Trace Detection of Organic Compounds in Complex Sample Matrixes

Anal. Chem. 2009, 81, 4456–4467

Trace Detection of Organic Compounds in Complex Sample Matrixes by Single Photon Ionization Ion Trap Mass Spectrometry: Real-Time Detection of Security-Relevant Compounds and Online Analysis of the Coffee-Roasting Process Elisabeth Schramm,†,‡ Andreas Ku¨rten,†,§,# Jasper Ho¨lzer,†,‡ Stefan Mitschke,† Fabian Mu¨hlberger,† Martin Sklorz,†,‡ Jochen Wieser,⊥ Andreas Ulrich,| Michael Pu¨tz,∆ Rasmus Schulte-Ladbeck,∆ Rainer Schultze,X Joachim Curtius,×,# Stephan Borrmann,§,× and Ralf Zimmermann*,†,‡,) Helmholtz Zentrum Mu¨nchen, Institute of Ecological Chemistry, Ingolsta¨dter Landstrasse 1, 85764 Neuherberg, Germany, University of Rostock, Dr.-Lorenz-Weg 1, 18051 Rostock, Germany, Max Planck Institute for Chemistry (Otto Hahn Institute), Particle Chemistry Department, Joh.-Joachim-Becherweg 27, 55128 Mainz, Germany, Coherent GmbH, Zielstattstrasse 32, 81379 München, Germany, Technische Universität München, Physik Department E12, James-Franck-Strasse 1, 85748 Garching, Germany, Federal Criminal Police Office (BKA), Forensic Science Institute (KTI), 65173 Wiesbaden, Germany, Optimare—Optische Messverfahren für Meeresforschung und Umweltüberwachung GmbH, 26382 Wilhelmshaven, Germany, Institute for Atmospheric Physics, University of Mainz, Becherweg 21, 55099 Mainz, Germany, and bifa Umweltinstitut GmbH, 86167 Augsburg, Germany An in-house-built ion trap mass spectrometer combined with a soft ionization source has been set up and tested. As ionization source, an electron beam pumped vacuum UV (VUV) excimer lamp (EBEL) was used for single-photon ionization. It was shown that soft ionization allows the reduction of fragmentation of the target analytes and the suppression of most matrix components. Therefore, the combination of photon ionization with the tandem mass spectrometry (MS/MS) capability of an ion trap yields a powerful tool for molecular ion peak detection and identification of organic trace compounds in complex matrixes. This setup was successfully tested for two different applications. The first one is the detection of security-relevant substances like explosives, narcotics, and chemical warfare agents. One test substance from each of these groups was chosen and detected successfully with single photon ionization ion trap mass spectrometry (SPI-ITMS) MS/MS measurements. Additionally, first tests were performed, demonstrating that this method is not influenced by matrix compounds. The second field of application is the detection of process gases. Here, exhaust gas from coffee roasting was analyzed in real time, and some of its compounds were identified using MS/ MS studies. Real-time trace detection of organic compounds in complex matrixes is a challenging task in the field of analytical * To whom correspondence should be addressed. E-mail: ralf.zimmermann@ helmholtz-muenchen.de. † Helmholtz Zentrum München. ‡ University of Rostock. § Max Planck Institute for Chemistry (Otto Hahn Institute). # Present address: Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt am Main, Altenhöferallee 1, 60438 Frankfurt am Main, Germany. ⊥ Coherent GmbH.

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chemistry. For a real-time detection system, various applications are possible; however, they have to meet various requirements. Probably one of the most challenging aspects is that the detection system has to offer low detection limits and high selectivity. Especially for the detection of security-relevant substances, such as explosives, low false positive and false negative rates are imperative despite the presence of complex matrixes. Furthermore, such a system has to be reliable, fast, mobile, and easy to operate enabling real-time field measurements. Mass spectrometry (MS) is a powerful analytical tool principally suited for real-time analytical applications as it generally has a high mass resolving power and low detection limits. However, with classical electron ionization (EI), the selectivity is rather poor particularly when complex matrixes are present as many organic substances easily fragment. Accordingly, a typical EI mass spectrum without separation of the analytes prior to the detection with the mass spectrometer would have many superimposing peaks in the low mass-to-charge (m/z) range.1 Additionally, all matrix components like air would be ionized as well resulting in an overcrowded mass spectrum, poor mass resolution, and possibly detector saturation. Thus, an EI mass spectrum of complex analyte matrix mixtures is very challenging to interpret. In order to circumvent these problems, two different strategies are possible. The first one is a separation of the analyte prior to the detection, e.g., by gas chromatography (GC) or liquid chromatography (LC). These analytical methods are widely used Technische Universität München. Federal Criminal Police Office (BKA), Forensic Science Institute (KTI). Optimare—Optische Messverfahren für Meeresforschung und Umweltüberwachung GmbH. × Institute for Atmospheric Physics, University of Mainz. ) bifa Umweltinstitut GmbH. (1) Gross, J. H. Mass Spectrometry; Springer Verlag: Heidelberg, Germany, 2004. |

