Single Photon Ionization Time-of-Flight Mass Spectrometry with a

Real-time trace detection of security-relevant compounds in complex sample matrices by thermal desorption–single photon ionization–ion trap mass ...
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Anal. Chem. 2005, 77, 7408-7414

Single Photon Ionization Time-of-Flight Mass Spectrometry with a Pulsed Electron Beam Pumped Excimer VUV Lamp for On-Line Gas Analysis: Setup and First Results on Cigarette Smoke and Human Breath F. Mu 1 hlberger,*,† T. Streibel,†,‡ J. Wieser,§ A. Ulrich,| and R. Zimmermann†,‡,⊥

Institute for Ecological Chemistry, GSF-National Research Center for Environment and Health, D-85764 Neuherberg, Germany, Institut fu¨r Physik, Universita¨t Augsburg, D-86159 Augsburg, Germany, TuiLaser AG, D-81379 Munich, Germany, Fakulta¨t fu¨r Physik E12, Technische Universita¨t Mu¨nchen, D-85748 Garching, Germany, and BIfA GmbH, D-86167 Augsburg, Germany

Single-photon ionization (SPI) using vacuum ultraviolet (VUV) light produced by an electron beam pumped rare gas excimer source has been coupled to a compact and mobile time-of-flight mass spectrometer (TOFMS). The novel device enables real-time on-line monitoring of organic trace substances in complex gaseous matrixes down to the ppb range. The pulsed VUV radiation of the light source is employed for SPI in the ion source of the TOFMS. Ion extraction is also carried out in a pulsed mode with a short time delay with respect to ionization. The experimental setup of the interface VUV light source/ time-of-flight mass spectrometer is described, and the novel SPI-TOFMS system is characterized by means of standard calibration gases. Limits of detection down to 50 ppb for aliphatic and aromatic hydrocarbons were achieved. First on-line applications comprised real-time measurements of aromatic and aliphatic trace compounds in mainstream cigarette smoke, which represents a highly dynamic fluctuating gaseous matrix. Time resolution was sufficient to monitor the smoking process on a puff-bypuff resolved basis. Furthermore, human breath analysis has been carried out to detect differences in the breath of a smoker and a nonsmoker, respectively. Several wellknown biomarkers for smoke could be identified in the smoker’s breath. The possibility for even shorter measurement times while maintaining the achieved sensitivity makes this new device a promising tool for on-line analysis of organic trace compounds in process gases or biological systems. The combination of photoionization (PI) and mass spectrometry (MS) has attracted great interest in recent years for the online analysis of organic species, since it has several advantages * Corresponding author. E-mail: [email protected]. † GSF-National Research Center for Environment and Health. ‡ Universita¨t Augsburg. § TuiLaser AG. | Technische Universita¨t Mu ¨ nchen. ⊥ BIfA GmbH.

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compared to other ionization techniques commonly used in MS approaches. Most important is the relatively low energy supplied by the utilized photons, which is in the range of the ionization potential of most organic molecules. Therefore, only little excess energy is deposited in the investigated molecules, causing almost no fragmentation of the produced molecular ions. This is of advantage for the analysis of complex mixtures due to the absence of overlapping fragment peaks. Furthermore, the choice of the excitation wavelength gives rise to high selectivity, and combined with an appropriate inlet system, high sensitivity can also be achieved. Together with resonance-enhanced multiphoton ionization,1-3 single-photon ionization (SPI) with vacuum ultraviolet (VUV) light4-8 is one of the most efficient photoionization methods. For the latter, selectivity is provided via the ionization potential (IP), since only compounds with an IP lower than the respective photon energy can be ionized. For instance, on-line PI-MS has been applied for detection and monitoring of organic trace compounds in gaseous matrixes,9-19 (1) Boesl, U.; Neusser, H. J.; Schlag, E. W. Z. Naturforsch. 1978, 33A, 15461548. (2) Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 54, 660-665. (3) Hager, J. W.; Wallace, S. C. Anal. Chem. 1988, 60, 5-10. (4) Miller, J. C.; Compton, R. N. J. Chem. Phys. 1982, 76 (8), 3967-3973. (5) Nir, E.; Hunziker, H. E.; Vries, M. S. Anal. Chem. 1999, 71, 1674-1678. (6) Pallix, J. B.; Schu ¨ hle, U.; Becker, C. H.; Huestis, D. L. Anal. Chem. 1989, 61, 805-811. (7) Van Bramer, S. E.; Johnston, M. V. J. Am. Soc. Mass Spectrom. 1990, 1, 419-426. (8) Butcher, D. J. Microchem. J. 1999, 62, 354-362. (9) Tanada, T. N.; Velazquez, J.; Hemmi, N.; Cool, T. A. Combust. Sci. Technol. 1994, 101, 333-348. (10) Tembreull, R.; Lubman, D. M. Anal. Chem. 1984, 56, 1962-1967. (11) Butcher, D. J.; Goeringer, D. E.; Hurst, G. B. Anal. Chem. 1999, 71, 489496. (12) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46-57. (13) Oser, H.; Thanner, R.; Grotheer, H.-H. Combust. Sci. Technol. 1996, 116117, 567-582. (14) Oudejans, L.; Touati, A.; Gullett, B. K. Anal. Chem. 2004, 76, 2517-2524. (15) Adam, T.; Streibel, T.; Mitschke, S.; Mu ¨ hlberger, F.; Cao, L.; Baker, R. R.; Zimmermann, R. J. Anal. Appl. Pyrolysis 2005, 74, 454-464. (16) Boesl, U. J. Mass Spectrom. 2000, 35, 289-304. 10.1021/ac051194+ CCC: $30.25

