ARTICLE pubs.acs.org/ac
Investigation of the Utility of Laser-Secondary Neutral Mass Spectrometry for the Detection of Polyaromatic Hydrocarbons in Individual Atmospheric Aerosol Particles Bonnie J. Tyler,*,† Steffen Dambach,‡ Sebastian Galla,‡ Richard E. Peterson,† and Heinrich F. Arlinghaus‡ † ‡
Departments of Chemical Engineering and Chemistry, University of the West Indies, St. Augustine, Trinidad and Tobago Physikalisches Institut, University of Muenster, Wilhelm-Klemm-Strausse 10, 48149 Muenster, Germany
bS Supporting Information ABSTRACT: The distribution of polyaromatic hydrocarbons (PAHs) in ambient aerosol particles is of importance to both human health and climate forcing. Although time-of-flight secondary ion mass spectrometry (ToF-SIMS) has proven useful for studying the distribution of organic compounds in individual aerosol particles, it is difficult to detect PAHs at relevant concentrations in individual aerosol particles because of their low ion yield. In this study, we explore the potential of using laser secondary neutral mass spectrometry (Laser-SNMS) to study three PAHs: pyrene, anthracene, and naphthalene. Because of the high volatility of PAHs, a cryostage was required for the analysis to prevent sublimation of the molecules into the vacuum chamber. We studied two laser systems, a 157 nm excimer laser, which is capable of single-photon ionization of the PAHs, and a 193 nm laser, which requires multiphoton ionization. Under optimized conditions for laser power density and primary ion pulse length, 193 nm postionization resulted in a 2 50-fold increase in ion yield over ToF-SIMS. Using the 157 nm laser, the yield was increased by more than 3 orders of magnitude for all 3 PAHs studied. The single-photon postionization process proved superior in terms of both yield enhancement and reduced fragmentation. By using the optimized 157 nm laser system and a cryostage, we were able to detect PAHs on the surface of 2 μm diameter ambient aerosol particles.
T
ime-of-flight secondary ion mass spectrometry (ToF-SIMS) has been successfully employed for the analysis of organic and inorganic compounds in individual atmospheric particles.1 7 It is perhaps the only currently available technique capable of analyzing the surface composition and three-dimensional structure of atmospheric particles. Although ToF-SIMS has excellent sensitivity to fatty acids and other aliphatic hydrocarbons, its sensitivity to aromatic hydrocarbons is poor. In this work we have investigated the utility of laser secondary neutral mass spectrometry (Laser-SNMS) with both a 157 nm and a 193 nm laser as a means of enhancing detection of polyaromatic hydrocarbons for the study of individual atmospheric aerosol particles. Polyaromatic hydrocarbons (PAHs), organic compounds with 2 or more fused aromatic rings, are near ubiquitous semivolatile atmospheric constituents. They are typically formed during incomplete combustion or other high temperature processes. PAHs have a long environmental history. Scrotal cancer in chimney sweeps, described by Sir Percival Pott in 1775, has been ascribed to occupational exposure to PAHs in chimney soot. Many other examples of PAH health concerns are given in the review by Nikolaou et al8 and by the World Health Organization.9 Their chemistry in the atmosphere is a function of their low volatility, leading to their presence largely in condensed phase, and by their nonpolar structures, yielding low aqueous solubility. In consequence, PAHs are lipophilic and r 2011 American Chemical Society
tend toward bioaccumulation. In the environment they are found on the surface of particles.10 This well-known surface behavior has been little applied to atmospheric aerosol research, despite the well-publicized importance of aerosol to critical issues including climate change11 and health.12 Interestingly, while most PAHs absorb strongly in the 300 420 nm range, which is a component of solar radiation, those with extensive pibond delocalization may also absorb visible light.13 Visible absorption by aerosol is, however, typically studied on earth only for black carbon14 or smoke.15 Light absorption by PAHs, especially cationized forms, seems most discussed in astrophysics as a potential contributor to heating of the interstellar medium.16 18 The effect of the atmospheric aerosol on both health and climate forcing is a function of particle size and chemistry. The analysis of individual aerosol particles has become much more common, even garnering special journal issues.19 Unfortunately, laser ionization aerosol spectrometry tends to fragment organic compounds, making speciation difficult. This problem is lessened in electron impact instruments.20 In practice, the low concentration of PAHs in aerosol makes measurements most commonly Received: April 11, 2011 Accepted: August 8, 2011 Published: August 08, 2011 76
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Table 1. Properties of PAHs name
a
chemical formula
source/purity
molecular weighta
vapor pressure at
ionization
number of rings
(g/mol)
20 °C28 (mbar)
potential29 (eV)
pyrene
C16H10
Merck, >96%
4
202.078
9.11 10
7
7.43
anthracene
C14H10
Sigma-Aldrich 97%
3
178.079
2.61 10
4
7.44
naphthalene
C10H8
Sigma-Aldrich, 99%
2
128.063
6.52 10
2
8.14
Most abundant isotope only.
