Anal. Chem. 2008, 80, 3393-3403
Thermogravimetry Coupled to Single Photon Ionization Quadrupole Mass Spectrometry: A Tool To Investigate the Chemical Signature of Thermal Decomposition of Polymeric Materials M. Saraji-Bozorgzad,† R. Geissler,† T. Streibel,*,†,‡ F. Mu 1 hlberger,† M. Sklorz,† E. Kaisersberger,§ § .†,‡,| T. Denner, and R. Zimmermann*
Helmholtz Zentrum Mu¨nchen, Institut fu¨r O ¨ kologische Chemie, Deutsches Forschungszentrum fu¨r Gesundheit und Umwelt (GmbH), D-85764 Neuherberg, Germany, Netzsch-Gera¨tebau GmbH, D-95100 Selb, Germany, Institut fu¨r Physik, Universita¨t Augsburg, D-86159 Augsburg, Germany, and BIfA GmbH, D-86167 Augsburg, Germany
Mass spectrometry (MS) is an established analytical technique to analyze evolved gas in thermogravimetry (TG). In this study, for the first time a novel SPI-MS technique using an electron beam pumped VUV excimer lamp as photon source (λ ) 126 nm) was employed in conjunction with thermogravimetry. The coupling was achieved with an improved heated interface and adjacent transfer capillary between TG and ion source of a quadrupole mass spectrometer. The feasibility of this approach was proven by investigating semivolatile substances such as long-chain alkanes (heptadecane C17H36), polymers, e.g., polystyrene, polycarbonate, and acrylonitrile-butadiene-styrene, polymer mixtures and blends. Mass spectra with almost no fragmentation were obtained, and quantification of selected substances could be achieved. Polymer mixtures could be distinguished by their SPI mass spectra, and the effect of premixing of polymers has been accessed. Its unique attributes render the TA-SPIMS method a promising new tool for quantitative and qualitative evaluation of complex organic thermal degradation products. In most fields of material research and industrial process controlling, especially polymer and polymer design industry, thermal analysis (TA) is used to characterize temperaturedependent material properties, evaluate thermodynamical conversions and thermo-physical parameters, as well as to observe chemical reactions. In addition to the differential methods, differential thermal analysis1 and differential scanning calorimetry,2-7 which are mainly used to increase the sensitivity of the quantitative * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. † Helmholtz Zentrum Mu ¨ nchen. ‡ Universita ¨t Augsburg. § Netzsch-Gera ¨tebau GmbH. | BIfA GmbH. (1) Fatu, D.; Geambas, G.; Segal, E.; Budrugeac, P.; Ciutacu, S. Thermochim. Acta 1989, 149, 181-187. (2) Sourour, S.; Kamal, M. R. Thermochim. Acta 1976, 14, 41-59. (3) Asaletha, R.; Kumaran, M. G.; Thomas, S. Polym. Degrad. Stab. 1998, 61, 431-439. (4) Flammersheim, H. J.; Opfermann, J. Thermochim. Acta 1999, 337, 141148. 10.1021/ac702599y CCC: $40.75 Published on Web 04/08/2008
© 2008 American Chemical Society
acquisition of caloric changes, Thermogravimetry (TG) has achieved a particular importance for the investigation of thermal decomposition.8-10 However, for more advanced applications, a chemical analysis of the evolved gases is required. This can be done either by coupling of TG to a sequentially working analytical device such as a gas chromatograph, with or without mass spectrometric detector, (TG-GC)11-13 or (TG-GC/MS)14-17 or by coupling of on-line analytical technology. Examples for the latter are thermogravimetry - Fourier-transform infrared spectroscopy18 for analysis of organic main products and thermogravimetry-mass spectrometry (TG-MS).19-24 Thereby, the coupling of MS as fast on-line gas analyzer with TA methods, predominately with TG, represents a well-established analytical method. (5) Covolan, V. L.; Fernandes, E. G.; D’Antone, S.; Chiellini, E. Thermochim. Acta 1999, 342, 97-103. (6) Li, Y.; Fan, Y.; Ma, J. Polym. Degrad. Stab. 2001, 73, 163-167. (7) Stack, S.; O’Donoghue, O.; Birkinshaw, C. Polym. Degrad. Stab. 2003, 79, 29-36. (8) di Cortemiglia, M. P. L.; Camino, G.; Costa, L.; Guaita, M. Thermochim. Acta 1985, 93, 187-190. (9) Faravelli, T.; Bozzano, G.; Scassa, C.; Perego, M.; Fabini, S.; Ranzi, E.; Dente, M. J. Anal. Appl. Pyrolysis 1999, 52, 87-103. (10) Yang, J.; Miranda, R.; Roy, C. Polym. Degrad. Stab. 2001, 73, 455-461. (11) Chiu, J. Anal. Chem. 1968, 40, 1516-1520. (12) Tsuge, S.; Sugimura, Y.; Nagaya, T. J. Anal. Appl. Pyrolysis 1980, 1, 221229. (13) Costa, L.; Camino, G.; Trossarelli, L. J. Anal. Appl. Pyrolysis 1985, 8, 1524. (14) Day, M.; Cooney, J. D.; Touchette-Barrette, C.; Sheehan, S. E. J. Anal. Appl. Pyrolysis 1999, 52, 199-224. (15) Jakab, E.; Blazso, M. J. Anal. Appl. Pyrolysis 2002, 64, 263-277. (16) Herrera, M.; Matuschek, G.; Kettrup, A. J. Anal. Appl. Pyrolysis 2003, 70, 35-42. (17) Bozi, J.; Czegeny, Z.; Meszaros, E.; Blazso, M. J. Anal. Appl. Pyrolysis 2007, 79, 337-345. PYROLYSIS 2006: Papers presented at the 17th International Symposium on Analytical and Applied Pyrolysis, Budapest, Hungary, 2226 May 2006 (18) Herrera, M.; Wilhelm, M.; Matuschek, G.; Kettrup, A. J. Anal. Appl. Pyrolysis 2001, 58-59, 173-188. (19) Chiu, J.; Beattie, A. J. Thermochim. Acta 1980, 40, 251-259. (20) Chiu, J.; Beattie, A. J. Thermochim. Acta 1981, 50, 49-56. (21) Jones, E. G.; D. L. P.; Goldfarb, I. J. Polym. Eng. Sci. 1988, 28, 1046-1051. (22) Behrens, R. J. Rev. Sci. Instrum. 1987, 58, 451-461. (23) Raemakers, K. G. H.; Bart, J. C. J. Thermochim. Acta Coupling Therm. Anal. Gas Anal. Methods 1997, 295, 1-58. (24) Maciejewski, M.; Baiker, A. Thermochim. Acta Coupling Ther. Anal. Gas Anal. Methods 1997, 295, 95-105.