© 2005 American Chemical Society Published on Web 10/01/2005

characterization of aerosols,20-22 peptide and protein analytics,23,24 and in atmospheric pressure ionization.25,26 In this regard, laser-based photoionization methods were often utilized. As a matter of fact, VUV light for SPI can be generated by intense short pulse lasers. For example, a wavelength of 118 nm (10.49 eV) is generated by frequency tripling of intensive 355nm third harmonic Nd:YAG UV laser pulses in a rare gas cell27-29 providing VUV light pulses with a time width of some nanoseconds. However, one major drawback of the application of laserbased instrumentation lies in the expensive and sophisticated devices needed for the on-line measurements. To overcome these disadvantages of laser-based devices, utilization of compact, rugged, and less expensive VUV lamps, e.g., deuterium lamps, as photon source has become feasible. However, such conventional VUV lamps often suffer from low spectral resolution in the required frequency range. In addition, for intensive VUV generation, these lamps need water-cooling. Since 1958, the VUV emission of rare gas excimers is known.30 A very efficient excimer light generation is via electron beam excitation of dense rare gases. This requires acceleration of energetic electrons in an evacuated chamber with subsequent guiding of the electron beam through a thin foil into a gas cell. This method has significant advantages such as high efficiency of the light sources. A 35% conversion of electron energy to light was reported for argon,31 and very clean excimer emission spectra were obtained due to an extremely low level of impurities in the gas and low gas temperature during operation. The gas purity is achieved by avoiding debris production, which is a problem in discharge-driven excimer lamps due to electrode erosion. Electron beam pumped excimer lamps are well suited to be operated in pulsed mode, which allows increasing the VUV light intensity significantly. Moreover, the required electron beams can be generated by very compact electron guns. Analysis of multiple species in a rapidly changing matrix requires a fast mass spectrometry method such as time-of-flight mass spectrometry (TOFMS). With TOFMS systems, it is possible to obtain several thousand mass spectra per second, each showing the full spectral information from one respective ionization pulse. (17) Nomayo, M.; Thanner, R.; Pokorny, H.; Grotheer, H.-H.; Stu ¨ tzle, R. Chemosphere 2001, 43, 461-467. (18) Lee, S.-H.; Kajii, Y.; Akimoto, H. Environ. Sci. Technol. 2000, 34, 44344438. (19) Mu ¨ hlberger, F.; Hafner, K.; Kaesdorf, S.; Ferge, T.; Zimmermann, R. Anal. Chem. 2004, 76, 6753-6764. (20) Sykes, D. C.; Woods, E.; Smith, G. D.; Baer, T.; Miller, R. E. Anal. Chem. 2002, 74, 2048-2052. (21) Johnston, M. V. J. Mass Spectrom. 2000, 35, 585-595. (22) Ferge, T.; Mu ¨ hlberger, F.; Zimmermann, R. Anal. Chem. 2005, 77, 45284538. (23) Edirisinghe, P. D.; Lateef, S. S.; Crot, C. A.; Hanley, L.; Pellin, M. J.; Callaway, W. F.; Moore, J. F. Anal. Chem. 2004, 76, 4267-4270. (24) Kim, T.-Y.; Thompson, M. S.; Reilly, J. P. Rapid Commun. Mass Spectrom. 2005, 19, 1657-1665. (25) Hanold, K. A.; Fischer, S. M.; Cormia, P. H.; Miller, C. E.; Syage, J. A. Anal. Chem. 2004, 76, 2842-2851. (26) Constapel, M.; Schellentra¨ger, M.; Scmitz, O. J.; Ga¨b, S.; Brockmann, K. J.; Giese, R.; Benter, T. Rapid Commun. Mass Spectrom. 2005, 19, 326-336. (27) Bjorklund, G. C. IEEE J. Quantum Electron. 1975, QE-11, No. 6, 287296. (28) Maker, P. D.; Terhune, R. W. Phys. Rev. 1965, 137, 801-818. (29) Vidal, C. R. In Tunable Lasers; Mollenauer, L. F., White, J. C., Eds.; SpringerVerlag: Berlin, 1987; Vol. 59, pp 56-113. (30) Tanaka, Y.; Jursa, A. S.; Blanc, F. J. J. Opt. Soc. Am. B 1958, 48, 304. (31) Wieser, J.; Murnick, D. E.; Ulrich, A.; Huggins, H. A.; Liddle, A.; Brown, W. L. Rev. Sci. Instrum. 1997, 68, 1360-1364.

