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Mass spectrometric analysis of large PAH in a fuel-rich ethylene flame
Proceedings of the
Proceedings of the Combustion Institute 31 (2007) 547–553
Combustion Institute www.elsevier.com/locate/proci
Mass spectrometric analysis of large PAH in a fuel-rich ethylene flame B. Apicella a,*, A. Carpentieri b, M. Alfe` a, R. Barbella a, A. Tregrossi a, P. Pucci b, A. Ciajolo a a
Istituto di Ricerche sulla Combustione, CNR, P.le V. Tecchio, 80-80125 Napoli, Italy Dipartimento di Chimica, Universita` ‘‘Federico II’’, Via Cintia, 80125 Napoli, Italy
b
Abstract Mass spectrometric analysis by laser desorption-time of flight-mass spectrometry (LDI-TOF-MS) was exploited to extend the detection of flame-formed polycyclic aromatic hydrocarbons (PAH) up to the mass limit of the first soot particles (>2000 Da) in the soot formation region of a premixed fuel-rich (C/O = 1) ethylene flame. The typical decreasing intensity of PAH ion peaks with increasing mass was found in the mass range m/z 500–1700 although a slight enrichment in the heavier part of PAH could be observed to occur along the flame axis. The separation by means of size exclusion chromatography (SEC) into two different classes of PAH followed by UV-visible spectroscopy corroborated the mass spectral identification of large mass PAH. Critical examination of mass spectral features and SEC separation was the starting point for speculation about the changes occurring in PAH growth from planar to concave structures which could be important for soot inception mechanisms. Ó 2006 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: PAH; Mass spectrometry; Soot inception
1. Introduction Polycyclic aromatic hydrocarbons (PAH), usually present in the soot nucleation zone of a flame, are considered precursors in soot formation, but their role in the soot formation process is not fully understood even though soot inception has a rich literature [1,2] and contributes significantly in round-table discussions [3–6]. The main problem in following PAH growth into the transition regime to soot inception is the analytical difficulty in detecting PAH with molecular weight (MW) higher than 300–400 Da and below the MW attrib*
Corresponding author. Fax: +39 081 593 6936. E-mail address: [email protected] (B. Apicella).
uted to the first soot particles (P2000 Da). Most work deals with PAH having molecular weights up to 300–400 Da detected by gas chromatography-mass spectrometry (GC–MS) and therefore named GC–PAH [7–11], although partial contribution of GC–PAH to the total mass of high molecular weight species [9] and from their UVvisible and fluorescence spectra [12,13] indicates the presence of unidentified PAH larger than 400 Da. The detection of PAH above 400 Da has been attempted using different kinds of mass spectrometric apparatus: laser microprobe mass spectrometry (LMMS) up to m/z 472 [14], resonance enhanced multi-photon ionization-mass spectrometry (REMPI-MS) up to m/z 900 [15] and
1540-7489/$ - see front matter Ó 2006 Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.proci.2006.08.014
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atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) up to m/z 1792 [16]. Measurements of positive ions in the mass range m/z 600–10,000 by on-line time of flight-mass spectrometry could be considered to fill the gap between GC–PAH and soot particles, but only by assuming ionic species representative of neutral counterparts [17]. In the present work laser desorption ionization-time of flight-mass spectrometry (LDI-TOFMS) has been used for the analysis of soot extracts sampled in a premixed ethylene flame. This technique, generally assisted by the presence of a matrix (Matrix Assisted, MALDI-TOF-MS), is largely used for synthetic polymer analysis and in biochemical fields especially for geonomic and proteomic analysis. By contrast relatively little work has been carried out in MW characterization of carbonaceous materials such as asphaltenes, coal and coal-derived samples [18–21]. Previously, we found LDI-TOF-MS more useful than MALDI-TOF-MS for the analysis of soot extracted from a flame [22]. In the present work, the performance of LDI-TOF-MS has been applied to soot extracts sampled from the soot formation region of a fuel-rich premixed ethylene flame to extend the detection of flame-formed PAH up to the highest limit typical of the first soot particles (>2000 Da). The data interpretation has been also supported by UV-visible spectroscopy and size exclusion chromatography analysis. 2. Experimental The experimental combustion system has been described in detail elsewhere [23,24]. Briefly, soot extract and dry soot were collected from a premixed laminar ethylene flame. The fuel-rich flame was established on a commercial McKenna burner in very sooting conditions (C/O = 1, i.e. U = 3.03). A stainless steel water-cooled probe was used to sample combustion products along the flame axis. Soot collected from the probe wall, on a teflon filter and in an ice-cooled trap placed in the sampling line was extracted with dichloromethane (DCM) until there was no detectable UV-fluorescence signal in the DCM washings. The extracted organic species were named soot extract. Polycyclic aromatic hydrocarbons up to 300 Da were analysed in the soot extract by gas chromatography-mass spectrometry on a HP5890 gas chromatograph coupled with a HP5970 mass spectrometer. Soot extract, dried and weighed, was dissolved again in DCM for LDI-TOF-MS analyses and in N-methyl pyrrolidone (NMP) for SEC analysis. The SEC analysis of soot extract was carried out on a PL-gel styrene-divinylbenzene individual pore column (Polymer Laboratories, Ltd., UK)
with a column particle size of 5 lm diameter and a pore dimension of 50 nm. This column is able to separate polystyrene standards in the molecular mass range 100–100,000 Da. The on-line detection of species eluted from the SEC column used an HP1050 UV-visible diode array detector measuring the absorbance signal at selected wavelengths. In the following discussion the relation between molecular mass of polystyrenes and of PAH, shown to hold for PAH standards [29], is used for the MW evaluation of soot extracts. Positive Reflectron LDI-TOF spectra were recorded on a Voyager DE STR Pro instrument (Applied Biosystems, Framingham, MA). The target was prepared by depositing about 1 ll of a solution of the sample dissolved in DCM on the metallic sample plate. Acceleration and reflector voltages set up were: target voltage 25 kV, first grid at 95% of target voltage, delayed extraction at 600 ns. The time-of-flight data were externally calibrated (two-point calibration algorithm) for each sample plate and different sample preparations. Calibration and further data processing used the computer software provided by the manufacturer. 