Atmospheric Solids Analysis Probe Mass Spectrometry: A New

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Letters to Analytical Chemistry Atmospheric Solids Analysis Probe Mass Spectrometry: A New Approach for Airborne Particle Analysis Emily A. Bruns, Ve´ronique Perraud, John Greaves, and Barbara J. Finlayson-Pitts* Department of Chemistry University of California, Irvine, Irvine, California 92697-2025 Secondary organic aerosols (SOA) formed in the atmosphere from the condensation of semivolatile oxidation products are a significant component of airborne particles which have deleterious effects on health, visibility, and climate. In this study, atmospheric solids analysis probe mass spectrometry (ASAP-MS) is applied for the first time to the identification of organics in particles from laboratory systems as well as from ambient air. SOA were generated in the laboratory from the ozonolysis of r-pinene and isoprene, as well as from NO3 oxidation of r-pinene, and ambient air was sampled at forested and suburban sites. Particles were collected by impaction on ZnSe disks, analyzed by Fourier transform-infrared spectroscopy (FT-IR) and then transferred to an ASAP-MS probe for further analysis. ASAP-MS data for the laboratory-generated samples show peaks from wellknown products of these reactions, and higher molecular weight oligomers are present in both laboratory and ambient samples. Oligomeric products are shown to be present in the NO3 reaction products for the first time. A major advantage of this technique is that minimal sample preparation is required, and complementary information from nondestructive techniques such as FT-IR can be obtained on the same samples. In addition, a dedicated instrument is not required for particle analysis. This work establishes that ASAP-MS will be useful for identification of organic components of SOA in a variety of field and laboratory studies.

which have sufficiently low vapor pressures to condense and form new particles or to partition between the gas phase and the organic phase of pre-existing particles.5-10 These products are known as secondary organic aerosols (SOA). Individual components in the complex mixtures that comprise SOA have been identified in a number of studies by their collection on filters or impactors, followed by solvent extraction and analysis using a variety of techniques, including gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/mass spectrometry (LC/MS).11-17 However, the identified components are dependent on the details of the analytical techniques used (e.g., nature of the solvent), and both positive and negative artifacts can occur during sampling. For example, reaction of some components of SOA with solvent in the electrospray process have been reported.18 While a number of real-time particle mass spectrometric analysis techniques have been developed for atmospheric measurements,19-24 complete speciation of all of the (5) (6) (7) (8) (9) (10) (11) (12) (13)

(14) (15)

Airborne particles negatively impact human health,1 degrade visibility,2 and affect climate directly by light scattering and indirectly by altering cloud properties.3,4 Organic compounds are oxidized in air to form a complex mixture of products, some of * To whom correspondence should be addressed. Phone: (949) 824-7670. Fax (949) 824-2420. E-mail: [email protected]. (1) Pope, C. A., III; Dockery, D. W. J. Air Waste Manage. Assoc. 2006, 56, 709–742. (2) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Chemistry of the Upper and Lower Atmosphere-Theory, Experiments, and Applications; Academic Press: San Diego, CA, 2000. (3) The IPCC 4th Assessment Report; Pauchauri, R. K., Reisinger, A., Eds.; World Meterological Organization: Geneva, Switzerland, 2007. (4) Ghan, S. J.; Schwartz, S. E. Bull. Am. Meteorol. Soc. 2007, 88, 1059–1083.

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(16)

(17) (18) (19)

(20) (21)

