Application of Infrared Laser Desorption Vacuum-UV Single-Photon

Anal. Chem. 2005, 77, 4528-4538

Application of Infrared Laser Desorption Vacuum-UV Single-Photon Ionization Mass Spectrometry for Analysis of Organic Compounds from Particulate Matter Filter Samples T. Ferge,†,‡ F. Mu 1 hlberger,† and R. Zimmermann*,†,‡,§

GSF Forschungszentrum, Institut fu¨r O ¨ kologische Chemie, Ingolsta¨dter Landstr. 1, 85764 Neuherberg, Germany, Analytische Chemie, Lehrstuhl fu¨r Festko¨rperphysik, Institut fu¨r Physik, Universita¨t Augsburg, Universita¨tsstrasse 1, 86159 Augsburg, Germany, and BifA - Bayerisches Institut fu¨r Umweltforschung und -technik, Abteilung Umwelt- und Prozesschemie, Am Mittleren Moos 46, 86167 Augsburg, Germany

A new built instrument suitable for laser desorption-single photon ionization time-of-flight mass spectrometry (LDSPI-TOFMS) with use of Vacuum-UV photons with a wavelength of 118 nm was used for the analysis of organic compounds. Fragmentation-free analysis of a variety of substances was achieved for desorption experiments with pure compounds desorbed from quartz glass filters applying low desorption energies. It was further found that the rate of fragmentation is strongly dependent on the desorption energy. Matrix effects were investigated by desorption experiments utilizing soot spiked with several organic compounds.The characteristics of the desorption process are assessed in more detail and the impact on the analysis of ambient particulate matter (PM) samples on filters are discussed. First results obtained from the application of the technique to the analysis of organic compounds from ambient PM are presented. Furthermore, possibilities of future developments of the method, in particular for analysis of ambient PM, are discussed. Several epidemiological studies to date show that there is a significant relevance of ambient particles in health effects.1 Particles with diameters of less than 2.5 µm (referred to as PM2.5) are strongly associated with mortality and other consequences such as cardio-pulmonary diseases.2-4 Recent studies suggest that ultrafine particles (DP < 100 nm) are more toxic than PM10.5 However, the number of ultrafine particles is often poorly * Corresponding author. E-mail: [email protected]. † GSF Forschungszentrum. ‡ Universita ¨t Augsburg. § BifA. (1) WHO. Health aspects of air pollution with particulate matter, ozone and nitrogen dioxide; WHO: Bonn, 2003. (2) Hoek, G.; Brunekreef, B.; Goldbohm, S.; Fischer, P.; van den Brandt, P. A. Lancet 2002, 360, 1203-1209. (3) Pope, C. A.; Burnett, R. T.; Thun, M. J.; Calle, E. E.; Krewski, D.; Ito, K.; Thurston, G. D. J. Am. Med. Assoc. 2002, 287, 1132-1141. (4) Zanobetti, A.; Schwartz, J. C.; Samoli, E.; Gryparis, A.; Touloumi, G.; Atkinson, R.; Le Tertre, A.; Bobros, J.; Celko, M.; Goren, A.; Forsberg, B.; Michelozzi, P.; Rabczenko, D.; Aranguez Ruiz, E.; Katsouyanni, K. Epidemiology 2002, 13, 87-93. (5) Brown, D. M.; Wilson, M. R.; MacNee, W.; Stone, V.; Donaldson, K. Toxicol. Appl. Pharmacol. 2001, 175, 191-199.

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correlated with PM2.5 or PM10, even though there is epidemiological evidence for the health effects of these extremely small particles.6 Up to now it has not been possible to establish a relationship between particle-related health effects and single components, although intensive research has been carried out in the past decade. So far, several investigations suggest a significant contribution of organic components to toxicity of particulate matter.7,8 The organic fraction of urban aerosols constitutes a complex mixture of a multitude of different compounds. For PM2.5 the overall concentration amounts to 1-12 µg/m3, which makes up roughly up to 50% of the total particle mass.9,10 Major compound classes in urban aerosols are for example aliphatic hydrocarbons, organic acids, and polycyclic aromatic hydrocarbons.11 For individual compounds the concentration is usually in the range from 0.1 to 10 ng/m3.12 The identification and analysis of organic compounds requires long sampling times, extensive sample preparation and clean up, and analysis time in the laboratory. For time-series studies aimed at the influence of organic aerosols on human health, a compilation of data of several compounds or compound classes is necessary at least on a daily basis. Various chromatographic methods such as gas chromatography/mass spectrometry (GC/MS)11,13 and comprehensive GC (GCxGC)14-16 are used for resolving these analytical challenges. However, the necessity for tedious sample preparation and (6) Peters, A.; Wichmann, H. E.; Tuch, T.; Heinrich, J.; Heyder, J. Am. J. Respir. Crit. Care Med. 1997, 155, 1376-1383. (7) Tolbert, P. E.; Klein, M.; Metzger, K. B.; Peel, J.; Flanders, W. D.; Todd, K.; Mulholland, J. A.; Ryan, P. B.; Frumkin, H. J. Exposure Anal. Environ. Epidemiol. 2000, 10, 446-460. (8) Monn, C.; Becker, S. Toxicol. Appl. Pharmacol. 1999, 155, 245-252. (9) Turpin, B. J.; Saxena, P.; Andrews, E. Atmos. Environ. 2000, 34, 29833013. (10) Tolocka, M. P.; Solomon, P. A.; Mitchell, W.; Norris, G. A.; Gemmill, D. B.; Wiener, R. W.; Vanderpool, R. W.; Homolya, J. B.; Rice, J. Aerosol Sci. Technol. 2001, 34, 88-96. (11) Rogge, W. F.; Mazurek, M. A.; Hildemann, L. M.; Cass, G. R.; Simoneit, B. R. T. Atmos. Environ. 1993, 27A, 1309-1330. (12) Schnelle-Kreis, J.; Sklorz, M.; Peters, A.; Cyrys, J.; Zimmermann, R. Atmos. Environ. 2005, in press. (13) Cass, G. R. Trends Anal. Chem. 1998, 17, 356-366. (14) Marriott, P. J.; Shellie, R. Trends Anal. Chem. 2002, 21, 573-583. 10.1021/ac050296x CCC: $30.25

