Anal. Chem. 2003, 75, 5639-5645
Accelerated Articles
Resonance-Enhanced Multiphoton Ionization and VUV-Single Photon Ionization as Soft and Selective Laser Ionization Methods for On-Line Time-of-Flight Mass Spectrometry: Investigation of the Pyrolysis of Typical Organic Contaminants in the Steel Recycling Process L. Cao,† F. Mu 1 hlberger,‡ T. Adam,‡,§ T. Streibel,‡,§ H. Z. Wang,† A. Kettrup,‡,| and R. Zimmermann*,‡,§,⊥
National Analysis Center for Iron and Steel, 76 Xue Yuan Nan Lu, Beijing 100081, P. R. China, Institute of Ecological Chemistry, GSFsNational Research Center for Environment and Health, D-85764, Neuherberg, Munich, Germany, Analytical Chemistry, Institute for Physics, University of Augsburg, Universitaetsstrasse 1, D-86159, Augsburg, Germany, Technical University of Munich, D-85748, Freising, Germany, and BIfA GmbH, Am Mittleren Moos 46, D-86167 Augsburg, Germany
A newly conceived compact and mobile time-of flight mass spectrometer (TOFMS) for real-time monitoring of highly complex gas mixtures is presented. The device utilizes two selective and sensitive soft ionization techniques, viz., resonance-enhanced multiphoton ionization (REMPI) and single-photon ionization (SPI) in a (quasi)-simultaneous mode. Both methods allow a fragmentationless ionization. The REMPI method selectively addresses aromatic species, while with SPI applying vacuum ultaviolet light (118 nm) in principle all compounds with an ionization potential below 10.5 eV are accessible. This provides comprehensive information of the chemical composition of complex matrixes. The combustion and pyrolysis behavior of five organic materials typically used in steel processing in China was studied. The trace amounts of organic compounds in the gas phase during combustion and pyrolysis were monitored selectively and sensitively by real-time SPI/REMPI-TOFMS. The measurements were carried out at several constant temperatures in the range from 300 to 1190 °C in both synthetic air and nitrogen. Timely resolved mass spectra reveal the formation and subsequent growth of aromatic molecules. At lower temperatures, highly alkylated PAHs predominate, while at temperatures above 800 °C, the more stable benzene and PAHs without side chains prevail. Potential hyphenation 10.1021/ac0349025 CCC: $25.00 Published on Web 10/04/2003
© 2003 American Chemical Society
of SPI/REMPI-TOFMS to methods of thermal analysis is discussed.
Monitoring of toxic products emitted from combustion and pyrolysis processes is of growing public interest due to the significant impact of such compounds on the environment and human health. Since combustion and pyrolysis products consist of highly complex mixtures containing several hundreds of different species, sophisticated analytical techniques are required to investigate their chemical composition. By applying conventional off-line methods such as high-resolution gas chromatography/mass spectrometry and high-performance liquid chromatography/UV detection or mass spectrometry1-3 the chemical signature of these mixtures can be determined. However, short time fluctuations of the combustion process, which might lead to * Corresponding author. E-mail:
[email protected]. † National Analysis Center for Iron and Steel. ‡ GSFsNational Research Center for Environment and Health. § University of Augsburg. | Technical University of Munich. ⊥ BIfA GmbH. (1) Herrera, M.; Matuschek, G.; Kettrup, A. Polym. Degrad. Stab. 2002, 78, 323-331. (2) Bjørset, A., Ed. Handbook of Polycyclic Aromatic Hydrocarbons; Marcel Dekker: New York, 1983; p 724. (3) Jay, K.; Stieglitz, L. Chemosphere 1995, 30, 1249-1260