3. Results and discussion The mass spectrometric analysis of large PAH in the present work was performed in a fuel-rich premixed ethylene flame burning in very fuel-rich conditions (C/O = 1, i.e. U = 3.03) much above the soot formation threshold (C/O = 0.66), in order to maximize the formation of pyrolytic products. The flame structure has been described in detail in previous work [24]. On the basis of the chemical structure of the flame, particulates were sampled at 6, 8 and 14 mm height in the soot inception region, at maximum soot formation rate and at the end of soot formation region, respectively. The particulate was extracted with DCM in order to separate the organic species adsorbed on it, named soot extract, from DCM insoluble soot. Soot extracts were analysed in DCM solution by GC–MS for the PAH analysis from naphthalene (128 Da) up to coronene (300 Da) which accounts for up to 30–40 wt% of the total soot extract [24]. Soot extracts have been analysed by LDITOF-MS which in principle can detect up to 100,000 Da in linear mode configuration. However, the main practical limitation of the linear TOF spectrometer is the relatively low mass resolution achievable because of the energy spread of the generated ion packet and other experimental factors such as delayed ion formation and space charge effects. The resolution can be markedly improved by adding an ion mirror (reflector) to the time-of-flight mass spectrometer, which
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doubles the length of the flight tube although it limits the detectable high MW range to 20,000 Da. In contrast to linear mode, reflector mode spectra can contain not only the molecular ion, but also fragment ions as fragmentation can occur after ions enter the flight tube. In the linear mode these metastable fragments are recorded at the same times as their precursors so that sensitivity for molecular weight measurements is higher than in the reflector mode where they are dispersed, generally unfocused, along the time axis [25]. As a result it is very important when dealing with unknown species to confirm the reflector mode spectrum with a linear mode spectrum. In the present investigation both the linear and reflector configurations of TOF-MS have been used without observing significant differences in terms of the number of peaks, which testifies to the absence of fragmentation, and in MW range detected. Therefore, only the spectra in reflector mode have been reported as the mass resolution is much higher than that obtained in linear mode. The mass spectra of soot extracts at 6, 8 and 14 mm flame height, reported in Figs. 1–4, extend from m/z 300 up to 1700. Between m/z 1000 and 1700 (Fig. 4) the signal/noise ratio only allows detection of signals, but not resolution of all the peaks around the parent peak because of the isotopic pattern and to the hydrogen loose (Figs. 1–3). The typical decreasing trend of intensity of PAH ion peaks with increasing molecular mass, already observed in GC–PAH up to 300 Da [11], appears to extend up to 1700 Da which is much above the mass limit of gas chromatographic (300 Da) and liquid chromatographic (450 Da) techniques. PAH attribution to the species in the mass range m/z 300–1700 are based on the typical garland-like structure of the PAH mass spectrum also observed in the mass range m/z 100–400 [11]. However, the attribution to singular PAH structures, is obtained with a degree of speculation
Fig. 1. LDI-TOF-MS spectrum of soot extract sampled at z = 6 mm of height above the burner in the range m/z 300–1000 (top) with zoom in the range m/z 600–1000 (inset) and in the range m/z 300–600 (bottom).
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Fig. 2. LDI-TOF-MS spectrum of soot extract sampled at z = 8 mm of height above the burner in the range m/z 300–1000 (top) with zoom in the range m/z 600–1000 (inset) and in the range m/z 300–600 (bottom).
Fig. 3. LDI-TOF-MS spectrum of soot extract sampled at z = 14 mm of height above the burner in the range m/z 300–1000 (top) with zoom in the range m/z 600–1000 (inset) and in the range m/z 300–600 (bottom).
Fig. 4. LDI-TOF-MS spectrum of soot extract sampled at z = 14 mm of height above the burner in the range m/z 1000–1700.
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because of the high number of isomers and the paucity of data for large masses. The mass spectra consist of a sequence of major ion peaks with a spacing of 24 Da superimposed on a sequence of minor ion peaks which present the same spacing of 24 Da; the spacing between the major and the minor ion peaks is 12 Da. Similar features of mass spectra have been observed in the mass range m/z 200–400 by particle-beam mass spectrometric (PB-MS) analysis of soot extract samples from premixed flames of benzene [11]. In the mass range m/z 200–300 the GC– MS analysis has allowed the attribution of the sequence of major ion peaks, spaced at 24 Da, mainly to PAH with an even number of carbon atoms (even-C-numbered PAH) where the molecular weight difference is due to net sequential addition of C2 as an ethylene bridge. By contrast, the sequence of minor peaks, spaced at 12 Da relative to the major peaks, were ascribed to odd-Cnumbered PAH containing cyclopenta-fused rings coming from the insertion of a methylene (–CH2–) into a bay region of angular PAH. The attribution of the minor odd-C-numbered peaks to cyclopenta-fused ring PAH is corroborated by the similar intensity of (M 1)+ and M+ mass peaks, due to the easy loss of hydrogen linked on methylene bridge, observed in the PB-MS spectra [11]. An extension of this finding to larger PAH leads to the attribution of the predominant peaks mainly to even-C-numbered PAH and the smaller ones to cyclopenta-fused ring odd-C-numbered PAH. Considering the concentration ratio between even- and odd-C-numbered PAH it is noteworthy to observe that the PAH present very different response factors to the mass spectrometric technique. The even-C-numbered PAH present a parent peak (M+) which prevails over the other peaks and is therefore representative of the concentration of the PAH. Odd-C-numbered PAH have a (M 1)+ peak comparable in intensity with M+ peak, whose intensity is consequently halved with respect to the M+ of even-C-numbered PAH. Hence, we can consider the M+ signal intensity as representative of concentration for even-C-numbered PAH, and twice the M+ signal intensity as representative of the concentration for odd-Cnumbered PAH. In effect, the ionization efficiencies should also be taken into account for transforming the signal intensity into concentration, however, it is possible to assume that PAH differing by only a few Daltons in mass present similar ionization efficiencies [15]. By examination of the spectra reported in Figs. 1–3, it can thus be inferred that even-C-numbered PAH are still largely prevalent at least up to 500 Da as previously found by GC–MS and PB-MS spectrometry up to 400 Da [11]. The contributions of even- and odd-C-numbered PAH become comparable for PAH with more than 40 C atoms (about 500 Da), consistent