© 2005 American Chemical Society Published on Web 05/27/2005

the collection of relatively large quantities renders the analysis of aerosol from sites with low concentrations (or when only limited time for sampling is available) an elaborate procedure. Hence, additional methods are desirable, which circumvent the timeconsuming sample preparation and enable the researcher to easily analyze smaller sample quantities. In the recent past, two-step laser desorption photoionization (LD-PI) mass spectrometry17-24 has proven to be a suitable candidate for such a purpose as it allows chemical analysis within minutes and without any sample preparation. LD-PI uses a first laser pulse for laser desorption (LD) of intact neutral molecules from the sample surface and a second laser pulse for ionization (PI) of desorbed species. Typically for desorption a pulsed CO2-laser (10.6 µm) or pulsed YAG-lasers (Er: YAG: λ ) 2.94 µm; Nd:YAG: λ ) 1.064 µm) are used; pulsed UV-lasers with fixed frequency wavelengths of 193 or 266 nm or OPO systems with tunable wavelengths (e.g., 273 and 290 nm24) are applied as ionization lasers. In this UV wavelength range, the resonance-enhanced multiphoton ionization (REMPI) is a selective and soft means of ionization, offering the detection of only those molecules with appropriate electronic transitions, thus representing an ideal method for analysis of aromatic trace compounds in complex samples.24-29 In the case of LD-REMPI, for polycyclic aromatic hydrocarbons quantitation in the low-nanogram to picogram range was achieved21,24,30 whereas limits of detection even in the attomole range have been reported.31,32 Also, the application of LD-REMPI to on-line single-particle analysis was accomplished for the analysis of organic model particles as well as aerosols from wood and cigarette combustion.33 However, REMPI, while providing high sensitivity for aromatic compounds, is not useful for analysis of aliphatic compounds, which are representing the vast majority of the organic mass in ambient PM. An alternative to multiphoton ionization is the single(15) Xu, X.; Williams, J.; Plass-Du ¨ lmer, C.; Berresheim, H.; Salisbury, G.; Lange, L.; Leliveld, J. Atmos. Chem. Phys. Discuss. 2003, 3, 1477-1513. (16) Welthagen, W.; Schnelle-Kreis, J.; Zimmermann, R. J. Chromatogr. A 2003, 1019, 233-249. (17) Haefliger, O. P.; Zenobi, R. Anal. Chem. 1998, 70, 2660-2665. (18) Haefliger, O. P.; Bucheli, T. D.; Zenobi, R. Environ. Sci. Technol. 2000, 34, 2178-2183. (19) Haefliger, O. P.; Bucheli, T. D.; Zenobi, R. Environ. Sci. Technol. 2000, 34, 2184-2189. (20) Morrical, B. D.; Zenobi, R. Atmos. Environ. 2002, 36, 801-881. (21) Kalberer, M.; Morrical, B. D.; Sax, M.; Zenobi, R. Anal. Chem. 2002, 74, 3492-3497. (22) Specht, A. A.; Blades, M. W. J. Am. Soc. Mass Spectrom. 2003, 14, 562570. (23) Elsila, J. E.; de Leon, N. P.; Zare, R. N. Anal. Chem. 2004, 76, 2430-2437. (24) Hauler, T. E.; Boesl, U.; Kaesdorf, S.; Zimmermann, R. J. Chromatogr. A 2004, 1058, 39-49. (25) Lubman, D. M., Ed. Lasers and Mass Spectrometry; Oxford University Press: New York, 1990. (26) Boesl, U. J. Mass Spectrom. 2000, 35, 289-304. (27) Grotheer, H.-H.; Nomayo, M.; Pokorny, H.; Thanner, R.; Gullett, B. K. Trends Appl. Spectrosc. 2001, 3, 181-206. (28) Cao, L.; Mu ¨ hlberger, F.; Adam, T.; Streibel, T.; Wang, H. Z.; Kettrup, A.; Zimmermann, R. Anal. Chem. 2003, 75, 5639-5645. (29) Dorfner, R.; Ferge, T.; Yeretzian, C.; Kettrup, A.; Zimmermann, R. Anal. Chem. 2004, 76, 1368-1402. (30) Emmenegger, C.; Kalberer, M.; Morrical, B. D.; Zenobi, R. Anal. Chem. 2003, 75, 4508-4513. (31) Clemett, S. J.; Zare, R. N. In Molecules in Astrophysic: Probes and Processes; van Dishoeck, E. F., Ed.; Kluwer Academic Publishers: Leiden, The Netherlands, 1997; Vol. 178, pp 305-320. (32) Zhan, Q.; Voumard, P.; Zenobi, R. Rapid Commun. Mass Spectrom. 1995, 9, 119-127. (33) Morrical, B. D.; Fergenson, D. P.; Prather, K. A. J. Am. Soc. Mass Spectrom. 1998, 9, 1068-1073.

photon ionization (SPI) with VUV photons.34,35 Due to medium selectivity by virtue of the ionization threshold, it is suitable for both aromatic and aliphatic organic compound classes. The typically used radiation (118 nm, 10.5 eV) is just above the ionization threshold of most organic compounds, making it a soft ionization method which produces little or no fragmentation. SPI time-of-flight mass spectrometry (SPI-TOFMS) has proven to be a method which can be applied in various analytical fields with limits of detection for single compounds down to the low ppb range.28,29,35-38 The combination of laser desorption with singlephoton ionization (LD-SPI) therefore offers the possibility for the analysis of organic compounds in aerosol samples. So far, SPI was employed for real-time detection of molecules desorbed from individual particles as well as depth profiling of organic compounds in single particles.39 Also characterization of thermo-desorbed organic compounds from particles impacted on a heated probe has been reported.40 However, with the currently used SPI-MS technologies individual particles with diameters less than 1 µm are difficult to analyze due to vanishingly small signal levels. For analysis of ambient aerosols this constitutes a problem as most organics are present in particles smaller than 1 µm.9,41,42 There are several approaches dealing with fast characterization of organic compounds in “bulk” aerosol samples. In each case particles are sampled through an aerodynamical lens and impacted on a heated probe where semivolatile components are vaporized and then ionized by 70-eV electron impact ionization (EI).43,44 With these techniques a very good sensitivity is achieved by integrating the signal over a large number of particles. However, due to the use of EI, mass spectra show extensive fragmentation so that a detection of molecular ions is difficult if possible at all. Recently, SPI was used for analysis of organic aerosols with laser desorption utilizing radiation from an Nd:YAG laser at 1064 nm.45 Aerosol particles are sampled through an aerodynamical lens on a deposition probe, laser-desorbed, and then ionized by VUV radiation. Here, a sensitivity of 50-500 ng/m3 was achieved for individual compounds in an aerosol sample. However, some degree of fragmentation still occurs in the mass spectra, most probably due to the desorption process utilizing 1064 nm photons. Even this rather low rate of fragmentation turns out to be a problem for compound identification of molecular peaks in the mass spectra of more complex samples. (34) Nir, E.; Hunziker, H. E.; Vries, M. S. Anal. Chem. 1999, 71, 1674-1678. (35) Butcher, D. J. Microchem. J. 1999, 62, 354-362. (36) Brown, A. L.; Dayton, D. C.; Nimlos, M. R.; Daily, J. W. Chemosphere 2001, 42, 663-669. (37) Mu ¨ hlberger, F.; Hafner, K.; Kaesdorf, S.; Ferge, T.; Zimmermann, R. Anal. Chem. 2004, 76, 6753-6764. (38) Mu ¨ hlberger, F.; Zimmermann, R.; Kettrup, A. Anal. Chem. 2001, 73, 35903604. (39) Woods, E.; Smith, G. D.; Miller, R. E.; Baer, T. Anal. Chem. 2002, 74, 16421649. (40) Sykes, D. C.; Woods, E.; Smith, G. D.; Baer, T.; Miller, R. E. Anal. Chem. 2002, 74, 2048-2052. (41) Liu, D. Y.; Wenzel, R. J.; Prather, K. A. J. Geophys. Res. D: Atmos. 2003, 108 (D7), 8426, doi: 10.1029/2001JD001562. (42) Alfarra, M. R.; Coe, H.; Allan, J. D.; Bower, K. N.; Boudries, H.; Canagaratna, M. R.; Jimenez, J. L.; Jayne, J. T.; Garforth, A. A.; Li, S.-M.; Worsnop, D. R. Atmos. Environ. 2004, 38, 5745-5758. (43) Tobias, H. J.; Ziemann, P. J. Anal. Chem. 1999, 71, 3428-3435. (44) Jayne, J. T.; Leard, D. C.; Zhang, X.; Davidovits, P.; Smith, K. A.; Kolb, C. E.; Worsnop, D. R. Aerosol Sci. Technol. 2000, 33, 49-70. (45) Oktem, B.; Tolocka, M. P.; Johnston, M. V. Anal. Chem. 2004, 76, 253261.