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with both high selectivity and high sensitivity.2-6 As different analytes reach the mass spectrometer at different times due to the separation, the EI spectra can be more easily interpreted and automatically compared to a library. Additionally, the elution time is a second, orthogonal indicator that helps identifying the substances. But, as the separation step usually takes several minutes, this method is not well suited for real-time measurements. Thus, another strategy can be pursued. Instead of using a separation step, a selective MS step is used. There are some concepts to overcome the problems discussed above. One possibility is to use very high mass resolution to distinguish substances due to their exact molecular weight. This is possible, e.g., with a Fourier transform ion cyclotron resonance mass spectrometer (FTICR MS),1,7 but these instruments are usually rather large and expensive. Another option is to use selective ionization such as “resonance enhanced multiphoton ionization” (REMPI).8 In the simplest case, a molecule absorbs one photon and thereby reaches an excited state, then a second photon can induce ionization. This method is highly sensitive and selective as the photon energy has to be in resonance with an excited state of the analyte. As for REMPI, photons in the ultraviolet (UV) range with a high photon density are needed; lasers are applied for this process. These required lasers, however, are sophisticated and relatively expensive. However, here we use another method by adopting a so-called “soft ionization” technique. It provides mass spectra of complex samples which can be well interpreted due to reduced fragmentation. Consequently, fewer peaks appear in the mass spectra and the abundance of signals at higher masses or even at molecular ion masses enhance which can more easily be assigned to the corresponding substances. Several soft ionization techniques have been developed in the past, e.g., chemical ionization (CI),1,9-11 electrospray ionization (ESI),1 field ionization (FI),1,12 and photoionization (PI).13 Each of these methods has its specific advantages and disadvantages published elsewhere.13 One aspect is that CI and ESI are based upon attaching or abstracting a charge, such as a proton or electron, to molecules. Because the affinity for charge can vary widely for different molecules, the ionization efficiencies can likewise vary over a wide range. Furthermore, molecules with a high ionization potential can ionize molecules with a lower ionization potential by charge transfer, which in some cases can render the latter molecule undetectable. We use a PI method which has no such dependency as the ionization occurs at rather low pressure so that collisions between (2) Karasek, F. W.; Clement, R. E. Basic Gas Chromatography-Mass Spectrometry: Principles and Techniques; Elsevier Science Publishing Company Inc.: New York, 1988. (3) Helmig, D.; Pollock, W.; Greenberg, J.; Zimmerman, P. J. Geophys. Res. 1996, 101, 14697–14710. (4) Yinon, J. J. Chromatogr., A 1996, 742, 205–209. (5) Niessen, W. M. A. J. Chromatogr., A 1999, 856, 179–197. (6) Yunhui, W. Biomed. Chromatogr. 2000, 14, 384–396. (7) Asamoto, B. FT-ICR/MS: Analytical Applications of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry; VCH Publishers: New York, 1991. (8) Boesl, U.; Neusser, H. J.; Schlag, E. W. Z. Naturforsch. 1978, 33a, 1546– 1548. (9) Munson, B. Anal. Chem. 1977, 49, 772A–778A. (10) Munson, M. S. B.; Field, F. H. J. Am. Chem. Soc. 1966, 88, 2621. (11) Lindinger, W.; Hansel, A.; Jordan, A. Chem. Soc. Rev. 1998, 27, 347–354. (12) Beckey, H. D.; Levsen, K.; Ro ¨llgen, F. W.; Schulten, H.-R. Surf. Sci. 1978, 70, 325–362. (13) Milne, G. W. A.; Lacey, M. J.; Arsenault, G. P. Crit. Rev. Anal. Chem. 1974, 4, 45–81.

two molecules are not very likely. This is rather favorable for reliably analyzing trace compounds in unknown, complex matrixes. The method that has been chosen in this study is single photon ionization (SPI).14 The SPI technique, unlike REMPI, uses only one photon in the vacuum ultraviolet (VUV) spectral range for ionization. This photon is absorbed by the molecule, and if the photon energy exceeds the ionization potential (IP) of the molecule, ionization occurs. Thus, SPI is less selective as REMPI and can ionize all substances with IPs lower than the photon energy. Accordingly, a large amount of substances can be detected without changing the setup. By using photon energies just above the ionization potential of the molecule, little excess energy is transferred to the molecule and fragmentation is minimized as the appearance energies (AE) of many fragments are not reached. However, especially for explosives, the AEs of the fragments are not much higher than the IPs of the molecules as they are very unstable by nature. Therefore, the photon energy has to be chosen accurately. Additionally, matrix components with higher ionization potential than the photon energy are not ionized resulting in suppression of the signal of these substances. Most matrix molecules such as nitrogen (IP ) 15.58 eV), oxygen (IP ) 12.06 eV), carbon dioxide (IP ) 13.77 eV), and water vapor (IP ) 12.62 eV)15 have relatively high IPs. Most organic compounds have an IP in the range of 7-12 eV, and if the IPs of the substances of interest are all below the IPs of the matrix elements, it is possible to use an intermediate photon energy in order not to ionize the matrix elements. The ionization cross section for different substances and therefore the sensitivity vary by about 1 order of magnitude.16 This has to be taken into account, but as the cross section is not influenced by matrix elements as in the case of CI due to the lower pressure, this factor can be taken into account by calculations. A commonly used photon energy for SPI is 10.5 eV (118 nm) generated by frequency-tripling of the third harmonic of a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser. Mullen, Pond, and co-workers17,18 accomplished SPI measurements with the explosive trinitrotoluene (TNT) among others. TNT is ionizable by 118 nm photons without mentionable fragmentation. An additional method for improving the selectivity of an MS system is the MS/MS technique. This method isolates ions with high masses, preferably the molecular ions, and fragments them afterward. The resulting MS/MS mass spectrum of the fragments provides a second, orthogonal indicator that helps identifying the substances. As the molecular ion peak increases when little fragmentation occurs, soft ionization increases the sensitivity and enables MS/MS measurements of the molecular ion peak which helps improving the selectivity. Our approach of providing a powerful measurement system is an ion trap mass spectrometer with MS/MS capability combined with single photon ionization (SPI-ITMS). In such an instrument, (14) Shi, Y. J.; Lipson, R. H. Can. J. Chem. 2005, 83, 1891–1902. (15) Mallard, W. G.; Linstrom, P. J. National Institute of Standards and Technology (NIST). http://webbook.nist.gov/chemistry, 2000; Vol. 2000 (accessed Jan 31, 2001). (16) Adam, T.; Zimmermann, R. Anal. Bioanal. Chem. 2007, 389, 1941–1951. (17) Mullen, C.; Irwin, A.; Pond, B. V.; Huestis, D. L.; Coggiola, M. J.; Oser, H. Anal. Chem. 2006, 78, 3807–3814. (18) Pond, B. V.; Mullen, C.; Suarez, I.; Kessler, J.; Briggs, K.; Young, S. E.; Coggiola, M. J.; Crosley, D. R.; Oser, H. Appl. Phys. B: Lasers Opt. 2007, 86, 735–742.