However, there are no reports yet on excimer VUV lamps, which are able to generate light pulses in the range of some nanoseconds (comparable to laser-based VUV sources) due to the time constants of the excimer formation process.32 Thus, excimer VUV lamp TOFMS systems have to be operated with pulsed ion extraction or alternatively equipped with an orthogonal ion source setup.33 In a previous publication,34 coupling of a pulsed electron beam pumped excimer lamp31,35 to a time-of-flight mass spectrometer has been reported. However, this prototype did show relatively high limits of detection in the low-ppm range for aromatic hydrocarbons such as benzene. In addition, the obtained mass resolution was rather low (R50% ) 88 at 85 m/z). Recently, coupling of the electron beam pumped excimer lamp to a quadrupole mass spectrometer (QMS) has been described.36 QMS systems can be operated with a continuous ion beam, which is easily generated with the excimer VUV lamp. On the other hand, they require scanning, which limits the time resolution of the measurement. This is often not a drawback, if the sample contains only some substances of interest. In this case, scan rates in the range of milliseconds are possible by just scanning these specific masses. In this paper, coupling of a pulsed electron beam pumped excimer lamp to a time-of-flight mass spectrometer with pulsed ion extraction is reported. Compared to the abovementioned first prototype,34 significant improvements concerning limits of detection and mass resolution could be achieved. Especially in these features the former device could not meet the requirements for on-line analysis of trace compounds. The technical solutions as well as the experimental setup are addressed, and first measurements demonstrating the instrument’s characteristics and its applicability for trace compound analysis in gaseous matrixes are shown. Instrumentation and Experimental Setup. Herein a TOFMS system with an optimized pulsed TOF ion source in combination with an improved pulsed excimer lamp to provide a compact analysis tool for single-photon ionization mass spectrometry is presented. The basic principle of the electron beam pumped excimer lamp for VUV light generation has been described previously31 as well as the interface between the lamp and the ion source;36 therefore, only a short description thereof is given here. In the lamp used here, excimer molecules are formed via electron beam excitation of a dense gas.31 By choosing various rare gases or gas mixtures, different wavelengths can be generated.31,37-39 The main innovation of the light source is a 0.7 × 0.7 mm2, 300-nm-thick SiNx foil, which separates the rare gas volume (p > 1 bar) from the vacuum chamber containing an (32) Ribitzki, G.; Ulrich, A.; Busch, B.; Kro ¨tz, W.; Wieser, J.; Murnick, D. E. Phys. Rev. E 1994, 50, 3973. (33) Laiko, V. V.; Dodonov, A. F. Rapid Commun. Mass Spectrom. 1994, 8, 720726. (34) Mu ¨ hlberger, F.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2002, 74, 3790-3801. (35) Ulrich, A.; Wieser, J.; Kro¨tz, W. Deutsches Patentamt: Deutschland, 1996. (36) Mu ¨ hlberger, F.; Wieser, J.; Morozov, A.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 2218-2226. (37) Fedenev, A.; Morozov, A.; Kru ¨ cken, R.; Schoop, S.; Wieser, J.; Ulrich, A. J. Phys. D: Appl. Phys. 2004, 37, 1586-1591. (38) Wieser, J.; Salvermoser, M.; Shaw, L. H.; Ulrich, A.; Murick, D. E.; Dahi, H. J. Phys. B: At., Mol. Opt. Phys. 1998, 31, 4589-4597. (39) Morozov, A.; Krylov, B.; Gerasimov, G.; Arnesen, A.; Hallin, R. J. Phys. D: Appl. Phys. 2003, 36, 1126-1134.