with the results obtained by REMPI-MS in the mass range m/z 100–900 [15]. Assuming that odd-C-numbered PAH are cyclopenta-fused ring PAH, the insertion of a pentagon in a PAH after 400 Da can be considered to cause curvature in the carbon skeleton (see for example corannulene, MW = 250 Da). It can be speculated that beyond 400 Da the progressive increase at larger masses of odd-C-numbered PAH relative to even-C-numbered PAH is a signature of a progressive structural change from planar two-dimensional (2D) PAH up to curved and/or concave PAH approaching compact, closed-shell 3D young soot particles. It is worth noting that mass spectrometric data support the presence of PAH with pentagon rings in the periphery, which lose a hydrogen atom from a methylene carbon, and these PAH are still planar whereas PAH are non-planar only if the 5-membered rings are embedded in the PAH structure rather than on the periphery. However, the rearrangement of a PAH with an externally fused cyclopenta group toward a PAH with a centrally cyclopenta fused group is a process which has been observed under conditions pertinent to combustion as reported by Scott [26] and Lafleur et al. [27]. Thus the presence of PAH with pentagon rings in the periphery detected by mass spectrometry can be considered as a signature of the presence of curved PAH with internally fused cyclopenta groups. These considerations appear to be corroborated by the evaluation of MW distribution in a soot extract obtained by means of size exclusion chromatography [28–30]. This technique allows the analysis in a wide MW range (m/z 100–100,000) on the basis of a comparison of the elution times of soot extract components with polystyrene and polycyclic aromatic standard molecules [29]. A typical SEC chromatogram of a soot extract sample, reported in Fig. 5, exhibits a first and most prominent peak (labelled as peak 1) due to GC–PAH (m/z 100–400); the second peak (2), is just contiguous with the predominant PAH peak, ranging from 400 to 3000 Da with a maximum at about 600 Da. A smaller peak, not reported here, was also found, corresponding to MW P 100,000 Da, i.e. beyond the exclusion limit of the column. The separation between the GC–PAH peak and the second peak is poor and corresponds to a MW of 400 Da. This is where the relative increase of odd-relative to even-C-numbered PAH is found in the mass spectra and is attributed to the progressive dominance of concave PAH species as MW rises. By contrast, any further increase of concentration beyond 400 Da was detectable in the LDI-TOF-MS spectra (Figs. 1–3) which shows the typical decreasing trend of PAH ion peak intensities up to 1700 Da. It can be hypothesized that the SEC elution times are
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Fig. 5. Height-normalized SEC chromatogram of soot extract sampled at z = 14 mm acquired at k = 350 nm and zoomed in the range m/z 100–4000.
Fig. 6. Height-normalized UV-visible absorption spectra of whole soot extract sample (WL, - - -), GC–PAH ). (peak 1, —) and peak 2 species (
also sensitive to structural changes as well as to the MW [31]. Hence, the appearance after 400 Da of a peak/shoulder contiguous with the main PAH peak could be due to structurally different PAH with cyclopenta-fused rings whose curved shape causes a delay in the elution of large masses (peak 2, Fig. 5). The separation of peak 1 and peak 2 could be mainly due to a structural change (from planar to curved structures) which causes these species to behave as if they were slightly larger than real molecular mass. If this were the case, in the absence of structural effects on the SEC separation, the second peak would be submerged by the main PAH peak and the mass range reduced from the apparent m/z 400 to 3000 to m/z 300–2000, consistent with the LDI-TOF-MS analysis. Deeper information on the nature and characteristics of large mass PAH eluted in the second peak and of the GC–PAH eluted in the first peak, can be obtained by UV-visible absorption spectra of both chromatographic PAH peaks acquired on-line by a diode array UV-visible detector. All the spectra are height-normalized and reported in Fig. 6 in comparison with the height-normalized spectrum of the whole soot extract. It can be noted that the absorbance spectrum acquired in the region of the 400– 3000 Da species (peak 2) is devoid of the fine structure detected in the main PAH peak, the latter being well reproduced in the whole soot extract spectrum (Fig. 5). The loss of fine structure and the larger visible absorption of high mass PAH is due to the overlapping of a larger number of isomers and to aromatic structures having a higher condensation degree relative to GC–PAH, in agreement with the LDI-TOF-
MS spectra interpretation. The larger size of the aromatic structures in the peak 2 was also demonstrated by the shift of the fluorescence spectra toward higher emission wavelengths not reported here for space limitations. Overall, it is noteworthy that the mass spectra of soot extracts sampled along the flame do not show significant differences. The unique observation that can be done by comparing the mass spectra in the crucial zones of soot formation region concerns the slight enrichment along the flame in larger mass species in the m/z region 600–1700 corresponding to species in a subnanometric– nanometric size range that is next to the size range of young soot individuated by microscopy [32] and LMMS techniques [33]. The unique features of PAH spectra in the MW range up to about 1700 Da can also allow some speculation on PAH growth and soot inception mechanisms. The presence of a 24 Da gap among ion peaks of the same PAH sequence suggests that the growth of PAH mainly occurs through the stepwise gain of two carbon atoms due to reaction with acetylene, as described in the HACA mechanism [34]. However, the attribution of the minor ion peak sequence to odd-C-numbered PAH (mainly PAH with 5-member rings) suggests the occurrence of another competitive mechanism. This mechanism, in agreement with the model proposed by Zhang et al. [35] for soot inception, assumes that the large polycyclic aromatic molecules tend to maximize carbon– carbon bonding by incorporating pentagons into their aromatic bonding network. These pentagons generate a curvature which brings the net back on itself to form a spheroidal shell which, in turn, can close to form a spherical soot particle.
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4. Conclusions The detection of large PAH up to 1700 Da was obtained by laser desorption mass spectrometric analysis of soot extracts collected in the soot formation region of a premixed fuel-rich ethylene flame. The typical decreasing intensity of PAH ion peak intensities with increasing mass was also found in the mass range m/z 500–1700 even though a slight enrichment in the heavier part of PAH could be observed along the flame axis. The SEC separation into two different classes of PAH followed by UV-visible spectroscopy corroborated the mass spectral identification of large mass PAH. Critical examination of mass spectral features and of SEC separation led to speculation about the structural changes occurring in the PAH growth from planar to curved PAH structures which could be important for soot inception mechanisms.