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soft ionization results in little fragmentation and in suppression of bulk gases like nitrogen or oxygen. This is very favorable as a high density of ions inside the ion trap causes space charge effects, a reduction of the duty cycle, an increase of the baseline, and a potential saturation of the detector resulting in an increase of the limit of detection (LOD) for the trace compounds. Additionally, these would cause an unwanted decrease in mass resolution due to peak broadening. The ITMS yields good sensitivity due to the possibility of accumulating of selected ions within the ion trap. Additionally, MS/MS studies can be performed that allow the reliable identification of unknown compounds.19 There are some examples where an SPI-ITMS technique has already been used with different light sources. Hanna et al.20 developed a laser-based VUV light source for aerosol mass spectrometry with a wide tuning range of the photon energy from 7.4 to 10.2 eV. They used resonance enhanced four wave difference mixing in xenon gas for photon generation and reflected them into an ion trap for internal ionization. In their study, first measurements were effectually accomplished that investigated the ionization threshold and the appearance energy of the fragments of some substances. Morii, Tsuruga, and co-workers21-23 built an ion trap mass spectrometer with internal SPI ionization. They utilized a 121.6 nm (10.2 eV) VUV lamp with a H2/He mixture irradiated by a microwave generator. With this setup, they successfully accomplished real-time measurements of incinerator exhaust for dioxin analysis. For some measurements, they additionally use a GC separation step and a concentration apparatus prior to the ITMS. This increases the sensitivity and selectivity at the expense of an increased detection time. Butcher et al.24 used VUV light at 118 nm (10.5 eV) generated by frequency-tripling of 355 nm pulses from a Nd:YAG laser within a xenon cell. They perform internal ionization within a modified commercial ion trap mass spectrometer. With this setup, they performed real-time determination of aromatics in automobile exhaust. Syage and co-workers25,26 built discharge lamps as SPI sources and offer atmospheric pressure photoionization (APPI)26-28 sources for commercial mass spectrometers. Additionally, they have tested low-pressure photoionization (LPPI) sources working at about 1 Torr. The photon energy can be varied between approximately 8 and 12 eV by changing the rare gas within the lamps. They performed experiments for the analysis of fatty acids and acylglycerol lipids and others. The APPI and LPPI sources work at relatively high pressure which favors unwanted ion reactions. The other setups mentioned above as (19) March, R. E. J. Mass Spectrom. 1997, 32, 351–369. (20) Hanna, S. J.; Campuzano-Jost, P.; Simpson, E. A.; Robb, D. B.; Burak, I.; Blades, M. W.; Hepburn, J. W.; Bertram, A. K. Int. J. Mass Spectrom. 2009, 279, 134–146. (21) Tsuruga, S.; Futami, H.; Yamakoshi, H.; Danno, M.; Yamashita, I.; Kuribayashi, S. J. Mass Spectrom. Soc. Jpn. 2004, 52, 295–300. (22) Kuribayashi, S.; Yamakoshi, H.; Danno, M.; Sakai, S.; Tsuruga, S.; Futami, H.; Morii, S. Anal. Chem. 2005, 77, 1007–1012. (23) Tsuruga, S.; Suzuki, T.; Takatsudo, Y.; Seki, K.; Yamauchi, U.; Kuribayashi, H.; Morii, S. Environ. Sci. Technol. 2007, 3684–3688. (24) Butcher, D. J.; Goeringer, D. E.; Hurst, G. B. Anal. Chem. 1999, 71, 489– 496. (25) Short, L. C.; Cai, S.-S.; Syage, J. A. J. Am. Soc. Mass Spectrom. 2007, 18, 589–599. (26) Syage, J. A. J. Am. Soc. Mass Spectrom. 2004, 15, 1521–1533. (27) Andrea Raffaelli, A. S. Mass Spectrom. Rev. 2003, 22, 318–331. (28) Syage, J. A.; Hanold, K. A.; Lynn, T. C.; Horner, J. A.; Thakur, R. A. J. Chromatogr., A 2004, 1050, 137–149.

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well as our setup ionize at lower pressure thereby minimizing ion reactions. In this work, an electron beam pumped excimer lamp (EBEL)29-32 described in the Experimental and Method Setup section is used to generate VUV photons. This light source is well suited for field use since no laser and laser safety specialist is needed for tuning and operating the lamp. It is also much more compact and robust than, e.g., a Nd:YAG laser within a xenon cell for frequency-tripling or microwave light sources. Additionally, these lamps have a good spectral stability, as the rare gas is easy to be kept clean as no sources of contamination are within the gas cell using a getter to remove potential contamination from the gas cell which would alter the emission spectrum due to excitation transfer. Several applications using EBEL in conjunction with online time-of-flight mass spectrometry and quadrupole mass spectrometry were published.33,34 Furthermore, the coupling of SPI MS with thermogravimetry35 and with gas chromatography36 were investigated. Here, the EBEL was has been coupled for the first time to an ion trap MS to enable the possibility of MS/MS measurements. This combination has a powerful potential, as the soft ionization yields peaks with higher masses (ideally molecular ion peaks) well suited for MS/MS studies. By applying different rare gases or gas mixtures photon energies in the range of about 7 and 12 eV are achievable. Thus, it is possible to tune the photon energy to a value which is necessary for the particular application. FIELDS OF APPLICATION Potential fields of application for such a device are, e.g., the analysis of chemical processes or environmental analysis. For developing and testing our system, two applications were selected. One is the detection of trace amounts of explosives, narcotics, and chemical warfare agents, e.g., at airport transit terminals, and the other one is the analysis of coffee roasting off-gas. The importance of detecting security-relevant substances is obvious after the recent terror events. Federal authorities are interested in detecting, e.g., substances from nitro-based explosives like TNT. Also security relevant is the detection of narcotics like heroine or chemical warfare agents (CWA) like sarin. The main difficulty is that many of these security-relevant substances appear in low concentrations due to their low vapor pressures. The vapor pressure of the explosive TNT, for example, is about (29) Ulrich, A.; Wieser, J.; Kro ¨tz, W. VUV-Lampe. German Patent 44 38 407, accorded Sept 19, 1996. (30) Ulrich, A.; Wieser, J.; Salvermoser, M.; Murnick, D. Phys. Bl. 2000, 56, 49–52. (31) Ulrich, A.; Morozov, A.; Kru ¨ cken, R.; Go ¨rtler, A.; Wieser, J.; Kornfeld, G.; Peters, A.; Steinhu ¨ bl, R.; Mu ¨ hlberger, F.; Zimmermann, R. In Light Sources 2004; Proceedings of the 10th International Symposium on the Science and Technology of Light Sources, Toulouse, France, July 18–22, 2004; Zissis, G., Ed.; CRC Press: Boca Raton, FL, 2004; ISBN 0750310073, 9780750310079; 664 pp. (32) Wieser, J.; Murnick, D. E.; Ulrich, A.; Huggins, H. A.; Liddle, A.; Brown, W. L. Rev. Sci. Instrum. 1997, 68, 1360–1364. (33) Muhlberger, F.; Zimmermann, R.; Kettrup, A. Anal. Chem. 2001, 73, 3590– 3604. (34) Mu ¨ hlberger, F.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2002, 74, 3790–3801. (35) Saraji-Bozorgzad, M.; Geissler, R.; Streibel, T.; Mu ¨ hlberger, F.; Sklorz, M.; Kaisersberger, E.; Denner, T.; Zimmermann, R. Anal. Chem. 2008, 80, 3393–3403. (36) Zimmermann, R.; Mu ¨ hlberger, F.; Fuhrer, K.; Gonin, M.; Welthagen, W. J. Mater. Cycles Waste Manage. 2008, 10, 24–31.

