largerPAHs, the fact that each individual isomer has a distinct UV (and fluorescence)spectrum represents the solution to the problem. HPLC, in combination with a full-spectrum UV detector, can be used to identify individual isomers [3]. These spectrahave even been used to identify PAHisomers that were previously unknown, becausethe patterns in the PAH UV spectra follow trends based on the arrangements of therings [4, 5]. One such example is shown in Figure 16.2, where the top and bottomspectra belonged to known compounds.

The UV spectra of a pair of very similarly structured PAHs are shown inFigure 16.3. UV spectral detection is especially powerful when the eluent is thenpassed to a mass spectrometer to provide the empirical formula for each UVspectrum.

The specifically prepared HPLC columns used for PAH analyses are invariablybased on the common octadecyl (C18) bonded phase, and their special preparationmakes themdissimilar tomost C18 commercial columns. These PAH columns havea great ability to separatemolecules by their shapes, and this results in the separationof PAHs simultaneously by both carbon number and isomer shape. An isomer setresults in a cluster of separated peaks in a time range which is different from that forother isomer sets.

With HPLC, the method of detection has one major advantage over that used inGC, in that UV methods are possible. Each PAH has, by definition, a specificarrangement of its rings: the number of rings; which are five-carbon and which six-carbon rings; and how the rings are joined. But this alsomeans that the arrangementofp electrons is discrete and individual, which in turn determines theUVabsorbancespectrum of a PAH.

HPLC can be used for PAHs of 12 or more rings, whereas GC is usually limitedto approximately seven rings; however, this range can be further extended and


improved by the choice of HPLC mobile phase. GC is also limited by the operatingtemperature and the thermal stability of the columns used. For smaller PAHs,mobiles phases using water and acetonitrile ormethanol are commonly used, but forlarger PAHs (more than six rings) solvents such as ethyl acetate, dichloromethane,chlorobenzene, and toluene have been used.

16.4Formation, Variation, and Occurrence of PAHs

16.4.1Formation

PAH formation is dominated by two factors – the thermodynamic stabilities and thekinetic mechanisms. Both of these are important, however, and the final distribution

Figure 16.2 UV spectra comparison of two known PAHs (top and bottom spectra) with a newlysynthesized PAH (middle spectrum). The vertical axes have been normalized for this comparison.

16.4 Formation, Variation, and Occurrence of PAHs j429

of PAHs in a combustion product usually is produced by pathways that involve bothmechanisms.

By sharing its p electrons throughout the carbon skeleton, a PAH gains thermo-dynamic stability; moreover, the more widespread and uniform the sharing, thegreater the gain in stability. Such stability gain throughp electron sharing is known asresonance.

These resonance stabilities can be determined theoretically by computations, andthe appropriate values may be obtained in tabular form in a variety of literaturesources. In brief, isomers with more compact structures are generally more stable.Likewise, structures with fewer hydrogen atoms for the same number of carbonatoms are also more stable, and therefore triphenylene is much more stable than itsisomer naphthacene. A PAHof formula C28H14, such as benzo[a]coronene, would be

Figure 16.3 TheUV absorbance spectra of two dinaphthocoronene isomers. The units of the y-axisare milliabsorbance units (mAU).


more stable than one of formula C28H16, such as benzo[a,n]perylene, which is in turnmore stable than one of formula C28H18, such as dibenzo[a,l]pentacene.

Kinetic pathways are those series of reactions that produce PAHs. The specificmechanisms may depend both on the starting reactants and the favorability ofspecific reactions due to thermodynamics or steric effects. Although PAHs aregenerally produced in a series of reactions that build up larger PAHs, one ring at atime, some reactions occur in a more �leapfrog� fashion. The simplest example ofthis is the condensation of two naphthalene molecules to form perylene or benzo[ j]fluoranthene or benzo[k]fluoranthene.

Onoccasion, a starting reactantwill have two ormore possible routeswith differentproduct PAHs, and generally when this is the case the product isomer with themorethermodynamic stability will be preferred and be seen in greater amounts. However,this does not mean that it is the sole product – only that its proportion compared tothe other isomer is greater. The proportion of each isomer often is similar to theproportion of the stabilities as measured by the heats of formation. The PAH-formation pathway of greatest stability is shown in Figure 16.4 [1].

16.4.2Variation in PAHs Due to Combustion Source

PAHs are produced at some level in every type of combustion, except for those whereonly water, carbon dioxide, and other small permanent gases are the products. PAHsare the precursors to soot and similar carbonaceous deposits, and their productiondepends on both thermodynamic stability and on the kinetics of the reactionpathways to a particular structure. These, in turn, depend on the heat in thecombustion zone and the residence time.