performed on bulk aerosol followed by chromatographic separation (e.g.,21), rather than analysis in individual particles. Polidori et al. have recently been able to infer real-time particle-bound PAH in vehicle exhaust22 using a photoelectric sensor. Thornhill et al,23 using a photoionization instrument (PAS) in Mexico City, found a real-time estimate of the total surface PAH concentration of particles. However, the distribution of constituents within a particle is important. Jacobson24 has pointed out the influence of mixing state of black carbon on radiative forcing, and surely this would apply to other compounds as well. Yordanov et al,25 in their examination of soot by EPR, note that PAHs are assumed to be absorbed on the surface of soot. Hygroscopic behavior, such as for cloud condensation nuclei, depend on the wetability of the surface and thus on particle age.26 Roelofs27 thus found mixing state important in a GCM study of organics in marine aerosol.
at the University of Muenster, Germany. The instrument is capable of operation both in ToF-SIMS mode and in LaserSNMS mode. The instrument is equipped with a Ga+ primary ion source and two excimer lasers, a 193 nm (6.4 eV) and a 157 nm (7.9 eV) laser for laser postionization of sputtered neutral species.28,29 The instrument detection system consists of a 10 keV post acceleration system, a multichannel plate, a scintillator and a photomultiplier. For signal registration in Laser-SNMS mode, an 8 bit digitizer was used to digitize the analog signal. For ToF-SIMS, a time-to-digital converter (TDC) was used. To directly compare the Laser-SNMS signal with the ToF-SIMS signal, the analog Laser-SNMS signal was converted into counts by using a calibration factor. This factor was determined by simultaneous measurement of the ion signals obtained from various standard samples with the TDC and the 8-bit digitizer. Laser-SNMS measurements were carried out using a 1500 ns pulse length focused Ga+ primary ion beam with a repetition rate from 20 Hz up to 200 Hz. The lasers were focused a distance of ca. 200 μm above the sample. The 193 nm laser has a pulse length of 10 ns and a 150 μm 450 μm focus spot size. The 157 nm laser has a pulse length of 17 ns and a 600 μm 1800 μm focus spot size. The timing sequence of the primary ion pulse, laser pulse and extraction field were optimized to maximize the detected ion signal. The instrument is also designed to allow introduction and analysis of samples at cryogenic temperatures and all samples were analyzed at 100 °C to prevent loss of semivolatile species in the vacuum. With both lasers, the laser power density was varied to determine optimal parameters for detection of PAHs. Analyses were performed in static mode with no pre-etching by dc sputtering, which would remove the surface layer.