References [1] B.S. Haynes, H.Gg. Wagner, Prog. Energy Combust. Sci. 7 (1981) 229–273. [2] B.S. Haynes, in: W. Bartok, A.F. Sarofim (Eds.), Fossil Fuel Combustion, John Wiley and Sons Inc., New York, 1991, p. 261. [3] D.C. Siegla, G.W. Smith (Eds.), Particulate Carbon Formation During Combustion, Plenum Press, New York and London, 1981. [4] J. Lahaye, G. Prado, Soot in Combustion Systems and its Toxic Properties, Plenum Press, New York and London, 1983. [5] H. Jander, H.G. Wagner (Eds.), Soot formation in Combustion. An International Round Table Discussion, Vandenhoech & Ruprecht, Gottingen, 1990. [6] H. Bockhorn (Ed.), Soot Formation in Combustion, Springer-Verlag, 1994. [7] H. Bockhorn, F. Fetting, H.W. Wenz, Ber. Bunsenges. Phys. Chem. 87 (1983) 1067–1073. [8] A. Ciajolo, R. Barbella, M. Mattiello, A. D’Alessio, Proc. Combust. Inst. 19 (1982) 1369–1377. [9] A. Ciajolo, A. D’Anna, R. Barbella, A. Tregrossi, A. Violi, Proc. Combust. Inst. 26 (1996) 2327–2333. [10] A. Ciajolo, B. Apicella, R. Barbella, A. Tregrossi, F. Beretta, C. Allouis, Energy Fuel 15 (4) (2001) 987–995. [11] A. Tregrossi, A. Ciajolo, R. Barbella, Combust. Flame 117 (3) (1999) 553–561. [12] A. Ciajolo, R. Barbella, A. Tregrossi, L. Bonfanti, Proc. Combust. Inst. 27 (1998) 1481–1487.
[13] A. Tregrossi, A. Ciajolo, R. Barbella, National Congress of Informatic Chemistry, Napoli, 1997, p. 295. [14] R.A. Dobbins, R.A. Fletcher, H.-C. Chang, Combust. Flame 115 (1998) 285–298. [15] A. Keller, R. Kovacs, K.-H. Homann, Phys. Chem. Chem. Phys. 2 (2000) 1667–1675. [16] A.L. Lafleur, K. Taghizadeh, J.B. Howard, J.F. Anacleto, M.A. Quilliam, J. Am. Soc. Mass spectrom. 7 (1996) 276–286. [17] K.H. Homann, Ber. Bunsenges. Phys. Chem. 83 (1979) 738–745. [18] J.T. Miller, R.B. Fisher, P. Thiyagarian, R.E. Winans, J.E. Hunt, Energy Fuel 12 (1998) 1290– 1298. [19] M. Domin, R. Moreea, M.J. Lazaro, A.A. Herod, R. Kandiyoti, Rapid Commun. Mass Spectrom. 11 (1997) 1845–1852. [20] A.A. Herod, M.J. Lazaro, M. Domin, C.A. Islas, R. Kandiyoti, Fuel 79 (2000) 323–337. [21] W.S. Edwards, L. Jin, M.C. Thies, Carbon 41 (2003) 2761–2768. [22] B. Apicella, A. Ciajolo, M. Millan, C. Galmes, A.A. Herod, R. Kandiyoti, Rapid Commun. Mass Spectrom. 18 (2004) 331–338. [23] A. Ciajolo, A. D’Anna, R. Barbella, Combust. Sci. Technol. 100 (1994) 271–281. [24] B. Apicella, R. Barbella, A. Ciajolo, A. Tregrossi, Combust. Sci. Technol. 174 (11–12) (2002) 309–324. [25] R.J. Cotter, Time of Flight Mass Spectrometry, Oxford University Press, 1997. [26] L.T. Scott, Pure Appl. Chem. 68 (2) (1996) 291–300. [27] A.L. Lafleur, J.B. Howard, K. Taghizadeh, E.F. Plummer, L.T. Scott, A. Necula, K.C. Swallow, J. Phys. Chem. 100 (1996) 17421–17428. [28] B. Apicella, A. Ciajolo, I.I. Suelves, T.J. Morgan, A.A. Herod, R. Kandiyoti, Combust. Sci. Technol. 174 (11–12) (2002) 403–417. [29] B. Apicella, R. Barbella, A. Ciajolo, A. Tregrossi, Chemosphere 51 (2003) 1063–1069. [30] B. Apicella, A. Ciajolo, R. Barbella, A. Tregrossi, T.J. Morgan, A.A. Herod, R. Kandiyoti, Energy Fuel 17 (2003) 565–570. [31] F. Karaca, C.A. Islas, M. Millan, M. Behrouzi, T. Morgan, A.A. Herod, R. Kandiyoti, Energy Fuel 18 (2004) 778–788. [32] B.L. Wersborg, J.B. Howard, G.C. Williams, Proc. Combust. Inst. 14 (1973) 929–940. [33] R.A. Dobbins, R.A. Fletcher, W. Lu, Combust. Flame 100 (1995) 301–309. [34] M. Frenklach, in: H. Jander, H.Gg. Wagner (Eds.), Soot Formation in Combustion. An International Round Table Discussion, Vandenhoeck & Ruprecht, Gottingen, 1990, p. 162. [35] Q.L. Zhang, S.C. O’Brien, J.R. Heath, Y. Liu, R.F. Curl, H.W. Kroto, R.E. Smalley, J. Phys. Chem. 90 (4) (1986) 525–528.
Comments Barry Dellinger, Louisiana State University, USA. A weakness of LDI-TOF-MS is fragmentation of larger species resulting in erroneous identification of lower molecular weight species. What type of controlled experiments have you performed to rule out fragmentation?
Reply. As already mentioned in the paper, the similarity between linear and reflector mode spectra was found that testifies to the absence of fragmentation. A further confirmation was given by the analysis of heavy standard PAH like coronene and ovalene that
B. Apicella et al. / Proceedings of the Combustion Institute 31 (2007) 547–553 presented no fragmentation under the LDI-TOF-MS conditions used in the work. Moreover, the samples were separated in classes on the basis of their molecular weights (MW) by using Size Exclusion Chromatography (SEC) and they were analyzed by LDI-TOFMS. The SEC fraction in the MW range 400– 3000 Da presented MS spectra similar to those reported in the paper, thus testifying that the ion peaks found are not due to the fragmentation of larger species.
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explained alternatively than PAHs with cyclopenta rays imbed in them? Reply. The attribution of odd-C numbered PAH to PAH with cyclo-penta rings came out from a comparison with GC–MS spectra as explained in the paper in the ‘‘Results and discussion’’ paragraph: ‘‘In the mass range m/z 200–300 the GC–MS analysis .[. . .] mainly to even-C-numbered PAH and the smaller ones to cyclopenta-fused ring odd-C-numbered PAH.’’
d
Hai Wang, University of Southern California, USA. Could odd-C-numbered PAHs be simply methyl-substituted PAHs and thus your MS spectra could be
We ruled out the attribution of odd-C numbered PAH to methyl-substituted PAH as the insertion of a methyl group on a PAH causes a mass increase of 14 Da whereas we mainly found gap of 12 Da.