16.4.3Conventional Combustion of Plant Matter

Wood and peat are still commonly used fuels for home heating. The burning of plantmatter in grassland or forest fires also produces similar PAHs. In some parts of theworld, slash-and-burn agriculture is common, whereby fire is used either to clear theland for planting or to remove the remnants of certain crops, such as grass, wheat, orrice stubble, or the leaves and other remains from sugar cane, sorghum, and corn. Asthe combustion temperature is lower and the residence time shorter than with mostother fuel types, the PAHs produced are smaller, with usually five rings or fewer [2].

16.4.4Motor Vehicles

Gasoline- (petrol) and diesel-fueled engines have a much higher combustiontemperature than is encountered in the conventional burning of plant material, andthis results in a greater production of nonalternant PAHs, such as fluoranthene.Although there are differences in the relative proportions of individual PAHs found

16.4 Formation, Variation, and Occurrence of PAHs j431

in gasoline anddiesel emissions, the variety of PAHs is similar. Almost every possiblecondensed alternant and nonalternant structure is found [6, 7]; for example, of the 33possible C24H14 isomers, all are found in diesel exhaust particulate mater [8].

Diesel engines do produce nitro-substituted PAHs, a class of compounds which isnot seen in many other combustion products, and which may be indicative of motorvehicle sources [9].

16.4.5Fuel Oil and Coal Burning

Fuel oil and coal burn less efficiently than natural gas when used to heat homes orpower industrial plants. In this situation, the soot is usually required to be scrubbed

Figure 16.4 The pathway leading to the most thermodynamically favored peri-fused PAHs.


or trapped in order to reduce its emission, though some nations have less strictregulations than others.Coal tar, which is the product of pyrolysis of bituminous coal,contains a very complexmixture of PAHs [10–14]. The combustion of anthracite coalproduces more PAHs, and these have more rings than bituminous coal burnedunder similar conditions [15]. In contrast, brown (lignite) coal additionally hadnumerous �ace� (fused cyclopenta) PAHs structures [16].

16.5Controlled Pyrolysis as a Means to Study Combustion

The mechanisms of PAH formation are impossible to study in the true-worldsituation, and therefore carefully conducted studies using simpler, controlled

Figure 16.5 HPLC chromatogram of productsof catechol pyrolysis (1000 �C and 0.3 s) elutingfrom 40 to 75min in the solvent program of theHPLC/UV. The rise in the baseline at �63mincorresponds to a change in HPLCmobile phasecomposition to UV-absorbingdichloromethane. Shown in light grey, theidentified C24H14 PAH product components,in order of elution from left to right, are: naphtho[1,2-e]pyrene, naphtho[1,2-b]fluoranthene,naphtho[2,3-e]pyrene, naphtho[1,2-a]pyreneeluting with dibenzo[a,e]pyrene, naphtho[1,2-k]fluoranthene, benzo[b]perylene, dibenzo[e,l]pyrene, dibenzo[b,k]fluoranthene, naphtho[2,3-b]fluoranthene, naphtho[2,1-a]pyrene, dibenzo[a,i]pyrene, naphtho[2,3-a]pyrene, naphtho[2,3-k]fluoranthene, and dibenzo[a,h]pyrene. Threeof these C24H14 PAHs – naphtho[2,1-a]pyrene,dibenzo[a,i]pyrene, and naphtho[2,3-a]pyrene –

have been identified in a previous catecholpyrolysis study in this series. Shown in black, thePAH product components whoseidentifications are demonstrated elsewhere, inorder of elution from left to right, are: benzo[a]pyrene, naphthacene, dibenz[a,j]anthracene,pentaphene, dibenz[a,h]anthracene, 4H-benzo[def ]cyclopenta[mno]chrysene, benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, dibenzo[a,h]fluorene, indeno[1,2,3-cd]fluoranthene, benzo[b]chrysene, 1H-benzo[ghi]cyclopenta[pqr]perylene eluting with benzo[b]perylene,anthanthrene, picene, benzo[ghi]cyclopenta[cd]perylene, 8H-dibenzo[a,jk]pyrene, coronene,dibenzo[b,ghi]perylene, 1-methylcoronene,phenanthro[2,3-a]pyrene, dibenzo[e,ghi]perylene, cyclopenta[bc]coronene, and naphtho[8,1,2-bcd]perylene.