’ MATERIALS AND METHODS Two sample systems, model PAHs and ambient atmospheric aerosol particles, have been studied. Three PAHs, naphthalene, anthracene and pyrene, were selected for detailed analysis. Table 1 summarizes the relevant properties of these PAHs. Note that the ionization potentials of pyrene and anthracene fall between the photon energy for the two lasers used in the experiments, hence a single-photon ionization process is possible for these species with the 157 nm (7.9 eV) laser but a multiphoton ionization process is required with the 193 nm (6.4 eV) laser. Ionization of naphthalene, in contrast, requires a multiphoton process with either laser. Thin films were prepared by spin-casting from 1% solution in xylene onto clean silicon. Both pure samples and 1:1 mixtures of the PAHs were studied. After spin-casting, samples were allowed to dry under ambient conditions for a minimum of 24 h. Because these compounds have relatively high vapor pressure, it was necessary to analyze the samples under cryogenic conditions. Prior to analysis, samples were cooled via submersion in a liquid nitrogen bath and then rapidly transferred to a vacuum preparation chamber. In the preparation chamber, samples were warmed to 70 °C for 5 min to allow sublimation of the adsorbed water ice and then cooled to 100 °C before introduction into the analysis chamber. Samples were then maintained at 100 °C throughout the analysis process. Ambient atmospheric aerosol samples were studied in addition to the model PAHs. Size segregated atmospheric aerosol samples were collected using a 6 stage Marple personal cascade impactor. Samples were collected on aluminum substrates for 4 h during peak traffic at a site along the primary traffic ring in Muenster, Germany. As with the model PAH samples, the aerosol samples were cooled in liquid nitrogen, transferred to vacuum, heated to 70 °C to sublime adherent ice and then analyzed at 100 °C. All experiments were carried out on a combined gridless reflectron-based Laser-SNMS instrument constructed in-house
’ RESULTS AND DISCUSSION SIMS and Laser-SNMS spectra of pyrene are shown in Figure 1 with intensity in arbitrary units (a.u.) The intact molecular ion (M+) for pyrene is evident in all spectra except the negative ion SIMS (Figure 1a). Although the M+ is clearly identifiable in the positive SIMS spectra, its intensity is an order of magnitude lower than intensities of lower mass fragments. With 193 nm laser postionization, the M+ peak for pyrene is the most intense peak over 50 m/z, but the strongest peaks in the spectra are those from CHx+ and C2Hx+ ions indicating significant fragmentation of the molecule in the postionization process. When the sputtered neutral species are ionized with the 157 nm laser, the molecular ion peak for pyrene is the most intense peak in the spectrum. Very similar trends are observed in the anthracene and naphthalene spectra included in the Supporting Information (Figures S-1 and S-2). Naphthalene, however, shows a distinctly different pattern in comparison to pyrene and anthracene for 157 nm laser ionization (Supporting Information, Figure S-1). The most intense peaks in the spectra are hydrocarbon fragments with 2 and 3 carbons. 77
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Figure 1. Mass spectra of pyrene taken with (a) negative ion SIMS, (b) positive ion SIMS, (c) 193 nm Laser-SNMS, and (d) 157 nm Laser-SNMS. Note that the y axis scale has been expanded in the high mass range in spectra a, b, and c.
Figure 2. 193-nm Laser-SNMS signal intensity of pyrene molecular ions and fragments as a function of laser power density.
In the higher mass region, the most intense peak is M H+ (m/z = 127) rather than M+ (m/z = 128). The intense M H+ peak is only observed when the naphthalene is sputtered and not when gas phase naphthalene is ionized with the 157 nm laser, indicating that the M H radical is formed in the sputtering process and not generated during laser postionization. Unlike pyrene and anthracene, the first ionization energy of naphthalene (8.14 eV) is greater than the energy of the 157 nm laser (7.90 eV) so the ground state molecule cannot be ionized in a single-photon process. Although the first ionization energy of the C10H7 (M H) free radical has never been measured, calculations by density function theory indicate single-photon ionization should
be possible for this species. This is consistent with the enhanced intensity of the C10H7+ peak in the 157 nm Laser-SNMS spectra. Figures 2 and 3 show the effect of laser power density on the pyrene molecular ion peak as well as key fragment ions for the two postionization lasers. For both lasers, the intensity of the molecular ion peak does not change significantly above 4 MW/cm2. Similar trends were observed for naphthalene and anthracene. Although the intensity of the molecular ion peak is nearly independent of laser power density, the degree of fragmentation shows opposite trends for the two lasers (Supporting Information, Figures S-3 and S-4) . With the 193 nm laser (multiphoton ionization), the ratio of the molecular ion to the 78
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Figure 3. 157-nm Laser-SNMS signal intensity of pyrene molecular ions and fragments as a function of laser power density.