Proceedings of the Combustion Institute 31 (2007) 547–553
Combustion Institute www.elsevier.com/locate/proci
Mass spectrometric analysis of large PAH in a fuel-rich ethylene flame B. Apicella a,*, A. Carpentieri b, M. Alfe` a, R. Barbella a, A. Tregrossi a, P. Pucci b, A. Ciajolo a a
Istituto di Ricerche sulla Combustione, CNR, P.le V. Tecchio, 80-80125 Napoli, Italy Dipartimento di Chimica, Universita` ‘‘Federico II’’, Via Cintia, 80125 Napoli, Italy
b
Abstract Mass spectrometric analysis by laser desorption-time of flight-mass spectrometry (LDI-TOF-MS) was exploited to extend the detection of flame-formed polycyclic aromatic hydrocarbons (PAH) up to the mass limit of the first soot particles (>2000 Da) in the soot formation region of a premixed fuel-rich (C/O = 1) ethylene flame. The typical decreasing intensity of PAH ion peaks with increasing mass was found in the mass range m/z 500–1700 although a slight enrichment in the heavier part of PAH could be observed to occur along the flame axis. The separation by means of size exclusion chromatography (SEC) into two different classes of PAH followed by UV-visible spectroscopy corroborated the mass spectral identification of large mass PAH. Critical examination of mass spectral features and SEC separation was the starting point for speculation about the changes occurring in PAH growth from planar to concave structures which could be important for soot inception mechanisms. Ó 2006 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: PAH; Mass spectrometry; Soot inception
1. Introduction Polycyclic aromatic hydrocarbons (PAH), usually present in the soot nucleation zone of a flame, are considered precursors in soot formation, but their role in the soot formation process is not fully understood even though soot inception has a rich literature [1,2] and contributes significantly in round-table discussions [3–6]. The main problem in following PAH growth into the transition regime to soot inception is the analytical difficulty in detecting PAH with molecular weight (MW) higher than 300–400 Da and below the MW attrib*
Corresponding author. Fax: +39 081 593 6936. E-mail address: [email protected] (B. Apicella).
uted to the first soot particles (P2000 Da). Most work deals with PAH having molecular weights up to 300–400 Da detected by gas chromatography-mass spectrometry (GC–MS) and therefore named GC–PAH [7–11], although partial contribution of GC–PAH to the total mass of high molecular weight species [9] and from their UVvisible and fluorescence spectra [12,13] indicates the presence of unidentified PAH larger than 400 Da. The detection of PAH above 400 Da has been attempted using different kinds of mass spectrometric apparatus: laser microprobe mass spectrometry (LMMS) up to m/z 472 [14], resonance enhanced multi-photon ionization-mass spectrometry (REMPI-MS) up to m/z 900 [15] and
1540-7489/$ - see front matter Ó 2006 Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.proci.2006.08.014
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atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) up to m/z 1792 [16]. Measurements of positive ions in the mass range m/z 600–10,000 by on-line time of flight-mass spectrometry could be considered to fill the gap between GC–PAH and soot particles, but only by assuming ionic species representative of neutral counterparts [17]. In the present work laser desorption ionization-time of flight-mass spectrometry (LDI-TOFMS) has been used for the analysis of soot extracts sampled in a premixed ethylene flame. This technique, generally assisted by the presence of a matrix (Matrix Assisted, MALDI-TOF-MS), is largely used for synthetic polymer analysis and in biochemical fields especially for geonomic and proteomic analysis. By contrast relatively little work has been carried out in MW characterization of carbonaceous materials such as asphaltenes, coal and coal-derived samples [18–21]. Previously, we found LDI-TOF-MS more useful than MALDI-TOF-MS for the analysis of soot extracted from a flame [22]. In the present work, the performance of LDI-TOF-MS has been applied to soot extracts sampled from the soot formation region of a fuel-rich premixed ethylene flame to extend the detection of flame-formed PAH up to the highest limit typical of the first soot particles (>2000 Da). The data interpretation has been also supported by UV-visible spectroscopy and size exclusion chromatography analysis. 2. Experimental The experimental combustion system has been described in detail elsewhere [23,24]. Briefly, soot extract and dry soot were collected from a premixed laminar ethylene flame. The fuel-rich flame was established on a commercial McKenna burner in very sooting conditions (C/O = 1, i.e. U = 3.03). A stainless steel water-cooled probe was used to sample combustion products along the flame axis. Soot collected from the probe wall, on a teflon filter and in an ice-cooled trap placed in the sampling line was extracted with dichloromethane (DCM) until there was no detectable UV-fluorescence signal in the DCM washings. The extracted organic species were named soot extract. Polycyclic aromatic hydrocarbons up to 300 Da were analysed in the soot extract by gas chromatography-mass spectrometry on a HP5890 gas chromatograph coupled with a HP5970 mass spectrometer. Soot extract, dried and weighed, was dissolved again in DCM for LDI-TOF-MS analyses and in N-methyl pyrrolidone (NMP) for SEC analysis. The SEC analysis of soot extract was carried out on a PL-gel styrene-divinylbenzene individual pore column (Polymer Laboratories, Ltd., UK)
with a column particle size of 5 lm diameter and a pore dimension of 50 nm. This column is able to separate polystyrene standards in the molecular mass range 100–100,000 Da. The on-line detection of species eluted from the SEC column used an HP1050 UV-visible diode array detector measuring the absorbance signal at selected wavelengths. In the following discussion the relation between molecular mass of polystyrenes and of PAH, shown to hold for PAH standards [29], is used for the MW evaluation of soot extracts. Positive Reflectron LDI-TOF spectra were recorded on a Voyager DE STR Pro instrument (Applied Biosystems, Framingham, MA). The target was prepared by depositing about 1 ll of a solution of the sample dissolved in DCM on the metallic sample plate. Acceleration and reflector voltages set up were: target voltage 25 kV, first grid at 95% of target voltage, delayed extraction at 600 ns. The time-of-flight data were externally calibrated (two-point calibration algorithm) for each sample plate and different sample preparations. Calibration and further data processing used the computer software provided by the manufacturer. 