A Mobile Mass Spectrometer for Comprehensive On-Line Analysis of

Anal. Chem. 2001, 73, 3590-3604

A Mobile Mass Spectrometer for Comprehensive On-Line Analysis of Trace and Bulk Components of Complex Gas Mixtures: Parallel Application of the Laser-Based Ionization Methods VUV SinglePhoton Ionization, Resonant Multiphoton Ionization, and Laser-Induced Electron Impact Ionization F. Mu 1 hlberger,†,‡ R. Zimmermann,*,†,‡,§ and A. Kettrup†,⊥ GSF-Forschungszentrum fu¨r Umwelt und Gesundheit, Institut fu¨r O ¨ kologische Chemie, D-85764 Oberschleissheim, Germany, Professur fu¨r Analytische Chemie, Lehrstuhl fu¨r Festko¨rperchemie, Universita¨t Augsburg, Universita¨tsstrasse 1, D-86159 Augsburg, Germany, BIfA-Bayerisches Institut fu¨r Umweltforschung und-technik, Abteilung Umwelt und Prozesschemie, Am Mittleren Moos 46, D-86167 Augsburg, Germany, and Technische Universita¨t Mu¨nchen, Lehrstuhl fu¨r O ¨ kologische Chemie und Umweltanalytik, D-85350 Freising, Germany

A newly developed compact and mobile time-of-flight mass spectrometer (TOFMS) for on-line analysis and monitoring of complex gas mixtures is presented. The instrument is designed for a (quasi-)simultaneous application of three ionization techniques that exhibit different ionization selectivities. The highly selective resonance-enhanced multiphoton ionization (REMPI) technique, using 266nm UV laser pulses, is applied for selective and fragmentationless ionization of aromatic compounds at trace levels (parts-per-billion volume range). Mass spectra obtained using this technique show the chemical signature solely of monocyclic (benzene, phenols, etc.) and polycyclic (naphthalene, phenathrene, indol, etc.) aromatic species. Furthermore, the less selective but still fragmentationless single photon ionization (SPI) technique with 118-nm VUV laser pulses allows the ionization of compounds with an ionization potential below 10.5 eV. Mass spectra obtained using this technique show the profile of most organic compounds (aliphatic and aromatic species, like nonane, acetaldehyde, or pyrrol) and some inorganic compounds (e.g., ammonia, nitrogen monoxide). Finally, the nonselective ionization technique laser-induced electron-impact ionization (LEI) is applied. However, the sensitivity of the LEI technique is adjusted to be fairly low. Thus, the LEI signal in the mass spectra gives information on the inorganic bulk constituents of the sample (i.e., compounds such as water, oxygen, nitrogen, and carbon dioxide). Because the three ionization methods (REMPI, SPI, LEI) exhibit largely different ionization selectivities, the isolated application of each method alone solely provides specific mass spectrometric information about the sample composition. Special techniques have been developed and applied which allow the quasi-parallel use of all three ionization techniques for on-line monitoring purposes. Thus, a comprehensive characterization of complex samples is feasible jointly using the characteristic advantages of the three ionization techniques. Laboratory applications show results on rapid overview characterization of mineral oil-based fuels and coffee headspace. The first reported field applications include timely resolved on-line monitoring results on automobile exhausts and of waste incineration flue gas. 3590 Analytical Chemistry, Vol. 73, No. 15, August 1, 2001

Mass spectrometry plays a key role in instrumental analytical chemistry. The development of mass spectrometry-based analytical instrumentation and techniques with enhanced sensitivity, selectivity, and measurement speed is of particular importance for on-line analytical applications such as, for example, industrial process analysis or environmental monitoring. The laser-based resonance-enhanced multiphoton-ionization time-of-flight mass spectrometry (REMPI-TOFMS) technique is known as a highly selective and sensitive analytical method,1-3 well-suited for online measurement of compounds in trace quantities from complex gas mixtures. The REMPI technique uses two or more UV photons for photoionization, utilizing an optical resonance absorption step. Because of this optical resonance absorption step, the selectivity of UV gas-phase laser spectroscopy is included in the ionization process. Depending on the molecular systems to be analyzed and the method of sample introduction (i.e., by jet expansion,4-7 which provides adiabatic cooling of the sample molecules or by effusive inlet, which generates a warm molecular beam), the selectivity of the REMPI technique can range from substance-class-specific ionization up to ionization of single isomers from a mixture of isomeric and isobaric compounds. REMPI-TOFMS is particularly well-suited for monitoring aromatic * Corresponding author. E-mail: [email protected]. † Institut fu ¨r O ¨ kologische Chemie. ‡ Universita ¨t Augsburg. § BIfA-Bayerisches Institut fu ¨ r Umweltforschung und -technik. ⊥ Technische Universita ¨t Mu ¨ nchen. (1) Hurst, G. S.; Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. Mod. Phys. 1979, 51, 767-819. (2) Lubman, D. M., Ed. Lasers and Mass Spectrometry; Oxford University Press: New York, 1990. (3) Boesl, U.; Weinkauf, R.; Weickhardt, C.; Schlag, E. W. Int. J. Mass Spectrom. Ion Proc. 1994, 131, 87-124. (4) Cool, T. A.; Williams, B. A. Combust. Sci. Technol. 1992, 82, 67-85. (5) Zimmermann, R.; Lermer, C.; Lenoir, D.; Boesl, U. 7th International Symposium on Resonance Ionization Spectroscopy; American Institute of Physics; AIP-Press: New York, 1994; 527-530. (6) Oser, H.; Thanner, R.; Grotheer, H.-H. Combust. Sci. Technol. 1996, 116117, 567-582. (7) Tembreull, R.; Lubman, D. M. Anal. Chem. 1984, 56, 1962-1967. 10.1021/ac010023b CCC: $20.00

© 2001 American Chemical Society Published on Web 06/29/2001

Figure 1. (top) Comparison of the applied ionization methods and their selectivity. (bottom) Schematic representation of the applied ionization processes: (A) Laser-induced electron impact ionization, LEI with accelerated electrons, released from the inlet steel, hollow needle by photoemission; (B) single photon ionization, SPI by absorption of 118-nm VUV photons; and (C) resonance-enhanced multiphoton ionization, REMPI, by absorption of two 266-nm UV photons.

species. Furthermore, in many cases, REMPI is a soft ionization technique that can be adjusted to be nearly fragmentation-free. Because of these properties, the combination of REMPI with mass spectrometric analysis of the formed ions has an outstanding position for on-line analyzing of traces of aromatic compounds in complex gas mixtures.6,8-12 Recently, the application of mobile REMPI-TOFMS instruments for on-line monitoring of PAH and other aromatic species in the flue gas of industrial incineration plants has been reported.13-17 This includes the on-line measurement of monochlorobenzene,18,19 which has been identified as a surrogate for the dioxin emission of waste incineration plants. Further applications for monitoring flavor-active compounds in the off-gas of the coffee-roasting process have been reported.12 (8) Gittins, C. M.; Castaldi, M. J.; Senkan, S. M.; Rohlfing, E. A. Anal. Chem. 1997, 69, 286-293. (9) Heger, H. J.; Boesl, U.; Zimmermann, R.; Dorfner, R.; Kettrup, A. Eur. Mass Spectrom. 1999, 5, 51-57. (10) Franzen, J.; Frey, R.; Holle, A.; Betzold, H.; Ulke, W.; Boesl, U. SAE Technol. Pap. 1993, 930082, 55. (11) Weickhardt, C.; Boesl, U.; Schlag, E. W. Anal. Chem. 1994, 66, 10621069. (12) Zimmermann, R.; Heger, H. J.; Yeretzian, C.; Nagel, H.; Boesl, U. Rapid Commun. Mass Spectrom. 1996, 10, 1975-1979. (13) Zimmermann, R.; Heger, H. J.; Kettrup, A.; Boesl, U. Rapid Commun. Mass Spectrom. 1997, 11, 1095-1102. (14) Zimmermann, R.; Heger, H. J.; Dorfner, R.; Boesl, U.; Blumenstock, M.; Lenoir, D.; Kettrup, A. Combust. Sci. Technol. 1998, 134, 87. (15) Thanner, R.; Oser, H.; Grotheer, H.-H. Eur. Mass Spectrom. 1998, 4, 215222. (16) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46-57. (17) Zimmermann, R.; Heger, H. J.; Kettrup, A.; Nikolai, U. Fresen. J. Anal. Chem. 2000, 366, 368-374.