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Figure 1. Schematic representation of the instrument. For desorption the beam of a pulsed CO2-laser is focused on the sample surface. After a time delay, the ionization laser is fired (here 118 nm generated in a frequency tripling cell). Formed ions are subsequently detected in a time-of-flight mass spectrometer.

In the present work first results from the application of LDSPI for analysis of organic compounds from filter samples is described. Individual compounds are desorbed from glass fiber filters by a CO2 laser (10.6 µm) and vaporized molecules are ionized with VUV radiation at 118 nm (10.5 eV). The influence of the desorption laser fluence as well as the influence of the sample matrix on the spectra are discussed. Furthermore, first results from measurements on real-world particulate matter samples are presented. EXPERIMENTAL SECTION Instrumental Setup. The LD-SPI-TOFMS instrument is shown schematically in Figure 1. The mass spectrometer consists of a hybrid laser desorption/molecular beam ion source, a vacuum ultraviolet frequency tripling cell, and a tunable OPO-laser system for application of various laser wavelengths and a reflectron mass spectrometer. The instrument was developed as a mobile system suitable for laboratory as well as field applications and is described in detail in a recent publication.24 The first configuration of the laser system described in there consisted of a µ-TEA CO2-laser for desorption and an Nd:YAG based OPO-system for ionization. The CO2-laser (µ-TEA, Laser Science Inc., Franklin, MA) generates 10.6 µm pulses with a peak energy of approximately 15 mJ and peak width of 100 ns. The energy of the laser can be adjusted by a variable focal aperture located directly in front of the focusing optics. The ionization laser system is based on an Nd:YAG-laser (Quanta Ray INDI 50-10, Spectra Physics, Mountain View, CA), which includes devices for generation of the third and fourth harmonic frequencies (355 and 266 nm). The 266 nm pulses can be directly used for REMPI, whereas the 355 nm radiation is used for pumping an optical parametric oscillator (OPO) with second harmonic generation generating tunable UV laser radiation in the range from 220 to 350 nm. The tunability renders the system convenient for a wide variety of Ld-REMPI-TOFMS applications. Generation of VUV Laser Radiation. For the generation of Vacuum-UV laser radiation a conversion cell was added to the instrumental setup, which is mounted directly onto the ion source of the mass spectrometer. The rare gas cell (xenon; purity 4.0; total pressure 30 mbar) was designed similar to corresponding cells in previously described instruments.37,38 Due to the original 4530 Analytical Chemistry, Vol. 77, No. 14, July 15, 2005

design of the instrument as a mobile device, the cell is relatively short. In this case phase-matching conditions in a rare gas mixture do not enhance the efficiency of the conversion too effectively. For this reason we did not implement rare gas mixing in this work. The 355 nm beam generated by the pump laser can be directed to the VUV cell by exchanging only one mirror in the optical setup of the laser system. The 355 nm pulses (∼30 mJ/pulse) are focused with a quartz lens (f ) 100 mm) through a quartz window into a 180 mm long stainless steel tube. Calculation of the beam parameters and beam paths was performed according to literature.46 Separation of the remaining 355 nm and the generated 118 nm radiation was done by off-axis irradiation of the 355/118 nm beams onto a MgF2 lens.34,47 This is necessary to avoid fragmentation of the formed ions of the analyte molecules by the high intensity of the 355 nm fundamental beam.38 The resulting VUVlaser pulse is focused on the center of the ion source. The VUV pulse energy is not measured, but is expected to be on the order of 0.3 µJ based on an approximate conversion efficiency of 10-5 reported in the literature.48 Laser Desorption/Ionization. The sample to be investigated is mounted on a probe tip (L ) 6 mm), which is introduced into the ion source via an airlock. The probe tip is located on level with the repeller electrode of the TOFMS.24 The desorption laser is focused on a spot with a diameter of 1.0 mm on the sample surface. Typically, the desorption energy is adjusted to 1 mJ/pulse, resulting in a power density of 1.2 × 106 W/cm2. The laser spot hits the target 1.5 mm off the center. The probe is rotated by a small stepper motor in order to partly refresh the LD-target surface for each laser shot. The plume of desorbed molecules expands into the center region of the ion source and is crossed by the beam path of the ionization laser (VUV). The distance between sample surface and the VUV laser beam was 2.5 mm, resulting in an optimal delay time between desorption laser pulse and VUV laser pulse of 12 µs (measured for oleic acid desorbed from glass fiber filter). This value was found optimal for the experiments made with oleic acid. Although this value is dependent on the material to be desorbed, this delay time was kept for all experiments, which were carried out in the frame of this work (see Results and Discussion section below). The ions generated by the VUV pulse are analyzed with a reflectron time-of-flight mass spectrometer. A more detailed description of the ion source, the desorption probe, and the mass spectrometer can be found elsewhere.24 Sample Preparation. In the experiments performed, we used different organic compounds, which are representing typical organic compound classes found in ambient aerosols: Oleic acid is present in tropospheric particles as a product of vegetative burn with an average concentration of 25 ng/m3 under ambient conditions.11 Cholesterol is found at approximately 2 ng/ m3 in ambient air and can be used as a tracer for meat cooking.49,50 Pyrene and 9,10-phenanthroquinone are representing the polycyclic aromatic hydrocarbons (PAH) and oxygenated PAH (Oxy(46) Maker, P. D.; Terhune, R. W. Phys. Rev. 1965, 137 (No. 3A), 801-818. (47) Steenvoorden, R. J. J. M.; Hage, E. R. E.; Boon, J. J.; Kistemaker, P. G.; Weeding, T. L. Org. Mass Spectrom. 1994, 29, 78-84. (48) Kung, A. H. Opt. Lett. 1983, 8, 24-26. (49) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R. Environ. Sci. Technol. 1991, 25, 1112-1125. (50) Nolte, C. G.; Schauer, J. J.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci. Technol. 1999, 33, 3313-3316.