Figure 4. Yield for pyrene molecular ion and fragments for negative SIMS, positive SIMS, Laser SNMS 193 nm, and Laser-SNMS 157 nm. The yield for the molecular ion is 3 orders of magnitude higher using 157 nm Laser-SNMS than in positive SIMS.
Figure 5. Yield for anthracene molecular ion and fragments for negative SIMS, positive SIMS, Laser-SNMS 193 nm, and Laser-SNMS 157 nm. The yield for the molecular ion is 3 orders of magnitude higher using 157 nm Laser-SNMS than in positive SIMS. 79
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Figure 6. Yield for naphthalene molecular ion and fragments for negative SIMS, positive SIMS, Laser-SNMS 193 nm, and Laser-SNMS 157 nm. The yield for the molecular ion is 2 orders of magnitude higher using 157 nm Laser-SNMS than in positive SIMS.
Figure 7. 75 75 μm2, Laser-SNMS (157 nm) images of ambient aerosol particles from impactor stage 6 (3.5 to 2 μm in aerodynamic diameter). The top row shows the total ion image, a red-green-blue overlay of MAF factors 1 3, and regions of interest corresponding to the 3 particle types identified in the analysis. The lower row shows the molecular ion images for naphthalene, anthracene and pyrene.
C6Hx fragments increases gradually with increasing laser power density for all three PAHs studied. In contrast, with the 157 nm laser (single-photon ionization), the ratio of the molecular ions to the sum of C6Hx fragments decreases abruptly when the laser power density is increased above 1 W/cm2. For the 193 nm laser, the optimal postionization condition for these PAHs is at the maximum power density obtainable with the laser (90 MW/ cm2). For the 157 nm laser, the optimal postionization condition is at ∼10 MW/cm2 since at this laser power density there is an optimal balance between high yield of the molecular ion and low fragmentation. The yield, defined as detected ions per primary ion, under optimal conditions for molecular ions and fragments, has been calculated for each of the PAHs and is shown in figures 4-6. For positive ion SIMS, the yield is ∼10 5. Using an ion dose of 1013 ions/cm2, this would result in only 1 detected ion from a micrometer sized particle consisting of a single pure PAH. Since pure PAH particles are not expected in ambient aerosol, where
PAHs are generally part of a complex organic mixture and their concentration is typically a few tenths of a percent,30,31 the SIMS yield is too low for the study of PAHs in individual atmospheric aerosol. In comparison to SIMS, the yield is enhanced for all PAHs with both laser systems; however enhancement is significantly greater for single-photon ionization with the 157 nm laser. Under optimized conditions for laser power density and primary ion pulse length, 193 nm postionization results in a 2 50 fold increase in yield. Using the 157 nm laser, the yield is increased by more than 3 orders of magnitude for all 3 PAHs studied. For anthracene (C14H10) (Figure 5) and naphthalene (C10H8) (Figure 6), the yield for the sum of all M Hx ions exceeds 10 2 for 157 nm. The greatest enhancement is observed for pyrene (C16H10) (Figure 4), for which the yield of the sum of all C16Hx ions exceeds 10 1. This dramatic enhancement in the yield should permit detection of PAHs in real atmospheric aerosol particles in the 80
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Figure 8. 157 nm Laser-SNMS spectra from particles identified. We were unable to detect PAHs in any of the SIMS or 193 nm Laser-SNMS images, however images of the aerosol taken with the 157 nm laser show PAH peaks associated with individual particles.