However, several aliphatic compounds or inorganic components are not detectable in a straightforward procedure by REMPI. Their detection requires complicated multilaser-wavelength REMPI schemes and high-power or tunable laser systems. Thus, although being an ideal method for detection of individual target compounds or aromatic species, REMPI-TOFMS cannot give a comprehensive overview characterization of gas samples. The application of direct inlet mass spectrometry with other ionization techniques, which exhibit a different ionization selectivity profile, allows a different view of the sample. Regarding the ionization selectivity and the induction of fragmentation, the single photon ionization (SPI) technique is located between the REMPI and the EI ionization techniques (Figure 1, top). The SPI technique uses vacuum ultraviolet (VUV) photons for a single photon absorption/ionization process.20 The selectivity is given as a result of the ionization energy (IE) threshold, that is, only those compounds are ionized that exhibit an IE lower than the photon energy. A typical photon energy for laser-based SPI is 10.5 eV (118 nm). Laser VUV photons can be generated by frequency tripling of intensive UV laser pulses in a rare gas cell.21-23 A continuously increasing number of publications (18) Zimmermann, R.; Heger, H. J.; Blumenstock, M.; Dorfner, R.; Schramm, K.-W.; Boesl, U.; Kettrup, A. Rapid Commun. Mass Spectrom. 1999, 13, 307-314. (19) Blumenstock, M.; Zimmermann, R.; Schramm, K.-W.; Kettrup, A. Chemosphere 2001, 42 (2001), 507-518. (20) Pallix, J. B.; Schu ¨ hle, U.; Becker, C. H.; Huestis, D. L. Anal. Chem. 1989, 61, 805-811. (21) Vidal, C. R. In Tunable Lasers; Mollenauer, L. F., White, J. C., Eds.; SpringerVerlag: Berlin, 1987; Vol. 59, pp 56-113.

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on generation of laser-induced VUV pulses and applications of SPI as ionization method for mass spectrometry demonstrate a growing interest in this method.20,24-36 Furthermore, conventional electron impact (EI) ionization can be applied for the analysis of gas mixtures; however, electron impact ionization is not suited for on-line mass spectrometry of trace compounds, because it is quite insensitive (i.e., low cross section of EI) and causes massive fragmentation. The overlapping fragmentation patterns of multiple compounds cannot be deconvoluted, so it is impossible to identify trace compounds from complex mixtures. On the other hand, electron impact ionization represents a valuable method for the ionization and detection of less fragile, major components of a gas mixture, such as N2, CO2, O2, and H2O in combustion flue gases. In conclusion, the three ionization techniques REMPI, SPI, and EI are complementary with regard to the ionization selectivity and some other properties. In the upper part of Figure 1, the ionization techniques are ordered concerning to their selectivity. However, if the three ionization techniques are applied in parallel or in rapid alternation, the information obtained via the different ionization techniques can be combined to give a comprehensive, overall picture of the sample composition. In this paper, a newly developed, compact (19-in. rack), and rugged gas analyzer based on a TOF mass spectrometer is presented which allows quasiparallel use of all three of the above-mentioned ionization techniques (SPI, EI, and REMPI) for a comprehensive on-line characterization of complex gas mixtures. The instrument uses a compact Nd:YAG laser for generation of 266- and 118-nm laser pulses for REMPI and SPI ionization, respectively. The electron pulses for EI are generated by laser-induced photoemission (i.e., from laser photons that are hitting a metal target). The emitted electrons are accelerated in the field of the ion source and hit the sample molecules, performing laser-induced electron impact ionization (LEI, Figure 1A). First application results are presented. These include laboratory results on calibration gases and complex mixtures as well as online measurements of combustion flue gases (municipal waste incineration plant and automotive exhaust gas). The concept and instrumentation for (quasi-) simultaneous application of VUV and UV for SPI and REMPI-ionization, respectively, have been submitted as a patent application.37 (22) Maker, P. D.; Terhune, R. W. Phys. Rev. 1965, 137, (3A), 801-818. (23) Bjorklund, G. C. IEEE J. Quantum Electron. 1975, 11, (6), 287-296. (24) Becker, C. H. Fresen. J. Anal. Chem. 1991, 341, 3-6. (25) Butcher, D. J.; Goeringer, D. E.; Hurst, G. B. Anal. Chem. 1999, 71, 489496. (26) Kornienko, O.; Ada, E. T.; Tinka, J.; Wijesundara, M. B. J.; Hanley, L. Anal. Chem. 1998, 70, 1208-1213. (27) Materer, N.; Goodman, R. S.; Leone, S. R. J. Vac. Sci. Technol. 1997, A 15 (4), 2134-2142. (28) McEnally, C. S.; Pfefferle, L. D.; Mohammed, R. K.; Smooke, M. D.; Colket, M. B. Anal. Chem. 1999, 71, 364-372. (29) Miller, J. C.; Compton, R. N. J. Chem. Phys. 1982, 76 (8), 3967-3973. (30) Shi, Y. J.; Hu, X. K.; Mao, D. M.; Dimov, S. S.; Lipson, R. H. Anal. Chem. 1998, 70, 4534-4539. (31) Steenvoorden, R. J. J. M.; Kistemaker, P. G.; Vries, A. E.; Michalak, L.; Nibbering, N. M. M. Int. J. Mass Spectrom. Ion Proc. 1991, 107, 475-489. (32) Trevor, J. L.; Hanly, L.; Lykke, K. R. Rapid Commun. Mass Spectrom. 1997, 11, 587-589. (33) Vries, M. S.; Hunziker, H. E. J. Photochem. Photobiol. A 1997, 106, 31-36. (34) Werner, J. H.; Cool, T. A. Chem. Phys. Lett. 1997, 275, 278-282. (35) Werner, J. H.; Cool, T. A. Chem. Phys. Lett. 1998, 290, 81-87. (36) Zoller, D. L.; Sum, S. T.; Johnston, M. V. Anal. Chem. 1999, 71, 866-872.