5.93 × 10-6 hPa at room temperature,37 resulting in a maximum concentration of approximately 6 parts per billion (ppb) in air. On the top of that, the matrix can be very multifaceted. Everything from dust and dirt to lotions and nail polish remover may be included. The state of the art for dealing with these substances is to use a thermal desorber for wipe pads to increase the concentration and then using detection with an ion mobility spectrometer (IMS).38 This device has a low detection limit and a short response time and does not need a vacuum system, which greatly reduces the size, cost, and complexity in comparison with an MS. However, the rather low resolution makes IMS vulnerable for false negative or positive responses. The consequences of a false negative alarm are obvious. The consequence of a false positive alarm is demonstrated, e.g., by the false alarm in the Swedish nuclear power plant Oskarshamn in May 2008 which was caused by a shaving cream and cost, according to the newspaper “The Local Europe AB”39 the company that runs the plant some 100 million kronor ($16.8 million). That clearly shows the need for optimization. As in an IMS the ions are dispersed by ion mobility, whereas a mass spectrometer can determine the ion mass-to-charge ratio, the selectivity of a mass spectrometer is generally higher than the one of an IMS.40 A recent report by the National Academy of Sciences noted that MS has a resolving power that is 10-10 000 times higher than IMS and recommended making MS the core technology to detect explosives for aviation security.41 By using an ion trap as MS, the selectivity is even higher because MS/MS studies can be performed. The other tested application is the analysis of the evolved gas during the coffee roasting process. This is one example of a continuous process which can be monitored in real time. Coffee is one of the main merchandises of trade worldwide, and the value of coffee beans is determined and increased by the roasting process. As during the roasting process complex physical and chemical reactions take place inside the beans leading to the formation of desired coffee flavor compounds,42,43 this is a very crucial part of getting coffee ready to consume. The knowledge of the formation of coffee flavor is fragmentary and sometimes speculative. This explains that in order to deliver a consistent quality, one trusts a dedicated and experienced roaster who uses a set of indicators, his senses, and his empirical knowledge to control the process. However, an automated real-time process control cannot be performed, so the determination of the actual roast degree cannot be determined during the process. Ideally, the roasting process would be characterized in real time by monitoring the formation and release of coffee flavor components (37) Pella, P. A. J. Chem. Thermodyn. 1977, 9, 301–305. (38) Ewing, R. G.; Atkinson, D. A.; Eiceman, G. A.; Ewing, G. J. Talanta 2001, 54, 515–529. (39) AFP. In The Local, Stockholm, 2008. (40) Syage, J. A.; Hanold, K. A. In Trace Chemical Sensing of Explosives; Woodfin, R. L., Ed.; Wiley: New York, 2007. (41) Committee on Assessment of Security Technologies for Transportation, N. R. C. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry; National Academies Press: Washington, DC, 2003. (42) Tressl, R. In Thermal Generation of Aromas; Parliment, T. H., McGorrin, R. J., Eds.; American Chemical Society: Washington, DC, 1989; Vol. 409, pp 285-301. (43) Yeretzian, C.; Jordan, A.; Brevard, H.; Lindinger, W. ACS Symp. Ser. 2000, 763, 112–123.

by sensory evaluation. The main difficulty is that the roasting process is rather fast (some 10 min), and the roaster off-gas is a complex mixture of primarily inorganic gases such as CO2, CO, N2, and H2O, and only about 1% of the gases are volatile organic compounds (VOCs), a fraction of which represents coffee flavor compounds.44 So this is a big challenge for the selectivity of the measurement system. Gas sensors or gas sensor arrays, sometimes termed “electronic nose” are not suited up to now as the process is too complex and too fast.45 Dorfner et al.46 showed that REMPI MS and SPI MS are possibilities to monitor some of the interesting compounds and can be used for online determination of the roasting degree. Lindinger et al.47 recently demonstrated that it is possible to predict sensory profiles of coffee with online proton transfer reaction mass spectrometry (PTR MS) analysis, whereas PTR is a kind of chemical ionization and therewith a soft ionization method. The drawback of the methods mentioned above is that the selectivity of MS is rather low like mentioned above. With the SPI-ITMS, however, MS/MS studies are possible to verify the assignments of SPI MS peaks online. EXPERIMENTAL METHOD AND SETUP The setup of the measurement system shown in Figure 1 consists of an inlet system, coupled to a heated capillary to an ion source for soft SPI, and an ITMS built at the University of Mainz48 within a collaboration. In the following subsections each component will be described in detail. Inlet System. For the two applications two different inlet systems were utilized. For detection of security-relevant substances on wipe pads a thermal desorber is used. It is an original thermal desorber from an ion mobility spectrometer (IONSCAN 400B, Smiths Detection, Watford, U.K.) used at many airports for wipe tests. The thermal desorber was coupled to a heated capillary inlet yielding to the ion source by a heated connector. Thereby, a small dead volume of only 1 mL was realized. The desorber and the adapter were heated to 200 °C, whereas the transfer line consisting of a deactivated fused-silica capillary was heated to 250 °C to avoid cold spots which would result in condensation of various substances. The used standard wipe pads from Smiths Detection consist of coated cotton. They were purged with a hot stream of nitrogen at 5 mL/min. The gas flow into the ion source through the capillary was about 1 mL/min. The difference prevented an unwanted gas stream of ambient air through the wipe pad into the capillary. During the change of the wipe pad, it is unavoidable that ambient air enters into the system so that there is always some water vapor present. The second setup for analyzing the off-gas from coffee roasting used a direct inlet. A coffee roast off-gas sample was generated by filling a vial with green coffee beans and heating the vial to a (44) Dorfner, R.; Ferge, T.; Yeretzian, C.; Kettrup, A.; Zimmermann, R. Anal. Chem. 2004, 76, 1386–1402. (45) Gardner, J. W., Bartlett, P. N., Eds. Sensors and Sensory Systems for an Electronic Nose; Springer: New York, 1991. (46) Dorfner, R.; Yeretzian, C.; Zimmermann, R.; Kettrup, A. In 18th International Scientific Colloquium on Coffee, Helsinki, Finland, Aug 2–6, 1999; ASIC: Paris, France, 2000. (47) Lindinger, C.; Labbe, D.; Pollien, P.; Rytz, A.; Juillerat, M. A.; Yeretzian, C.; Blank, I. Anal. Chem. 2008, 80, 1574–1581. (48) Ku ¨ rten, A.; Curtius, J.; Helleis, F.; Lovejoy, E. R.; Borrmann, S. Int. J. Mass Spectrom. 2007, 265, 30–39.