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electron gun (EG). The EG generates a 13-15-keV electron beam, which is directed into the rare gas through the SiNx foil with low energy loss. With the intended casing of 0.5-mm sheet iron installed, no increase of the dose rate in a distance of 10 cm from the lamp could be measured. In the dense rare gas, the energetic electrons collide with gas atoms, resulting in the excitation and ionization of these atoms. In successive gas kinetic steps, excited diatomic rare gas molecules (excimers) are formed. The radiative decay of these excimers provides intense VUV radiation. As a result of the high gas pressure, the excimer formation only occurs in a restricted volume behind the electron entrance foil.40 For all measurements presented here, the lamp was filled with 2 bar of argon, which provides an 10-nm broad VUV emission peaked at 126 nm (9.8-eV photon energy).31 With a continuous 10-µA, 13-keV electron beam from the electron gun and a conversion efficiency of beam power of 0.35,31 the argon excimer lamp generates on the order of 45 mW (2.9 × 1016 photons/s) of VUV radiation at 126 nm. Due to a small utilized solid angle, 1% of the totally produced VUV light can be collected and sent through two MgF2 lenses into the ionization zone. It has to be considered that each of the lenses is reflecting and absorbing ∼40% of the radiation at this wavelength. Using this estimate, the ionization zone should be irradiated with 0.16 mW (1.0 × 1014 photons/s). With a semiconductor radiation sensor (type AXUV100, IRD Inc., Torrance, CA) the VUV light intensity in the ionization zone (Figure 1A) was measured to 0.024 mW (1.5 × 1013 VUV photons/s).36 Besides general problems concerning absolutely calibrated measurements of VUV light, the missing factor of ∼6 of light in the ionization zone can be attributed to the following loss mechanisms: imperfect focusing of the electron beam, backscattering of electrons in the gas and the membrane, impurities on the optical surfaces, and limited purity of the lightemitting gas, which can have a huge influence on the efficiency value that is actually achieved. The thermal stability of the ceramic entrance foil for the electron beam represents the limiting factor for increasing the emitted light intensity. The time-averaged thermal stress applied to the entrance foil is proportional to the average electron flux. In pulsed mode of operation, the instantaneous electron current can, however, be increased drastically up to a factor of 80.000 or more.41 For the experiments presented here, the 15-keV electron current was increased by a factor of 550 from 10 µA to 5.5 mA by switching from continuous to pulsed mode. With these settings, the argon excimer lamp produces on the order of 30 W (1.9 × 1019 VUV photons/s) of pulsed VUV power in total. Assuming the same rather unfavorable ratio between calculated light output and semiconductor measurement as mentioned above, this results in a light pulse intensity of 16 mW (∼1.0 × 1016 VUV photons/s) for VUV radiation at the ionization zone. In combination with TOFMS (Figure 1B), the excimer lamp was pulsed with a frequency of 50 Hz to match the ability of the data acquisition. The width of the electron pulses was 1 µs. The repeller and the extraction electrode were pulsed 0.7 µs after the (40) Valkealahti, S.; Schou, J.; Nieminen, R. M. J. Appl. Phys. 1988, 65, 22582271. (41) Morozov, A.; Ulrich, A.; Wieser, J.; Steinhu ¨ bl, R.; Kru ¨ cken, R. Annual Report of the Maier-Leibnitz-Laboratorium fu ¨ r Kern- and Teilchenphysik der Ludwig-Maximilians-Universita¨t Mu ¨ nchen und der Technischen Universita¨t Mu ¨ nchen: Munich, 2003; p 52.

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Figure 1. (A) Schematic drawing of the SPI-TOFMS ion source. (B) Schematic drawing of the electron beam pumped rare gas excimer VUV lamp-TOFMS device.