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EXPERIMENTAL AND INSTRUMENTAL SETUP The mobile laser mass spectrometer (155 × 57 × 78 cm) has a modular setup. It consists of two separable units (19-in. racks), the mass spectrometer unit, and the laser unit (Figure 3A). Within this unit, the components are mounted in 19-in. drawers. The time-of-flight (TOF) mass spectrometer itself consists of two 19-in. drawers. The first one contains a custom-built, compact, in-line reflectron time-of-flight tube (Kaesdorf Instruments, Germany) (flight path, 63 cm) with the gas inlet system, two turbo molecular pumps (Pfeiffer, Germany; 60 and 210 L/s N2) and highvoltage supplies for the ion source and the ion detectors. The second drawer contains a membrane rough pump as well as the steering electronics for vacuum generation and measurement (Pfeiffer, Germany). Furthermore, the MS unit contains a drawer with a 19-in. PC computer for data acquisition and instrument control and a 19-in. drawer with a home-built standard gas generation device that supplies a mixture of different compounds in parts-per-billion volume concentrations for mass calibration and external quantification purposes. The laser unit consists of a compact Nd:YAG laser (Continuum, CA, Minilight-II), laser head and power supply) and nonlinear optical elements (frequency doubling crystals, frequency mixing crystals, and a rare gas cell for frequency tripling) for generation of UV and VUV laser pulses. The compact laser mass spectrometer system needs an ordinary 220V/16A power supply, and no external gas or water cooling supply is required. The system is designed for operation under rough environmental conditions at industrial facilities. First field application results were obtained during a flue gas measurement campaign at a municipal waste incineration plant as well as during measurements of automotive exhaust and are partially reported below. In the following sections, the setup and operation of the subunits of the mobile MS system are described in more detail. The Compact 19-in. Time-of-Flight Mass Spectrometer. The compact time-of-flight mass spectrometer (Figure 3C) is designed as a linear in-line reflectron with a field free-drift region of 634 mm and a mass resolution R50% of ∼400 at 128 m/z. The ion source and the flight tube are differentially pumped by a 210 L/s and a 60 L/s turbo molecular pump. The Wiley-McLarentype ion source38 can be operated with static or pulsed acceleration fields. Similar to a preceding instrument,16 the repeller electrode and the first extraction electrodes have the same potential with opposite polarity to ensure ground potential for the central ionization region (i.e., the position of the needle inlet). The field free-drift region is on the same negative potential as the second extraction electrode (liner). After the second extraction electrode, the ion beam passes an electrostatic lens unit as well as an x-y deflection plate unit for controlling the ion trajectories. Typical HV values for the ion source are repeller electrode, 500 V; first extraction electrode, -500 V; and second extraction electrode and liner, -3000 V. The ions enter the field free-drift region through a small tube leading through the central hole of the microsphere plate detector (L ) 40 mm, d ) 1.5 mm, central hole L ) 8 mm, L microspheres ) 60 µm; typ 050D, El-Mull, Israel). The doublestaged ion mirror consists of 13 equidistant electrodes over a depth (37) Zimmermann, R.; Heger, H. J.; Kettrup, A.; Mu¨hlberger, F.; Hafner, K.; Boesl, U. Patent application, Deutsches Patentamt: Germany, 2000. (38) Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150-1157.

Figure 2. Instrumentation employed for SPI- and REMPI-TOFMS, which consists of a Nd:YAG laser, a VUV generator, an ion source, and a time-of-flight mass spectrometer with a MCP detector. The VUV generator itself consists of a xenon-filled tripling cell, a moveable quartz focusing lens, a quartz entrance window, and a MgF2 lens for collimating and separating the third harmonic of the Nd:YAG laser from the 118-nm beam.

of 117 mm (two grids). In addition, a further detector (dual channel plate detector, chevron setup) is mounted behind the ion mirror for operation of the mass spectrometer as linear TOFMS38 (i.e., by deactivating the ion mirror potential). In the linear mode, the field free-drift region is 430 mm. A mass resolution (R50%) of 150 is achieved in the linear mode. The operation as linear TOFMS, however, allows an increased sensitivity as a result of enhanced transmission. The Sample Inlet System. The sample inlet system is similar to a capillary-based inlet system described in the literature.13,16 Briefly, it consists of a heated, hollow, stainless steel needle (o.d., 1 mm) reaching into the center of the TOFMS ion source. Within the heated, hollow, steel needle runs a deactivated fused-silica capillary of 200 µm i.d. The opening of the fused-silica capillary is aligned with the end of the inlet needle and is located 2 mm above of the center of the ion source (i.e., the ionization region, Figure 2: ). The other end of the fused-silica capillary, which has a total length of ∼1 m, runs through a heated vacuum seal and a heated transfer line tube and is connected to the sampling system. The capillary itself acts as a restriction step between the vacuum of

the ion source (10-5 Torr) and the ambient pressure at the sampling system. The gas flow through the capillary is ∼2 mL/ min. Behind the orifice of the fused-silica capillary in the ion source, an effusive molecular beam is formed.39 The ionization zone is directly underneath the capillary orifice in the center of the TOF-MS to ensure a maximum density of analyte molecules in the ionization region. The design of the sampling system depends on the application. For sampling of ambient air or headspace measurements, the end of the fused-silica inlet capillary is simply mounted in a heated metal support (connected to the flexible heated transfer line tubing). For on-line analysis of combustion flue gases, a complex heated sampling train with a dust filter and a flow-controlled sampling pump is used. A small fraction of the gas flow in the sampling train is sampled by the fused-silica inlet capillary, which is mounted in a heated needle, placed orthogonal to the main gas flow. Details of the sampling train for combustion off-gases are given in the literature.13,16 (39) Wegner, P. P., Ed. Molecular Beams and Low-Density Gas Dynamics; Marcel Dekker: New York, 1974.


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Figure 3. Photographs of the SPI-/REMPI-TOFMS device: (A) the total instrument, (B) close-up view on the THG rare gas cell for VUV generation, and (C) close-up view on the 19-in. TOFMS unit.

The Laser Unit for Alternating Generation of UV and VUV Laser Pulses for REMPI, LEI, and SPI Ionization. The developed mass spectrometer for trace gas analysis uses two different laser-based photoionization methods: the single-photon ionization technique with VUV photons and the resonanceenhanced two-photon ionization (REMPI) with UV photons. To use both ionization methods (see description below) nearly simultaneously, a special laser unit, based on a Nd:YAG laser head, was designed. The fundamental Nd:YAG laser pulses (wavelength, 1064 nm) pass a nonlinear optical crystal for second harmonic generation (SHG). The resulting laser pulse consists of collinear 1064- and 532-nm radiation. Via a motorized and computercontrolled fast and precise moving mirror switch, two optical pathways of the collinear 1064- and 532-nm radiation can be selected: (i) First Optical Pathway: If the mirror is tilted into the beam path, the collinear 1064- and 532-nm radiation is deflected through an additional SHG crystal for generation of 266-nm UV laser pulses (fourth harmonic Nd:YAG wavelength). All other harmonic Nd: YAG wavelengths (1064 and 532 nm) are precipitated via dichroidic beam separators. The 266-nm laser pulses are directed via an entrance window into the ion source of the TOFMS for REMPI. The typical achieved pulse energy at 266 nm (4 ns width) was 5 mJ. (ii) Second Optical Pathway: If the mirror is tilted out of the beam path, the collinear 1064- and 532-nm radiation passes a nonlinear optical crystal for frequency mixing. Here, the third harmonic wavelength of the Nd:YAG, 355 nm, is generated. The other harmonic Nd:YAG wavelengths (1064 and 532 nm) again are precipitated via dichroidic beam separators. The 355-nm pulses are focused into the third harmonic generation (THG) rare gas 3594


cell for generation of 118-nm VUV laser pulses. The typical achieved pulse energy at 355 nm was 7 mJ, resulting in an estimated pulse energy of ∼10-6 mJ for the 118-nm VUV pulse.40In the following, the implementation of the ionization methods is described in detail. Resonance Enhanced Multiphoton Ionization (REMPI) and Laser-Induced Electron Impact Ionization (LEI). For the resonance-enhanced multiphoton ionization (REMPI) detection of aromatic compounds, the 266-nm (4.65 eV) photons from the 10 Hz Nd:YAG laser are used (Figure 1C). The 266-nm laser beam (L, 4 mm) is irradiated without focusing into the ionization chamber where it hits the molecular beam. If a molecule is excited by photon absorption (resonance absorption, UV spectroscopic step), it may absorb a second photon during the lifetime of the primary excitation; however, this is only likely if the photon density is sufficiently high, like for example, in intense laser pulses. If a second photon is absorbed and if the total energy of the two photons exceeds the ionization potential, the molecule will be ionized (ionization step). At the given wavelength of 266 nm, this one-color, two-photon REMPI-ionization scheme is particularly efficient for a large number of aromatic species. In addition to the REMPI process for ionizing selectively aromatic species, the laser pulses can also be used for the induction of electron impact ionization (EI) of major compounds. The optical path of the 266 nm (photon energy, 4.66 eV) beam could be tuned such that the outer part of the laser beam hits the stainless steel inlet needle (Figure 1A). The metal surface absorbs the UV photons under photoemission of electrons (work functionFe, 4.5 eV). The emitted electrons subsequently are accelerated (40) Hilbig, R.; Wallenstein, R. Appl. Optics 1982, 21, (5), 913-917.