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Figure 1. Setup of the measurement system; not to scale.

constant temperature of 200 °C. A deactivated fused-silica capillary, heated to 250 °C, transfers the gas to the ion source. VUV Light Source and Ion Source. The heated transfer capillary is inserted into a heated inlet where the gas effusively flows into the ionization region. The VUV photons for SPI are focused from the light source into the ion source by two parabolic mirrors. If the photon energy of a single photon that is absorbed by the molecule is higher than its IP, an electron is removed.13 4460


Such an ionized molecule can then be detected by the MS. An EBEL was used as a VUV photon source. The assembly of the EBEL is shown in Figure 1. Its dimensions are approximately 25 cm × 10 cm × 10 cm; the figure is not true to scale. The main innovation is a 0.7 × 0.7 mm2, 300 nm thick silicon nitride foil that separates the rare gas volume (p > 1 bar) from the vacuum chamber containing an electron gun (e-gun). The e-gun generates a 12 keV electron beam which is directed into the

Figure 2. Panel 2.1: chronological sequence of an ion trap measurement [according to Ku¨rten et al., ref 48; see there for definition of the ion trap parameter u, also]. Panel 2.2: SPI spectra of 10 ppm BTX with pulsed and nonpulsed EBEL. (Note that the poor mass resolution in these and the spectra in the following figures is due to the improper mass range extension of the self-constructed ion trap mass spectrometer.)

rare gas through the silicon nitride foil with low energy loss. In the dense rare gas the high-energetic electrons collide with gas atoms, resulting in their excitation and ionization. In successive gas kinetic steps, excited diatomic rare gas molecules (excimers) are formed. The radioactive decay of these excimers provides intense VUV radiation.49,50 By using different rare gases, it is possible to generate photons with different energies. First measurements were performed using argon resulting in a photon energy of 9.8 eV (126 nm) and krypton yielding a photon energy of 8.4 eV (147 nm). Currently, EBEL with neon/ argon mixtures resulting in a photon energy of 11.7 eV (106 nm) are being developed. It is planned to connect all three light sources at the same time to the ion trap using parabolic or elliptic mirror optics. This will enable the use of different photon energies and therefore provide further information about the compounds of interest. The sensitivity of the system depends strongly on the photon flux. For the measurements presented in this manuscript an emission current of the e-gun of normally 3 µA was used, yielding on the order of 1013 photons/s in the ionization region. Actually, EBEL can be driven by about 10 µA resulting in a 3 times higher photon flux, and the development of the EBEL is still in progress to enhance the yield further. That will improve the sensitivity accordingly. For coupling the EBEL to a mass spectrometer the brilliance is very important. As the light spot is only a few millimeters in diameter, it is possible to focus the light into the ion source with little scattering. Accordingly, no photoelectrons are emitted from the walls which would lead to analyte fragmentation. Ion Trap Mass Spectrometer. The ion trap mass spectrometer used in this study was designed and built in cooperation with the University of Mainz and the Max Planck Institute for Chemistry in Mainz, Germany. The setup is very similar to the one described by Ku¨rten et al.48 and was originally developed for the chemical analysis of aerosol particles. Here we describe the setup only shortly and focus more on the modifications that have been made to the instrument; a more general and detailed

description can be found in the above-mentioned literature. Whereas the ion trap aerosol mass spectrometer (IT-AMS) utilizes electron ionization only so far, the ion trap system used here was adjusted for the application of the VUV light source described in the previous section. One of the main changes compared to the original setup is the modification of the ion source to allow for the ionization of gas-phase compounds exclusively. For this, the cartridge heater that was used for the flash-vaporization of aerosol particles was removed, thereby leaving room within the ion source for the VUV photon beam. Another modification to the original setup consisted of adding the EBEL to the vacuum chamber and focusing the VUV radiation into the cross beam ion source (Pfeiffer Inc., Asslar, Germany, part no. BN846481-T). The sampled gas is introduced into the ion source through a capillary. The capillary is aligned with the ion optical axis defined by the symmetry axis of the trap and focusing electrodes. The generated ions are accelerated toward the ion trap by means of an extraction lens at about -200 V (relative to the potential on the ion source cage). Afterward, they are focused into the ion trap through the 1.5 mm entrance hole of the first end-cap electrode by three electrostatic lenses. This modified angle Paul trap (Θ ) 1.9, r0 ) 1 cm, z0 ) 0.725 cm; a detailed explanation of these parameters can be found elsewhere51) consists of a hyperbolic ring electrode between two hyperbolic end-caps. The ions are trapped by a high-frequency voltage applied to the ring electrode. Helium is used as a damping gas to improve trapping efficiency and to stimulate collision-induced dissociation (CID) during MS/MS studies. Figure 2.1 shows the timing diagram for the measurements. Note that the duration of the different periods can be varied by (49) Mu ¨ hlberger, F.; Streibel, T.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 7408–7414. (50) Fedenev, A.; Morozov, A.; Kru ¨ cken, R.; Schoop, S.; Wieser, J.; Ulrich, A. J. Phys. D: Appl. Phys. 2004, 37, 1586–1591. (51) March, R. E., Todd, J. F. J., Eds. Practical Aspects of Ion Trap Mass Spectrometry/Fundamentals of Ion Trap Mass Spectrometry; CRC Press Inc.: Boca Raton, FL, 1995.


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changing parameters in the control software (see below). In addition, the periods in the figure are not drawn to scale and some parts, like the MS/MS step, are optional and can be enabled and adjusted by the software user as well as various other parameters. By applying so-called filtered noise fields (FNF) at the end-cap electrodes (ac voltage waveforms generated by an arbitrary waveform generator, type PCI-5412, National Instruments, Austin, TX) it is possible to eject all ions with selected mass-to-charge ratios during or after trapping. This helps increasing the sensitivity and the identification of unknown ions during MS/MS studies. By expelling all ions with mass-to-charge ratios that are lower than the m/z that has been selected for the MS/MS study, fragments resulting from CID can be unambiguously related to the parent ion. Due to the geometry and the maximum amplitude the radio frequency (rf) generator for the ring electrode voltage (rf amplitude) can provide, the ion trap used here is only capable of detecting ions with masses larger than ∼120 m/z by applying mass range extension. This means that an additional ac voltage has to be applied to the end-cap electrodes in order to eject ions already at lower rf amplitudes. Mass range extension was performed here with a frequency of 325 kHz, one-fourth of the 1.3 MHz of the ring electrode voltage, resulting in an ejection of ions having a βz-value of 0.5 (see literature for terminology52). When applying mass range extension with this value we have found that a rather poor mass resolution results. This problem did not occur with the original ion trap developed by Ku ¨ rten et al.48 where a βz-value of 0.125 was used for the mass range extension. This value, though, was not applicable for the measurements described here, as the mass range extension is therewith up to m/z ) 1006 which is quite too much especially for the applied MS/MS studies. However, for the applications presented here this is not really relevant and we did not further investigate this problem. Nevertheless, it has to be noted here that the detection limit of the EBEL-ITMS combination is possibly influenced also. This means that the sensitivity values given in the following section are a conservative estimate, and although the values are satisfactory in most cases, it is expected that the detection limits with of a commercial ion trap might be substantially better. Therefore, in future studies we plan on using a commercial ion trap which most likely does not show this problem to such an extent. A general problem of ion traps, however, is that it is impossible to detect MS/MS fragments with high efficiency that differ largely in m/z to their original ion. Nevertheless, as will be shown in the following section, the identification of substances is still possible, because MS/MS fragments with relatively high masses are more significant, as they allow for a more unambiguous assignment to their original molecules. The ion trap is controlled by a LabView program (National Instruments Corporation, Austin, TX). Currently, there exists no automated version of data analysis software. However, for forthcoming studies, a commercial ion trap with automated data analysis will be used. CHARACTERIZATION OF THE SETUP In Figure 2.2 spectra of a standard gas mixture of nitrogen with 10 ppm benzene, toluene, and xylene (BTX), respectively, (52) March, R. E. J. Mass Spectrom. 1997, 32, 351–369.