Pankow, J. F. Atmos. Environ. 1987, 21, 2275–2283. Pankow, J. F. Atmos. Environ. 1994, 28, 189–193. Po ¨schl, U. Angew. Chem., Int. Ed. 2005, 44, 7520–7540. Rudich, Y.; Donahue, N. M.; Mentel, T. F. Annu. Rev. Phys. Chem. 2007, 58, 321–352. Kroll, J. H.; Seinfeld, J. H. Atmos. Environ. 2008, 42, 3593–3624. Pankow, J. F.; Chang, E. I. Environ. Sci. Technol. 2008, 42, 7321–7329. Doskey, P. V.; Andren, A. W. Atmos. Environ. 1986, 20, 1735–1744. Jang, M.; Kamens, R. M. Atmos. Environ. 1999, 33, 459–474. Simoneit, B. R. T.; Schauer, J. J.; Nolte, C. G.; Oros, D. R.; Elias, V. O.; Fraser, M. P.; Rogge, W. F.; Cass, G. R. Atmos. Environ. 1999, 33, 173– 182. Jaoui, M.; Kamens, R. M. J. Geophys. Res. 2001, 107, DOI: 10.1029/ 2001JD900005. Zheng, M.; Ke, L.; Edgerton, E. S.; Schauer, J. J.; Dong, M. Y.; Russell, A. G. J. Geophys. Res. 2006, 111, DOI: 10.1029/2005JD006777. Chowdhury, Z.; Zheng, M.; Schauer, J. J.; Sheesley, R. J.; Salmon, L. G.; Cass, G. R.; Russell, A. G. J. Geophys. Res. 2007, 112, DOI: 10.1029/ 2007JD008386. Laskin, A.; Smith, J. S.; Laskin, J. Environ. Sci. Technol. 2009, 43, 3764– 3771. Bateman, A. P.; Walser, M. L.; Desyaterik, Y.; Laskin, J.; Laskin, A.; Nizkorodov, S. A. Environ. Sci. Technol. 2008, 42, 7341–7346. 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, 185– 222. Prather, K. A.; Hatch, C. D.; Grassian, V. H. Annu. Rev. Anal. Chem. 2008, 1, 485–514. Zelenyuk, A.; Imre, D. Aerosol Sci. Technol. 2005, 39, 554–568. 10.1021/ac101028j  2010 American Chemical Society Published on Web 06/22/2010

particle constituents remains a challenge, and in addition, such techniques require specialized and dedicated instruments. The relatively recent development of a number of different ambient ionization techniques25,26 is promising for identifying the organic component of SOA. For example, desorption electrospray ionization (DESI) has been applied to atmospheric aerosol samples collected on filters either with or without extraction.27-29 We report here what is apparently the first application of another relatively new ambient ionization technique, atmospheric solids analysis probe mass spectrometry (ASAP-MS)30,31 to SOA formed from the oxidation of R-pinene and isoprene in laboratory studies, as well as to samples collected in forested and suburban areas. This method uses a heated stream of N2 to thermally desorb components of SOA in the ionization region of an atmospheric pressure chemical ionization mass spectrometer. This technique has been used successfully to identify components of biological systems such as ergosterol31 (MW 396) and other steroids,30 as well as drug molecules such as erythromycin (MW 733).32 ASAP-MS provides mass spectra that are comparable to those obtained by laser desorption techniques but without the use of a dedicated instrument. Because SOA samples can be collected on ZnSe disks, it has the additional advantage that infrared spectra can be obtained on the same samples, providing additional information. Of particular relevance is the fact that samples can be transferred directly onto the ASAP probe without any sample preparation. EXPERIMENTAL SECTION SOA were formed from the reaction of R-pinene (0.3-1.1 ppm) or isoprene (2.5 ppm) with O3 (0.5-2.1 ppm for R-pinene, 4.9 ppm for isoprene) in Teflon chambers (260-300 L) under dry conditions (RH e 5%) at 295 K in 1 atm of synthetic air (Oxygen Services Company, blend of O2 and N2, THC < 0.01 ppm, H2O < 2.0 ppm, CO < 0.5 ppm, CO2 < 0.5 ppm). For the isoprene reaction, NaCl seed particles (6.7 × 103 particles cm-3, 200 nm electrical mobility diameter) were added to provide surfaces for condensation of low-volatility products. SOA were also generated by the NO3 oxidation of R-pinene (1 ppm) where the thermal decomposition of N2O5 (1 ppm) was used as the source of NO3, as described elsewhere.33 Authentic samples of cis-pinonic acid (Sigma Aldrich, 98%) and NH4NO3 (Fisher (22) Tobias, H. J.; Ziemann, P. J. Environ. Sci. Technol. 2000, 34, 2105–2115. (23) Dreyfus, M. A.; Adou, K.; Zucker, S. M.; Johnston, M. V. Atmos. Environ. 2009, 43, 2901–2910. (24) Dreyfus, M. A.; Johnston, M. V. Aerosol Sci. Technol. 2008, 42, 18–27. (25) Cooks, R. G.; Ouyang, Z.; Takats, Z.; Wiseman, J. M. Science 2006, 311, 1566–1570. (26) Venter, A.; Nefliu, M.; Cooks, R. G. Trends Anal. Chem. 2008, 27, 284– 290. (27) Li, M.; Chen, H.; Wang, B. F.; Yang, X.; Lian, J. J.; Chen, J. M. Int. J. Mass Spectrom. 2009, 281, 31–36. (28) Li, M.; Chen, H.; Yang, X.; Chen, J.; Li, C. Atmos. Environ. 2009, 43, 2717– 2720. (29) Laskin, J.; Laskin, A.; Roach, P. J.; Slysz, G. W.; Anderson, G. A.; Nizkorodov, S. A.; Bones, D. L.; Nguyen, L. Q. Anal. Chem. 2010, 82, 2048–2058. (30) McEwen, C. N.; McKay, R. G.; Larsen, B. S. Anal. Chem. 2005, 77, 7826– 7831. (31) McEwen, C.; Gutteridge, S. J. Am. Soc. Mass Spectrom. 2007, 18, 1274– 1278. (32) Petucci, C.; Diffendal, J. J. Mass Spectrom. 2008, 43, 1565–1568. (33) Bruns, E. A.; Perraud, V.; Zelenyuk, A.; Ezell, M. J.; Johnson, S. N.; Yu, Y.; Imre, D.; Finlayson-Pitts, B. J.; Alexander, M. L. Environ. Sci. Technol. 2009, 44, 1056–1061.