PAH) respectively present in urban aerosols.51 These compound classes are known air toxics due to their highly carcinogenic activity52 and their capability to induce oxidative stress in cells.53,54 6-Nitrochrysene was chosen as a member of the highly toxic substance group of nitropolycyclic aromatic hydrocarbons, which are released into ambient air through direct emission or due to secondary reactions in the atmosphere.55 Long-chain alkanes such as triacontane (C30H62) are present in ambient aerosols in relatively high individual concentrations of up to 20 ng/m3 12 and originate from unburnt fuel (diesel, heating fuel, and vehicle emissions) and vaporized lubricants.56,57 Polar organic compounds have been analyzed in ambient particulate samples with n-alkanoic acids such as triacontanoic acid being present in high individual concentrations (1.5-6.5 ng/m3).58,59 All chemicals were purchased from Sigma Aldrich (Sigma Aldrich, Taufkirchen, Germany) and used without further treatment. The individual compounds were dissolved in organic solvents (cholesterol, pyrene, 9,10-phenanthrenedione, and oleic acid in ethanol, triacontane and triacontanoic acid in hexane, and 6-nitrochrysene in dichloromethane - 5 µg/mL respectively) and 5 µL of the solutions were applied to pieces of glass fiber filters (Glass Microfiber Filters, Whatman, Brentford, UK). In addition, samples of elemental carbon particles (spark generated soot GFG 1000, Palas GmbH, Karlsruhe, Germany) were spiked with the prepared solutions. For this purpose, 2 µL of the respective solution was added per milligram of soot to the particles, allowing the solvent to dry overnight. For analysis the filter samples were fixed on the probe tip with double-sided adhesive tape, and the samples of spiked soot were fixed in the same way using the adhesive tape with approximately 1 mg of sample on the probe tip. For determination of the exact amount of soot adhered to the tip, the probe tip was weighed before and after the application of the soot. RESULTS AND DISCUSSION The primary mechanism for vaporization of analyte molecules by a desorption laser pulse is thought to be fast heating of the probe surface and subsequent heat transfer to the material deposited on the substrate, which then is desorbed and released into the gas phase. Figure 2 shows LD-SPI mass spectra of the seven pure compounds investigated in this study as well as those of the blank filter. The spectrum of oleic acid (Figure 2a) is dominated by the peak of the molecular ion at 282 m/z. The fragment ion peaks that originate from loss of H2O and OH (264 and 265 m/z) are visible. A very small peak at 222 m/z probably is due to loss of (51) Schnelle-Kreis, J.; Gebefu ¨ gi, I.; Welzl, G.; Ja¨nsch, T.; Kettrup, A. Atmos. Environ. 2001, 35, S71-S81. (52) Denissenko, M. F.; Pao, A.; Tang, M.; Pfeiffer, G. P. Science 1996, 274, 430-432. (53) Hiura, T. S.; Kaszubowski, M. P.; Li, N.; Nel, A. E. J. Immunol. 1999, 163, 5582-5591. (54) Bolton, J. L.; Trush, M. A.; Penning, T. M.; Dryhurst, G.; Monks, T. J. Chem. Res. Toxicol. 2000, 13, 135-160. (55) Helmig, D.; Arey, J.; Atkinson, R.; Harger, W. P.; McElroy, P. A. Atmos. Environ. 1992, 26A, 1735-1745. (56) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R. Environ. Sci. Technol. 1993, 27, 636-651. (57) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R. Environ. Sci. Technol. 1997, 31, 2731-2373. (58) Schauer, J. J.; Cass, G. R. Environ. Sci. Technol. 2000, 34, 1821-1832. (59) Yue, Z.; Fraser, M. P. Atmos. Environ. 2004, 38, 3253-3261.