16.5 Controlled Pyrolysis as a Means to Study Combustion j433

conditions are useful. An example of this is provided by the combined researchefforts of Prof. M. J. Wornat and colleagues, formerly of Princeton University and ofLouisiana State University [17–22]. Since combustion fuels are usually complexmixtures, the first step in simplification is to combust individual compounds and toobserve the resultant PAHs, such that a comparison can be obtained of the differentconditions required for each fuel. When a variety of fuels is compared, the thermo-dynamic and kinetic factors emerge.

Figure 16.6 Reversed-phase HPLCchromatogram of products of1-methylnaphthalene pyrolyzed at 585 �C,110 atm, and 140 s. The rise in baseline at63min corresponds to a change in mobilephase composition to UV-absorbingdichloromethane. Identified products arelisted by class, in order of elution. Classes 1 and2 (in black): naphthalene; 1-methylnaphthalene;2-methylnaphthalene; 1,8-dimethylnaphthalene; 1-ethylnaphthalene;1,2-dimethylnaphthalene; 1,4- and1,5-dimethylnaphthalene; 1,3- and1,7-dimethylnaphthalene;2,3-dimethylnaphthalene;1,6-dimethylnaphthalene; 2,6-dimethylnaphthalene; 2,7-dimethylnaphthalene; trimethylnaphthalene;

1,10-bi-naphthyls; 1,20-bi-naphthyls; 2,20-bi-naphthyls. Class 3 (blue): benzo[j]fluoranthene;perylene; benzo[k]fluoranthene; methylbenzo[k]fluoranthene; methylbenzo[j]fluoranthene.Class 4 (green): dibenzo[a,i]fluorene; dibenzo[a,g]fluorene; methyldibenzo[a,i]fluorene;methyl-dibenzo[a,i]fluorene; methyldibenzo[a,h]fluorene; methyldibenzo[a,g]fluorene;methyldibenzo-[a,i]fluorene; dibenzo[a,h]fluorene; methyldibenzo[a,h]fluorene.Class 5 (red): benzo[c]-chrysene; dibenz[a,j]anthracene; dibenz[a,h]anthracene; picene.Class 6 (black): naphtho[2,1-a]pyrene;methylnaphtho[2,1-a]pyrene; naphtho[2,3-a]pyrene; methylnaphtho[2,1-a]pyrene;methylnaphtho[2,1-a]pyrene; and dibenzo[cd,lm]perylene. Data are taken from Ref. [18]and Ref. [17].


Such as approach is far from straightforward, not only in terms of the finalcomparisons but also in the initial data gathering. As noted above, PAHmixtures canbe complex, with many similar isomers occurring; hence the differentiation of eachstructure must be carried out in order to assess the reaction pathways and prefer-ences. Yet, modern chemical analytical methods can be used to achieve this relatively

Figure 16.8 UV absorbance spectra of the reference standard of dibenzo[a,h]pyrene (dashed line)and of a catechol pyrolysis product component (solid line) eluting at 74.5min in Figure 16.5.

Figure 16.7 UV absorbance spectra of the reference standard of naphtho[2,3-a]pyrene (dashedline) and of a catechol pyrolysis product component (solid line) eluting at 72.1min in Figure 16.5.

16.5 Controlled Pyrolysis as a Means to Study Combustion j435

easily, with Prof.Wornat�s group employing one of themore powerful combinations,namely HPLC-UV-MS. This, as described above, allows for numerous specific PAHsto be identified quantitatively. Two example chromatograms from these studies areshown in Figures 16.5 and 16.6, while Figures 16.7–16.10 show examples of thecomparisons of UV spectra used for identification [22].

Figure 16.10 UV absorbance spectra of the reference standard of dibenzo[a,i]pyrene (dashed line)and of a catechol pyrolysis product component (solid line) eluting at 70.5min in Figure 16.5.

Figure 16.9 UV absorbance spectra of the reference standard of naphtho[2,1-a]pyrene (dashedline) and of a catechol pyrolysis product component (solid line) eluting at 64.2min in Figure 16.5.


References

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2 Fetzer, J.C. (1989) Gas and liquidchromatographic techniques, in ChemicalAnalysis of Polycyclic Aromatic Compounds(ed. T. Vo-Dinh), John Wiley & Sons,pp. 59–109.

3 Fetzer, J.C. and Biggs, W.R. (1996) Theanalysis of large polycyclic aromatichydrocarbons. Trends Anal. Chem., 15,196–206.

4 Fetzer, J.C. and Biggs, W.R. (1994)Identification of a new eight-ringcondensed polycyclic aromatichydrocarbon. Polycyclic Aromat. Compd.,5, 193–199.