Scientific, Certified ACS grade, 99.7%), common components of atmospheric samples,2 were also analyzed. After 20 min of reaction, laboratory-generated SOA were collected by drawing 250-290 L (R-pinene oxidation) or 515 L (isoprene oxidation, where SOA from two experiments were aggregated) of the reaction mixture through a Sioutas impactor containing a stage with a 50% cut-point of 0.25 µm holding a ZnSe disk.33,34 After acquiring transmission infrared spectra of the samples on a ZnSe disk (Mattson Cygnus 100, 128 scans at 4 cm-1 resolution), some of the SOA was manually transferred to the tip of a glass melting point tube (mp tube) attached to an ASAPMS probe (Waters) by drawing it across the disk. The mass of material collected on the ZnSe disk was estimated from the aerosol concentrations measured in similar experiments, and the sampling rate and time, to be 50-130 µg, depending on the experiment. The mp tube was drawn across 20-30% of the sample for each run which would give approximately 20 µg on the mp tube if complete transfer occurred. Three to five samples from the same ZnSe disk were analyzed for each experiment. Prior to use, the mp tube was cleaned by baking at 450 °C for at least 1 h. The probe was then inserted into an LCT Premier time-of-flight mass spectrometer (Waters) operating in positive ionization mode. The source temperature was held constant at 150 °C, and the temperature of the N2 elution gas (500 L h-1 flow rate) was ramped stepwise from 50 to 450 °C to desorb components of SOA directly into the ionization region. The presence of a corona discharge (5 µA) and a small container of liquid water in the source compartment resulted in proton transfer from H3O+ to the volatilized species to give [M + H]+ ions. Data were acquired using MassLynx software (Waters) over a mass range of 100-1000 Da. A mixture of three poly(ethylene glycol) preparations (Sigma Aldrich, average molecular weights 200, 400, and 1000) in methanol (Fisher Scientific, Optima LC/MS grade, >99.9%) was analyzed daily for instrument calibration. Caffeine (Sigma Aldrich), with an [M + H]+ peak at m/z 195.0882, was used as a lock mass in each replicate for minor calibration adjustments. Samples were also collected at two field sites in Southern California: (1) a high elevation (6500 ft) pine and oak forest and (2) a suburban site approximately 5 miles inland from the Pacific Ocean. Particles were collected from 36 m3 of ambient air using a sampling train that had an impactor with a 2.5 µm stage coated with high vacuum grease (Dow Corning, MI) at the entrance to remove the largest particles (e.g., dust). The airstream then passed through a carbon monolith denuder (Novacarb, Mast Carbon Ltd., U.K.) to remove organic gases and then through a second impactor with two stages, each holding a ZnSe disk, having 50% cut-points of 1.0 and 0.25 µm, respectively. For the field samples, we focus here on the second stage as SOA are commonly found in the smaller particle size range. RESULTS AND DISCUSSION Figure 1 shows infrared spectra of the laboratory and ambient air samples and, for comparison, NH4NO3. In the laboratory(34) Hinds, W. C. Aerosol Technology: Properties, Behavior and Measurement of Airborne Particles; John Wiley & Sons Inc.: New York, 1999.