electron pulses within 10 ns to (820 V for 2 µs to extract the generated ions into the TOFMS (fast push-pull switches, HTS 31-GSM, Behlke Electronic GmbH, Kronberg, Germany). It turned out that a short delay between the light pulse and the extraction reduces the electron impact signal by generated photoelectrons significantly, whereas the photoionization signal is almost not affected. For the measurements presented here, the excimer lamp was coupled to a custom-built reflectron TOFMS system (Stefan Kaesdorf, Munich, Germany). The TOFMS can be used in linear and reflectron modes. When used in reflectron mode, the folded field-free drift region is 380 mm long. The ion source and the flight tube are differentially pumped by two 210 L/s (N2) turbo molecular pumps (TMU 261, Pfeiffer Vacuum, Aslar, Germany). The Wiley-McLaren type ion source42 can be operated with static or pulsed acceleration fields. The repeller electrode and the first extraction electrode are on the same potential with opposite polarity to maintain ground potential at the central ionization region thus minimizing the influence of the grounded inlet needle.12,43 Ions are detected using a two-stage multichannel plate detector (40-mm diameter) with extended dynamic range. The inlet system is based on a continuous molecular beam inlet technique19,44 with a 2-m-long fused-silica capillary of 200-µm i.d, providing a 2 mL/min gas flow into the ion source. This molecular (42) Wiley: W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150-1157. (43) Hafner, K.; Zimmermann, R.; Rohwer, E. R.; Dorfner, R.; Kettrup, A. Anal. Chem. 2001, 73, 4171-4180.

Figure 3. SPI-TOFMS spectrum using the argon excimer lamp for photoionization of a calibration gas mixture containing benzene, toluene, and m-xylene. The concentrations of the standard compounds in nitrogen were ∼10 ppm. Table 1. Limits of Detection for Selected Compounds with the Argon Excimer SPI-TOFMSa compound benzene toluene m-xylene

u

IP (eV)

calcd detection limit (ppb, S/N ) 2)

78 92 106

9.24 8.83 8.55

27 30 40

a Nominal mass, ionization potential,60 and calculated limit of detection (average of 50 spectra, recorded in 1 s, S/N ) 2) are shown.

Figure 2. Photograph of the mobile SPI-TOFMS system.

gas beam is hit by the VUV light pulses directly underneath the capillary tip43,45 (Figure 1A). The ion source of the TOFMS has slits in the extraction electrodes (Figure 1A). The flight path of the gas molecules is parallel to the alignment of the exit slit in the extraction electrode. The ionization takes place at the top level of the slit (compare Figure 1A) while the extraction field is turned off. Generated ions fly within the gas stream parallel to the slit. After the lamp has been turned off, the extraction field is switched on, and all positive ions are accelerated toward the extraction electrode. The ions that were located in front of the slit will exit the ion source and enter the TOF region. The advantage of this setup is the extension of the extraction volume, which allows a short accumulation of ions in the ion source resulting in an improvement of the detection limit. All parts of the excimer lamp TOFMS system are mounted in a mobile 19-in. rack (59 × 77 × 150 cm, width × length × height) as shown in Figure 2. RESULTS AND DISCUSSION Characterization of the Instrument. The instrument was characterized with a calibration standard gas (10-L, 200-bar steel cylinder, Messer Griessheim GmbH) containing benzene (78 (44) Zimmermann, R.; Heger, H. J.; Kettrup, A.; Boesl, U. Rapid Commun. Mass Spectrom. 1997, 11, 1095-1102. (45) Mu ¨ hlberger, F.; Zimmermann, R.; Kettrup, A. Anal. Chem. 2001, 73, 35903604.

m/z), toluene (92 m/z), and m-xylene (106 m/z) with a respective concentration of 10 ppm in nitrogen. An example of a single-photon ionization time-of-flight mass spectrum is shown in Figure 3, depicting the response signal of the substances in the gas standard. Fifty consecutive single spectra (generated with 50 Hz) have been averaged, resulting in a measurement period of 1 s. It is obvious from the spectrum that only the molecular peaks are visible and no fragment peaks appear. Parts of the ion source are irradiated by VUV light and emit photoelectrons. Electrons between the extraction plates are accelerated by the electric field and cause electron impact ionization. This effect has been considerably reduced with the current setup. Consequently, the remaining signal resulting from molecular nitrogen (28 m/z) is negligible compared to the hydrocarbon signals, albeit the content of nitrogen in the mixture exceeds the concentration of the calibration gases by a factor of 105. The spectra of the calibration gas were also used to calculate the respective limits of detection (LOD) for benzene, toluene, and m-xylene, assuming a minimum detection limit for a signal (S) twice as high as the statistic noise (N) of the baseline (S/N ) 2). Results for the investigated substances are given in Table 1 along with their ionization potentials. Limits of detection were found to be in the 50 ppb region. This is an improvement by 2 orders of magnitude compared to the first excimer lamp TOFMS system reported in 2002.34 Linearity of single-photon ionization with VUV light over several orders of magnitude has been proven with various systems46,47 and is therefore assumed for the current setup, too. Analytical Chemistry, Vol. 77, No. 22, November 15, 2005