micrometer size range. To test this, positive ion SIMS, 193 nm Laser-SNMS and 157 nm Laser-SNMS images were taken of the atmospheric aerosol described earlier. Use of a cryostage was critical for the image measurements because the semivolatile PAHs are lost with time in vacuum at ambient temperature. In our early attempts to measure PAHs in the aerosol without cooling, we found that the PAH signal in the chamber residual gas increased rapidly with time. Although we could occasionally see PAHs in individual particles in the first few minutes of analysis, this signal was quickly swamped out by the signal from the residual gas and image contrast disappeared. Figure 7 shows a 157 nm Laser-ToF-SNMS image taken using the cryostage. The image is from impactor stage 6, which contains particles with an aerodynamic diameter of 2 3.5 μm. The molecular ion images for pyrene, anthracene, and naphthalene (Figure 7, bottom row) show clear association of these PAH peaks with several particles in the image. One of the key motivations for this work is to be able to measure differences in the organic content of individual aerosol particles that may be relevant to the particles history as well as their health and climate impacts. Toward this end, a stack of 215 mass peak images was further analyzed using maximum auto correlation factors (MAF), a close relative of the more common principal components analysis (PCA) which has proven superior for reduction of low signal-to-noise ratio ToF-SIMS images.32,33 MAF analysis revealed 3 different types of particles in the image. A red - green-blue overlay of the first three MAFs showing the 3 particle types can be seen in Figure 7, top center. Spectra from each of the particle types, identified in the regions of interest (ROIs) image (Figure 7, top right) are shown in Figure 7. The molecular ions from pyrene (m/z 202), anthracene (m/z 178), and naphthalene (m/z 128) are evident in different quantities on the surface of all 3 particle types. A series
of highly unsaturated hydrocarbon peaks, likely arising from other PAHs, are correlated with the pyrene, anthracene and naphthalene. The presence of these peaks on the surface of all the particle types is consistent with condensation of PAHs on a variety of existing aerosol nuclei, which would occur when PAHs are generated from a combustion source such as a diesel engine or grill. Although further work needs to be done to verify these results, the very low fragmentation observed with the 157 nm laser strongly suggests the presence of a mixture of PAHs on the surface of the aerosol. Prominent peaks also occur in the spectra at masses 105 and 115. These peaks were not observed in analysis of either pure or mixed PAHs and so arise from other yet to be identified compounds present in the aerosol sample.
’ CONCLUSIONS Single-photon postionization with a 157 nm Vacuum UV laser can dramatically enhance detection of PAHs. Analysis of pure pyrene, anthracene and naphthalene shows a 3 orders of magnitude increase in yield over SIMS. Single-photon postionization is superior to multiphoton ionization both in terms of increasing the yield of the intact molecular species and reducing the generation of fragment ions. Use of a cryostage is critical for this measurement, as the semivolatile PAHs are lost with time in vacuum under ambient conditions. This is particularly critical in Laser-SNMS since the increase in PAHs in the residual gas can swamp out the signal from sputtered molecules and degrade image contrast. By using 157 nm Laser-SNMS with a cryostage, we have been able to image PAHs in individual atmospheric aerosol particles. Because of the very low fragmentation obtained with the lower laser power density using the 157 nm laser, there is great promise for identifying PAHs in complex mixtures. 81