C3H6O, a peak also observed in two-step desorption/ionization spectra of oleic acid by other researchers.45 Figure 2b shows the mass spectrum of cholesterol. Here also the molecular ion is the dominating species of the spectrum together with some small high m/z fragments at 368, 353, and 275, which are known fragments in EI-mass spectra of cholesterol. All other spectra (Figure 2cg) show the same fundamental characteristic of the molecular ions being by far the dominant if not only species visible. The only fragmentation that occurs are the mentioned small signals of fragments in the spectrum of cholesterol, the water loss from the carboxylic group in oleic acid, and the loss of CO in 9,10phenanthroquinone (208 m/z and 180 m/z, Figure 2d). Even the long-chain aliphatic hydrocarbons as triacontane and triacontanoic acid do not show any fragmentation pattern. Astonishingly, triacontanoic acid does not even show fragmentation due to loss of OH and/or H2O as is the case for oleic acid. Furthermore, it is possible to detect even 6-nitrochrysene as molecular ion without any fragmentation. This is a very interesting aspect as the detection of nitroaromatic compounds usually is hampered by several factors such as the loss of NO and/or NO2 as well as the need of other UV wavelengths for detection of minor molecular peaks among typical fragment patterns (e.g., 213 nm60) or the application of negative ion mass spectrometry.61 Recently, we could also detect molecular ions of 2,4- and 2,6-dinitroanilin without fragmentation (data not shown here). For the analysis of complex sample matrixes the model of pure compounds on filters is certainly not appropriate. In urban aerosols the fraction of black carbonaceous matter can account for up to 20-50%.11,62 A more convenient sample matrix therefore would be carbonaceous material. To exclude interfering effects of other compounds, we used the spark-generated soot as stated in the Experimental Section. This soot is known to be composed mainly of elemental carbon (EC/TC ) 89.9% with EC: elemental carbon, TC: total carbon including organic content).63 Consequently, samples of spiked soot were investigated with LD-SPI. Figure 3 shows the resulting mass spectra. In principle, the spectra do not show major differences when compared to the spectra recorded by desorbing directly from filter material. Only some minor peaks, which obviously stem from the soot matrix, are additionally visible and marked with asterisks. That these peaks originate from the soot is shown in Figure 3h, which depicts a spectrum of pure soot substrate obtained under the same experimental conditions. Yet the nature of these peaks remains to be unraveled. In general, with the soot matrix, higher signal intensity is observed for all compounds in the LD-SPI spectra, which supports the assumption that black carbon increases the yield of desorbed material due to the higher incoupling rate of the laser energy (see also the discussion below). Differences are observed in the spectra of oleic acid, triacontane andsto a lesser extentsof triacontanoic acid. Here, an apparent number of peaks with the typical fragment pattern of aliphatic hydrocarbons suggest some rate of fragmentation of the compounds in the LD-SPI process. (60) Dotter, R. N.; Smith, C. H.; Young, M. K.; Kelly, P. B.; Jones, A. D.; McCauley, E. M.; Chang, D. P. Y. Anal. Chem. 1996, 68, 2319-2324. (61) Bezabeh, D. Z.; Allen, T. M.; McCauley, E. M.; Kelly, P. B.; Jones, A. D. J. Am. Soc. Mass Spectrom. 1997, 8, 630-636. (62) Chow, J. C.; Watson, J. G.; Lowenthal, D. H.; Solomon, P. A.; Magliano, K. L.; Ziman, S. D.; Richards, L. W. Aerosol Sci. Technol. 1993, 18, 105-128. (63) Ferge, T.; Karg, E.; Schro ¨ppel, A.; Tobias, H. J.; Frank, M.; Gard, E. E.; Zimmermann, R., Submitted to Environ. Sci. Technol.


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Figure 2. LD-SPI mass spectra of (a) oleic acid, (b) cholesterol, (c) pyrene, (d) 9,10-phenathroquinone, (e) triacontane, (f) triacontanoic acid, and (g) 6-nitrochrysene on glass fiber filter; 1 mJ CO2-laser energy, 30 mJ/pulse @ 355 nm for VUV generation. All spectra show molecular ions as predominant species. No fragmentation occurs, not even with ionization of long-chain aliphatic compounds.

The fragmentation we observe in the spectra of aliphatic hydrocarbons is influenced by the desorption process as is illustrated in Figure 4. Mass spectra of oleic acid on soot were recorded with varying desorption energy, applying 1.0, 1.5, and 4532 Analytical Chemistry, Vol. 77, No. 14, July 15, 2005

2.0 mJ, respectively. These pulse energies correspond to laser fluences of approximately 1.2 × 106, 1.9 × 106, and 2.5 × 106 W/cm2 (Figure 4a, b, and c, respectively). In the same way the peaks stemming from substrate material show stronger signals

Figure 3. LD-SPI mass spectra of (a) oleic acid, (b) cholesterol, (c) pyrene, (d) 9,10-phenathroquinone, (e) triacontane, (f) triacontanoic acid, and (g) 6-nitrochrysene on soot (derived from a spark generator - see text); 1 mJ CO2-laser energy, 30 mJ/pulse @ 355 nm for VUV generation. (h) LD-SPI mass spectrum of the soot itself. Typical peaks stemming from the soot underground are marked with an asterisk. Oleic acid, triacontane, and to a much lower extent also triacontanoic acid show some fragmentation.

as the energy used for desorption is raised and the overall signal intensity of desorbed material is higher in the spectra with

increased desorption energy. Thus, we deal with two effects upon the increase of the desorption energy: (1) higher signal intensities Analytical Chemistry, Vol. 77, No. 14, July 15, 2005

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Figure 4. LD-SPI mass spectra of oleic acid on soot with different desorption laser energies. With increasing desorption laser energy, a higher rate of fragmentation occurs. Higher desorption laser power leads to higher energy transfer to desorbed molecules and presumably to higher desorption temperatures and thus rather high excess energy.

(i.e., more material is desorbed per laser shot) and (2) increasing rate of fragmentation of long-chain organic hydrocarbons. The higher power density in the desorption laser spot consequently boosts the deposition of laser energy in the sample matrix, resulting in a higher yield of desorbed material. In the same way desorbed molecules gather excess thermal energy, which for some species leads to fragmentation of the desorbed neutral molecules upon ionization. With higher CO2 laser fluence, the desorbed molecules have a higher amount of thermal excess energy, thus the formed ions have enough energy to dissociate to produce fragment ions.64 To gain some more insight into the desorption process, we measured the signal intensity of the signal of the molecular ion of oleic acid as a function of the delay time between desorption and ionization laser pulse. The plume of molecules desorbed from (64) Woods, E.; Smith, G. D.; Dessiaterik, Y.; Baer, T.; Miller, R. E. Anal. Chem. 2001, 73, 2317-2322.