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The coupling interface between a thermobalance and a mass spectrometer has two main functions:25(1) efficient transfer of a representative part of the evolved gases from the thermobalance to the MS; (2) pressure reduction between the TG system and the mass spectrometer The conditions for the gas flow in coupling interfaces, e.g., pressure, temperature, and flow rate, heavily influence the results in a coupled gas analyzer. Loss of semivolatile compounds by condensation at cold spots, low detection sensitivity because of dilution with the purge gas, and low time and temperature resolution because of long transfer times and mixing with the purge gas, by diffusion and by uncontrolled flow conditions, can appear and should be avoided. The mass spectrometry (MS) method commonly used in conjunction with TG is quadrupole mass spectrometry (QMS) with electron impact ionization (EI). The coupling between the TG oven and the mass spectrometer is realized via either a transfer capillary25 or a skimmed supersonic expansion.26 Although MS is a very powerful analytical technique for molecular analysis, the currently available TA-MS systems have problems in showing the composition of the organic trace species in the evolved gases.15 For on-line detection of compounds in a complex gas mixture using a mass spectrometer, the target compounds should preferably not fragment due to the ionization step. As the overlapping fragmentation of multiple organic molecules precludes the identification of the original products, the almost exclusively used EI in the already available measuring devices involves a considerable drawback for the on-line analysis of organic molecules in gaseous mixtures containing dozens of substances. Therefore, to detect these molecules with a low degree of fragmentation, a soft ionization method, e.g., chemical ionization,27,28 metastable atom bombardment,29-32 resonance-enhanced multiphoton ionization (REMPI),33,34 and single photon ionization (SPI)35,36 becomes essential. First thermal desorption studies, pyrolysis studies, and TG studies using laser-based soft ionization mass spectrometry as detector, revealed that highly valuable molecular information on the thermal decomposition processes can be achieved.37-40 The generation of VUV photons for SPI can be realized by frequency tripling of intensive 355-nm third (25) Kaisersberger, E.; Post, E. Thermochim. Acta Coupling Therm. Anal. Gas Anal. Methods 1997, 295, 73-93. (26) Kaisersberger, E.; Post, E. Thermochim. Acta 1998, 324, 197-201. (27) Lindinger, W.; Hirber, J.; Paretzke, H. Int. J. Mass Spectrom. Ion Processes 1993, 129, 79-88. (28) Lindinger, W.; Hansel, A.; Jordan, A. Chem. Soc. Rev. 1998, 27, 347-354. (29) Faubert, D.; Paul, G. J. C.; Giroux, J.; Bertrand, M. J. Int. J. Mass Spectrom. Ion Processes 1993, 124, 69-77. (30) Moore, S. Chemosphere 2002, 49, 121-125. (31) Boutin, M.; Lesage, J.; Ostiguy, C.; Bertrand, M. J. J. Anal. Appl. Pyrolysis 2003, 70, 505-517. (32) Boutin, M.; Lesage, J.; Ostiguy, C.; Bertrand, M. J. J. Am. Soc. Mass Spectrom. 2004, 15, 1315-1319. (33) Boesl, U.; Neusser, H. J.; Schlag, E. W. Chem. Phys. 1981, 55, 193-204. (34) Streibel, T.; Hafner, K.; Mu ¨ hlberger, F.; Adam, T.; Zimmermann, R. Appl. Spectrosc. 2006, 60, 72-79. (35) Burtscher, H.; Schmidt-Ott, A.; Siegmann, H. C. Aerosol Sci. Technol. 1988, 8, 125-132. (36) Mitschke, S.; Adam, T.; Streibel, T.; Baker, R. R.; Zimmermann, R. Anal. Chem. 2005, 77, 2288-2296. (37) Zoller, D. L.; Sum, S. T.; Johnston, M. V. Anal. Chem. 1999, 71, 866-872. (38) Zoller, D. L.; Johnston, M. V.; Tomic, J.; Wang, X.; Calkins, W. H. Energy Fuels 1999, 13, 1097-1104. (39) Adam, T.; Mu ¨ hlberger, F.; Mitschke, S., Norfolk, VA, 2003. (40) Adam, T.; Streibel, T.; Mitschke, S.; Mu ¨ hlberger, F.; Baker, R. R.; Zimmermann, R. J. Anal. Appl. Pyrolysis 2005, 74, 454-464.