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6 Schmidt, W., Grimmer, G., Jacob, J., andDettbarn, G. (1986) Relevance ofpolycyclic aromatic hydrocarbons asenvironmental carcinogens. Toxicol.Environ. Chem., 13, 1–16.

7 Schmidt, W., Grimmer, G., Jacob, J.,Dettbarn, G., and Naujack, K.W. (1987)Polycyclic aromatic hydrocarbons withmass number 300 and 302 in hard-coalflue gas condensate. Fresenius Z. Anal.Chem., 326, 401–413.

8 Bergvall, C. and Westerholm, R. (2006)Determination of dibenzopyrenes instandard reference materials (SRM)1649a, 1650, and 2975usingultrasonicallyassisted extraction and LC–GC–MS.Anal. Bioanal. Chem., 384, 438–447.

9 Lindner, W., Pusch, W., Wolfbeis, O.S.,and Tritthart, P. (1985) Analysis of nitro-PAHs in diesel exhaust particulateextracts with multicolumn HPLC.Chromatographia, 20, 213–218.

10 Fetzer, J.C. and Kershaw, J.R. (1995) Theidentification of large PAHs in a coal tarpitch. Fuels, 74, 1533–1536.

11 Suzuki, S., Kaneko, T., and Tsuchiya, M.(1996) Hyphenated techniques forchromatographic detection. KankyoKagaku, 6, 511–520.

12 Senthilnathan, V.P. and Stein, S.E. (1986)Hydrogen transfer in the formation anddestruction of retrograde products in coalconversion. J. Org. Chem., 53, 3000–3007.

13 Colmsjo, A. and Ostman, C. (1982)Polynuclear Aromatic Hydrocarbons:Physical and Biological Chemistry(eds M. Cooke, A.J. Dennis, and G.L.Fisher), Batelle Press, Columbus OH,USA, pp. 201–210.

14 Wise, S.A., Benner, B.A., Liu, H.,Byrd, G.D., and Colmsjo, A. (1988)Separation and identification of polycyclicaromatic hydrocarbon isomers ofmolecular weight 302 in complexmixtures. Anal. Chem., 60, 630–637.

15 Wornat, M.J., Vriesendorp, F.J.J., LaFleur,A.L., Plummer, E.F., Necula, A., andScott, L.T. (1999) The identification of newethynyl-substituted and cyclopenta-fusedpolycyclic aromatic hydrocarbons in theproducts of anthracene pyrolysis.Polycyclic Aromat. Compd., 13, 1563–0000.

16 LaFleur, A.L., Taghizadeh, K., Howard,J.B., Anacleto, J.E., and Quilliam, M.A.(1996) Characterization of flame-generated C10 to C160 polycyclic aromatichydrocarbons by atmospheric-pressurechemical ionization mass spectrometrywith liquid introduction via nebulizerinterface. J. Am. Soc. Mass Spectrom., 7,276–286.

17 Somers, M.L., McClaine, J.W., andWornat, M.J. (2007) The formation ofpolycyclic aromatic hydrocarbons fromthe supercritical pyrolysis of 1-methylnaphthalene. Proc. Combust. Inst.,31 (I), 501–509.

18 Somers, M.L. and Wornat, M.J. (2007)UV spectral identification of polycyclicaromatic hydrocarbon products ofsupercritical 1-methylnaphthalenepyrolysis. Polycyclic Aromat. Compd.,27 (4), 261–280.

19 Oña, J.O. and Wornat, M.J. (2007)Identification of the C30H16 polycyclicaromatic hydrocarbon benzo[cd]naphtho[1,2,3-lm]perylene as a product of thesupercritical pyrolysis of a synthetic jetfuel. Polycyclic Aromat. Compd., 27 (3),165–183.

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20 McClaine, J.W., Oña, J.O., and Wornat,M.J. (2007) Identification of a newC28H14 polycyclic aromatic hydrocarbonas a product of supercritical fuel pyrolysis:Tribenzo[cd,ghi,lm]perylene.J. Chromatogr. A, 1138 (1–2), 175–183.

21 Marsh, N.D., Wornat, M.J., Scott, L.T.,Necula, A., La Fleur, A.L., andPlummer, E.F. (2000) The identification

of cyclopenta-fused and ethynyl-substituted polycyclic aromatichydrocarbons in benzene dropletcombustion products. Polycyclic Aromat.Compd., 13 (4), 379–402.

22 Thomas, S. and Wornat, M.J. (2008)C24H14 polycyclic aromatic hydrocarbonsfrom the pyrolysis of catechol. Int. J.Environ. Anal. Chem., 88, 825–840.


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