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the probe surface expands from the desorption laser spot toward the center of the ion source and crosses the ionization laser beam focus 2.5 mm above the target surface. Figures 5a and 5b show the peak area, which is a measure of the number of molecules ionized, of the molecular ion peak (m/z 282) as a function of applied delay time when desorption takes place from a glass fiber filter and soot, respectively. In the case of the glass fiber filter the peak area reaches a maximum at a delay time of 12 µs, whereas with soot as substrate the optimal delay time is considerably shorter with approximately 7 µs. With the desorptionionization geometry of the setup these delay times can be translated into molecular velocities of the desorbed species. Figure 5c,d shows the peak areas as a function of the molecular velocity. Support for the assumption of a thermal desorption process is given by the fact that the velocity distribution in Figure 5c (glass fiber filter) roughly fits a Maxwell-Boltzmann distribution with a translational temperature of 650 ( 50 K. These results, which indicate a thermal process, are in principle agreement with recent reports on the vaporization of molecules by laser desorption with a pulsed Nd:YAG laser.45 Figure 5d shows the velocity distribution of oleic acid molecules desorbed from soot. Here, the fit to a Maxwell-Boltzmann distribution is not possible any longer for the whole velocity range. Only the increasing part of the velocity distribution can be fitted with a distribution according to a translational temperature of 1200 ( 50 K, whereas the declining part shows higher values in the measurement than expected from the pure thermal Maxwell-Boltzmann distribution. The considerable amount of molecules, which are ejected with very high velocities (and thus kinetic energies), suggest an explosive process rather than a thermal vaporization. Thus, with the current setup the desorption mechanism can be mainly explained by thermal desorption but in the case of the soot matrix an overlay with explosive vaporization occurs. Since the pulse length of the CO2 laser (100 ns) is much shorter than the time for thermal equilibration (approximately 10 µs), the desorption process occurs within the limit of thermal confinement,65,66 resulting in a heating of the surface well beyond its regular boiling temperature. The evaporation mechanism depends on the laser fluence and changes from thermal desorption from the surface to an explosive ablation of the overheated surface above the threshold fluence. Both mechanisms are known in the literature, the threshold value for the transition from thermal to explosive desorption being in the range of 1.0 × 106 W/cm2.67 The here applied pulse energy of 1 mJ/pulse, which results in a laser fluence of 1.2 × 106 W/cm2, is in the range of this threshold value, suggesting that with the current setup we are close to this transition. However, this threshold fluence depends on a number of factors such as the identity of the material from which the compounds of interest are desorbed. The favored thermal desorption requires uniform heating and low laser fluence. One prerequisite for uniform heating is that the material is optically thin, meaning that the product of the material thickness (penetration depth of the laser light) d and the absorption coefficient κ multiplied by the material density F meet the following inequality, dκF , 1. Under these circumstances the laser beam is not noticeably attenuated, allowing all points in the sample layer to (65) Zhigilei, L. V.; Garrison, B. J. Appl. Phys. Lett. 1999, 74, 1341-1343. (66) Zhigilei, L. V.; Garrison, B. J. J. Appl. Phys. 2000, 88, 1281-1298. (67) Woods, E.; Miller, R. E.; Baer, T. J. Phys. Chem. A 2003, 107, 2119-2125.

Figure 5. (a) Signal intensity vs delay time for LD-SPI measurements of oleic acid desorbed from glass fiber filter. (b) Signal intensity vs delay time for LD-SPI measurements of oleic acid desorbed from soot. (c) Approximation of signal intensity distribution (glass fiber) with a MaxwellBoltzmann velocity distribution. For a distance of 2.5 mm between desorption plate and ionization focus, a maximum in molecule velocity of 187 m/s corresponding to a temperature of 650 K can be approximated. (d) Approximation of signal intensity distribution (soot) with a MaxwellBoltzmann velocity distribution. Only the increasing part of the distribution can be approximated relatively well with a distribution corresponding to a temperature of 1200 K (maximum in velocity distribution of 265 m/s). The high fraction of very fast molecules leads to the assumption of two conquering desorption mechanisms: thermal desorption and explosive evaporation.

experience the same laser fluence. In comparison to pure organic compounds and glass fiber filter material, soot certainly is an optically thicker material; therefore, the threshold value for the laser fluence above which explosive evaporation occurs is lower. This explains the observed explosive characteristics of the desorption process from the soot matrix. These characteristics certainly have an impact on the analysis of ambient aerosol samples (see discussion below). For estimation of limits of detection (LOD) of the compounds on the soot matrix, the known amounts of the pure standard compounds, which we applied as described in the Experimental Section in a concentration of 2 µL/mg to the soot, were taken as basis. By assuming an even distribution of compounds on the soot particles after evaporation of the solvent and with the application of a fixed amount of spiked soot (1 mg) to the probe tip, we can estimate the quantity of adsorbed substance to be 10 ng per sample. The irradiated surface (IR laser desorption) was used as a basis for further calculation. The laser is focused 1.5 mm offcenter to a spot with a diameter of 1 mm on the probe tip. Thus, a ring surface representing 33.3% of the entire probe surface is irradiated by the IR laser. Therefore, 3.33 ng of the respective compounds are available for laser desorption. The 200 LD-SPI TOF mass spectra for the measurement of a sample (corresponding

to one full turn of the probe tip) were averaged and the LOD for a signal-to-noise (S/N) ratio of 2 was calculated according to the formula68

LODS/N-2 )

2cσ (p-m)

where c is the absolute amount of irradiated molecules (3.33 ng), σ the standard deviation of the noise, p the signal height, and m the baseline level. The LODs are in the low pg range for the investigated compounds when desorbed from the pure soot matrix. In detail the following LODs (S/N ) 2) were determined: Oleic acid 5 pg; cholesterol 15 pg; pyrene 7 pg; 9,10phenathroquinone 11 pg; triacontane 5 pg; triacontanoic acid 12 pg; 6-nitrochrysene 10 pg. These absolute values compare well with those reported from other LD-SPI experiments.45 The linearity of the LD-PI method was tested and shown in a recent publication.24 The obtained LODs are in principle sufficient for detection of organic compounds in ambient samples. A recent study12 dealing (68) Williams, B. A.; Tanada, T. N.; Cool, T. A. In 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 1587-1596.


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Table 1. Peak Assignment of Ambient Aerosol Sample (Figure 6); Aliphatic Hydrocarbon Acids Are the Prevailing Species in the Mass Spectrum m/z 17 30 43/57/71/85/99/113 (peak groups) 60 74 98 122 198 144/228/368/396/424/452 270/298

Figure 6. LD-SPI mass spectrum of an ambient aerosol sample. The low mass region of the spectrum is dominated by signals from fragmented aliphatic hydrocarbons and some peaks, which can be assigned to aliphatic hydrocarbon acids. Even a tentative assignment to long-chain aliphatic acids and/or esters is possible in the high mass range. The inset exemplarily shows the mass region from 220 to 285 m/z in an expanded view.