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harmonic Nd:YAG UV pulses in a rare gas cell,41-44 providing VUV light pulses with a time width of several nanoseconds at λ ) 118 nm. Laser-based instrumentation often involves big drawbacks such as high costs and complexity of the laser devices. To reduce this disadvantage, VUV lamps such as deuterium or krypton discharge lamps can be an attractive option.45,46 However, such conventional VUV lamps consist in the case of deuterium lamps of a weak broadband continuum beside the narrow Lyman R atomic line at 121.567 nm. Generally, these lamps have a rather low spectral intensity in the required spectral range and a low optical brilliance. High photon densities and optical brilliance were reported in the past, using an electron beam pumped excimer VUV lamp (EBEL) as photon source.47,48 The working principles of the EBEL and the EBEL-QMS system were previously described in the literature47,49 and will be illustrated in Instrumentation and Experimental Setup. Many different applications using EBEL for SPI-MS were achieved and reported in the past.47,49,50 Mu¨hlberger et al. reported about measurements of petrochemical samples, e.g., petrol and diesel as well as on-line monitoring of automotive exhaust gases with an EBEL-QMS system. For highly time-resolved monitoring applications such as the puff-resolved monitoring of mainstream cigarette smoke or inhalation cycleresolved human breath analysis, a single photon ionization - timeof-flight mass spectrometer (SPI-TOFMS) was applied using an EBEL as photon source.48 INSTRUMENTATION AND EXPERIMENTAL SETUP For the thermal degradation and decomposition studies, a STA 409 PG Luxx thermobalance (Netzsch Geraetebau GmbH, Selb, Germany) was applied. The samples, heptadecane and several polymers listed in Table 1 were heated in an Al2O3 sample pan from ambient temperature of the furnace (40 °C) to 1000 °C at a heating rate of 10 K min-1 in an N2 atmosphere. The gas flow through the STA was set to 60 mL min-1 during all measurements. A self-made EBEL was connected to a compact QMS system (QMS Prisma 200, 6-mm rod system, Pfeiffer, Germany) using a cube as recipient as presented schematically in Figure 1A. The main innovation of the EBEL-VUV light source is a 0.7 × 0.7 mm2 ceramic silicon nitride (SiNx) foil of only 300-nm thickness that separates the rare gas volume (p > 1 bar) from a vacuum chamber containing an electron gun (EG). The EG generates a 13-keV electron beam, which is shot into the rare gas through the SiNx foil with low energy loss. The energetic electrons excite and ionize the rare gas atoms. In successive processes, short-lived excited diatomic rare gas molecules (excited dimers/excimers) with a (41) Maker, P. D.; Terhune, R. W. Phys. Rev. 1965, 137 (3A), 801-818. (42) Bjorklund, G. C. IEEE J. Quantum Electron. 1975, QE-11 (6), 287-296. (43) Vidal, C. R. In Tunable Lasers; Mollenauer, L. F., White, J. C., Eds.; SpringerVerlag: Berlin, 1987, Vol. 59, pp 56-113. (44) Mu ¨ hlberger, F.; Zimmermann, R.; Kettrup, A. Anal. Chem. 2001, 73, 35903604. (45) Arii, T.; Otake, S.; Takata, Y.; Matsuura, S. J. Mass Spectrom. Soc.Jpn. 2006, 54, 243-249. (46) Arii, T.; Otake, S. J. Therm. Anal. Calorim. 2007, 91, 419-426. (47) Muhlberger, F.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2002, 74, 3790-3801. (48) Muhlberger, F.; Saraji-Bozorgzad, M.; Gonin, M.; Fuhrer, K.; Zimmermann, R. Anal. Chem. 2007, 79, 8118-8124. (49) Mu ¨ hlberger, F.; Wieser, J.; Morozov, A.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 2218-2226. (50) Mu ¨ hlberger, F.; Streibel, T.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 7408-7414.
Table 1. Labeling of the Applied Polymer Samples sample
polymer
PS PC ABS
polystyrene polycarbonate acrylonitrile-butadiene-styrene
PCABS
commercial PC ABS blend
PC/ABS
50/50% PC and ABS blend
PC&ABS
50/50% PC and ABS heated in different samples pans
monomer(s) styrene acrylonitrile butadiene styrene acrylonitrile butadiene styrene acrylonitrile butadiene styrene acrylonitrile butadiene styrene
trade mark
av sample wt, mg
PS 143E0020 PC0018 Lexan144 14R/11 ABS 011 P2MC Novodur
19 27 28
PC0006 Bayblend T45 PG
26 28.9 14.6 ABS 14.3 PC 24.4 12.4 ABS 12 PC
bond length rex are formed.51 The excimers decay in two free rare gas atoms under emission of the difference energy as a photon. In detail, the photon energy corresponds to the difference of the lowest bound state of the excimer and the energy of two free rare gas atoms with a distance of rex from each other.52,53 The energy difference ∆E, however, varies for excimers formed from different rare gases, with heavier rare gases exhibiting lower ∆E (i.e., longer wavelengths) and lighter rare gases exhibiting larger ∆E (i.e., shorter wavelengths). By using high gas pressure, it is possible to make the excimer formation occur in a small volume in the proximity of the electron entrance foil. That leads to high brilliance of this light source.54 For all measurements presented here, the EBEL was filled with argon (gas purity 5.0) to a pressure of ∼2 bar. Due to the connection of a gas getter (Saes Getters Group, Milano, Italy) to the gas cell, the argon purity is higher than 99.9990% during operation of the lamp (emission maximum: 126 nm, bandwidth: 9 nm, 9.8 eV center photon energy). A specific interface has been developed for combining the VUV excimer lamp with the compact QMS as described by Mu¨hlberger et al. in detail.49 It consists of two equal spherical MgF2 lenses (planoconvex, r ) 29.7 mm, f ) 49.42 mm121 nm, i.d. 1 in., Korth Kristalle GmbH, Altenholz, Germany) forming a 1:1 imaging system. The first lens is located at a distance of 46 mm from the light emitting volume and is mounted vacuum tight to act as exit window between gas cell and MS vacuum. It collects ∼1-2% of the isotropically emitted VUV light and forms a parallel beam. The second lens, mounted closely behind the first, focuses the light in the center of the cross-beam ion source with a focal diameter of 3 mm. For measuring the ion currents, the QMS system is equipped with a secondary electron multiplier detector. The QMS can be operated either in the scan mode, to scan through an m/z range, or in the multi-ion detection (MID) mode, to monitor the ion current of selected m/z values. The original EI cross-beam ion source of the QMS in which the light beam crosses the sample gas stream orthogonally was modified by removing one of its filaments. The sample inlet system consists of a heated hollow stainless steel needle (o.d. ) 1 mm) and is similar to a capillary(51) Wieser, J.; Murnick, D. E.; Ulrich, A.; Huggins, H. A.; Liddle, A.; Brown, W. L. Rev. Sci. Instrum. 1997, 68, 1360-1364. (52) El-Habachi, A.; Schoenbach, K. H. Appl. Phys. Lett. 1998, 72, 22-24. (53) El-Habachi, A.; Schoenbach, K. H. Appl. Phys. Lett. 1998, 73, 885-887. (54) Ulrich, A.; Wieser, J.; Salvermoser, M.; Murnick, D. Physikalische Bla ¨ tter 2000, 56, 49-52.