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003 5639
dramatic changes of the chemical pattern and concentrations of single compounds, respectively, cannot be observed.4 Therefore, the combustion or pyrolysis behavior of the material is often studied in model experiments by on-line techniques such as thermal analysis/mass spectrometry (TA/MS) and thermogravimetry/Fourier transformation infrared spectroscopy.5-11 Despite the capability for real-time analysis, it is difficult to detect numerous organic compounds by these analytical techniques due to the complexity and low concentration of the gaseous products. In addition, most conventional mass spectrometers use the nonselective electron impact ionization technique (EI, usually with 70-eV kinetic energy electrons), which in many cases leads to massive fragmentation of organic analyte molecules. Thus, interpretation of such mass spectra obtained with EI ionization is often impossible due to the high number of peaks, which results in superpositions of mother and fragment ion signals. In contrast, the highly selective laser-based resonanceenhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) technique is known as a fragmentationless, soft, and sensitive analytical method12-14 for on-line measurement of trace amounts of organic compounds in complex gas mixtures down to the ppb or even ppt range.15,16 Thereby, using a supersonic jet inlet system for adiabatic cooling of the molecular beam in combination with a fine-tunable laser, even isomer selective ionization is possible. However, the more robust effusive beam inlet with a fixed frequency laser is more convenient for group selective ionization, allowing an overview of the occurring compound pattern. Mass spectra obtained using this soft ionization technique with an effusive beam inlet account for the chemical signature solely of monocyclic and polycyclic aromatic hydrocarbons. Furthermore, the less selective, but also fragmentationless single-photon ionization (SPI) using vacuum ultraviolet (VUV) photons combined with time-of-flight mass spectrometry can also be applied for real-time analysis of complex gas mixtures.17,18 In (4) Weckhardt, C.; Boesl, U.; Schlag, E. W. Anal. Chem. 1994, 66, 10621069. (5) Ohrbach, K.-H.; Matuschek, G.; Kettrup, A. Thermochim. Acta 1987, 112, 107-110. (6) Kettrup, A.; Ohrbach, K.-H.; Matuschek, G.; Joachim, A. Thermochim. Acta 1990, 166, 41-52. (7) Wunsch, P.; Matuschek, G.; Kettrup, A. Thermochim. Acta 1995, 263, 95100. (8) Herrera, M.; Wilhelm, M.; Matuschek, G.; Kettrup, A. J. Anal. Pyrolysis 2001, 58-59, 173-188. (9) Post, E.; Rahner, S.; Mohler, H.; Rager, A. Thermochim. Acta 1995, 263, 1-6. (10) Matuschek, G.; Stoffers, H.; Ohrbach, K.-H.; Kettrup, A. Thermochim. Acta 1994, 234, 127-137. (11) Ohrbach, K.-H.; Matuschek, G.; Kettrup, A.; Joachim, A. Thermochim. Acta 1990, 166, 277-289. (12) Zimmermann, R., Heger, H. J.; Kettrup, A.; Boesl, U. Rapid Commun. Mass Spectrom. 1997, 11, 1095. (13) Zimmermann, R.; Heger, H. J.; Yeretzian, C.; Nagel, H.; Boesl, U. Rapid Commun. Mass Spectrom. 1996, 10, 1980. (14) Gittins, C. M.; Castaldi, M. J.; Senkan, S. M.; Rohlfing, E. A. Anal. Chem. 1997, 69, 286-293. (15) Williams, B. A.; Tanada, T. N.; Cool, T. A. 24th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992; p 1587. (16) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46-57. (17) Pallix, J. B.; Schuhle, U.; Becker, C. H.; Huestis, D. L. Anal. Chem. 1989, 61, 805-811. (18) Muhlberger, F.; Zimmermann, R.; Kettrup, A. Anal. Chem. 2001, 73, 35903604.