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To further improve the sensitivity, various optimizations of the experimental setup may be considered such as the application of a special Al/MgF2-coated spherical mirror (reflectivity 50-70%) to reintroduce the VUV beam into the ionization source to increase the number of photons.48 A second advantage of such a mirror would be the decrease in number of photoelectrons generated in the ion source responsible for a part of the background noise by guiding the VUV beam out of the ion source back through the special VUV optics into the lamp. Furthermore, the utilization of different optical elements to utilize a larger solid angle of the emitted VUV radiation of the lamp would lead to a decrease of LOD. For instance, a system consisting of lenses with large curvature as well as elements to correct the aberration or the application of an ellipsoidal mirror to image the emission point of the lamp into the ionization region could serve this purpose. Nevertheless, implementation of an improved data acquisition based on a high-speed signal-averaging card, which can record full spectra with frequencies of more than 1 kHz, should enable us to reach either much better detection limits within the same measurement time as the values given in Table 1 or the same values with a significantly increased time resolution (recording more spectra within the same time period). Mass resolution of the system is a critical point, since the excimer lamp cannot be pulsed in the range of nanoseconds, which enforces a pulsed ion extraction. The calculated mass resolution for the argon excimer lamp TOFMS system presented here is R50% ) 650 at 112 m/z, which is a considerable improvement to the first excimer lamp TOFMS system described in ref 34. On-Line Cigarette Smoke Measurements. A challenging application for the newly developed VUV lamp time-of-flight mass spectrometer system consists of the investigation of cigarette smoke, since this is a highly complex and dynamic matrix containing many chemical species,49 most of them trace compounds, which undergoes rapid changes in concentration levels and overall composition. Determination of compound levels in cigarette mainstream smoke, i.e., the part of the smoke drawn through the filter or mouth end of the cigarette, which is inhaled by the smoker, is usually carried out by means of a smoking machine. This device enables standardized and thus comparable measurements according to the International Organization of Standardization (ISO). Cigarettes are stored for several days under controlled conditions of 60% relative air humidity and 22 °C and smoked by complying with the standardized puff scheme (puff volume 35 mL, puff duration 2 s, 1 puff/min).50 However, for puffby-puff resolved on-line measurements, the standard Borgwaldt single-port smoking machine had to be slightly modified to reduce interpuff contamination and memory effects caused by its large dead volume. Details of the modification can be found in ref 46. Consequently, the heated transfer line (250 °C) of the excimer lamp SPI-TOFMS device was connected to the outlet of the modified smoking machine (see Figure 4A). A commercially (46) Mitschke, S.; Adam, T.; Streibel, T.; Baker, R. R.; Zimmermann, R. Anal. Chem. 2005, 77, 2288-2296. (47) Tonokura, K.; Nakamura, T.; Koshi, M. Anal. Sci. 2003, 19, 1109-1113. (48) Suzuki, Y.; Maeno, M.; Ikehata, T.; Kitada, N.; Kirihara, N.; Ozaki, T.; Kimura, H. Anal. Sci. 2001, 17, i563-i566. (49) Dube, M. F.; Green, C. R. Recent Adv. Tobacco Sci. 1982, 8, 42-102. (50) Plunkett, S.; Parrish, M. E.; Shafer, K. H.; Nelson, D.; Shorter, J.; Zahniser, M. Vib. Spectrosc. 2001, 27, 53-63.

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Figure 4. (A) Sampling setup for the cigarette smoke measurements. (B) Sampling setup for breath analysis.

Figure 5. SPI-TOFMS spectrum of mainstream cigarette smoke obtained by averaging 130 consecutive single spectra (measurement time 2.6 s) thus covering one single puff.