Figure 1. (A) Schematic drawing of the TG-EBEL-QMS system. (B) Photograph of the TG-EBEL-QMS system.
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Figure 2. (A) Measured temperature distribution inside the thermobalance at various positions in the gas flow tube at various furnace temperatures. (B) the thermal distribution inside the adapter. (C) A nonuniform temperature distribution along the transfer induces a tailed heptadecane ion signal. (D) The tailing of the heptadecane ion signal could be reduced to an acceptable degree using the new designed hyphenation.
based inlet system described in the literature.55 It ends ∼1 mm in front of the QMS ion source, where the filament has been removed. The heated needle (T ) 250 °C) contains a deactivated fused-silica capillary of 150-µm i.d. The orifice of the fused-silica capillary is aligned with the end of the inlet needle and is located 5 mm above the center of the ion source (i.e., the ionization region). Behind the orifice of the capillary, an effusive molecular beam is formed in the ion source.56 The formed ions are extracted perpendicularly to the light beam and the gas stream into the rod system of the QMS. The whole QMS is evacuated with a 210 Ls-1(N2) turbomolecular pump. The other end of the capillary, which has a total length of 2.2 m, is guided through a heated vacuum sealing and a heated transfer line (T ) 250 °C) to the thermobalance. The capillary itself acts as restrictor between the vacuum of the ion source (∼5 × 10-5 mbar) and the ambient pressure in the TA system (∼1 bar). All parts of the excimer lamp QMS system are mounted in a mobile 19-in. rack (59 × 77 × 130 cm, width × length × height) as shown in Figure 1B. The performance of the hyphenation of thermal analysis device and mass spectrometer is crucial for the quality of the measurement results. A nonuniform temperature distribution along the transfer device promotes the formation of so-called cold spots. Already evaporated sample molecules can condense at these cold areas between their point of origin and the analysis zone in the mass spectrometer. Figure 2A demonstrates the measured temperature distribution inside the thermobalance at various positions in the gas flow tube (see colored lines in Figure 2A). There is (55) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46-57. (56) Wegner, P. P., Ed. Molecular Beams and Low Density Gas Dynamics; Marcel Dekker: New York, 1974.
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apparently a difference between the adjusted heating rate of the furnace and the monitored temperature changes in the upper section of the gas flow tube. In particular, temperatures below 100 °C occur over a considerable time span. As a consequence, already evaporated sample material condenses at the gas outlet and volatilizes again when the evaporation temperature at this point has been reached several minutes later. A tailed ion signal therefore occurs as presented in Figure 2C for heptadecane. The new conceptual designed hyphenation device is made of aluminum and has a copper core. Due to the excellent thermal conductivity of copper and aluminum, the thermal distribution inside the adapter is almost constant above 200 °C as pictured in Figure 2B. Measurements of heptadecane with the new device show that the tailing of the ion signal could be reduced to an acceptable degree (Figure 2D). The remaining delay is basically caused by the transport time of the volatiles from the TG to the MS. RESULTS AND DISCUSSION In the following, first the obtained analytical figures of merit, e.g., mass resolution, dynamic range of the TA-EBEL-QMS, yield of the mass discrimination, and the limit of detection for toluene, are presented. Subsequently, first results of the new TA-EBELQMS system are presented. In detail, the thermal decomposition of polymers was exemplarily investigated. Analytical Figures of Merit. (a) Mass Resolution, and Dynamic Range. Quadrupole analyzers are generally operated at so-called “unit resolution”. The resolution is basically limited by the mechanical accuracy with which the rods are constructed
and supported.57 The maximum reachable resolution is mostly expressed by the equation R ) mk, where k is a constant, typically in the range of 2, 3, or 4, and m is the relevant mass. The QMS 200 Prisma reaches a resolution of 2 m50%valley (50% valley definition). The dynamic range of the here presented TA-EBEL-MS is linear in the range of 3 orders magnitude, operating the QMS in scan mode with a scan time of 20 (m/z) s-1. The dynamic range could be extended when using the MID mode for longer scan periods. Mitschke et al. and Tonokura et al. reported about similar dynamic ranges for SPI-MS using a time-of-flight mass spectrometer (VUV/SPI-TOFMS).36,58 Using REMPI-TOFMS, even dynamic ranges of 6 orders of magnitude for benzene59 were reported by Oser et al. (b) Correction for Mass Discrimination. Generally, for the quadrupole mass spectrometer, the mass discrimination effect (peak height variability as a function of mass) is inevitable and is not a strictly linear function with the mass number. As pointed out by Masuda et al., the mass discrimination raises mostly from the condition of electrode bias supplied for the ion optical system60 and is affected by the response of electron multipliers, which are dependent upon the mass, energy, charge, and chemical nature of the impinging ions.61 In practice, multiplier gain variation with ionic mass and other parameters is usually determined by calibration. This is normally only a minor problem, which is indeed a modest price to pay for the increased sensitivity.61 When accurate quantitative measurements are needed, calibration may become more critical. As this is very important in isotopic work, mass discrimination correction can be done by comparing measurement results and calculating a correction factor.62 Using perfluorotributylamine (FC-43) as calibration substance for EI measurements, a decrease of signal intensity with increasing m/z, compared to NIST63 EI spectra of FC-43 could be observed. The correction for the mass discrimination is inevitable for the quadrupole-type mass spectrometer and was carried out by dividing the normalized measured ion intensities by normalized NIST ion intensities according to the equation
K ) Im/INIST
Figure 3 plots an example of the mass discrimination factor K against m/z measured for FC-43. (c) Detection Limit Measurements. Limits of detection (LODs) of the EBEL-QMS system for different compounds, e.g., benzene, toluene, and styrene, were determined in previous studies.49 Mu¨hlberger et al. operated the EBEL with a continuous 10-µA, 13-keV electron beam. The QMS scanning speed was set to 1 (m/z) s-1. Using these parameters for many compounds, detection limits in the 50 ppb region were found. To characterize (57) Gross, J. H. Mass Spectrometry A Textbook; Springer-Verlag Heidelberg: Heidelberg, 2004. (58) Tonokura, K.; Nakamura, T.; Koshi, M. Anal. Sci. 2003, 19, 1109-1113. (59) Oser, H.; Thanner, R.; Grotheer, H.-H.; Richters, U.; Walter, R.; Merz, A. In Combustion Diagnostics; Tacke, M., Stricker, W., Eds.; 1997; Vol. 3108, pp 21-29. (60) Masuda, A.; Hirata, T.; Shimizu, H. Geochem. J. 1986, 20, 233-239. (61) Roboz, J. Introduction to Mass Spectrometry: Instrumentation and Techniques; John Wiley & Sons, Inc.: New York, 1986. (62) Shimizu, O.; Tanaka, T.; Senda, R. J. Mass Spectrom. Soc. Jpn. 2004, 52, 189-195. (63) NIST, N. I. o. S. a. T., 69 ed., 2005; Vol. 2005.