5640
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003
addition to the REMPI technique, the SPI method allows the detection of aromatic and aliphatic hydrocarbons. The softness of both ionization techniques (REMPI and SPI) is due to the low excess energy, i.e., the energy difference between the ionization potential and the energy supplied in the ionization process. Therefore, no fragmentation of organic molecules occurs and the mass spectra solely reflect the distribution of molecular masses in the sample. Thus, combining both the SPI and REMPI techniques, a comprehensive, overall information on the organic composition of the gas mixtures could be obtained.18 This is a prerequisite for real-time monitoring of the organic chemical signature of combustion and pyrolysis processes. In the framework of this study, organic pollutants emitted from the combustion and pyrolysis of different oils occurring in the steel recycling process were investigated by the on-line (quasi-) simultaneous SPI/REMPI-TOFMS method. Recycling of steel by melting in an electric arc furnace usually results in high emissions of organic pollutants, as waste steels are coated with residues of oils, plastics, and paints. These environmentally harmful organic compounds can be produced by reaction, reformation, and pyrolysis. In the growing, developing Asian economies, e.g., in China, these emissions need to be regulated in the future. Thus, investigation of the pyrolysis and combustion behavior of these organic materials is highly significant for the optimization of the furnaces’ operation parameters in order to reduce the emission of these species. Five organic samples collected from Chinese steel plants were investigated by on-line (quasi-) simultaneous SPI/REMPI-TOFMS. Different temperatures and two carrier gas flows (synthetic air and nitrogen) were adjusted. The results presented in this work demonstrate the applicability of the SPI/REMPI-TOFMS method for studying combustion and pyrolysis processes in the laboratory. The potential of the hyphenation of the technique to TA methods is discussed. EXPERIMENTAL SECTION The schematic diagram (Figure 1A) of the pyrolysis-SPI/ REMPI-TOFMS system illustrates the experimental setup. A photograph of the home-built mobile SPI/REMPI-TOFMS device with data acquisition facilities is shown on the left side of Figure 1B. Fundamental Nd:YAG laser pulses (1064 nm, repetition rate 10 Hz) are used for nonlinear generation of UV and VUV laser pulses for REMPI and SPI ionization, respectively. The principle of SPI/REMPI is described elsewhere;12,13,18,19 therefore, only a brief description is given here. The initial laser beam is frequency tripled, and the resulting 355-nm beam is split into two parts. The first one is used to pump an β-BBO crystal (of a thermally stabilized OPO laser) to generate UV laser pulses for the REMPI ionization. It generates OPO laser pulses of tunable wavelengths in the range from 205 to 2500 nm. Results presented in this work were carried out by adjusting the crystal to provide 266-nm laser pulses, because this wavelength has been proven to be well suited for REMPI ionization of aromatic compounds in a sequential twophoton absorption process via an electronic transition state.16,20 Experiments using other wavelengths are not shown here, because they did not provide additional insights with respect to (19) Zimmermann, R.; Heger, J. H.; Kettrup, A. Fresenius J. Anal. Chem. 1999, 363, 720-730. (20) Boesl, U.; Zimmermann, R.; Weickhardt, C.; Lenoir, D.; Schramm, K.-W.; Kettrup, A.; Schlag, E. W. Chemosphere 1994, 29, 1429-1440.
Figure 1. Schematic diagram (A) and photograph (B) of the pyrolysis-SPI/REMPI-TOFMS system.