by the strong electric fields present in the ionization zone (E ∼ 500 V/cm) and are readily accelerated to sufficient kinetic energies for effective EI ionization of molecules. This laser-induced electron impact ionization (LEI) method has been previously described in the literature.41 THG Unit for Generation of VUV Laser Pulses for Single Photon Ionization. The 355-nm beam (L, 3 mm; divergence < 3 mrad; 7 mJ/pulse) of the Nd:YAG laser system that was used is focused by a quartz lens (f, 100 mm) through a quartz window into a 150-mm-long stainless steal cell for third harmonic generation. The dimensions of the beam parameters and the parts used were calculated according to literature data.23 Special care was taken for minimization of possible contamination sources for the THG’s rare gas filling. Only stainless steel parts were used, and they were cleaned carefully before assembly. The o-ring seal for the entrance window and the exit lens were made of Kalrez. To remove residual contaminants, that could compromise the tripling efficiency due to gas breakthrough or multiphoton ionization processes21 within the focus of the 355-nm laser beam, the rare gas is cleaned by a heated getter pump (Figure 3B; getter material: 84% Zr, 16% Al42). After these precautions are taken, the expensive xenon gas filling can be used for weeks without a decrease in the conversion efficiency. The cell was filled with some millibars of xenon gas (purity 4.0).24,27,32,43-45 Phase-matching conditions for the THG process were not optimized for the measurements shown below. The produced 118-nm radiation is focused by a plain convex MgF2 lens (radius of curvature R ) 35 mm) directly into the ionization region. As shown in Figure 4B, the intense 355-nm radiation, although being defocused at the ionization region as a result of the differences of the dispersion coefficient of MgF2 for 355 (n ) 1.39) and 118 nm (n ) 1.68), can cause significant fragmentation of the analyte ions. Therefore, the 118-nm beam was separated from the 355-nm beam. This was performed by an off-axis irradiation of the 355/118-nm laser beam onto the MgF2 lens in a manner similar to that described in the literature20,31,46,47. Because of the dispersion difference, the foci of the 355- and 118-nm beams are transversely relocated, and the 355-nm beam can be blocked by a beam dump. In the following, the calibration unit and data acquisition system are described. The Compact Calibration Gas Unit. Quantification and calibration is an important issue for laser-based analytical techniques, because highly nonlinear processes are involved. Moreover, different analytes often have largely different responses. Similarly to preceding work,13,16 we use a standard gas containing relevant analytes in parts-per-billion volume quantities for external calibration and quantification. The standard gas is generated after the “defined leak” principle using the permeation tube approach.48,49 A compact calibration gas generation unit was devel(41) Rohwer, E. R.; Beavis, R. C.; Ko¨ster, C.; Lindner, J.; Grotemeyer, J.; Schlag, E. W. Z. Naturforsch. 1988, 43a, 1151-1153. (42) Barosi, A.; Rabusin, E. Jap. J. Appl. Phys. Suppl. 1974, 2, (1), 49-52. (43) Boyle, J. G.; Pfefferle, L. D.; Gulcicek, E. E.; Colson, S. D. Rev. Sci. Instrum. 1991, 62, 323-333. (44) Kung, A. H.; Young, J. F.; Harris, S. E. Appl. Phys. Lett. 1973, 22, 301303. (45) Kung, A. H.; Young, J. F.; Harris, S. E. Appl. Phys. Lett. 1976, 28, 239. (46) Nir, E.; Hunziker, H. E.; Vries, M. S. Anal. Chem. 1999, 71, 1674-1678. (47) Steenvoorden, R. J. J. M.; Hage, E. R. E.; Boon, J. J.; Kistemaker, P. G.; Weeding, T. L. Org. Mass Spectrom. 1994, 29, 78-84. (48) Namiesnik, J. Chromatographia 1983, 17, 47-48. (49) Namiesnik, J. J. Chromatogr. 1984, 300, 79-108.

Figure 4. Influence of the ionization techniques on fragmentation in mass spectra of n-nonane: the upper trace (A) shows a conventional 50 eV EI-MS spectrum; the middle trace (B), a 118-nm SPITOFMS spectrum without separation of the primary 355-nm radiation; and the bottom trace (C), a 118-nm SPI-TOFMS spectrum with separation of the primary 355-nm radiation.

oped for the mobile mass spectrometer. The unit is mounted in a 19-in. drawer (Figure 3A). The main piece of the calibration unit is a temperature-controlled (26 ( 0.1 °C) aluminum block containing a glass tubing system through which a constant flow of 10 mL/min precleaned and prethermalized (26 °C) air is drawn. The air flow is generated by a small membrane pump and a flow controller system. The temperature control is performed by a Peltier element-based heating/cooling system. The permeation tubes, which are filled with the calibration standard compounds, are placed in the thermalized glass tubing system. The standard compounds permeate at a defined and constant permeation rate through the Teflon membranes of the permeation tubes and are seeded in the air flow, forming the calibration standard gas. The concentrations of the different components in the calibration standard gas can be determined by taking into account the gravimetrically determined weight loss rate of the respective permeation tube (approximately a few micrograms-per-hour range) and the air flow rate. The Instrument Steering and Data Acquisition System. A 400 MHz computer in a 19-in. drawer installed in the mass spectrometer controls all of the electric potentials for the TOFMS. Furthermore, the same computer controls the motor used for the tiltable mirror (Tarm, Germany; for selection between the REMPI Analytical Chemistry, Vol. 73, No. 15, August 1, 2001

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Figure 5. 118-nm SPI-TOFMS spectrum of a calibration gas mixture containing 6 ( 0.6 ppmv of benzene, toluene, and p-xylene in nitrogen for estimation of the detection limit (see Table 1).

and SPI ionization method) and is used for TOFMS data acquisition and analysis. The TOF mass spectra are recorded via a 250 MHz/1GS/s 128 k transient recorder PC card (Aquiris, Switzerland) at a repetition rate of up to 10 Hz (laser repetition frequency). Using a home-written software package (National Instruments, LabView), the mass spectra can be stored in real time on the hard disk and displayed on the monitor. One hour of acquisition of full mass spectra at a repetition rate of 10 Hz requires about 2 GB of hard disk space. For measurement campaigns, changeable 18GB hard disks and a 24-GB DAT streamer were used to handle the large amount of data. Analysis of the MS data was performed by means of several home-written LabView data analysis software program codes. The total program package, for example, includes subroutines which perform mass calibration, quantification of target compounds via external standardization as well as plotting of mass spectra, ion profiles (versus time), or 3-dimensional plots (see Figure 10). RESULTS AND DISCUSSION In the following, first application results of the developed instrument are presented. In the first part of this section, laboratory results are given and discussed. In the second part, first field application results are shown. This includes headspace measurements of coffee and gasoline samples as well as real-time, on-line monitoring results of waste incineration flue gas and automobile exhaust. Laboratory Results. The performance of the mobile TOFMS system using the single-photon ionization (SPI) option is evaluated by acquiring spectra of pure substances seeded in nitrogen or air. In addition, SPI detection sensitivities for selected compounds have been determined by means of calibrated gas mixtures. Finally, the ability of both the SPI and the REMPI technique for characterization of complex mixtures is compared. For this purpose, headspace spectra of complex samples (gasoline, coffee) were recorded. For evaluation of the SPI performance, n-nonane was chosen as test substance. n-Nonane has an ionization energy (IE) of 9.7 eV,50 and standard EI spectra are available. Alkanes usually undergo massive fragmentation upon EI ionization. This makes on-line analysis of alkane mixtures by EI-MS very difficult. In Figure 4, three mass spectra of n-nonane are given. In Figure 4A, 3596 Analytical Chemistry, Vol. 73, No. 15, August 1, 2001