Figure 3. Plot of mass discrimination factor K vs m/z. Perfluorotributylamine (FC-43) as calibration substance for EI measurements was employed here.
the TA-SPI-QMS system, different detection limits were investigated by changing the VUV lamp intensity at a constant scanning speed of 20 (0.05 s (m/z)-1), 5 (0.2 s (m/z)-1), and 1 (m/z) s-1 (1 s (m/z)-1). The detection limit for the TA-SPI-QMS system was determined according to the method of Williams et al.64 from an SPI mass spectrum of a calibration standard gas containing 10 ppm of benzene, toluene, and p-xylene (BTX), described in Figure 4A. The noise level was determined using the variance σ of the amplitude of the signal between 35 and 75 m/z; the mean value of the noise was defined as baseline B. The variance was compared to the signal peak height S of the target molecule in the mass spectrum. Where c is the concentration of the target molecule (toluene), the LOD is calculated for a signal/noise ratio of 2 according to55
LOD ) 2σc/(S - B) The calculated detection limits for toluene are diagrammed in Figure 4B. As described, higher photon intensities and the decrease of the scanning rate lead to lower detection limits. The lowest approachable detection limits for toluene with the here presented TA-EBEL-QMS system are 191 ( 80 ppb scanning with a rate of 20 (m/z) s-1, 114 ( 4 ppb scanning with a rate of 5 (m/z) s-1, and 61 ( 0.3 ppb scanning with a rate of 1 (m/z) s-1, when operating the EBEL with a 5-µA, 13-keV electron beam. Investigation of Heptadecane by TG-SPI-QMS. (a) Qualitative Considerations. The operability of the TA-SPI-MS for organic compounds was proven with a long-chain alkane and is illustrated in Figure 4C,D. Heptadecane (C17H36) shows a typical fragment pattern (Figure 4C) when ionized by electron impact with a kinetic energy of the electrons of 70 eV. To assign the actual chain length, only the identification of the molecular ion [M]+ can provide the information. The low fragmentation character of the SPI method is demonstrated in the EBEL SPI mass spectrum (Figure 4D). In contrast to the EI mass spectrum, the molecular ion at m/z ) 240 with almost no fragments is depicted. Furthermore, the absence of fragments in the lower m/z region of the SPI mass spectrum leads to the conclusion that thermal fragmentation has not taken place during the measurements. This (64) Williams, B. A.; Tanada, T. N.; Cool, T. A. In 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 1587-1596.
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Figure 4. (A) SPI spectra (scan rate 1 (m/z) s-1) of calibration standard gas containing 10 ppm benzene, toluene and p-xylene (BTX). (B) The calculated detection limits for toluene for different scan rates vs e-gun intensity of the EBEL. (C) EI (Eelectron ) 70 eV) spectra of heptadecane (C17H36). (D )EBEL SPI mass spectrum of heptadecane containing the mother ion [M]+ with almost no fragments.
information cannot be deduced from an EI mass spectrum, as there is no way to differ between a thermal decomposition and the fragmentation by the EI step itself. (b) Quantitative Considerations. The SPI mass spectrum (3D) can be used to determine the LOD for heptadecane, measured in scan mode, as described in the previous paragraph. Subjected to the condition that the concentration change of heptadecane is negligible during one scan (15 s), the concentration of heptadecane can be calculated from the mass loss data of the TA during the relevant cycle (1.67 mg), scan time (0.05 s (m/z)-1), mass range (300 m/z), and the gas flow through the TA (60 mL min-1). With the calculated concentration of 16‰, the measured ion signal at 240 m/z, and the noise variance (180230 m/z), an LOD of 25 ppm could be evaluated. As the noise range and the ion signal are adapted by almost the same range of mass discrimination, a correction of the signal can be neglected. However, compared to the LOD for aromatic compounds, e.g., benzene or toluene, the LOD of heptadecane is in good agreement. Note that, due to the mass discrimination, the value of the ion signal of heptadecane is 30 times lower, furthermore, the cross section of alkanes is almost 10 times lower than the cross section value for aromatic compounds in the VUV/UV wave length region.65 For the MID operation mode (Figure 2D), the LOD can be determined the same way. The noise level can be defined by the variance of the signal value at lower temperatures as heptadecane has not volatilized yet or even at higher temperatures where the whole mass has been evaporated. Due to the condition of the (65) Adam, T.; Zimmermann, R. Anal. Bioanal. Chem. 2007, 389, 1941-1951.