the obtained conclusions. The other part of the 355-nm beam is guided into a gas cell filled with xenon, where it is frequency tripled due to a nonlinear polarization effect in the isotrope gas medium.21 Subsequently, the formed VUV beam with a wavelength of 118 nm passes the ionization chamber and is used for the singlephoton ionization of the organic molecules in the gas sample. Since only 0.001% of the 355-nm beam is converted into 118 nm, the remaining 355-nm beam has to be separated from the generated VUV beam. This is achieved by utilization of the different refraction indices with respect to a MgF2 lens and subsequent dumping of the deflected 355-nm beam.18 Two computer-controlled beam blockers are used to select between the REMPI and SPI beams, allowing alternating application of REMPI and SPI with the corresponding frequency of 5 Hz. Both beams are focused curtly beneath the inlet needle of the sampling line by means of appropriate optical elements. The inlet needle is made of stainless steel. Within this heated, hollow needle runs a deactivated fusedsilica capillary of 350-µm i.d. Behind the orifice of the fused-silica capillary in the ion source, an effusive molecular beam is formed. The generated molecular ions of the aromatic and aliphatic hydrocarbons, respectively, are extracted into the flight tube of the reflectron time-of-flight mass spectrometer (Kaessdorf Instruments). The TOF mass spectra are recorded via a transient recorder PC card (Aquiris, 250 MHz, 1 GS/s, 128K). The recorded mass range reached from 0 to 300 m/z. Data processing is done by a LabView (National Instruments)-based home-written software. Quantification of the analyzed compounds is principially possible by applying commercial calibration gases or home-built gas standards. However, in this study, only a qualitative evaluation was performed, because this was sufficient for the purpose of comparing the observed emission behavior when different samples and pyrolysis conditions were applied. Details on the design of the home-built REMPI/SPI-TOFMS device (which is also able to perform electron impact ionization in the time span between two consecutive SPI/REMPI shots using an electron gun with a repetition rate of 20 kHz) will be published elsewhere.22 A rotary furnace (Carbolite GmbH) with an inserted quartz tube (i.d. 10 mm), was used for the combustion and pyrolysis experiments. The samples were placed into the quartz tube, and the furnace temperature was set to 300, 500, 700, 800, 1000, and (21) Zimmermann, R.; Heger, H. J.; Kettrup, A.; Muhlberger, F.; Hafner, K.; Boesl, U. Patent application, Deutsches Patentamt, Germany, 2000.
1190 °C, respectively. A flow rate of 1 L/min of either synthetic air (80% nitrogen, 20% oxygen) or nitrogen was provided, leading to a residence time of ∼3 s. A quartz fiber filter was placed at the outlet of the furnace to separate particles from the gas stream. The sampling fused-silica capillary, placed orthogonally to the main gas flow, was used to extract a small portion into the SPI/REMPITOF mass spectrometer as described before. All the connection parts outside of the furnace and the transfer line were constantly heated to 250 °C to prevent condensation and memory effects. Five samples (anticorrosion oil, cooling oil, lubricant, car paint, PVC coating material) typically found on recyclable waste steels were collected from Chinese steel plants. A thin layer of anticorrosion oil is usually coated on steel plates to prevent them from corrosion during storage. Machinery, tools, and engines are often contaminated with lubricant and cooling oil. Paints usually originate from used cars and machines, and PVC materials are normally derived from car decorations and colored steel plates, the latter representing a new and popular building material in China, on which a layer of PVC material is coated. Therefore, these contaminants are transferred to the electric arc furnace along with the recyclable waste steels. The content of the oils in the recycling furnace is in the range of tens of grams per ton of steel, while the PVC content is approximately hundreds of grams, and the content of paints is even several kilograms per ton of steel. The sample size for each measurement was 15 µL for liquid and 15 mg for solid samples. The decomposition of the samples usually took different time spans according to the varying conditions. On average, 80 s after the beginning of the experiment, no pyrolysis products could be observed anymore. Ten single laser shot mass spectra were averaged for the REMPI and SPI spectra, respectively, resulting in corresponding full-range mass spectra every 2 s. RESULTS AND DISCUSSION The three oil samples and the paint sample revealed quite similar characteristics in the combustion and pyrolysis experiments throughout the whole temperature range. Therefore, Figure 2 shows exemplarily the three-dimensional graph acquired during the combustion of cooling oil at 1000 °C in air representing the time to intensity profiles of various pyrolysis products. While smaller compounds such as benzene (78 m/z) and naphthalene (128 m/z) show a fast increase and decrease in concentration, phenanthrene (178 m/z) exhibits a steep concentration increase Analytical Chemistry, Vol. 75, No. 21, November 1, 2003
5641
Figure 2. Three-dimensional graph (mass number-time-abundance) of a REMPI analysis of cooling oil at 1000 °C in air.