with analysis of organic compounds in ambient PM in Augsburg, Germany, on a daily basis revealed concentrations of pyrene of 0.5-1.8 ng/m3 as well as of triacontane of 3.1-7.3 ng/m3. The filter stripes (2.1 × 27 mm) analyzed in this work by direct thermal desorption GC-TOFMS represented 1 m3 of sampled air. With the above-mentioned characteristics of the desorption laser spot the irradiated sample surface with these filter samples corresponds to 0.166 m3 air. By assuming an even distribution of compounds on the filter stripe, this accounts in the case of pyrene for a total accessible amount of 83-298 pg and in the case of triacontane for 514-1211 pg. With the above-discussed LOD, the analysis of these compounds should be possible by application of LD-SPI. Figure 6 shows a LD-SPI TOF mass spectrum of such a PM filter sample of ambient air collected in Augsburg, Germany (1 mJ CO2-laser pulse energy, 30 mJ per pulse 355 nm radiation). Panel a shows the low mass region up to m/z ) 150 whereas 4536 Analytical Chemistry, Vol. 77, No. 14, July 15, 2005

species NH3 (from NH4NO3) NO (from NH4NO3) alkyl fragments acetic acid propionic acid maleic anhydride benzoic acid naphthalic anhydride long-chain aliphatic hydrocarbon acids (even number of C-atoms) long-chain aliphatic esters (uneven number of C-atoms)

panel b shows the whole measured mass region up to m/z ) 500. The spectrum is dominated by several large peaks in the low mass region and exhibits a large number of peaks in the medium mass region ranging from m/z ) 140 to m/z ) 350. In the high mass region of the spectrum some peaks noticeably jut out of the spectral noise. The dominant peaks at m/z ) 17 and 30 are due to NH3 and NO stemming from ammoniumnitrate (NH4NO3) present in high amounts in ambient aerosols. The peak groups emerging around m/z ) 43, 57, 71, 85, 99, and 113 can be assigned to alkyl-fragments from aliphatic compounds, which are fragmented during the desorption/ionization process (see above). The spectra of oleic acid show the same peak groups (see also Figure 4c), when higher laser power is applied. It has to be assumed that there are several fragmentation pathways accessible for the desorbed molecules (which carry high excess energy) when ionized by the VUV laser beam. However, some prominent mass signals can be assigned to organic molecules present in the sample. In detail, organic acids are the prevailing species with acetic acid (m/z ) 60), propionic acid (m/z ) 73/74), maleic anhydride (from maleic acid, m/z ) 98), and benzoic acid (m/z ) 122) showing the most prominent signals. Even some stronger signals in the medium and high mass range can be tentatively assigned to aliphatic organic acids. A detailed peak assignment is given in Table 1. Even numbered hydrocarbon acids and their esters are usually strongly present in aerosol samples when these are influenced by biogenic sources.59,69-71 The here investigated aerosol sample was an urban background aerosol, which is also influenced by biogenic emissions from plants in the vicinity of the sampling site. Although the mass assignment is tentative, the here given interpretation with aliphatic hydrocarbon acids (even number of carbons) and esters (uneven number of carbons) is also backed by conventional gas chromatographic analysis of the same sample, which showed these compounds being present in relatively high concentrations.72 In addition to the assignment of single outstanding peaks, representing compounds present in high concentrations, it should also be possible to compare different (69) Simoneit, B. R. T.; Mazurek, M. A. Atmos. Environ. 1982, 16, 2139-2159. (70) Simoneit, B. R. T. Appl. Geochem. 2002, 17, 129-162. (71) Bin Abas, M. R.; Rahman, N.; Omar, N. Y. M. J.; Maah, M. J.; Samah, A. A.; Oros, D. R.; Otto, A.; Simoneit, B. R. T. Atmos. Environ. 2004, 38, 42234241. (72) Schnelle-Kreis, J. Personal communication, 2005.

Figure 7. Two LD-SPI mass spectra of ambient aerosol samples on glass fiber filters with induced fragmentation (see text). The Friday sample with a high load of aliphatic hydrocarbons shows a noticeable higher amount of fragments from aliphatic hydrocarbons than the Saturday sample. This can be explained with the differing traffic situation. Even without molecular information accessible, the overall amount of aliphatic hydrocarbons can be estimated via the fragment peaks.

homologue rows of aliphatic hydrocarbons as alkanes and aldehydes, alkenes and cycloalkanes, alkynes and dienes, and alcohols, esters, and acids from the spectra. This is obvious from the inset in Figure 6, where the mass region from 220 to 285 m/z is enlarged. One can easily see that the peaks present in the spectra are only present on even numbered mass-to-charge ratios as it is expected for hydrocarbon compounds. For example, in this enlarged mass region the peaks with m/z ) 226, 240, 254, 268, and 282 can be assigned as a sum value to the homologue row of alkanes and aldehydes, the signals at m/z ) 224, 238, 252, 266, and 280 to the row of alkenes and cycloalkanes, the signals at m/z ) 222, 236, 250, 264, and 278 to alkynes or dienes, and finally the signals at m/z ) 228, 242, 256, 270, and 284 to the row of alcohols, esters, and acids. Even if with this assignment no information on a molecular level is available for sure, it can give an overview of the relative amount of different organic compound classes present in the samples. With the current setup single-species investigation seems difficult as only major components are clearly visible in the spectrum. However, in addition to species identification and comparison of compound classes via the homologue rows, LDSPI mass spectra can give information on the relative organic carbon content of filter samples when one takes advantage of the fragmentation, which is induced by the desorption process. For this, we applied 1.5 mJ CO2-laser pulse energy for desorption of ambient air filter samples (30 mJ per pulse 355 nm radiation was kept constant as in previous experiments). Figure 7 shows the low mass region of two mass spectra from two ambient aerosol

samples on glass fiber filters sampled on two different days (PM2.5). The samples were taken near a street in northern Munich with heavy traffic during workdays. As expected, intense fragmentation of hydrocarbon compounds occurs, which is obvious due to the typical peak pattern and prominent peak groups at m/z ) 43, 57, 71, 85, and 99, corresponding to alkyl-fragments, which are dominating both spectra. In the higher m/z region there is no molecular information available for both cases under the applied LD conditions. As is indicated in Figure 7, the samples were taken on a Friday and Saturday, respectively. The main difference between these 2 days is the completely changed traffic flow on Saturday. Traffic-related emissions are known to contain a high fraction of organics and this difference is visible in the spectra, as the overall amount of detected compounds (fragments) is strongly reduced for the Saturday sample when car traffic was negligible compared to Friday. Even though, due to the strong fragmentation, no identification of compounds is possible, LDSPI gives information on the overall content of aliphatic hydrocarbons (which make up almost all organic carbon matter) when the integrated signal strength of the alkyl fragment peaks is taken into account. Even though this does not yield absolute values in terms of organic carbon content (like conventional EC/OC measurements), LD-SPI allows the direct comparison of several samples without extensive sample preparation by use of this induced fragmentation. Considering these results, it becomes obvious that LD-SPITOFMS is a valuable analytical technique for analysis of aerosol samples in two ways: First, it is possible to identify several aliphatic organic compounds present on filter samples (Figure 6) as well as to compare the relative amounts of certain organic compound classes. Second, with the application of higher desorption laser power, a fast, comparative analysis of the relative OC content is possible by the integration over the alkyl-fragment peaks. In combination with LD-REMPI, which is also possible with the new instrument, one can gain insights into the composition of the organic fraction of the aerosol sample regarding both the aliphatic and aromatic hydrocarbons. Thus, LD-SPI is a feasible analytical tool for identification of compounds and compound classes present in ambient aerosols. However, some considerations about the two main factors, which determine the efficiency of the LD-PI method for ambient measurements (the desorption and the ionization process), remain to be taken into account. One has to take into account that ambient aerosols are composed of a high degree of inorganic matter (e.g., alkali salts, NH4NO3, which is also present in the spectra). First experiments with NaSO4 as substrate, which was spiked with small amounts of cholesterol, did not show detectable signals. Recent studies concerning the one-step LDI process for organic species have shown that desorption/ionization efficiency strongly depends on the sample matrix. For example, it is not possible to detect PAHs in an LDI process from fly ash particles, which consist of mainly inorganic matter. No PAH signals were obtained even from spiked fly ash samples containing up to 1% of a single PAH species (Pyrene).73 In contrast, PAH adsorbed on black carbon (e.g., soot (73) Zimmermann, R.; Van Vaeck, L.; Davidovic, M.; Beckmann, M.; Adams, A. Environ. Sci. Technol. 2000, 34, 4780-4788.