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are shown. For these spectra, the e-gun emission current was set to 5 µA and the averaged measurement time was 160 s (corresponding to 1500 spectra, each with an ionization time of 40 ms). For the spectra on the top, the EBEL was switched on continuously over the whole measurement (alternative A as shown in Figure 2.1). Thereby, scattered photons reached the detector during the readout period resulting in a high noise level. Therefore, the VUV lamp was operated in a pulsed mode instead (alternative B as shown in Figure 2.1). For this, an electromagnet coupled with a pulse/delay generator (S.M.V., PDG 204) deflects the electron beam while ions are not being accumulated in the trap. Thus, photons were only generated during 40 ms ionization time. Doing so, background noise level in is reduced significantly as can be seen in the lower part of Figure 2.2. In this spectrum the 13C1-isotope peaks are clearly visible (625-818 ppb), and the 13C2-isotope peaks with concentrations between about 3.8 and 4.4 ppb are noticeably above the noise. Therewith, the limit of detection is roughly in the same order of magnitude like achieved with coupling of an EBEL with a quadrupole mass spectrometer35 considering the averaged time. For all further measurements, the EBEL was pulsed, too. There is a huge variety of potential applications and interesting substances to detect with such a setup. Accordingly, the IPs of the substances of interest cover a relatively broad range. They vary in the case of security-relevant substances in the range from 7.5 eV for the narcotic 3,4-methylenedioxy-N-methamphetamine (MDMA) to 10.7 eV for the explosive ethylene glycol dinitrate (EGDN).53 Because the photon energy that is high enough to ionize EGDN will already fragment MDMA to a too large extent, different photon energies are necessary. First experiments with argon excimer (9.8 eV) and krypton excimer (8.4 eV) EBEL were performed in this work. The photon energy is distributed over a spectrum with a full width at half-maximum (fwhm) of about 0.8 eV54 (see Figure 3.1). In order to demonstrate the influence of the photon energy, MDMA was examined. In Figure 3.2 the intensities of the molecular ion signal and the main photoinduced fragmentation peaks versus the ionization energy are shown as a photoionization efficiency plot (PIE plot) for MDMA.53 The molecular ion peak appears at the IP of 7.5 eV, but at 8.5 eV the molecular ion peak is already smaller than the peak of the main fragment at m/z ) 58 (C3H8N group). It is obvious that the fragmentation is higher if the ionization energy is higher as more excess energy is transferred to the molecule. SPI spectra for MDMA with the two wavelengths mentioned above as well as the EI spectrum from NIST15 at 70 eV ionization energy are shown in Figure 3.3. For the SPI spectra, 100 measurements were averaged resulting in an ionization time of 5 s. These spectra (regarding the peak area) are similar to the ones expected due to the PIE: At 70 eV ionization energy, the fragment with m/z ) 58 (C3H8N group) dominates the spectrum while the other peaks are hardly visible. At 9.8 eV the peak at m/z ) 58 still dominates the spectrum, but the peaks at m/z ) 135/136 rise to about 40% of the base peak area. In addition the molecular ion peak appears. When ionization energy of 8.4 eV is used, the (53) Schramm, E.; Mu ¨ hlberger, F.; Mitschke, S.; Reichardt, G.; Schulte-Ladbeck, R.; Pu ¨ tz, M.; Zimmermann, R. Appl. Spectrosc. 2008, 62, 238–247. (54) Ulrich; A.; Heindl; T.; Kru ¨ cken, R.; Morozov, A.; Skrobol, C.; Wieser, J. Eur. Phys. J. Appl. Phys., in press.

Figure 3. Panel 3.1: wavelengths distribution of EBEL photons [according to Ulrich et al., ref 54]. Panel 3.2: PIE of MDMA [according to Schramm et al., ref 53]. Panel 3.3: mass spectra of MDMA with 70, 9.8, and 8.4 eV ionization energy.

integrated area of the broad (see above) molecular ion signal is comparable to the peak area of the fragments. Therefore, for the detection of MDMA the krypton-EBEL is used, as the fragmentation is lower than for the other wavelength resulting in a more intense molecular ion peak which can be used for MS/MS studies. RESULTS AND DISCUSSION FOR SECURITY-RELEVANT SUBSTANCES With the measurement setup described above, first measurements were performed in order to test the method with one CWA precursor (dimethyl methylphosphonate, DMMP), one synthetic drug (MDMA), and two explosives (2,4-dinitrotoluene and 3,4dinitrotoluene, 2,4-DNT and 3,4-DNT) respectively. For sample introduction into the MS several milligrams of these substances were applied as solution on a wipe pad and heated inside the thermal desorber to 60 °C for DMMP and 180 °C for 2,4-DNT, 3,4-DNT, and MDMA, respectively. These large amounts of substances were used to achieve a steady-state equilibrium concentration in the gas phase allowing a longer time for testing and adjusting the trap parameters. For every mass spectrum, 100 measurements with 50 ms ionization time each were averaged. Neither nitrogen nor oxygen was detected due to their high ionization potentials. MDMA was provided by the German Federal Criminal Police Office with a purity of more than 99%, both DNTs were obtained from Sigma-Aldrich Corporation (St. Louis, MO) with a purity of more than 99%, and DMMP was supplied by ABCR GmbH & Co. KG (Karlsruhe, Germany) with a purity of 97%. Figure 4.1 displays the results for the synthetic drug MDMA. MDMA belongs to the group of amphetamine-type stimulants (ATS) and is the main active substance in most seized Ecstasy tablets. Its chemical structure contains a β-phenylethylamine moiety like some endogenous neurotransmitters. Its molecular mass is 193 amu. In the EI mass spectrum shown in Figure 4.1a, the fragment peak with m/z ) 58 is the base peak and the molecular ion peak is not visible. Accordingly, a detection of MDMA with EI would not be possible as the fragment at m/z ) 58 cannot be clearly assigned to MDMA because it could be associated with acetone from nail polish remover also or many other compounds. The mass spectrum recorded with SPI at 8.4