at the beginning of the measurement followed by a slightly less steep decrease. The concentration curves of pyrene (202 m/z) and other four-ring PAHs (228 m/z) demonstrate a totally different behavior. The broad emission peak of pyrene lasts ∼20 s with a slight time delay with respect to the smaller aromatic compounds. Even a much broader emission peak of the four-ring PAHs is observed, which shows an additional shift in time compared to the onset of the emission peak of pyrene, revealing the growth of PAHs after the formation of benzene and other intermediates. Benzene acts as precursor for the PAHs, which are formed by acetylene addition and subsequent ring closure reactions.23,24 In addition, PAHs with higher molecular masses could be formed by coagulation of smaller PAHs.25 Typical examples of such intermediates are phenylacetylene (102 m/z) and indene (116 m/z). In the following, time-averaged mass spectra will be discussed. This ensures the comparability of the individual measurements with respect to their different time spans, until no further signal could be detected. Figure 3 depicts exemplarily the REMPI mass spectra of cooling oil for all the investigated temperatures in both air and nitrogen atmosphere. In addition, a headspace measurement of the cooling oil is shown. Considering the samples in air atmosphere (left-hand side of Figure 3), at 300 °C, the alkylated benzenes (106, 120, 134 m/z) are present due to the vaporization of these volatile compounds in the sample (see Figure 3a). This was proved further by comparing the mass spectra at 300 °C with headspace analysis of the sample at room temperature, revealing that these compounds are components of the cooling oil. The alkylated naphthalenes (for all corresponding values of m/z see Table 1) and alkylated phenanthrenes or anthracenes dominated the products of the pyrolysis at this temperature. (Mass 178 m/z corresponds to the (22) Mu ¨ hlberger, F.; Hafner, K.; Ferge, T.; Zimmermann, R., in preparation. (23) Wang, H.; Frenklach, M. Combust. Flame 1997, 110, 173-221. (24) Bittner, J. D.; Howard, J. B. Eighteenth Symposium (International) on Combustion, The combustion Institute, Pittsburgh, PA, 1981; pp 11051116. (25) Siegmann, K.; Hepp, H.; Sattler, K. Combust. Sci. Technol. 1995, 109, 165181.
5642
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003
isobaric compounds phenanthrene and anthracene. However, note that in combustion and pyrolysis products phenanthrene is usually the preponderant compound and almost no anthracene would contribute to the signal.16 Therefore, in the following, 178 m/z is referred to as phenanthrene alone.) In contrast, nonalkylated PAHs are hardly visible. Nevertheless, by increasing the temperature to 500 °C, homologue rows of alkylated acenaphthenes and indenes (as well as phenol and its alkylated derivatives) were formed (see Figure 3b). Homologue rows of alkylated phenanthrenes, pyrenes, and other four-ring PAHs, e.g., chrysene, were the main compounds formed at 700 °C. In addition, even homologue rows of alkylated five-ring PAHs were observed. A minor amount of alkylated styrenes or indanes occurred, while alkylated indenes and phenols still appeared (Figure 3c). Naphthalene (128 m/z) was represented by a peak in significant higher intensity compared to that at 300 and 500 °C. Generally, up to a temperature of 700 °C, the mass spectra were dominated by high alkylated PAHs with masses larger than 180 m/z. By increasing the temperature to 800 °C, the situation changes. PAHs with less degree of alkylation prevailed, whereas the high alkylated ones could not be observed anymore (see Figure 3d). Naphthalene and methylnaphthalene (142 m/z), acenaphthene (154 m/z) and methylacenaphthene (168 m/z), phenanthrene (178 m/z), and its alkylated derivatives predominated the mass spectra, and out of the oxygen-containing compounds, only phenol and the isomeric cresoles (94 and 108 m/z) survived. This trend to form more stable, less alkylated PAHs with the increase of temperature was even more pronounced at 1000 (Figure 3e) and 1190 °C (Figure 3f), respectively. Benzene, naphthalene, phenanthrene, pyrene, and other four- and five (252 m/z)-ring PAHs were the most abundant compounds, while almost none of their alkylated derivatives as well as no phenols were found anymore at both temperatures. This indicates more complete combustion with the increase of temperature up to 1190 °C. Comparing the corresponding mass spectra