a conventional EI mass spectrum is shown (50 eV EI, after ref. 50). The spectrum is dominated by fragment ion peaks, and the relative intensity (i.e., the abundance relative to the highest mass peak) of the molecular ion (128 m/z) is below 10%. The lower spectra (Figure 4B,C) were recorded by the developed mass spectrometer system using the SPI technique. The mass spectrum shown in Figure 4B was recorded using SPI (118 nm) without direct separation of the fundamental 355-nm beam (see Experimental and Instrumental Setup section). In this spectrum, the fragmentation is considerably reduced with respect to EI ionization (Figure 4A). The molecular peak now represents the base peak (100% relative abundance). The observed fragmentation is caused by n-nonane cations that have absorbed an additional 355-nm photon. This process is favored by the fact that the excitation energy for cations is red-shifted with respect to the neutral species. To avoid or reduce the fragmentation due to 355-nm photon absorption, a special optical setup for precipitation of the primary 355-nm radiation was used. In Figure 4C, the effect of the removal of the majority of the 355-nm radiation is shown. Only one single fragment remains in the spectrum. Most likely, this residual fragmentation can be suppressed by further optimization of the 355-nm removal efficiency. In addition, a supersonic jet expansion for cooling of the analyte molecules may further reduce fragmentation tendencies of labile molecular species. A low degree of fragmentation is essential for direct on-line analysis of complex mixtures. Thus, for all following SPI-TOFMS measurements, the optical setup where the 355-nm photons are mostly separated from the 118-nm laser pulse was chosen. For a first evaluation of the detection sensitivity of the compact TOFMS in the SPI mode, spectra of calibration gases were recorded. In Figure 5, the 118-nm SPI-TOFMS spectrum of 10 ( 1 ppmv toluene standard gas in nitrogen (Messer-Griessheim, Germany) is shown. 100 single laser shot transients were averaged. A detection limit (DL) of 135 ppbv was estimated for toluene (S/N ) 2) using the formula DL ) 2σc/h, where c is the concentration of the toluene standard; σ, the standard deviation of the noise between 82 and 90 m/z; and h, the toluene peak height in the spectrum (see Figure 5). In analogy, the detection limits of benzene, p-xylene, and nitrogen monoxide were estimated and are listed in Table 1. These results were obtained using the compact Nd:YAG laser, supplying 355-nm pulses of only 7 mJ. As the yield of 118-nm photons from the frequency tripling process goes to the cube of the 355-nm laser pulse intensity, the detection efficiency will be enhanced by at least 2 orders of magnitude if a YAG laser that is 10 times more powerful is used. For a more accurate determination of the detection limits, calibration gases with parts-per-billion volume concentrations will applied in the future. In summary, the developed, compact TOFMS system with VUV source allows efficient but soft SPI ionization and, thus, is well-suited for on-line trace analysis in the parts-per-billion volume region. In the following, exemplary results on complex gas mixtures are shown. Both laser ionization methods, SPI and REMPI, are used to characterize headspace samples of standard gasoline (Figure 6), diesel fuel (Figure 7), and coffee powder (Figure 8). (50) NIST Chemistry WebBook, NIST Standard Reference Database (http:// webbook.nist.gov); February 2000; National Institute of Standards and Technology (NIST); p 69.

Figure 6. SPI- and REMPI-TOFMS headspace measurement of gasoline. The homologue rows are marked as follows: alkanes, asterisk (*); alkenes, 0; methylated benzenes, O; naphthalene, x (see Table 2).

Figure 7. SPI- and REMPI-TOFMS headspace measurement of diesel fuel. The homologue rows are marked as follows: alkanes, *; alkenes, 0; methylated benzenes, O; methylated naphthalenes, x (see Table 3).

The headspace measurement of the automobile fuels (gasoline and diesel fuel) were performed as follows. A few milliliters of the fuel were poured into a beaker. The beaker was sealed by aluminum foil and kept at a temperature of 25 °C. After a few minutes, the tip of the sampling probe was pressed through the aluminum foil. About 2 mL of headspace gas was sampled per minute. The TOFMS instrument was operated in the alternating SPI and REMPI mode (i.e., alternated between REMPI and SPI every second); the laser repetition rate was 10 Hz. Fifty single transients were averaged for the SPI and for the REMPI spectra, respectively. In Figure 6, the headspace results for a standard gasoline are shown. Because the selectivity of the SPI ionization method is restricted, it is important to have additional information on the sample composition for the interpretation of the mass spectra. Gasoline consists of mixtures of mostly alkylated benzenes and aliphatic hydrocarbons (alkanes, alkenes etc.). The concentration of oxygenated compounds (aldehydes, alcohols etc.)

in crude oil-based fuels is relatively low. Taking this into account, a straightforward assignation of the observed peaks in the SPI mass spectrum can be performed (Table 2). The SPI spectrum shows the homologue rows of the alkanes [marked with an asterisk (*), Figure 6] and alkenes (i.e., monoenes, marked with 0 in Figure 6). The homologue row of the alkanes starts with pentane (72 m/z) and leads over hexane (86 m/z) and heptane (100 m/z) to octane (114 m/z). The homologue row of the alkenes starts with butene (56 m/z) and leads over pentene (70 m/z), hexene (84 m/z) and heptene (98 m/z) to octene (112 m/z). The enlarged inset in Figure 6 (upper trace) additionally shows nonene (126 m/z) and nonane (128 m/z). Note that the absence of the butane peak (58 m/z) is due to the ionization energy selectivity. The ionization energy (IE) of butane is 10.57 eV, but the photon energy of the 118-nm photon is only 10.5 eV. Thus, butane is excluded from detectability. Butene, which is visible in the spectrum, exhibits an IE of 9.55 eV (1-butene,50), and thus, it is Analytical Chemistry, Vol. 73, No. 15, August 1, 2001

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Figure 8. SPI- and REMPI-TOF mass spectra of headspace gas over instant coffee powder (see Table 4).