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Figure 1. FT-IR spectra from (a-c) laboratory-generated SOA, (d, e) SOA collected from two field sites, and (f) an NH4NO3 standard.

generated samples, sCsH stretching absorptions at 2800-3000 cm-1 are seen, as well as peaks between 1710-1750 cm-1 characteristic of the sCdO groups expected to be formed during oxidation. Organic nitrate peaks at 1630, 1280, and 855 cm-1 are observed from the NO3 reaction, as reported earlier.33 Unique peaks at 980 and 1070 cm-1 in the isoprene reaction have been previously identified35 as sCsOH and sCsOC stretches from oligomers formed from glyoxal, an isoprene oxidation product. In the ambient samples, NH4NO3 is clearly a major component, along with organics similar to those from the laboratory-generated SOA. The left-hand column of Figure 2 shows ASAP-MS data obtained at 200 °C while the column on the right gives the total ion count profiles (TIC) as the temperature of the N2 over the probe increased. The TIC typically started to rise between 150 and 200 °C, with most of the volatile components desorbing below 300 °C. This pattern of volatilization is consistent with observed changes in mass of ambient particles upon heating.36-39 There was some variability between samples, which was attributed to the positioning of the tip of the mp tube relative to the corona discharge and MS inlet. Peaks observed in the mass spectra were generally [M + H]+ ions and their associated fragments (e.g., [M + H - H2O]+). Peaks corresponding to the [M + H]+ ion of di-isooctyl phthalate (m/z 391) and unidentified peaks at m/z (35) Loeffler, K. W.; Koehler, C. A.; Paul, N. M.; De Haan, D. O. Environ. Sci. Technol. 2006, 40, 6318–6323.

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219 and 279 were commonly observed contaminants that were removed from the spectra. Peaks below 300 Da in the NO3 reaction (Figure 2a) are similar to those observed recently using atmospheric pressure ionization chemical ionization.40 Accurate masses and assignments are shown in Table 1, where error is defined as the difference between the observed and theoretical masses. Two previously identified organic nitrate products,40–44 3-oxopinane-2-nitrate and 2-hydroxypinane-3-nitrate, were observed by ASAP-MS. Peroxyacyl nitrate (PAN) parent peaks that were previously reported40 were (36) Clarke, A. D.; Shinozuka, Y.; Kapustin, V. N.; Howell, S.; Huebert, B.; Doherty, S.; Anderson, T.; Covert, D.; Anderson, J.; Hua, X.; Moore, K. G.; McNaughton, C.; Carmichael, G.; Weber, R. J. Geophys. Res. 2004, 109, DOI: 10.1029/2003JD004378. (37) Paulsen, D.; Weingartner, E.; Alfarra, M. R.; Baltensperger, U. J. Aerosol. Sci. 2006, 37, 1025–1051. (38) Chow, J. C.; Yu, J. Z.; Watson, J. G.; Ho, S. S. H.; Bohannan, T. L.; Hays, M. D.; Fung, K. K. J. Environ. Sci. Health, Part A: Environ. Sci. Eng. Toxic Hazard. Subst. Control 2007, 42, 1521–1541. (39) Pratt, K. A.; Prather, K. A. Environ. Sci. Technol. 2009, 43, 8276– 8282. (40) Perraud, V.; Bruns, E. A.; Ezell, M. J.; Johnson, S. N.; Greaves, J.; FinlaysonPitts, B. J. Environ. Sci. Technol. In press. (41) Wa¨ngberg, I.; Barnes, I.; Becker, K. H. Environ. Sci. Technol. 1997, 31, 2130–2135. (42) Berndt, T.; Boge, O. J. Chem. Soc., Faraday Trans. 1997, 93, 3021–3027. (43) Hallquist, M.; Wa¨ngberg, I.; Ljungstro¨m, E.; Barnes, I.; Becker, K. H. Environ. Sci. Technol. 1999, 33, 553–559. (44) Spittler, M.; Barnes, I.; Bejan, I.; Brockmann, K. J.; Benter, T.; Wirtz, K. Atmos. Environ. 2006, 40, S116–S127.

Figure 2. The left-hand column shows ASAP-MS data of SOA from (a) NO3 oxidation of R-pinene, (b,c) ozonolysis of R-pinene and isoprene, and (d, e) ambient air at two field sites. Spectra were acquired at an N2 temperature of 200 °C. The right-hand column shows the thermal desorption profiles of the TIC as a function of time as the temperature was increased stepwise from 50 to 450 °C. Initial concentrations were as given in Figure 1.