of nitrogen and air atmosphere, respectively, it was found that the combustion patterns at 300, 500, 1000, and 1190 °C are quite similar. However, more alkylated aromatic compounds were formed during the pyrolysis process at 700 and 800 °C in nitrogen atmosphere than in air. Furthermore, from 500 to 800 °C, in nitrogen, unlike in air, the alkylated benzenes were visible in the mass spectra. The homologue row of alkylated naphthalenes showed a bigger increase in concentration from 500 to 700 °C (Figure 3b,c) in nitrogen compared to air. The same holds for the homologue rows of alkylated indenes and alkylated phenanthrenes. This reveals that, in nitrogen, PAHs with a high degree of alkylation are preferably formed (Figure 3c) with respect to the measurement in air atmosphere. Moreover, the number of PAHs produced in the pyrolysis process is higher in nitrogen atmosphere, whereby most PAHs were formed at 700 °C. These trends can also be observed at 800 °C, although the amount of PAHs with masses larger than 200 m/z have been considerably reduced in nitrogen, too. Nevertheless, pyrolysis temperatures of 1000 and 1190 °C, respectively, resulted in similar peak patterns for both atmospheres. At these high temperatures, only the most stable compounds survived, i.e., the PAHs reduced to their skeletal structure. No phenol and phenol derivatives were found through-
Figure 3. REMPI mass spectra of cooling oil at (a) 300, (b) 500, (c) 700, (d) 800, (e) 1000, and (f) 1190 °C in both air and nitrogen and headspace (on the top). Table 1. Listing of All Corresponding Values of m/z m/z
compound
m/z
compound
m/z
compound
78 92 94 104 106 108 116 118 120 122 128 130 132 134 136 142 144 146
benzene toluene phenol styrene xylenes cresoles indene methylstyrene C3-alkylbenzene C2-alkylphenol naphthalene methylindene C2-alkylstyrene C4-alkylbenzene C3-alkyl phenol methylnaphthalene C2-alkylindene C3-alkylstyrene
154 156 158 160 168 170 172 174 178 182 184 186 192 196 198 202 206 210
acenaphthene C2-alkylnaphthalene C3-alkylindene C4-alkylstyrene methylacenaphthene C3-alkylnaphthalene C4-alkylindene C5-alkylstyrene phenanthrene C2-alkylacenaphthene C4-alkylnaphthalene C5-alkylindene methylphenanthrene C3-alkylacenaphthene C5-alkylnaphthalene pyrene C2-alkylphenanthrene C4-alkylacenaphthene
212 216 220 226 228 230 234 240 242 244 248 252 254 256 266 270 282
C6-alkylnaphthalene methylpyrene C3-alkylphenanthrene C7-alkylnaphthalene 4-ring PAH C2-alkylpyrene C4-alkylphenanthrene C8-alkylnaphthalene methyl 4-ring PAH C3-alkylpyrene C5-alkylphenanthrene 5-ring PAH C9-alkylnaphthalene C2-alkyl 4-ring PAH methyl 5-ring PAH C3-alkyl 4-ring PAH C2-alkyl 5-ring PAH
out the whole temperature range in nitrogen, which reveals the necessity of the presence of oxygen for the formation of phenols during the pyrolysis of the cooling oil. Consideration of mass numbers 170 (C3-alkylnaphthalene) and 128 (naphthalene) as examples for alkylated and nonalkylated PAHs, respectively, allows a closer look at the different behavior of these compound classes. Figure 4 shows their temperaturenormalized signal intensity profiles. From 300 to 1190 °C, a gradual decrease of the concentration of C3-alkylnaphthalene could be observed in air (Figure 4a). However, in nitrogen, a slight decrease of its concentration between 300 and 700 °C is followed by a sharp decline from 700 to 1000 °C (Figure 4b). In the temperature range from 1000 to 1190 °C, C3-alkylnaphthalene was hardly visible in
both atmospheres. On the other hand, naphthalene reached the highest concentration at 1190 °C, whereas it could be rarely found below 500 °C in both air and nitrogen. The concentration of naphthalene increased strongly from 500 to 700 °C in air, while a gradual rise of the concentration occurred in the same temperature range in nitrogen. The figure exhibits the decrease of the concentration of alkylated PAHs and increase of the concentration of nonalkylated PAHs with the increase of temperature. Despite the fact that this trend holds for both atmospheres, the curve progression for both compounds is slightly different in nitrogen compared to air. Especially for 500, 700, and 800 °C, the concentration of C3-alkylnaphthalene is higher in nitrogen. An explanation for this behavior is the formation of oxygen-containing Analytical Chemistry, Vol. 75, No. 21, November 1, 2003
5643
Figure 4. Temperature profiles of mass number 128 and 170 m/z in air (a) and nitrogen (b) measured by REMPI.