eV with our setup looks different (Figure 4.1b). Therein, the molecular ion peak with m/z ) 193 is clearly visible although the mass resolution of this custom-built ion trap is rather poor due to the mass range extension (see the Experimental Method and Setup part). Nevertheless, it is possible to isolate the molecular ion with FNF (see Figure 4.1c) and record an MS/MS spectrum displayed in Figure 4.1d. Therein, the fragments with m/z ) 58 and m/z ) 135 are clearly visible caused by a loss of the C3H8N group. Accordingly, it is possible to identify MDMA with an MS/MS spectrum of m/z ) 193 when m/z ) 58 and 135 appear as fragments. In Figure 4.2 the results for DMMP are displayed. DMMP is a colorless liquid used as precursor for sarin, a CWA. As DMMP is a nontoxic compound with a chemical structure similar to many phosphonic acid ester based CWAs, it is used as a surrogate for the development of CWA detectors. DMMP has a molecular mass of 124 amu and an IP of 10.2 eV. The molecular ion peak is clearly visible in the EI mass spectrum shown in Figure 4.2a. In the SPI mass spectrum with 9.8 eV, however, nearly no fragmentation is observed. The photon energy of the EBEL has a distribution with an fwhm of about 0.8 eV (see Figure 3.1). So, with an average photon energy of 9.8 eV, a sufficient amount of photons (about one-sixth) have an energy of more than 10.2 eV so that they can ionize DMMP. In the SPI spectrum, the molecular ion peak is the base peak together with a peak at m/z ) 125. The height of this peak depends on the amount of water inside the trap, and it is assumed that it results from a reaction of the molecular ion with a proton from water during and after the trapping period. This is a common reaction within an ion trap if water molecules are present.26 As described above it is impossible to get rid of humidity inside the trap with the thermal desorber, so this peak cannot be avoided. In Figure 4.2c the FNF was used to eject all ions except the molecular ions at m/z ) 124 and m/z ) 125. In a subsequent step, CID was used to fragment the molecular ions and to record an MS/MS mass spectrum of DMMP shown in Figure 4.2d. Therein, the main fragments are at m/z ) 79 and 94. These fragments are EI fragments, too, resulting from the loss of two CH3 groups and the CH3O2P + H fragment, respectively. By detecting the fragment at m/z ) 79 as MS/MS fragment from the 124/125 peak it is clear that the peaks result from Analytical Chemistry, Vol. 81, No. 11, June 1, 2009

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Figure 4. Various MS and MS/MS mass spectra with EI (70 eV) and SPI (9.8 and 8.4 eV) from DNT, DMMP, and MDMA.

DMMP and not from another substance or fragment with the same mass-to-charge ratio. Figure 4.3 shows the results for the explosive 2,4-DNT. This is also a decomposition product of the more common explosive TNT, which can only be detected by VUV radiation of the neonfilled EBEL, which is actually in the stage of testing, and therefore no results are shown here. The molecular mass of 2,4-DNT is 182 amu. The EI mass spectrum is shown in Figure 4.3a. The main peaks are at m/z ) 165 (loss of OH) and at m/z ) 89 (C7H5 moiety). As the ionization potential of 2,4-DNT is 10.0 eV,53 for SPI measurements the argon excimer lamp with a photon energy of 9.8 eV and an fwhm of about 0.8 eV was used. In the SPI mass spectrum in Figure 4.3b, the molecular ion peak is much more intense. In Figure 4.3c, the molecular ion peak was isolated with FNF. And in Figure 4.3d the MS/MS mass spectrum of 2,4-DNT is shown. The main MS/MS fragments are m/z ) 165 and m/z ) 119 as they were measured by EI. It is again possible to clearly identify 2,4-DNT with an MS/MS spectrum for m/z ) 182 that shows fragments with m/z ) 165 and 119. MS spectra of 3,4-DNT are shown in Figure 4.4. The 3,4-DNT is an isomer of 2,4-DNT but cannot be a decomposition or byproduct of TNT as one nitro group is in meta position. The EI MS spectrum (Figure 4.4a) as well as the SPI MS/MS spectrum of 3,4-DNT (Figure 4.4d) show different fragments compared to the ones of 2,4-DNT. As there is no nitro group positioned next 4464


to the methyl group no loss of OH resulting in an m/z ) 165 fragment is detected (ortho effect). The signal at m/z ) 166 indicates a loss of an oxygen only instead. Especially in the MS/ MS spectrum signals at m/z ) 119 and m/z ) 165 are missing, clearly indicating that the measured substance is not a 2,4,6-TNT derivative. This example demonstrates that the differentiation of the isomers 2,4-DNT and 3,4-DNT is possible with an MS/MS study. The thermal desorber was characterized by measurements where short residence times of the wipe pads inside the desorber have been used. For this, first an empty wipe pad and afterward 5 µL of a solution of DMMP in dichloromethane (1:500) on a wipe pad were used. The blank sample as well as the sample with the DMMP solution containing 11 µg of DMMP were desorbed for 60 s with the thermal desorber heated up to 200 °C. The chronological sequence of the MS/MS peak at m/z ) 79 was recorded with a measurement time of 5 s (averaging of 41 spectra with 50 ms ionization time, respectively) and is shown in Figure 5.1. The rapidly increasing signal due to the fast heating at the beginning followed by its slower decrease is clearly visible. The very fast decrease at the end of the heating period indicates that there are no cold spots in the inlet. For a first valuation of the detection limit, the counts were integrated during blind desorption (14 counts) and during desorption of 11 µg of DMMP (626 counts)

Figure 5. Panel 5.1: chronological sequence of the m/z ) 79 MS/MS fragment of m/z ) 124/125 during desorption of 11 µg of DMMP. Panel 5.2: SPI MS and MS/MS mass spectra of wipe pads with much background signals.

for this measurement. Out of this, a limit of detection of about 750 ng with a signal-to-noise ratio of 3 is ascertainable. The susceptibility to matrix compounds was investigated in another study by choosing the common organic solvent dimethyl sulfoxide (DMSO) as matrix. Because DMSO increases the rate of absorption of some compounds through organic tissues including skin, it can be used as a drug delivery system, and it is also contained in many ointments. Its molecular weight is 78 amu. As the main MS/MS fragment of DMMP is at m/z ) 79, it is likely that DMSO can influence the detection of DMMP. To get a more complex matrix we choose to wipe a dusty surface of an old computer monitor with a DMSO soaked pad. This was chosen, as it represents the matrix present on objects mainly used indoors like laptops. Particularly the wiping of laptops at check points in airports is a common strategy for detection of explosives. However, more measurements with different matrixes are planned for the future. In Figure 5.2a the SPI spectrum of the wipe pad soaked with DMSO and wiped over the computer monitor is shown. (For the spectra in Figure 5.2, the same settings as for the measurements for Figure 5.1 were used.) The peak at m/z ) 78 is dominating the mass spectrum. In addition, the 13C-DMSO peak at m/z ) 79 is rather high. A second wipe pad treated as above was measured adopting the MS/MS method for DMMP resulting in the spectrum shown in Figure 5.2b). There was no significant peak visible at m/z ) 78 or m/z ) 79 from 13C-DMSO. The matrix can therefore be regarded as totally shielded. Afterward, some DMMP was added to a third wipe pad alike and the measurement was repeated. As shown in Figure 5.2c, the presence of the matrix does not prevent the detection of the substance of interest as the expected peaks at m/z ) 79 and 94 are clearly visible. RESULTS AND DISCUSSION FOR COFFEE ROASTING OFF-GAS As a demonstration for monitoring a continuous process with our experimental setup, the off-gas from coffee roasting was investigated. Figure 6 shows how the mass spectrum during the roasting process changes with time in a three-dimensional plot.