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constant concentration, the ion signal corresponding to the highest value of the DTG curve was chosen. With a scan time of 10 s for each cycle, and the same conditions as described above, an LOD of 14 ppm for heptadecane could be calculated. Applications of TG-SPI-QMS. The novel TG-EBEL-SPI-QMS device was applied to analyze the thermal decomposition and evolved gaseous products of common industrial polymers, namely, polystyrene (PS), acrylonitrile butadiene styrene copolymer (ABS), and polycarbonate (PC). Furthermore, mixtures of ABS and PC have been investigated by thermogravimetry single photon ionization-mass- spectrometry. This includes a commercial copolymerized blend, named Bayblend T45 PG (PCABS) as well as ABC and PC blends, heated in different pans without contact (PC&ABS) and premixed in the same sample pan (PC-ABS). For sample details, see Table 1. The three-dimensional SPI mass spectra of ABS, PC, and PCABS are illustrated in Figure 5. TG and DTG curves of all six samples are presented in Figure 6. Investigation of Simple Polymeric Materials by TG-SPIQMS. (A) Polystyrene. Polystyrene is a homopolymer of the styrene monomer. The thermal decomposition of PS typically occurs predominantly in the temperature range of 360-440 °C.15 The maximum rate of the decomposition (i.e., weight loss), which is described by the peak of the DTG curve as shown in Figure 6A, was at 420 °C in this study. (1) Qualitative Considerations. The EI mass spectrum (Figure 7A) at the point of maximal weight loss depicts strong signals at 104 and 91 m/z. Furthermore, prominent mass signals are registered at 78, 77, 65, 63, 51 and 39 m/z. The signals at 104, 78, and 51 m/z obviously are originating from the styrene
Figure 5. Comparison of three-dimensional plots of ABS, PCABS, and PC. All samples were measured in N2 atmosphere with a heating rate at 10 K min-1.
monomer; the peaks at 117, 115, 91, 77, 65, 63, and 52 m/z belong to the styrene dimer. Comparing the obtained mass spectrum with a standard 70-eV NIST EI mass spectrum of styrene and styrene dimer,63 we can see that the signal at 91 m/z belongs to styrene dimer, but the value is too high. According to Bouster et al., the thermal degradation value of styrene monomer is more than 10 times higher than the one of the styrene dimer and ∼100 times higher than the value of toluene.66 In contrast to the EI spectrum (Figure 7A), the SPI mass spectrum (Figure 7B) solely presents a weak molecular ion signal of toluene (92 m/z), methylstyrene (118 m/z), and phenylbutene (130 m/z) beside the two main products, the styrene monomer (m/z 104) and the styrene dimer
(208 m/z), which are listed in Table 2. Although the styrene homopolymer is a very simple polymeric system, the EI mass spectrometry fails to reveal the existence of dimers in the evolved gas analysis. Furthermore, SPI-mass spectrometry could show that there are no smaller aliphatic pyrolysis products (olefinic compounds) generated. The compounds detected by TG-EBEL-SPIQMS have been investigated previously by several research groups using pyrolyis-GC/MS.15,66 Furthermore, some kinetic modeling of the thermal degradation of polystyrene and polystyrene mixtures were published.9,67,68 Jakab et al. reported about the difficulties investigating the styrene dimer and styrene trimer by using the TG-MS technique.15 It was suggested that those compounds would condense in the interface, so that they could not reach the mass spectrometer. As the styrene dimer can be detected by TG-EBEL-SPI-QMS, but not by TA-EI-MS, an additional reason obviously lies in the EI fragmentation behavior of the styrene dimer. Note also that the implemented improved heatable coupling should have paid off in an improved transfer of styrene oligomers. However, with the used small quadrupole mass spectrometer, which is limited to 300 m/z, we could detect the dimer, but due to the mass range of the mass spectrometer, the styrene trimer (312 m/z) could not be detected. (2) Quantitative Considerations. Using conventional TG for quantification of sample compounds may only be made if a stoichiometric relationship exists between the original material and its decomposition products. Also, the combination of the TG with mass spectrometry using EI is almost not suited for quantitative measurements as the EI spectra often contain overlapping fragments of the decomposition products. Only the combination of a separation method, e.g., GC, can find a remedy. As SPI with its characteristic soft ionization is partially selective, quantitative conclusion can be done. Quantitative measurements can only be carried out if molecular flow conditions exist in the TG-MS apparatus. Moreover, it is necessary to have exact information about the flow rate through the TA system. As far as these conditions are presented, the basic assumption is made that the ion currents are proportional to the rate of release of any volatile product.23 Here are two different ways presented to quantify samples. The easiest way for quantification is to use calibration gases. To quantify the amount of toluene during the PS decomposition, the BTX standard gas was used. As the ion current for 10 ppm toluene yielded 1.4 ×10-11A, a conversion factor for the 10 ppm toluene yield was calculated. The ion current of toluene (0.4 × 10-9A) released during a scan (Figure 7B) corresponds to an amount of 150 ng of toluene considering the flow rate of 60 mL min-1 through the TA and the gas flow distribution between the exhaust (58.5 mL min-1 ) 97.5%) of the TA and the MS (1.5 mL min-1 ) 2.5%). As already noted, mass spectrometers need calibration to establish the exact relation between the concentration of the gas sample supplied and the measured ion signal. The knowledge about the total ionization cross section is essential for quantitative (66) Bouster, C.; Vermande, P.; Veron, J. J. Anal. Appl. Pyrolysis 1989, 15, 249259. (67) Bouster, C.; Vermande, P.; Veron, J. J. Anal. Appl. Pyrolysis 1980, 1, 297313. (68) Faravelli, T.; Bozzano, G.; Colombo, M.; Ranzi, E.; Dente, M. J. Anal. Appl. Pyrolysis 2003, 70, 761-777.