radicals in air, which leads generally to a faster decomposition of alkylated aromatics. This process is accelerated with increasing temperature, leading to an increase in concentration of the thermodynamically more stable compounds without any alkyl side chains. To compare the pyrolysis behavior of all samples, the timeaveraged REMPI mass spectra of cooling oil, anticorrosion oil, lubricant, car paint, and PVC coating material, acquired at 700 °C in either air or nitrogen atmosphere is presented in Figure 5. Basically, the three oil samples show quite similar pyrolysis patterns (Figure 5a-c): in nitrogen; more alkylation of PAHs happened and larger PAHs were formed. In contrast, phenol and its alkylated derivatives were produced in air rather than in nitrogen. However, some exceptions were observed. Phenol was found during the pyrolysis of car paint in both air and nitrogen (Figure 5d). The reason is that the paint is mainly composed of polyester, in which 25.6% (w/w) of oxygen content was determined by elemental analysis. This high concentration of oxygen in the sample can produce many radicals such as •O, •OH, and •O2H during the pyrolysis at high temperature even in a nitrogen atmosphere, which are the major reactants for the formation of phenol.26 In addition to phenol, homologue rows of styrene or indane and their respective alkylated derivatives, as well as alkylated naphthalenes, dominated the products of pyrolysis in both air and nitrogen, revealing that the high oxygen content in the sample decreases the formation rate of larger PAHs containing more rings. Unlike the other samples, PVC demonstrated a different pyrolysis behavior in some aspect, as a large amount of benzene was generated (see Figure 5e). It has to be considered that the sensitivity of REMPI at 266 nm for benzene is relatively low, which will be discussed in more detail later on. The high benzene content can be explained by cyclization reactions after the loss of HCl from PVC and the formation of acetylene.23 Moreover, naphthalene, acenaphthene, fluorene, phenanthrene, (26) Hucknell, D. J. Chemistry of Hydrocarbon Combustion; Chapman and Hall: London, 1985.