available cigarette was smoked according to the conditions described above. Between two consecutive puffs two cleaning puffs were inserted, whereby merely pure air was sucked through the system to account for any substance left over in the smoking machine. Figure 5 shows a SPI-TOFMS spectrum of the second puff obtained by averaging 130 consecutive single spectra resulting in a measurement time of 2.6 s, thus covering the whole puff. The mass spectrum is on one hand dominated by unsaturated aliphatic hydrocarbons, e.g., propene (42 m/z), butadiene (54 m/z), butene (56 m/z), cyclopentadiene (66 m/z), and pentene (70 m/z). On the other hand, various aromatic hydrocarbons, e.g., benzene (78 m/z) and its alkylated derivatives toluene (92 m/z), xylene (106 m/z), and mesitylene (120 m/z) as well as phenol (94 m/z) and its alkylated derivatives cresol (108 m/z) and xylenol (122 m/z), are abundant. Furthermore, there is the carbonylic species acetone (58 m/z) and some oxygen heterocyclic aromatic compounds prevalent such as furan (68 m/z), methylfuran (82 m/z), and 2,5-dimethylfuran (96 m/z). Finally, nicotine (162 m/z) can also be detected. However, it has to be noted that with the available mass resolution isobaric compounds cannot be distinguished, unless they can be excluded by their ionization potential. In the case of species exhibiting several isomeric structures, the peak represents the sum of all isomers. For instance, the peak at 68 m/z is also probably due to isoprene, which is a well-known

Figure 6. Puff-by-puff resolved SPI-TOFMS mass spectrum of butadiene in mainstream smoke. Ten consecutive single spectra have been averaged, resulting in a mass resolution of 200 ms.

constituent of mainstream smoke. As a matter of fact, Hoffmann et al.51 found from GC/MS analysis of mainstream smoke that the content of isoprene is 10 times higher compared to furan. Consequently, the majority of the peak at 68 m/z is due to isoprene. In principle, for this spectrum, peak assignment is carried out and justified by referring to the literature on the composition of cigarette smoke.51-53 The advantage of the new system for fast mass spectroscopic measurements is demonstrated in Figure 6, which shows the puffby-puff resolved SPI-TOFMS spectrum of butadiene in cigarette mainstream smoke of the same cigarette. The figure depicts the whole smoking process of the cigarette for the selected compound. At any time, 10 consecutive single spectra (generated by 50 Hz) were averaged, resulting in a time resolution of 200 ms. Quantification was carried out by means of the measured benzene calibration standard and the relative cross section of butadiene with respect to benzene (see Table 1). The respective two cleaning puffs inserted during the 60-s interval between actual puffs are marked by circles. Overall, the cigarette yielded eight puffs, each indicated by a steep increase in butadiene concentration. It is striking that there are still remnants of butadiene remaining in the smoking machine outlet zone. This issue has been discussed thoroughly in ref 46. The observed trend of butadiene concentration with increasing puff number is in good agreement with the previously reported behavior of several unsaturated aliphatic hydrocarbons,46 i.e., depicting the highest concentration level in the first puff. Subsequently, the concentration decreases significantly for the second puff followed by a steady increase toward the end of the smoking process. Breath Analysis. Several studies conducted in the past54-57 revealed that cigarette smoking directly affects the concentration (51) Hoffmann, D.; Hoffmann, I.; El-Bayoumy, K. Chem. Res. Toxicol. 2001, 14, 767-790. (52) Stedman, R. L. Chem. Rev. 1968, 68, 153-207. (53) Baker, R. R. In Tobacco: production, chemistry, and technology; Davis, L. D., Nielsen, M. T., Eds.; Blackwell Science: Oxford, U.K., 1999. (54) Wallace, L. A.; Pellizzari, E. D.; Hartwell, T. D.; Sparacino, C.; Whitmore, R.; Sheldon, L.; Zelon, H.; Perrit, R. Environ. Res. 1987, 43, 290-307. (55) Gordon, S. M. J. Chromatogr. 1990, 511, 291-302. (56) Hansel, A.; Jordan, A.; Holzinger, R.; Prazeller, P.; Vogel, W.; Lindinger, W. Int. J. Mass Spectrom. Ion Processes 1995, 149/150, 609-619. (57) Jo, W.-K.; Pack, K.-W. Environ. Res. 2000, A83, 180-187.

Figure 7. SPI-TOFMS spectra of breath from a smoker and a nonsmoker obtained by averaging 2500 consecutive single spectra (50 s measurement time).