Figure 6. Three-dimensional plot of the SPI mass spectrum of the evolved gas during a simulated coffee roasting process.

It can be seen that the signals can be followed over the roasting process, so real-time process analysis is possible. A rate of one measurement/min (average of 600 spectra with 50 ms ionization time, respectively) is sufficient to follow the roasting process, and several substances can be detected. Exemplarily, some compounds were labeled. Caffeine (m/z ) 194) and furfuryl alcohol (m/z ) 98) were discussed below in more detail. The peak at m/z ) 80 can be either pyrazine or protonated pyridine. Additionally, 4-vinyl-guaiacol (m/z ) 150), guaiacol (m/z ) 124), and dihydroxybenzene (m/z ) 110) were identified referring to Dorfner et al.55 According to them,44 one major reaction during coffee roasting is the degradation of 5-feruloyl quinic acid (5-FQA) yielding formation of phenolic compounds. In their study, two connected reaction channels are described. The first one is called “low activation energy channel”, which occurs below 120 °C within the early stage of roasting. It consists of ester hydrolysis of 5-FQA, (55) Dorfner, R.; Ferge, T.; Kettrup, A.; Zimmermann, R.; Yeretzian, C. J. Agric. Food Chem. 2003, 51, 5768–5773.


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Figure 7. SPI MS and MS/MS mass spectra and EI mass spectra of coffee roasting off-gas and standards.

followed by decarboxylation of the ferulic acid to form 4-vinylguaiacol, and finally polymerization of the vinyl group occurs that forms polymers with proteins (melanoidins) which is a Maillardtype reaction. The other described path is the so-called high activation energy that occurs at a later roasting state at higher temperatures, when the beans have dried. Then 4-vinylguaiacol oxidizes which leads sequentially to guaiacol and phenol. Thus, as it is possible with our measurement setup to monitor furfuryl alcohol, 4-vinylguaiacol, and guaiacol (m/z ) 124), these reactions can be observed in real time during roasting. This eventually makes it feasible to monitor the current status of the roasting process. For verification of the SPI MS peak assignment, MS/MS studies can be used as shown in Figure 7. The spectrum at the top (Figure 7a) shows the spectrum after 10 min of roasting from Figure 6. In the second row, two signals were isolated with FNF, and MS/MS spectra of these compounds were recorded afterward 4466


(third row). For these spectra, 600 measurements, each with an ionization time of 50 ms, were averaged. On the left-hand side, m/z ) 98 was investigated supposedly resulting from furfuryl alcohol.44,55 It is a volatile substance suitable for monitoring the roasting process.56 On the right-hand side m/z ) 194 was investigated supposed to be caffeine or ferulic acid.44,55 The best known substance in coffee beans is caffeine; ferula acid is one reactant occurring during roasting. Both substances were present within the beans, but caffeine is more likely to be detected as ferula acid is less volatile and more thermal labile. For comparison, SPI MS and MS/MS spectra of the pure substances were recorded with the same settings and shown in rows four and five, and the EI mass spectra of these substances from NIST15 are shown in the last row. The two substances were supplied by Sigma-Aldrich with a purity of 90%. (56) Bock, J. Ph.D. Dissertation, Justus-Liebig-Universita¨t Giessen, Giessen, Germany, 2000.

When these spectra, and especially the MS/MS fragments, which should be similar to the EI fragments, are compared, it can be concluded that the peak at m/z ) 98 results from furfuryl alcohol and the one at m/z ) 194 from caffeine as expected. On the left-hand side the unknown substance within the spectrum of the roasting process as well as furfuryl alcohol has a peak at m/z ) 98 (see Figure 7, parts b and d), and the MS/MS spectra of these peaks (Figure 7, parts c and e) as well as the EI spectrum of furfuryl alcohol at Figure 7f) show fragments at m/z ) 70 (loss of CO or C2H4) and 80/81 (loss of OH or H2O). On the right-hand side the unknown substance within the spectrum of the roasting process as well as caffeine has a peak at m/z ) 194 (see Figure 7, parts g and j), and the MS/MS spectra of these peaks (Figure 7, parts h and k) as well as the EI spectrum of caffeine at Figure 7l) show fragments at m/z ) 109 (C5H7N3), 137 (C6H7N3O), and 165 (C7H7N3O2). This example shows that not only the real-time measurement of MS spectra is possible but also the identification of unknown substances within complex matrixes through MS/MS studies. This could be shown here particularly for the coffee roasting process, but it is likely that this method can be adopted for other roasting processes as well, e.g., for nuts, cocoa beans, or malt, and eventually even for some continuous chemical processes in general. CONCLUSION AND OUTLOOK It has been shown that real-time detection of trace substances in various matrixes is possible with SPI-ITMS. Several applications were possible. In particular, the trace detection of security-relevant

substances on wipe pads and the real-time analysis of coffee roasting off-gas were successfully tested. Thereby, some advantages of the method were shown. First, most bulk matrix elements were not ionized due to their high ionization potential resulting in good selectivity. Furthermore, the fragmentation of the substances of interest was rather low resulting in an easy and sensitive identification of the peaks with MS/MS studies. Thus, unknown substances can be identified and very low false positive and false negative rates can be achieved. The limits of detection are sufficient for most applications. However, for some applications like trace detection of securityrelevant substances, the sensitivity will have to be further improved. In the future, an enhanced system with a commercial ion trap (240-MS Ion Trap, Varian Inc. Palo Alto, CA) will be field-tested for the detection of security-relevant substances. Additionally, light at a third wavelength with a photon energy higher than 10 eV will be developed to enable the detection of wider range of substances. ACKNOWLEDGMENT Support from the Helmholtz Zentrum Mu¨nchen, the University of Mainz, and the Max Planck Institute for Chemistry in Mainz as well as funding from the Federal Ministry of Education and Research (FKZ 13N8820) are gratefully acknowledged. Received for review February 6, 2009. Accepted April 2, 2009. AC900289R


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Trace Detection of Organic Compounds in Complex Sample Matrixes

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