readily detectable by SPI with 118 nm. Note that the IE selectivity can also be used for the discrimination of isomeric compounds if tunable VUV radiation is used. For example, cycloalkanes are isobaric to the respective alkenes. However, the alkenes generally have a lower IE with respect to the corresponding cycloalkanes (e.g., cyclohexane, 9.88 eV and 1-hexene, 9.46 eV50). Thus, a selective detection of hexenes can be performed in the photon energy range of 9.46-9.87 eV (131.1-125.6 nm). In addition to the aliphatic compounds, the homologue row of the alkylated benzenes (marked with O Figure 6) is clearly visible, too. The REMPI method, however, is by far more selective and shows solely aromatic species at the chosen wavelength of 266 nm. Besides the alkylated benzene derivatives (marked with O in Figure 6), naphthalene (marked with x in Figure 6, see inset) is also visible. Note that the peak at 128 m/z is not due to the isobaric nonane molecule, as it is the case of the SPI spectrum. The REMPI sensitivity at 266 nm for detection of naphthalene is extremely high.16 In Figure 7, the SPI and REMPI headspace spectra of diesel fuel are shown. In diesel fuel, the relative concentration of the more volatile methylated benzenes (marked with O in Figure 7) is lower, as in gasoline. On the other hand, the concentration of the semivolatile polycyclic aromatics is increased. In addition, the 3598


homologue pattern of the aliphatic compounds is shifted in comparison to gasoline. The higher homologues of the aliphatic compounds are more abundant. The differences in the chemical composition and the volatility of the main constituents reflect the different fractionation of gasoline and diesel fuel in the refining process. The SPI spectra of the diesel fuel (Figure 7) and of gasoline (Figure 6) clearly show these differences. In the case of the SPI mass spectrum of diesel fuel, the homologue row of the alkanes [marked with an asterisk (*) in Figure 7] could be followed up to dodecane (170 m/z). In addition, clearly the homologue row of the alkenes is visible (marked with 0 in Figure 7). In the corresponding diesel headspace REMPI mass spectrum also, the homologue rows of the methylated indanes and naphthalenes are clearly visible. Note that the homologue row of the methylated naphthalenes (m/z 128, 142,- 156, etc., marked with x in Figure 7) is isobaric to the higher members of the homologue row of the alkanes [marked with an asterisk (*), starting from octone, m/z 114, in Figure 7], which are visible only in the SPI spectrum. Because of the selectivity of REMPI, it is possible to discriminate between these compound classes, because the REMPI spectrum shows only the naphthalene derivatives, but the SPI spectrum shows the sum of the respective naphthalene derivative and the corresponding isobaric alkane. However, in the

Figure 9. Photograph of the SPI-REMPI-TOFMS device during on-line measurements of automotive exhausts and during a measurement campaign at a 24 MW waste incineration plant.

presented application, the concentration of naphthalene derivatives in the headspace of the fuels is in the low parts-per-billion volume region and, thus, will show no recognizable SPI signal. The assignation of the peaks observed in the diesel fuel headspace REMPI and SPI mass spectra is given in Table 3. In summary, the REMPI/SPI application for the analysis of crude oil-based fuel vapors demonstrates the high potential of methods and instrumentation for fast routine analysis and on-line process analysis of hydrocarbon mixtures in the chemical-, coal-, and mineral oil-based industries. In this context, on-line monitoring-feedback process control applications for cracking/refining processes or for oxidation and reduction processes are particular promising. The combination of the highly selective REMPI ionization process and the semi selective SPI ionization process for mass spectrometry is also feasible for fast characterization of complex mixtures in food chemistry. REMPI has been previously applied for characterization of instant coffee powder headspace and for the monitoring of target compounds in the coffee roasting process.12 The REMPI technique allows the selective and sensitive detection of aromatic species. In the case of coffee headspace or processing gas, this includes phenolic compounds such as cresols and guaiacols as well as nitrogen heterocyclic compounds like caffeine and indole derivatives. In Figure 8B, a headspace REMPI mass spectrum of an instant coffee powder sample is shown. Several hundred compounds are present in the headspace in the parts-per-million volume to parts-per-billion volume concentration

range; however, as mentioned above, only aromatic species are visible in the spectrum as a result of the selectivity of the resonance ionization process.12,51 The spectrum shows homologue rows of methylated phenols (m/z 94, 108, 122, and 136), methylated styrenes (m/z 104, 118, 132, and 146), methylated dihydroxybenzenes/guaiacols (m/z 110, 124, and 138), and methylated vinylphenols (m/z 120, 134, 148, and 162) as well as methylated dihydroxystyrenes/vinylguaiacols (m/z 136 and 150). Note that mass peaks may represent an ensemble of isomeric or isobaric compounds. However, it is known from conventional analyses and from the genesis of the phenolic and vinylic compounds (i.e., from decarboxilation of organic acids such as t-ferulaacid upon the roasting process) that only a few or even just one of the isobaric species is dominant by far. For example, the peak at 150 m/z is dominated by 4-vinylguiacol (decarboxylation product of t-ferulaacid). The SPI mass spectrum of the same sample shows a totally different picture. With 10.5 eV photons, most organic compounds are ionizable; however, the sensitivity of the SPI technique is lower in comparison to the sensitivity reached by REMPI for the detection of selected aromatics. Thus, the SPI spectrum in Figure 8A shows the major organic composition of the headspace gas. The spectrum is dominated by a homologue row of carbonylic compounds (m/z 44, 58, 72, 86, 100, and 114) and some smaller nitrogen and oxygen heterocyclics (e.g., pyridine, pyrazine, and furane derivatives). In addition, some phenolics and caffeine are (51) Dorfner, R.; Yeretzian, C.; Zimmermann, R.; Kettrup, A. 18th International Conference on Coffee Science (ASIC ‘99), 2000; 136-142.


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Figure 10. On-line-recorded TOF mass spectra of waste incineration flue gas with simultaneous use of the REMPI-, SPI-, and LEI ionization methods: (A) Single mass spectrum; compounds ionized by LEI and REMPI. (B) Single mass spectrum recorded with SPI- and LEI ionization methods. (C) 3D plot of a 25-min measurement sequence; compounds ionized by LEI and REMPI are shown. Two transient disturbances of the combustion conditions cause a variation of the peak pattern. The LEI oxygen signal (32 m/z) shows a drop of the oxygen concentration during the disturbed combustion conditions.

visible in the SPI spectrum. A tentative assignation of the detected species, as given in Table 4, again requires knowledge of coffee chemistry. The composition of coffee has been investigated by several researches and the results are available in the literature52-55 or electronic databases.56 A more detailed, combined REMPI- and SPI-TOFMS study on on-line analysis of coffee roasting effluents will be published soon.57 (52) Vitzthum, O. G.; Werkhoff, P. Z. Lebensm. Unters. Forsch. 1976, 160, 277291. (53) Tressl, R.; Gru ¨nwald, K. G.; Ko¨ppler, H.; Silwar, R. Z. Lebensm. Unters. Forsch. 1978, 167, 108-110. (54) Czerny, M.; Mayer, F.; Grosch, W. J. Agric. Food Chem. 1999, 47, 695699. (55) Grosch, W. Flavour Fragrance J. 1994, 9, 147-158. (56) Database: Volatile Compounds in Food; TNO: The Netherlands, 1996. (57) Dorfner, R.; Ferge, T.; Yeretzian, C.; Zimmermann, R.; Kettrup, A. Anal. Chem. 2001, in preparation.

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Table 1: Estimated SPI-TOF Detection Limits for Selected Compounds with 7 mJ 355-nm Pulse Energya compound

detection limit, S/N ) 2

nitrogen monoxide benzene toluene p-xylene

1.1 ppmv 160 ppbv 135 ppbv 150 ppbv

a Estimated 118-nm VUV pulse energy, 10-6 mJ. The detection limits (DL) were estimated from calibration gas mass spectra (see text and Figure 5 for detailed procedure). One hundred single laser shot transients were averaged (10-s measurement time) for the DL estimation.

In summary, the shown headspace measurements of mineral oil products and coffee powder demonstrate the ability of the methods used and the instrumentation for a fast, comprehensive

Table 2: Assigned Peaks in the Headspace REMPI and SPI-mass Spectra of a Gasoline Samplea,b mass m/z 56 68 70 72 78 82 84 86 92 98 100 106 112 114 120 128 134

assigned compds in the REMPI spectrum

benzene

toluene xylenes, ethylbenzene C3-benzene (trimethylbenzene) naphthalene C4-benzene (tetramethylbenzene)

Table 3: Assigned Peaks in the Headspace REMPI and SPI Mass Spectra of a Diesel Fuel Samplea,b

assigned compounds in the SPI spectrum

mass m/z

butene pentadiene pentene, cyclopentane pentane benzene hexadiene, cyclohexene hexene, cyclohexane hexane toluene heptene, cycloheptane heptane xylenes, ethylbenzene octene, cyclooctane octane C3-benzene (trimethylbenzene) nonane C4-benzene (tetramethylbenzene)

56 70 78 82 84 86 92 98 100 106 112 114 118 120

Figure 6. b The gray row shows an example for monitoring of different isobaric substances with REMPI and SPI, respectively.