3. Results and discussion The mass spectrometric analysis of large PAH in the present work was performed in a fuel-rich premixed ethylene flame burning in very fuel-rich conditions (C/O = 1, i.e. U = 3.03) much above the soot formation threshold (C/O = 0.66), in order to maximize the formation of pyrolytic products. The flame structure has been described in detail in previous work [24]. On the basis of the chemical structure of the flame, particulates were sampled at 6, 8 and 14 mm height in the soot inception region, at maximum soot formation rate and at the end of soot formation region, respectively. The particulate was extracted with DCM in order to separate the organic species adsorbed on it, named soot extract, from DCM insoluble soot. Soot extracts were analysed in DCM solution by GC–MS for the PAH analysis from naphthalene (128 Da) up to coronene (300 Da) which accounts for up to 30–40 wt% of the total soot extract [24]. Soot extracts have been analysed by LDITOF-MS which in principle can detect up to 100,000 Da in linear mode configuration. However, the main practical limitation of the linear TOF spectrometer is the relatively low mass resolution achievable because of the energy spread of the generated ion packet and other experimental factors such as delayed ion formation and space charge effects. The resolution can be markedly improved by adding an ion mirror (reflector) to the time-of-flight mass spectrometer, which
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doubles the length of the flight tube although it limits the detectable high MW range to 20,000 Da. In contrast to linear mode, reflector mode spectra can contain not only the molecular ion, but also fragment ions as fragmentation can occur after ions enter the flight tube. In the linear mode these metastable fragments are recorded at the same times as their precursors so that sensitivity for molecular weight measurements is higher than in the reflector mode where they are dispersed, generally unfocused, along the time axis [25]. As a result it is very important when dealing with unknown species to confirm the reflector mode spectrum with a linear mode spectrum. In the present investigation both the linear and reflector configurations of TOF-MS have been used without observing significant differences in terms of the number of peaks, which testifies to the absence of fragmentation, and in MW range detected. Therefore, only the spectra in reflector mode have been reported as the mass resolution is much higher than that obtained in linear mode. The mass spectra of soot extracts at 6, 8 and 14 mm flame height, reported in Figs. 1–4, extend from m/z 300 up to 1700. Between m/z 1000 and 1700 (Fig. 4) the signal/noise ratio only allows detection of signals, but not resolution of all the peaks around the parent peak because of the isotopic pattern and to the hydrogen loose (Figs. 1–3). The typical decreasing trend of intensity of PAH ion peaks with increasing molecular mass, already observed in GC–PAH up to 300 Da [11], appears to extend up to 1700 Da which is much above the mass limit of gas chromatographic (300 Da) and liquid chromatographic (450 Da) techniques. PAH attribution to the species in the mass range m/z 300–1700 are based on the typical garland-like structure of the PAH mass spectrum also observed in the mass range m/z 100–400 [11]. However, the attribution to singular PAH structures, is obtained with a degree of speculation
Fig. 1. LDI-TOF-MS spectrum of soot extract sampled at z = 6 mm of height above the burner in the range m/z 300–1000 (top) with zoom in the range m/z 600–1000 (inset) and in the range m/z 300–600 (bottom).
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Fig. 2. LDI-TOF-MS spectrum of soot extract sampled at z = 8 mm of height above the burner in the range m/z 300–1000 (top) with zoom in the range m/z 600–1000 (inset) and in the range m/z 300–600 (bottom).
Fig. 3. LDI-TOF-MS spectrum of soot extract sampled at z = 14 mm of height above the burner in the range m/z 300–1000 (top) with zoom in the range m/z 600–1000 (inset) and in the range m/z 300–600 (bottom).
Fig. 4. LDI-TOF-MS spectrum of soot extract sampled at z = 14 mm of height above the burner in the range m/z 1000–1700.
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because of the high number of isomers and the paucity of data for large masses. The mass spectra consist of a sequence of major ion peaks with a spacing of 24 Da superimposed on a sequence of minor ion peaks which present the same spacing of 24 Da; the spacing between the major and the minor ion peaks is 12 Da. Similar features of mass spectra have been observed in the mass range m/z 200–400 by particle-beam mass spectrometric (PB-MS) analysis of soot extract samples from premixed flames of benzene [11]. In the mass range m/z 200–300 the GC– MS analysis has allowed the attribution of the sequence of major ion peaks, spaced at 24 Da, mainly to PAH with an even number of carbon atoms (even-C-numbered PAH) where the molecular weight difference is due to net sequential addition of C2 as an ethylene bridge. By contrast, the sequence of minor peaks, spaced at 12 Da relative to the major peaks, were ascribed to odd-Cnumbered PAH containing cyclopenta-fused rings coming from the insertion of a methylene (–CH2–) into a bay region of angular PAH. The attribution of the minor odd-C-numbered peaks to cyclopenta-fused ring PAH is corroborated by the similar intensity of (M 1)+ and M+ mass peaks, due to the easy loss of hydrogen linked on methylene bridge, observed in the PB-MS spectra [11]. An extension of this finding to larger PAH leads to the attribution of the predominant peaks mainly to even-C-numbered PAH and the smaller ones to cyclopenta-fused ring odd-C-numbered PAH. Considering the concentration ratio between even- and odd-C-numbered PAH it is noteworthy to observe that the PAH present very different response factors to the mass spectrometric technique. The even-C-numbered PAH present a parent peak (M+) which prevails over the other peaks and is therefore representative of the concentration of the PAH. Odd-C-numbered PAH have a (M 1)+ peak comparable in intensity with M+ peak, whose intensity is consequently halved with respect to the M+ of even-C-numbered PAH. Hence, we can consider the M+ signal intensity as representative of concentration for even-C-numbered PAH, and twice the M+ signal intensity as representative of the concentration for odd-Cnumbered PAH. In effect, the ionization efficiencies should also be taken into account for transforming the signal intensity into concentration, however, it is possible to assume that PAH differing by only a few Daltons in mass present similar ionization efficiencies [15]. By examination of the spectra reported in Figs. 1–3, it can thus be inferred that even-C-numbered PAH are still largely prevalent at least up to 500 Da as previously found by GC–MS and PB-MS spectrometry up to 400 Da [11]. The contributions of even- and odd-C-numbered PAH become comparable for PAH with more than 40 C atoms (about 500 Da), consistent