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’ ASSOCIATED CONTENT
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bS
Supporting Information. Additional material as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ ACKNOWLEDGMENT This work was generously funded by NIH grant EB-002027 and the European Commission under contract 005045 (FP6). ’ REFERENCES (1) Peterson, R. E.; Nair, A.; Dambach, S.; Arlinghaus, H. F.; Tyler, B. J. Appl. Surf. Sci. 2006, 252 (19), 7006–7009. (2) Peterson, R. E.; Tyler, B. J. Atmos. Environ. 2002, 36 (39 40), 6041–6049. (3) Peterson, R. E.; Tyler, B. J. Appl. Surf. Sci. 2003, 203, 751–756. (4) Tervahattu, H.; Juhanoja, J.; Kupiainen, K. J. Geophys. Res., [Atmos.] 2002. (5) Laskin, A.; Cowin, J. P.; Iedema, M. J. J. Electron Spectrosc. Relat. Phenom. 2006, 150 (2 3), 260–274. (6) Holmes, H. A.; Pardyjak, E. R.; Tyler, B. J.; Peterson, R. E. Atmos. Environ. 2009, 43 (28), 4348–4358. (7) King, M. D.; Rennie, A. R.; Thompson, K. C.; Fisher, F. N.; Dong, C. C.; Thomas, R. K.; Pfrang, C.; Hughes, A. V. Phys. Chem. Chem. Phys. 2009, 11 (35), 7699–7707. (8) Nikolaou, K.; Masclet, P.; Mouvier, G. Sci. Total Environ. 1984, 32 (2), 103–132. (9) Godec, R.; Sisovifa, A.; Beslifa, I.; Vadifa, V. Arch. Ind. Hyg. Toxicol. 2008, 59 (3), 191–196. (10) ATSDR Toxicological Profile for Polycyclic Aromatic Hydrocarbons (PAHs); U.S. Department of Health and Human Services: Atlanta, GA, 1995. (11) Solomon, S. Intergovernmental Panel on Climate Change.; Intergovernmental Panel on Climate Change. Working Group I. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press: Cambridge, U.K., 2007; p Chapter 2, pp 153 171. (12) Anair, D.; Monahan, P. Sick of Soot: Reducing the Health Impacts of Diesel Pollution in California; Union of Concerned Scientists: Cambridge, MA, 2004; p xvi, 54 p. (13) Mallakin, A.; Dixon, D. G.; Greenberg, B. M. Chemosphere 2000, 40 (12), 1435–1441. (14) Babu, √ S. S.; Moorthy, K. K. Curr. Sci. 2001, 81 (9), 1208–1214. (15) Kl °ser, L.; Rosenfeld, D.; Macke, A.; Holzer-Popp, T. Atmos. Chem. Phys. Discuss. 2008, 8 (1), 549–568. (16) Gingell, J. Absorption Spectroscopy of Polyaromatic Hydrocarbons. http://www.chem.ucl.ac.uk/cosmicdust/pah.htm#how (accessed 7/1/2010). (17) Ruiterkamp, R.; Halasinski, T.; Salama, F.; Foing, B. H.; Allamandola, L. J.; Schmidt, W.; Ehrenfreund, P. Astron. Astrophys. 2002, 390 (3), 1153–1170. (18) Salama, F.; Allamandola, L. J. J. Chem. Soc., Faraday Trans. 1993, 89 (13), 2277–2284. (19) Wexler, A.; Prather, K. Aerosol Sci. Technol. 2000, 33 (1 2), 1–2. (20) Canagaratna, M. R.; Jayne, J. T.; Jimenez, J. L.; Allan, J. D.; Alfarra, M. R.; Zhang, Q.; Onasch, T. B.; Drewnick, F.; Coe, H.; Middlebrook, A.; Delia, A.; Williams, L. R.; Trimborn, A. M.; Northway, M. J.; DeCarlo, P. F.; Kolb, C. E.; Davidovits, P.; Worsnop, D. R. Mass Spectrom. Rev. 2007, 26 (2), 185–222. (21) Hock, N.; Schneider, J.; Borrmann, S.; Rompp, A.; Moortgat, G.; Franze, T.; Schauer, C.; Poschl, U.; Plass-Dulmer, C.; Berresheim, H. Atmos. Chem. Phys. 2008, 8 (3), 603–623. (22) Polidori, A.; Hu, S.; Biswas, S.; Delfino, R. J.; Sioutas, C. Atmos. Chem. Phys. 2008, 8 (5), 1277–1291. 82
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