124 126 128 132 134

a

characterization of complex samples. On the one hand, the pattern of major compounds can be recorded by SPI mass spectrometry, while on the other hand, specific minor and trace compounds can be monitored by the REMPI mass spectrometry technique. Note that the REMPI-TOFMS selectivity can be tuned by tuning the laser wavelength and eventually the use of a supersonic jet inlet system. For on-line monitoring of waste incineration flue gases, for example, monochlorobenzene, representing a surrogate for the emission of highly toxic polychlorinated dibenzodioxins and dibenzofurans, was recorded at a laser wavelength of 269.82 nm.18 First Field Applications. As mentioned above, industrial field application of the REMPI-TOFMS method have been performed since 1996.13 REMPI-TOFMS measurements of aromatic species in automotive exhaust were started in 1993.58 Recently, a first application of SPI with ion trap mass spectrometry (ITMS) for on-line detection of aromatic compounds (benzene, toluene, and xylene) from automotive exhaust was reported.25 In this paper, the first field applications of the newly developed instrumentation for simultaneous monitoring of SPI, REMPI, and EI mass spectra are presented. This includes results from a measurement campaign at a municipal waste incinerator as well as on-line automotive exhaust measurements. In Figure 9, photographs of the developed mobile REMPI-SPI-LEI-TOFMS instrument at the measurement sites in the waste incinerator and the automotive testing area are shown. The developed SPI-REMPI-LEI-TOFMS instrument was tested for flue gas monitoring at a German 23 MW municipal waste incineration plant. The flue gas was sampled at about 700 °C and transferred by a heated transfer line (200 °C) to the inlet system (200 °C) of the mass spectrometer. Details on the flue gas sampling technique for highly time-resolved, sensitive, and selective detection of traces of aromatic species by REMPI-TOFMS have been described in the literature.16-18 Because of the high (58) Boesl, U.; Weickhardt, C.; Zimmermann, R.; Schmidt, S.; Nagel, H. SAE Technol. Pap. 1993, 930083, 61-69.

138 140 142 146 148 152 154 156 160 162 166 168 170


benzene

toluene xylenes, ethylbenzene indane C3-benzene (trimethylbenzene) naphthalene methylindane C4-benzene (tetramethylbenzene) methylnaphthalene C2-indane C5-benzene C2-naphthalene C3-indane C6-benzene

assigned compds in the SPI spectrum butene pentene, cyclopentane benzene cyclohexene hexene, cyclohexane hexane toluene heptene, cycloheptane heptane xylenes, ethylbenzene octene, cyclooctane octane C3-benzene (trimethylbenzene) nonadien nonene nonane C4-benzene (tetramethylbenzene) decadiene decaene decane C5-benzene undecadiene undecene undecane dodecadiene dodecene dodecane

a Figure 7. b The gray rows show examples for monitoring of different isobaric substances with REMPI and SPI, respectively.

temperature in the flue gas channel and the nearly complete combustion achieved in modern waste incineration plants, the concentrations of prominent aromatic products of incomplete combustion (PIC) like naphthalene or benzene usually are remarkably low. Naphthalene, for example, is present in the low parts-per-billion volume or high parts-per-trillion volume range in the flue gas. Thus, the detection of the aromatic PIC compounds is beyond the capabilities of the SPI method in the present setup and feasible by the REMPI method only. The SPI approach, however, can be used readily for highly time-resolved detection of nitrogen monoxide. Furthermore, the LEI technique, which can be combined with either the SPI or the REMPI method, allows the collection of relevant data on the primary combustion chemistry by monitoring the concentration of water, oxygen, and carbon dioxide. In Figure 10A, an on-line mass spectrum of waste incineration flue gas is shown, which was recorded with combined REMPI and LEI ionization. The higher mass region (m/z g 78) of the spectrum shows peaks due to REMPI ionization of aromatic compounds, and a REMPI “pattern” of the aromatic compounds present in the flue gas is obtained. In addition to monocyclic aromatics, polycyclic aromatic hydrocarbons (PAH) are also prominent. The pattern of the aromatic compounds reflects the trace chemistry of fire, showing the efficiency of the combustion process, and the effects Analytical Chemistry, Vol. 73, No. 15, August 1, 2001

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Table 4: Assigned Peaks in the Headspace REMPI and SPI Mass Spectra of Instant Coffee Powdera,b mass, m/z


44 48 58 72 74 79 80 82 86

suggested assignation of peaks in the SPI spectrum acetaldehyde methanthiol acetone, propanal butanone, butanal butanol pyridine pyrazine methylfuran 2,3-butadione, pentanone, pentanal

92 94 96 98 100

toluene phenol

104 106 108 110 118 120 122 124 132 134 136 148 150 164 194

styrene xylenes cresoles dihydroxybenzenes methylstyrenes phenylacetaldehyde dimethylphenols guaiacol dimethylstyrene ? ? ? 4-vinylguaiacol dimethoxystyrene caffeine

phenol, methylpyrazin furfural, dimethylfuran furfuryl alcohol 2,3-pentadione, hexanone, hexanal cresoles dihydroxybenzenes dimethylphenols

caffeine

a Figure 8. b The assignation of the SPI spectrum is tentatively and supported by literature results on the occurrence of volatile compounds in coffee.56

of process control measures and malfunctions.16,59 For example, the PAH patterns indicate periods of increased formation of pollutants (PAHs themselves and chlorinated dibenzodioxins and -furans) after disturbed combustion conditions.59,60 Further highly transient changes of the combustion conditions can be detected by the fluctuation/peaking of species such as toluene or naphthalene.17 In the lower mass range (m/z e 44), the laser-induced electron impact (LEI) signals of the bulk flue gas components are visible. This includes combustion products such as water (18 m/z) and carbon dioxide (44 m/z) as well as the “combustion educt” oxygen (32 m/z) and the inert matrix nitrogen (28 m/z). The particularly interesting aspect of the spectrum is that combination of the LEI and REMPI techniques allows a simultaneous monitoring of the fundamental combustion products that are present in the percentage range as well as the relevant trace chemicals which occur in the parts-per-billion volume to partsper-trillion volume concentration range in one spectrum. This is relevant for combustion diagnostic applications, because the behavior of the bulk combustion products, such as CO and CO2, often shows considerable differences from the behavior of the trace chemicals (which often are quite relevant as pollutants). In Figure 10C, a combined LEI- and REMPI-mass spectrometry (59) Zimmermann, R.; Blumenstock, M.; Schramm, K.-W.; Kettrup, A. Organohalogen Compd. 2000, 46, 78-81. (60) Zimmerman, R.; Blumenstock, M.; Heger, H. J.; Schramm, K.-W.; Kettrup, A. Environ. Sci. Technol. 2001, 35, 1019-1030.