with the results obtained by REMPI-MS in the mass range m/z 100–900 [15]. Assuming that odd-C-numbered PAH are cyclopenta-fused ring PAH, the insertion of a pentagon in a PAH after 400 Da can be considered to cause curvature in the carbon skeleton (see for example corannulene, MW = 250 Da). It can be speculated that beyond 400 Da the progressive increase at larger masses of odd-C-numbered PAH relative to even-C-numbered PAH is a signature of a progressive structural change from planar two-dimensional (2D) PAH up to curved and/or concave PAH approaching compact, closed-shell 3D young soot particles. It is worth noting that mass spectrometric data support the presence of PAH with pentagon rings in the periphery, which lose a hydrogen atom from a methylene carbon, and these PAH are still planar whereas PAH are non-planar only if the 5-membered rings are embedded in the PAH structure rather than on the periphery. However, the rearrangement of a PAH with an externally fused cyclopenta group toward a PAH with a centrally cyclopenta fused group is a process which has been observed under conditions pertinent to combustion as reported by Scott [26] and Lafleur et al. [27]. Thus the presence of PAH with pentagon rings in the periphery detected by mass spectrometry can be considered as a signature of the presence of curved PAH with internally fused cyclopenta groups. These considerations appear to be corroborated by the evaluation of MW distribution in a soot extract obtained by means of size exclusion chromatography [28–30]. This technique allows the analysis in a wide MW range (m/z 100–100,000) on the basis of a comparison of the elution times of soot extract components with polystyrene and polycyclic aromatic standard molecules [29]. A typical SEC chromatogram of a soot extract sample, reported in Fig. 5, exhibits a first and most prominent peak (labelled as peak 1) due to GC–PAH (m/z 100–400); the second peak (2), is just contiguous with the predominant PAH peak, ranging from 400 to 3000 Da with a maximum at about 600 Da. A smaller peak, not reported here, was also found, corresponding to MW P 100,000 Da, i.e. beyond the exclusion limit of the column. The separation between the GC–PAH peak and the second peak is poor and corresponds to a MW of 400 Da. This is where the relative increase of odd-relative to even-C-numbered PAH is found in the mass spectra and is attributed to the progressive dominance of concave PAH species as MW rises. By contrast, any further increase of concentration beyond 400 Da was detectable in the LDI-TOF-MS spectra (Figs. 1–3) which shows the typical decreasing trend of PAH ion peak intensities up to 1700 Da. It can be hypothesized that the SEC elution times are
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Fig. 5. Height-normalized SEC chromatogram of soot extract sampled at z = 14 mm acquired at k = 350 nm and zoomed in the range m/z 100–4000.
Fig. 6. Height-normalized UV-visible absorption spectra of whole soot extract sample (WL, - - -), GC–PAH ). (peak 1, —) and peak 2 species (
also sensitive to structural changes as well as to the MW [31]. Hence, the appearance after 400 Da of a peak/shoulder contiguous with the main PAH peak could be due to structurally different PAH with cyclopenta-fused rings whose curved shape causes a delay in the elution of large masses (peak 2, Fig. 5). The separation of peak 1 and peak 2 could be mainly due to a structural change (from planar to curved structures) which causes these species to behave as if they were slightly larger than real molecular mass. If this were the case, in the absence of structural effects on the SEC separation, the second peak would be submerged by the main PAH peak and the mass range reduced from the apparent m/z 400 to 3000 to m/z 300–2000, consistent with the LDI-TOF-MS analysis. Deeper information on the nature and characteristics of large mass PAH eluted in the second peak and of the GC–PAH eluted in the first peak, can be obtained by UV-visible absorption spectra of both chromatographic PAH peaks acquired on-line by a diode array UV-visible detector. All the spectra are height-normalized and reported in Fig. 6 in comparison with the height-normalized spectrum of the whole soot extract. It can be noted that the absorbance spectrum acquired in the region of the 400– 3000 Da species (peak 2) is devoid of the fine structure detected in the main PAH peak, the latter being well reproduced in the whole soot extract spectrum (Fig. 5). The loss of fine structure and the larger visible absorption of high mass PAH is due to the overlapping of a larger number of isomers and to aromatic structures having a higher condensation degree relative to GC–PAH, in agreement with the LDI-TOF-
MS spectra interpretation. The larger size of the aromatic structures in the peak 2 was also demonstrated by the shift of the fluorescence spectra toward higher emission wavelengths not reported here for space limitations. Overall, it is noteworthy that the mass spectra of soot extracts sampled along the flame do not show significant differences. The unique observation that can be done by comparing the mass spectra in the crucial zones of soot formation region concerns the slight enrichment along the flame in larger mass species in the m/z region 600–1700 corresponding to species in a subnanometric– nanometric size range that is next to the size range of young soot individuated by microscopy [32] and LMMS techniques [33]. The unique features of PAH spectra in the MW range up to about 1700 Da can also allow some speculation on PAH growth and soot inception mechanisms. The presence of a 24 Da gap among ion peaks of the same PAH sequence suggests that the growth of PAH mainly occurs through the stepwise gain of two carbon atoms due to reaction with acetylene, as described in the HACA mechanism [34]. However, the attribution of the minor ion peak sequence to odd-C-numbered PAH (mainly PAH with 5-member rings) suggests the occurrence of another competitive mechanism. This mechanism, in agreement with the model proposed by Zhang et al. [35] for soot inception, assumes that the large polycyclic aromatic molecules tend to maximize carbon– carbon bonding by incorporating pentagons into their aromatic bonding network. These pentagons generate a curvature which brings the net back on itself to form a spheroidal shell which, in turn, can close to form a spherical soot particle.
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4. Conclusions The detection of large PAH up to 1700 Da was obtained by laser desorption mass spectrometric analysis of soot extracts collected in the soot formation region of a premixed fuel-rich ethylene flame. The typical decreasing intensity of PAH ion peak intensities with increasing mass was also found in the mass range m/z 500–1700 even though a slight enrichment in the heavier part of PAH could be observed along the flame axis. The SEC separation into two different classes of PAH followed by UV-visible spectroscopy corroborated the mass spectral identification of large mass PAH. Critical examination of mass spectral features and of SEC separation led to speculation about the structural changes occurring in the PAH growth from planar to curved PAH structures which could be important for soot inception mechanisms.