levels of several volatile organic compounds (VOC) in breath. In this context, species are of particular interest that exhibit ample differences regarding their content in a smoker’s and a nonsmoker’s breath, respectively. Such components could serve as definitive markers of smoking. Gordon55 evaluated the gas-phase constituents of smokers’ breath and identified 2,5-dimethylfuran as the most promising candidate. In addition, butadiene and benzene are mentioned as possible target compounds in this regard.58 Figure 7 depicts SPI-TOFMS measurements of the breath of a smoker (consuming more than 20 cigarettes/day) and a nonsmoker, respectively. The analysis of the smoker’s breath took place approximately in the middle between smoking two cigarettes. However, the overall compound pattern of the smoker’s breath did not change significantly when measured immediately before or after the smoking of a cigarette. The volunteering test persons exhaled into a small reservoir (see Figure 4B), by which it was ensured that the exhaled breath could be sampled continuously with 10 mL/min, even when the test person had to inhale. The additional short tube at the other outlet of the reservoir prohibited the intrusion of ambient air. In each case, 2500 consecutive single spectra are averaged commensurate with a measurement period of 50 s. The spectrum of nonsmoker’s breath is confined to two major peaks, which can be assigned to acetone (58 m/z) and isoprene (68 m/z), both well known as among the most abundant species in human breath.59 Furthermore, a small amount of phenol (94 m/z) could be detected. In contrast, the breath of the smoker contains a larger variety of compounds. In particular, the aforementioned marker substances 2,5-dimethylfuran (96 m/z), butadiene (54 m/z), and benzene (78 m/z) emerge. Additionally, other unsaturated hydrocarbons such as butene (56 m/z), cyclopentadiene (66 m/z), pentene (70 m/z), toluene (92 m/z), and xylene (106 m/z) appear solely in the spectrum of the smoker’s breath. Especially the aromatic hydrocarbons are well-established to exhibit elevated contents in (58) Gordon, S. M.; Wallace, L. A.; Brinkman, M. C.; Callahan, P. J.; Kenny, D. V. Environ. Health Perspect. 2002, 110, 689-698. (59) Phillips, M.; Herrera, J.; Krishnan, S.; Zain, M.; Greenberg, J.; Cataneo, R. N. J. Chromatogr., B 1999, 729, 75-88. (60) Mallard, W. G.; Linstrom, P. J.; National Institute of Standards and Technology (NIST): http://webbook.nist.gov/chemistry, 2000; Vol. 2000.

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smokers’ breath.54 Since the concentration level of these species for nonsmokers’ breath is in the very low-ppb range, they are below the LOD of the system. One has to keep in mind, however, that isobaric compounds such as furfural may contribute to the peak at 96 m/z. On the other hand, 1-penten-3-yne is also mentioned showing ample differences between breath of smoker and nonsmoker, respectively;55 thus, the peak at 66 m/z could partly be assigned to this compound. Regarding acetone and isoprene, both present in nonsmoker’s breath, a significant increase of the signal at 68 m/z can be observed, whereas the signal of acetone seems not affected. However, at least part of the peak at 68 m/z may be due to furan, since it is known as a constituent of cigarette smoke51 and its methylated homologues appear in the spectrum. Nevertheless, furan was not detected in human breath by GC/MS analysis.59 The novel SPI-TOFMS device apparently has the ability to monitor VOC in human breath. However, this task is aided by the relatively high averaging rate to increase the LOD of the device. Utilizing an improved system with higher data acquisition frequency (up to 1 kHz instead of 50 Hz) should give the same results in a considerably reduced measurement time. CONCLUSION A novel device for time-of-flight mass spectrometry based on electron beam pumped excimer VUV light for single-photon ionization has been proven to be a promising tool for on-line analysis of complex gas mixtures. The system presented in this work achieved detection limits down to the 50 ppb range with a measuring time of 1 s for a number of typical organic trace compounds in combustion and pyrolysis processes. Since the technique allows soft ionization, fragile compounds such as

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aliphatic hydrocarbons are ionized without producing considerable fragmentation. The compact and rugged prototype was applied for on-line measurements of cigarette smoke, which represents a complex gaseous matrix. In addition, breath analysis has been carried out to detect distinct biomarker compounds. Further work will comprise improvement of the optical components and the ion source. Utilizing new data acquisition tools is expected to reduce the measurement period without losing sensitivity. Possible applications for this relatively simple and robust system might comprise several areas, where detection of trace species is necessary, such as industrial processes and quality control, environmental monitoring of various toxic substances, and medical purposes. The potential of the device to be implemented as a continuous emission monitor should also be addressed in the future. ACKNOWLEDGMENT The authors thank T. Adam and S. Mitschke for assistance during the cigarette smoke measurements and M. Sklorz for contribution to the breath analysis. Financial support by the BFS (Bayerische Forschungsstiftung) as well as the Institute for Science and Health, St. Louis, MO, is gratefully acknowledged. Thales Electron Devices GmbH, Ulm, Germany, is acknowledged for provision of the electron gun for the excimer lamp.

Received for review July 6, 2005. Accepted September 2, 2005. AC051194+