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Figure 11. On-line-recorded SPI- and REMPI-signals of NO (SPI) and naphthalene (REMPI) from waste incineration flue gas.

measurement sequence with a duration of 25 min is shown as a 3-dimensional plot (mass × log ion signal × time). In the front, at low mass numbers, the LEI signals of oxygen (32 m/z) and CO2 (44 m/z) are visible. In the background at higher mass numbers, the REMPI signals of the aromatic flue gas contaminants are visible. The oxygen trace shows two dips representing transient oxygen shortages. Corresponding to these oxygen shortages, peaks in the CO2 concentration are observed. The fluctuation of the CO2/O2 concentrations is caused by transient increases in the heating value (e.g., resulting from spikes of highly calorific gas from the grate). Considering the aromatic compounds, it is obvious that the two observed oxygen shortages have a different impact on the trace compounds, respectively. The first transient phase of oxygen shortage induces a highly dynamic emission spike of the polyaromatic species indane and naphthalene. The second oxygen shortage event has no impact on the formation of aromatic combustion byproducts. If the flue gas is analyzed by SPI, as shown in Figure 10B and mentioned above, only nitrogen monoxide (30 m/z) can be detected by SPI with the low-intensity compact Nd:YAG laser that was used. Note that the broader appearance of the LEI signals in the SPI mass spectrum (Figure 10B) with respect to the LEI signals in the REMPI mass spectrum (Figure 10A) is due to the differences in adjustment of the 118- and 266-nm beam. The instrument allows a quasi-parallel use of the REMPI-LEI and SPI ionization techniques (mirror-switching technique, see Figure 2). Figure 11 shows a parallel SPI and REMPI measurement of waste incineration flue gas. The concentration curve of NO is taken from the SPI spectra; the one of naphthalene, from the corresponding REMPI spectra. The quantification was performed by means of an external standardization as described in the Experimental Section. The figure shows that increased concentrations of aromatic species due to fluctuations of the fuel/ oxygen ratio show no influence on the nitrogen monoxide concentration at the investigated measurement point (beginning of boiler section, 700 °C). In addition to the field application at the waste incinerator, automotive exhaust was also investigated. The combustion conditions in internal combustion engines are highly instationary. This causes significantly higher emissions of unwanted combustion byproducts and of fuel residues in comparison to industrial combustion processes. Because of their selectivity, low rate of fragmentation, sensitivity, and speed, the REMPI laser mass spectrometric methods are well-suited for the analysis of the exhaust of combustion engines.10,11,58,61 In this work, the first on-

Figure 12. On-line-recorded SPI- and REMPI-TOF mass spectra of automotive exhaust (see Table 5).

line automotive exhaust SPI measurements using a TOF mass analyzer are presented. Further REMPI and SPI spectra were recorded quasi-parallel. A Volkswagen (VW) Golf car, built in 1990 with a 55 PS gasoline engine, equipped with a regulated threeway catalytic converter for exhaust gas cleaning, was used for on-line exhaust gas measurements. Figure 12 shows a SPI and a REMPI mass spectrum of the exhaust gas, recorded during enhanced speed with no load. One hundred single laser shot transients were averaged for the spectra. The REMPI spectrum (top) shows the homologue rows of methylated benzene derivatives (78, 92, 106, 120, and 134 m/z) as well as naphthalene (128 m/z) and methylnaphthalene (142 m/z). The SPI spectrum (bottom) shows the more abundant aromatic species (i.e., the methylated benzene derivatives at 78, 92, 106, and 120 m/z), too. In addition to the prominent aromatic species, nitrogen monoxide (30 m/z) and the homologue rows of several aliphatic hydrocarbons could be observed; however, as in exhaust gases, oxygenated species also occur frequently, and mass interferences cannot be a priori excluded. With 118 nm, for example, the isobaric compounds hexane, pentanone, and pentanal can be ionized. A more specific determination of the oxygenated compounds would require VUV pulses with lower photon energies. The observed peaks and assigned compounds for the REMPI and the SPI spectrum of the automotive exhaust gas are given in Table 5. By using the SPI technique with the full 10 Hz laser repetition frequency, it is possible to record the emission dynamic of the aromatic species. The intense signals of the aromatic compounds induce a high-frequency detector-ringing signal. This is due to (61) Boesl, U.; Zimmermann, R.; Nagel, H. In Analytiker Taschenbuch; Gu ¨ nzler, H., Ed.; Springer: Berlin, 1998; Vol. 19, pp 163-213.

Table 5: Assigned Peaks in the On-Line Recorded REMPI and SPI Mass Spectra of Automobile Exhaust Gasa,b mass m/z 28 30 40 54 56 66 68 70 72 78 84 86 92 98 100 106 112 114 118 120 126 128 132 134 142


benzene toluene xylenes, ethylbenzene indane C3-benzene (trimethylbenzene) naphthalene methylindane C4-benzene (tetramethylbenzene) methylnaphthalene

assigned compds in the SPI spectrum nitrogen (LEI signal) nitrogen monoxide propyne butadiene acrolein, butene ? pentadiene butenal, pentene, cyclopentane butanale, butanone, pentane benzene pentenal, hexene, cyclohexane pentanale, pentanone, heptane toluene hexenal, heptene hexanal, hexanone, heptane xylenes, ethylbenzene heptenal, octene heptanal, heptanone, octane C3-benzene (trimethylbenzene) octenal, nonene octanal, octanone, nonane C4-benzene (tetramethylbenzene)

a Figure 12. b The gray row shows an example for monitoring of different isobaric substances with REMPI and SPI, respectively.

the impedance mismatch between the 50 Ω signal cable and the electron collector in the multisphere detector with the central hole. Analytical Chemistry, Vol. 73, No. 15, August 1, 2001

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TOFMS on-line sequence. The start of the engine results in a steep increase in the BTX compounds; however, with increasing catalyst temperature, the concentrations level off, reaching a constant level of about 10 ppmv benzene and 20 ppmv toluene after ∼3 min (idle run at no load and slightly increased revolutions). Short acceleration pulses at the end of the measurement sequence cause transient emission peaks. The right side of Figure 13 shows a similar measurement; however, at the end of the measurement time, a short high-speed acceleration phase was applied in this case (no load). The high-speed acceleration causes a relatively prominent emission of BTX compounds. In summary, the SPI-TOFMS gives similar information on the prominent aromatic species as achievable by REMPI-TOFMS. Furthermore, SPI-TOFMS gives results on nitrogen monoxide and on aliphatic compounds; however, monitoring of traces of polyaromatic compounds requires the more sensitive REMPI-TOFMS method.

Figure 13. On-line-recorded SPI-TOFMS measurement of automotive exhaust: concentration-time profile of benzene/toluene/xylene during transient changes in engine revolution.

Recently, a new approach for the avoidance of detector ringing in in-line reflection TOFMS instruments with central hole detectors was described.62 In Figure 13, the concentration profiles of benzene, toluene, and the xylene isomers (BTX compounds) were recorded by SPITOFMS during dynamic operation conditions. The benzene and toluene traces are quantified via external calibration.16 The sampling was performed in a manner similar to that described in the literature.13 The left side of Figure 13 shows a 450-s SPI(62) Cornish, T. J.; Ecelberger, S.; Brinckerhoff, W. Rapid Commun. Mass Spectrom. 2000, 14, 2408-2411.

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ACKNOWLEDGMENT The authors thank the members of the GSF laser mass spectrometry group, H. J. Heger, R. Dorfner, K. Hafner, T. Ferge, J. Maguhn, T. Hauler, and M. Blumenstock for contributions during the field measurement campaign and U. Boesl for continuous interest in their work. Funding from the Deutsche Bundesstiftung Umwelt, Osnabru¨ck, Germany (grant no. 12447), the German Ministry of Research, BMBF (HGF-Strategiefondsprojekt “Stickoxidminderung”, grant no. 01SF9920/7), and from Nestec Ltd., Lausanne, Switzerland (Nestle´ Research Center) is gratefully acknowledged.

Received for review January 8, 2001. Accepted May 23, 2001. AC010023B

A Mobile Mass Spectrometer for Comprehensive On-Line Analysis of

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