References [1] B.S. Haynes, H.Gg. Wagner, Prog. Energy Combust. Sci. 7 (1981) 229–273. [2] B.S. Haynes, in: W. Bartok, A.F. Sarofim (Eds.), Fossil Fuel Combustion, John Wiley and Sons Inc., New York, 1991, p. 261. [3] D.C. Siegla, G.W. Smith (Eds.), Particulate Carbon Formation During Combustion, Plenum Press, New York and London, 1981. [4] J. Lahaye, G. Prado, Soot in Combustion Systems and its Toxic Properties, Plenum Press, New York and London, 1983. [5] H. Jander, H.G. Wagner (Eds.), Soot formation in Combustion. An International Round Table Discussion, Vandenhoech & Ruprecht, Gottingen, 1990. [6] H. Bockhorn (Ed.), Soot Formation in Combustion, Springer-Verlag, 1994. [7] H. Bockhorn, F. Fetting, H.W. Wenz, Ber. Bunsenges. Phys. Chem. 87 (1983) 1067–1073. [8] A. Ciajolo, R. Barbella, M. Mattiello, A. D’Alessio, Proc. Combust. Inst. 19 (1982) 1369–1377. [9] A. Ciajolo, A. D’Anna, R. Barbella, A. Tregrossi, A. Violi, Proc. Combust. Inst. 26 (1996) 2327–2333. [10] A. Ciajolo, B. Apicella, R. Barbella, A. Tregrossi, F. Beretta, C. Allouis, Energy Fuel 15 (4) (2001) 987–995. [11] A. Tregrossi, A. Ciajolo, R. Barbella, Combust. Flame 117 (3) (1999) 553–561. [12] A. Ciajolo, R. Barbella, A. Tregrossi, L. Bonfanti, Proc. Combust. Inst. 27 (1998) 1481–1487.
[13] A. Tregrossi, A. Ciajolo, R. Barbella, National Congress of Informatic Chemistry, Napoli, 1997, p. 295. [14] R.A. Dobbins, R.A. Fletcher, H.-C. Chang, Combust. Flame 115 (1998) 285–298. [15] A. Keller, R. Kovacs, K.-H. Homann, Phys. Chem. Chem. Phys. 2 (2000) 1667–1675. [16] A.L. Lafleur, K. Taghizadeh, J.B. Howard, J.F. Anacleto, M.A. Quilliam, J. Am. Soc. Mass spectrom. 7 (1996) 276–286. [17] K.H. Homann, Ber. Bunsenges. Phys. Chem. 83 (1979) 738–745. [18] J.T. Miller, R.B. Fisher, P. Thiyagarian, R.E. Winans, J.E. Hunt, Energy Fuel 12 (1998) 1290– 1298. [19] M. Domin, R. Moreea, M.J. Lazaro, A.A. Herod, R. Kandiyoti, Rapid Commun. Mass Spectrom. 11 (1997) 1845–1852. [20] A.A. Herod, M.J. Lazaro, M. Domin, C.A. Islas, R. Kandiyoti, Fuel 79 (2000) 323–337. [21] W.S. Edwards, L. Jin, M.C. Thies, Carbon 41 (2003) 2761–2768. [22] B. Apicella, A. Ciajolo, M. Millan, C. Galmes, A.A. Herod, R. Kandiyoti, Rapid Commun. Mass Spectrom. 18 (2004) 331–338. [23] A. Ciajolo, A. D’Anna, R. Barbella, Combust. Sci. Technol. 100 (1994) 271–281. [24] B. Apicella, R. Barbella, A. Ciajolo, A. Tregrossi, Combust. Sci. Technol. 174 (11–12) (2002) 309–324. [25] R.J. Cotter, Time of Flight Mass Spectrometry, Oxford University Press, 1997. [26] L.T. Scott, Pure Appl. Chem. 68 (2) (1996) 291–300. [27] A.L. Lafleur, J.B. Howard, K. Taghizadeh, E.F. Plummer, L.T. Scott, A. Necula, K.C. Swallow, J. Phys. Chem. 100 (1996) 17421–17428. [28] B. Apicella, A. Ciajolo, I.I. Suelves, T.J. Morgan, A.A. Herod, R. Kandiyoti, Combust. Sci. Technol. 174 (11–12) (2002) 403–417. [29] B. Apicella, R. Barbella, A. Ciajolo, A. Tregrossi, Chemosphere 51 (2003) 1063–1069. [30] B. Apicella, A. Ciajolo, R. Barbella, A. Tregrossi, T.J. Morgan, A.A. Herod, R. Kandiyoti, Energy Fuel 17 (2003) 565–570. [31] F. Karaca, C.A. Islas, M. Millan, M. Behrouzi, T. Morgan, A.A. Herod, R. Kandiyoti, Energy Fuel 18 (2004) 778–788. [32] B.L. Wersborg, J.B. Howard, G.C. Williams, Proc. Combust. Inst. 14 (1973) 929–940. [33] R.A. Dobbins, R.A. Fletcher, W. Lu, Combust. Flame 100 (1995) 301–309. [34] M. Frenklach, in: H. Jander, H.Gg. Wagner (Eds.), Soot Formation in Combustion. An International Round Table Discussion, Vandenhoeck & Ruprecht, Gottingen, 1990, p. 162. [35] Q.L. Zhang, S.C. O’Brien, J.R. Heath, Y. Liu, R.F. Curl, H.W. Kroto, R.E. Smalley, J. Phys. Chem. 90 (4) (1986) 525–528.
Comments Barry Dellinger, Louisiana State University, USA. A weakness of LDI-TOF-MS is fragmentation of larger species resulting in erroneous identification of lower molecular weight species. What type of controlled experiments have you performed to rule out fragmentation?
Reply. As already mentioned in the paper, the similarity between linear and reflector mode spectra was found that testifies to the absence of fragmentation. A further confirmation was given by the analysis of heavy standard PAH like coronene and ovalene that
B. Apicella et al. / Proceedings of the Combustion Institute 31 (2007) 547–553 presented no fragmentation under the LDI-TOF-MS conditions used in the work. Moreover, the samples were separated in classes on the basis of their molecular weights (MW) by using Size Exclusion Chromatography (SEC) and they were analyzed by LDI-TOFMS. The SEC fraction in the MW range 400– 3000 Da presented MS spectra similar to those reported in the paper, thus testifying that the ion peaks found are not due to the fragmentation of larger species.
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explained alternatively than PAHs with cyclopenta rays imbed in them? Reply. The attribution of odd-C numbered PAH to PAH with cyclo-penta rings came out from a comparison with GC–MS spectra as explained in the paper in the ‘‘Results and discussion’’ paragraph: ‘‘In the mass range m/z 200–300 the GC–MS analysis .[. . .] mainly to even-C-numbered PAH and the smaller ones to cyclopenta-fused ring odd-C-numbered PAH.’’
d
Hai Wang, University of Southern California, USA. Could odd-C-numbered PAHs be simply methyl-substituted PAHs and thus your MS spectra could be
We ruled out the attribution of odd-C numbered PAH to methyl-substituted PAH as the insertion of a methyl group on a PAH causes a mass increase of 14 Da whereas we mainly found gap of 12 Da.