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particles) can be detected efficiently by LDI mass spectrometry.73,74 This phenomenon was interpreted by the high coupling efficiency of the laser energy due to the photon absorption of black carbon. A similar effect is known as graphite-assisted MALDI (matrixassisted laser desorption/ionization).75 Therefore, it seems reasonable that, also in the case of LD-PI, one has to deal with two concurrent processes: (1) enhancement of the desorption due to black carbon, which is accompanied by an increasing rate of fragmentation, and (2) a suppression of an effective desorption due to inorganic content of the sample matrix, both influencing the achievable LOD of compounds and thus mass spectral information. Increasing the laser power for desorption is impeded by the fact that one then changes the desorption from a rather thermal process to an explosive ablation, leading to increased fragmentation of the formed ions. However, for analysis of ambient aerosols with low concentrations or when shorter sampling times are required, the sensitivity has to be increased. In principle, this can be accomplished by desorbing more material from the sample surface. One possible way of keeping the laser fluence at the sample surface constant and at the same time increasing the accessible amount of material would be to broaden the incident CO2-laser beam with increased pulse power. However, this causes also a broader desorption plume and therefore also requires a modification of the ionization beam path (see below). Another promising desorption technique is the Laser Induced Acoustic Desorption (LIAD).76 With this technique long-chain aliphatic hydrocarbons also can be desorbed as neutrals into the gas phase and subsequently ionized by several techniques. A titanium foil holding the analytes is fired upon from the back with a pulsed Nd:YAG laser at 532 nm. The pulses induce ultrasonic acoustic waves that propagate through the foil, resulting in desorption of the molecules from the opposite side. Nevertheless, this requires major rearrangements of the ion source and laser setup of the current instrument. As stated above, the broadening of the desorption laser beam requires also a rearrangement of the UV-laser setup to generate 118 nm laser radiation. Currently, we are using pure xenon in the conversion cell and do not apply phase-matching conditions in a gas mixture. As a result, the incident 355 nm beam has to be tightly focused into the conversion cell. Simultaneously, this accounts for a small beam waist in the ionization region. However, when the spot size of the desorption laser is expanded, the density of desorbed molecules (when an even distribution is assumed) in the ionization beam waist will stay constant. This means that by only expanding the desorption spot and thus desorbing more material the achievable LOD will not improve. To again increase the sensitivity, the ionization focus has to be broadened likewise. This can be done as the ionization efficiency of the SPI-processs in contrast to multiphoton processessdepends only on the absolute number of photons and not on the photon density. One rather simple method to achieve this goal is to relocate the ionization focus by moving the focal point in the conversion cell. If even higher photon densities are required, also phase-matching (74) Zimmermann, R.; Ferge, T.; Ga¨lli, M.; Karlsson, R. Rapid Commun. Mass Spectrom. 2003, 17, 851-859. (75) Dale, M. J.; Knochenmuss, R.; Zenobi, R. Anal. Chem. 1996, 68, 33213329. (76) Campbell, J. L.; Crawford, K. E.; Kenttamaa, H. I. Anal. Chem. 2004, 76, 959-963.

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conditions in the conversion cell can be applied as this is necessary when higher energies of the 355 nm laser beam are used to avoid gas breakthrough. First work on these topics is currently carried out at the GSF research center. CONCLUSION LD-SPI-TOFMS allows the fragmentation free desorption/ ionization of a great variety of organic compounds. This includes aliphatic hydrocarbons such as long-chain alkanes and alkanoic acids as well as aromatic hydrocarbons, oxygenated PAH, and nitroaromatic compounds. Limits of detection for single compounds in the range of concentrations comparable to ambient aerosols were achieved in experiments with spiked soot. However, it has to be noted that these LODs are valid only for pure compounds desorbed from a pure soot matrix and cannot be translated one-to-one to ambient aerosols because of interfering effects of the inorganic content of the aerosol. The observed matrix effects on the desorption process strongly affect the resulting mass spectra. Soot as optically thick material has very high absorption efficiency and therefore features high rates of energy transfer to adsorbed species. This results in the transition of the evaporation mechanism from the favorable thermal vaporization to an explosive one, which results in molecules carrying high thermal excess energy, favoring fragmentation upon ionization. On the other hand, a high amount of inorganic material seems to suppress efficient desorption. These concurring properties of the matrixes are currently the main drawback when analyzing real world samples. Additionally, information on the overall amount of organic hydrocarbons is available by virtue of the total ion current in the mass spectra when one makes use of the induced fragmentation due to high desorption laser power. In conclusion, LD-SPI-TOFMS is a promising tool for fast and sensitive analysis of a wide variety of organic compounds. It requires no tedious sample preparation and cleanup and constitutes an easy method for obtaining a general view on sample composition and in combination with LD-REMPI can give an overview of the organic composition including aliphatic and aromatic organic compounds. ACKNOWLEDGMENT This work was carried out within the scope of the GSF-Focus “Health relevance of aerosols” which coordinates aerosol-related research within the GSF Research Center. The authors like to thank T. Streibel, T. Adam, S. Mitschke, W. Welthagen, M. Bente, M. Sklorz, and J. Schnelle-Kreis for valuable discussions as well as support with the sampling of ambient aerosols and E. Karg for providing the soot particles used in this study. Funding of the DFG (Grant Number ZI 764 1-1) are gratefully acknowledged.

Received for review February 17, 2005. Accepted April 27, 2005. AC050296X

Application of Infrared Laser Desorption Vacuum-UV Single-Photon

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