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Figure 6. Normalized TG and DTG curves of PS, ABS, PC, and the mixed PC/ABS, PC&ABS, and PCABS.
measurements,21 if there is no possibility to use standard gas for some substances. The relative cross section value to a well-known substance can also be used. Mu¨hlberger et al. reported about many relative cross sections compared to the cross section of benzene for VUV radiation at 126 nm.49 A relative quantification factor for styrene can be calculated as presented:
Istyrene ) 1.14‚Itoluene where Istyrene is the ion current of styrene monomer (104 m/z) and Itoluene is the measured toluene ion current (92 m/z) from the standard BTX gas. Using the ion current of the styrene monomer (4.9 × 10-8A) and the above relation, an amount of 18.5 µg of styrene could be calculated for this SPI spectrum (Figure 7B). Bouster et al. reported about a toluene/styrene monomer value of 0.01 (0.7% toluene to 65% styrene monomer at 500 °C).66 In this study, a toluene/styrene monomer value of ∼0.008 (150 ng of toluene to 18.5-µg styrene monomer at 420 °C) could be reached, which is in good agreement with the reported results of Bouster et al. (B) Polycarbonate. Polycarbonates are polyesters of carbonic acid. The most important commercial PC is that based on 2,2′bis(4-hydroxyphenyl)propane (bisphenol A) and can be synthesized by the reaction of the dihydric phenol with phosgene or by ester interchange with diphenyl carbonate,69 commercialized under the trade names Lexan, Merlon, and Calibre. Although it can be crystallized, most polycarbonates are amorphous. PC finds many possible fields of applications because of its exceptional transparency and impact resistance, as well as good dimensional (69) Odian, G. Principles of Polymerization, 3rd ed.; John Wiley & Sons Inc.: new York, 1991.
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and creep resistance. Applications include food contact, medical, glazing, optical, and other household, consumer, and automotive uses.69 PC0018 decomposes between 515 and 525 °C. Panels G and H in Figure 7 illustrate the EI mass spectrum of PC at 517 °C as well as the SPI mass spectrum of PC at 522 °C. The major volatile degradation products from PC consisted of phenolic compounds such as phenol (94 m/z), methylphenol (108 m/z) and ethylphenol (122 m/z); significant peaks at 120, 134, 136, and 228 m/z could also be detected and addressed to the chemical compounds listed in Table 3. Investigation of Complex Polymeric Materials, Including Copolymers and Blends, by TG-SPI-QMS. Most polystyrene products are not homopolystyrene since the latter is relatively brittle with low impact and solvent resistance. Various combinations of copolymerization and blending are used to improve the properties of polystyrene.69 Copolymerization of styrene with 1,3butadiene imparts flexibility to yield elastomeric products, e.g., styrene-1,3-butadiene rubbers (SBR). Radical copolymerization of styrene with acrylonitrile yields styrene-acrylonitrile (SAN) polymers. Acrylonitrile, by increasing the intermolecular forces, imparts solvent resistance, improves tensile strength, and raises the upper use temperature of polystyrene. (A) ABS. Acrylonitrile-butadiene-styrene polymers (trade names, e.g., Abson, Blendix, and Novodur) combine the properties of SAN with greatly improved resistance to impact. ABS is produced by emulsion, suspension, or bulk copolymerization of SAN in the presence of a rubber. The rubber is either poly(1,3butadiene) or SBR. The product of the reaction is a physical mixture SAN copolymer and the graft copolymer of styreneacrylonitrile onto the rubber. Additionally, SAN is often blended into the mixture. The final product, ABS, consists of a glassy
Figure 7. Comparison of the EI spectra (left) and the SPI spectra (right) of PS, ABS, PC0018, and the blend PC0006. Table 2. Degradation Products15,66 from the Thermal Decomposition of PS at 420 °C no. 1 2 3 4 5 6 a
compound
m/z
IP (eV)
QMS signal (10-9 A)
toluene styrene ethylbenzene R-methylstyrene phenylbutadiene diphenylbutene
92 104 106 118 130 208
8.83 8.46 8.75 8.35 7.95 naa
0.39 48.8 0.11 0.11 0.34 15.4
na, not available.
Table 3. Degradation Products14,17 from the Thermal Decomposition of PC at 522 °C
no. 1 2 3 4 5 6 7 8 a
polymer (SAN) dispersed in a rubbery matrix. Applications for ABS include household appliances, transportation, and business machine housing.69 ABS has its maximum thermal decomposition in the range of 420-440 °C. Note that the decomposition temperature is slightly affected by the specimen weight and therefore varies within a small interval. Figure 7D presents the SPI-mass spectrum of ABS. The pyrolyzed products of ABS have been investigated with the method of Py-GC/MS by several groups.16,70 The results of the ABS thermal analysis experiments are listed in Table 4. The main products of ABS beside the typical fractions of PS are benzenebutanenitrile (145 m/z) and n-phenyl4-pentenenitrile (157 m/z). (B) Blends of ABS and Polycarbonate. Polymer blends and interpenetrating polymer networks are different from copolymers (70) Day, M.; Cooney, J. D.; Shen, Z. J. Anal. Appl. Pyrolysis 1996, 37, 49-67.
compound
m/z
IP (eV)
phenol methylphenol vinylphenol ethylphenol methylvinylphenol methylethylphenol/propylphenol methylphenylethylphenol methylethylidene bisphenol
94 108 120 122 134 136 212 228
8.49 8.35 naa 7.8 (EI) na na na na
QMS signal (10-9 A) 2.04 5.43 0.94 2.67 0.70 0.57 0.13 0.08
na, not available.
but like copolymers are used to combine the properties of different polymers.71 PC is blended with a number of polymers including poly(butylene terephthalate), poly(ethylene terephthalate), and ABS rubber. The blends have lower costs compared to PC and, in addition, show improved properties. ABS, for example, imparts better processability69 and ABS/PC blends have lower impact strength compared to pure ABS.72 In thermal degradation of polymers, some relevant effects may occur caused by two polymers thermally decomposed as copolymer or even pyrolyzed side by side.14 Day et al. reported about changes in the pyrolyzed (71) Paul, D. R.; Barlow, J. W.; Keskkula, H. “Polymer Blends” in “Encyclopedia of Polymer Science and Engineering”, 2 ed.; Wiley-Interscience: New York, 1988. (72) Xiaodong Liu, H. B. J. Appl. Polym. Sci. 1999, 74, 510-515.