5644
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003
and their alkylated derivatives predominated the pyrolysis products. The results of the PVC pyrolysis experiments including evaluation of chlorine-containing species will be discussed in more detail in a future publication. SPI mass spectra provide additional information for the explanation of pyrolysis behavior. Figure 6 shows exemplarily time-averaged SPI mass spectra of cooling oil in air at different temperatures. The concentration of benzene largely increased with the increase of temperature from 300 to 1190 °C, a tendency that could not be reflected in such detail in REMPI mode. In the temperature range from 700 to 1000 °C, aliphatic compounds such as propyne (40 m/z), propene (42 m/z), butyne (54 m/z), butene (56 m/z), cyclopentadiene (66 m/z), pentyne or pentadiene (68 m/z), and pentene (70 m/z) were formed. These species could act as precursors of benzene and PAHs,22,23 which were also observed in SPI mode (Figure 6b,c). In particular, cyclopentadiene could play a very important role in the growth of PAHs.27 When the temperature was raised to 1190 °C, no aliphatic compounds were observed due to their oxidative decomposition at this temperature. Only stable PAHs such as benzene, naphthalene, acenaphthylene, phenanthrene, pyrene, and other four-ring PAHs survived the combustion process (Figure 6d), which is in good agreement with the REMPI results. Comparing the corresponding mass spectra of cooling oil in air, which are obtained by REMPI and SPI ionization, respectively, some differences are striking. First, the benzene peaks in the SPI mass spectra are rather high compared to the REMPI mass spectra. The wavelength chosen for the here presented REMPI measurements was 266 nm, which is known to be very appropriate for the detection of trace amounts of PAHs. However, the ionization efficiency for benzene is relatively low at this wavelength.28 This limitation could be overcome by tuning the OPO laser to a wavelength that is in resonance with an excited electronic state of the benzene molecule, e.g. 258,9 nm.28 Nevertheless, for the purpose of this work it was sufficient to perform the analysis of benzene by SPI. Moreover, the high sensitivity of REMPI at 266 nm with respect to PAHs was required, because the concentrations of these compounds are very low. This can also be noticed by looking at the SPI mass spectra, as the signals for the PAHs are hardly visible. In contrast, aliphatic species are only detectable with SPI, because the excited states of these molecules needed for the REMPI process are not within the laser wavelength range. Thus, both ionization techniques complement one another, yielding a comprehensive overview of the chemical signature of the investigated samples. In summary, the combined application of the soft REMPI and SPI ionization techniques with TOFMS provides comprehensive information of organic compounds formed by combustion and pyrolysis of different samples. In the temperature range from 300 to 700 °C, the alkylated PAHs are the main products for oil and paint samples, while the less alkylated PAHs dominate the gas phase at 800 °C. The tendency to form stable PAH molecules without side chains is observed at temperatures above 800 °C. The time to intensity profile exhibits the subsequent growth of (27) Marinov, N. M.; Pitz, W. J.; Westbrook, C. K.; Castaldi, M. J.; Senkan, S. M. Combust. Sci. Technol. 1996, 116-117, 211-287. (28) Boesl, U.; Heger, H.-J.; Zimmermann, R.; Nagel, H.; Pu ¨ffel, P. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley & Sons Ltd.: Chichester, 2000; pp 2087.
Figure 5. REMPI mass spectra of (a) cooling oil, (b) anti corrosion oil, (c) lubricant, (d) car paint, and (e) PVC at 700 °C in air and nitrogen.
Figure 6. SPI mass spectra of cooling oil at (a) 300, (b) 700, (c) 1000, and (d) 1190 °C in air.
PAHs after the initial formation of benzene and other intermediates. Some of the precursors of benzene and PAHs were detected by SPI mode, which provides additional information for the interpretation of the formation and growth of PAHs. The incomplete combustion most likely takes place in the temperature range from 700 to 800 °C. Varieties of PAHs formed during the combustion and pyrolysis process were largely reduced above 1000 °C, and only a few PAHs without branch groups can resist
these high temperatures. This reflects a more complete combustion at high temperatures. The trend to form phenol in air by increasing the combustion temperature up to 700 °C was also shown. However, no phenol was formed in nitrogen with the exception of car paint, which is due to its original high content of oxygen. A more detailed discussion of the combustion and pyrolysis behavior of the materials will be given in a future publication in a combustion-related journal. The implication of the herepresented work is to show the potential of soft and selective laser ionization mass spectrometry methods in combination with pyrolysis experiments. Future work needs to address coupling of the SPI/REMPI-TOFMS technique to TA, differential TA, and other thermoanalytical methods. In doing so, information on the chemical reactions, phase transformations, and structural changes of complex mixtures could be obtained and the corresponding pattern of aliphatic and aromatic hydrocarbons could be monitored on a real-time basis. Comprehensive mass spectra could map the chemical signature of materials during the pyrolysis process as a function of temperature. Thus, the dynamic processes occurring as the pyrolysis proceeds could be followed, and even rapid changes of the pattern of organic compounds could be observed. Received for review August 4, 2003. Accepted September 4, 2003. AC0349025
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003
5645