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Figure 8. (A) ABS and PC0018 at a ratio of 50/50 heated in different sample pans during the same TG run (PC&ABS). (B) Time profiles of volatile compounds (94, 108, 122, and 145 m/z) of the polymer blend PC0006 Bayblend (PCABS), decomposing during the TG run. (C) A 50/50 blend mixture of ABS and PC was pyrolyzed side by side in the thermobalance (PC/ABS). Table 4. Degradation Products14,16 from the Thermal Decomposition of ABS at 428 °Ca QMS signal (10-9 A)
no.
compound
m/z
IP (eV)
1 2 3 4 5 6 7 8 9 10 11
toluene styrene ethylbenzene/hexendinitrile R-methylstyrene phenylbutadiene benzenebutanenitrile isomeric phenyl C5-nitrilea phenylhexenenitrile diphenylpropane diphenylbutene 2-(2-phenylethyl)-4methylenepentanedinitrile diphenylbutylpropenenitrile
92 104 106 118 130 145 157 171 196 208 210
8.83 8.46 8.75/nab 8.35 7.95 na na na na na na
1.22 9.39 1.04 1.35 0.13 1.26 0.71 0.07 0.25 0.14 0.09
261
na
0.03
12
a The different isomers were detected by Py-GC/MS.14 available.
b
na, not
products of ABS when PVC was added. The yields of ethylbenzene, methylstyrene, toluene, and 1,3-diphenylpropane increased when ABS was pyrolyzed in the presence of PVC. The Bayblend sample causes an increase of the temperature of maximum weight loss compared to pure ABS to ∼440 °C. The temperature shift is not the only effect that could be observed. The SPI-mass spectrum of PC0006 depicted in Figure 7F clarifies how the mixture affects the obtained products. The yield of the phenolic compounds decreases considerably in the case of methylphenol (108 m/z) and ethylphenol (122 m/z), compared to pure PC (Figure 7H). Table 5 provides all identified degradation products of PC0006 by the TA-EBEL-QMS system. 3402 Analytical Chemistry, Vol. 80, No. 9, May 1, 2008
Table 5. Degradation Products from the Thermal Decomposition of PCABS at 440 °Ca QMS signal (10-9 A)
no.
compound
m/z
IP (eV)
1 2 3 4 5 6 7 8 9 10 11 12 13
toluene phenol styrene ethylbenzene R-methylstyrene vinylphenol methylvinylphenol methylethylphenol/propylphenol benzenebutanenitrile isomeric phenyl C5-nitrilea phenylhexenenitrile diphenylpropane 2-(2-phenylethyl)-4-methylenepentanedinitrile methylethylidene bis-phenol diphenylbutylpropenenitrile
92 94 104 106 118 120 134 136 145 157 171 196 210
8.83 8.49 8.46 8.75 8.35 nab na na na na na na na
0.78 0.82 4.1 0.89 0.81 0.22 0.29 0.29 0.53 0.17 0.05 0.08 0.09
228 261
na na
0.03 0.02
14 15
a The different isomers were detected by Py-GC/MS.14 available.
b
na, not
To further investigate this effect, ABS, PC0018, and PC0006 were thermally decomposed under three different conditions. First, ABS and PC0018 at a ratio of 50/50 were heated in different sample pans during the same TG run so they could not affect each other during the melting phase (PC&ABS). Second, a 50/ 50 blend mixture of ABS and PC0018 was pyrolyzed side by side in the thermobalance (PC/ABS), and last the PC0006 Bayblend was investigated (PCABS). For this, the QMS was running at MID mode and the mass to charge ratios of 94, 108, 122, and 145 m/z
were recorded, the latter representing benzenebutanenitrile (145 m/z), a typical degradation product of ABS. Its yield was used as a comparison signal, which was not affected by addition of PC0018. All three resulting ion traces are pictured in Figure 8. In the first case (Figure 8A), the same characteristics could be observed as with thermal analysis of pure ABS and pure PC. The yield of the phenolic group was distributed as expected with the highest range for methylphenol (108 m/z) followed by ethylphenol (122 m/z) and phenol (94 m/z) at approximately T ) 525 °C. The temperature of maximum ion current of the side-by-side degradation, which is equivalent to the maximum mass loss, is shifted to 470 °C in the case of phenol (94 m/z) and actually to 490 °C in the case of methylphenol (108 m/z) and ethylphenol (122 m/z) (Figure 8B). The signal intensities of methylphenol and ethylphenol are reduced as in the case of PC0006, while the yield of phenol increased slightly (Figure 8C). Obviously the degradation of ABS takes place prior to that of PC. Degradation products of ABS could undergo secondary chemical reactions, which influence PC species prior to their release, causing a shift in PC degradation products toward an increase in phenol and a decrease in alkylated phenols. Moreover, the phenol profile is different in the case of PC&ABS compared to PCABS, showing two distinct maxima instead of one. A reason for this behavior might be that the PCABS pellets are mixed homogenously compared to two granulates lying side by side. In all three experiments, benzenebutanenitrile (m/z 145) showed almost the same behavior with slight changes within the experimental variability, indicating that ABS degradation is not affected by the presence of PC. CONCLUSION In this study, the potential of combining the advantages of thermogravimetry with the benefits of mass spectrometry using
a soft ionization method has been demonstrated. The here presented TA-EBEL-MS was characterized regarding its sensitivity and selectivity. The functionality of the connection was proved with a semivolatile long-chain alkane. Using the newly designed hyphenation, the tailing of the ion signal resulting in cold spots in the flue could be reduced to an acceptable degree, which is basically limited by the transport time of the substances between TA and MS. The decomposition products of two polymers (PS, PC) were investigated and showed a very good agreement with the previously published results. Furthermore, we could clarify the decrease of phenolic compounds in the thermally degraded products of PC, when mixed or respectively degraded side by side with ABS. The TA-EBEL-MS is well suited for monitoring and analysis of thermal behavior of polymers. Future developments will include the use of a TOFMS for more sensitivity, speed, and mass resolution. A commercial version of the EBEL with more intense VUV radiation will also be available very soon, which will lead to lower detection limits for other fields of application. ACKNOWLEDGMENT The authors thank the members of the HMGU laser mass spectrometry group, S. Mitschke, T. Adam, J. Maguhn, and W. Welthagen, for contributions during the measurements. Financial support by the BFS (Bayerische Forschungsstiftung) is gratefully acknowledged.
Received for review December 21, 2007. Accepted February 21, 2008. AC702599Y
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