Table 1. Accurate Mass Measurements from ASAP-MS Analysis of SOA Generated from the NO3 Oxidation and from the Ozonolysis of r-Pinene measured m/z

theoretical m/z

error (ppm)

formula

214.1082 168.1150 201.1138 183.1031 199.0982 181.0875 198.1133 185.1180 167.1078 169.1217 151.1129

214.1079 168.1150 201.1127 183.1021 199.0970 181.0865 198.1130 185.1178 167.1072 169.1229 151.1123

1.2 -0.2 5.5 5.4 5.9 5.7 1.4 1.2 3.6 -6.9 4.0

R-Pinene + NO3 C10H16NO4 C10H16O2 C10H17O4 C10H15O3 C10H15O4 C10H13O3 C10H16NO3 C10H17O3 C10H15O2 C10H17O2 C10H15O

201.1113 183.1018 199.0961 181.0865 185.1165 167.1072 169.1228 151.1125

201.1127 183.1021 199.0970 181.0865 185.1178 167.1072 169.1229 151.1123

-6.9 -1.7 -4.7 0.2 -6.9 0.0 -0.3 1.4

R-Pinene + O3 C10H17O4 C10H15O3 C10H15O4 C10H13O3 C10H17O3 C10H15O2 C10H17O2 C10H15O

not observed by ASAP-MS in experiments at higher N2O5 concentrations, where they were expected, due to their thermal instability.45 (45) Roberts, J. M. In Volatile Organic Compounds in the Atmosphere; Koppmann, R., Ed.; Blackwell Publishing: Oxford, U.K., 2007; pp 221-268.

parent compound

peak assignment

3-oxopinane-2-nitrate 3-oxopinane-2-nitrate peroxy-pinonic acid peroxy-pinonic acid keto-pinonic acid keto-pinonic acid 2-hydroxypinane-3-nitrate pinonic acid pinonic acid pinonaldehyde pinonaldehyde

[M [M [M [M [M [M [M [M [M [M [M

+ + + + + + + + + + +

H]+ HH]+ HH]+ HHH]+ HH]+ H-

peroxy-pinonic acid peroxy-pinonic acid keto-pinonic acid keto-pinonic acid pinonic acid pinonic acid pinonaldehyde pinonaldehyde

[M [M [M [M [M [M [M [M

+ + + + + + + +

H]+ HH]+ HH]+ HH]+ H-

NO2]+ H2O]+ H2O]+ H2O]+ H2O]+ H2O]+

H2O]+ H2O]+ H2O]+ H2O]+

ASAP-MS provides spectra with less fragmentation than those observed using single particle laser ablation time-of-flight mass spectrometry (SPLAT II) and high resolution time of flight aerosol mass spectrometry (HR-ToF-AMS). For example, previously reported aerosol mass spectra from the NO3 oxidation of R-pinene Analytical Chemistry, Vol. 82, No. 14, July 15, 2010

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showed no significant peaks above m/z 160 using those methods.33 High molecular weight products can be seen in all of the systems studied here (Figure 2), including from the NO3 reaction. To the best of our knowledge, this has not been reported previously. Peaks in the ASAP-MS data from the ozonolysis of R-pinene (Figure 2b) represent known products such as pinonic acid, pinonaldehyde, keto-pinonic acid, and peroxy-pinonic acid, as shown in Table 1. The pattern of grouped peaks between approximately 300 and 600 Da that are separated by 14, 16, or 18 Da are characteristic of oligomeric products identified in other studies of R-pinene oxidation by O3 or NOx photooxidation.46-58 For example, ASAP-MS data shows [M + H]+ peaks at m/z 337.2000 (C19H29O5, -4.4 ppm) and 353.1990 (C19H29O6, 7.3 ppm), which are in excellent agreement with m/z 359.18349 and 375.17835 assigned by Tolocka et al.49 as the Na+ adducts of C19H28O5 and C19H28O6. Figure 2d,e show ASAP-MS data from samples collected from a forested region and a suburban area. Although the spectra are much more complex than the laboratory samples, similar patterns can be seen, with masses approaching 700 Da. The underlying broad signal in the sample from the forested region is similar to that observed in GC/MS analysis of organics in air and has been attributed to contributions from high molecular weight unidentified organics.11 The ASAP-MS data are also similar to laser desorption mass spectra of aqueous extracts of filter samples from Zurich, Switzerland.52 Here, there was a broad background with somewhat larger peaks typically separated by 14 and 16 Da in addition to more intense peaks, many of which are separated by 28 Da. The detection limit was determined based on the total ion signal from the estimated mass of material transferred to the mp (46) Gao, S.; Keywood, M.; Ng, N. L.; Surratt, J.; Varutbangkul, V.; Bahreini, R.; Flagan, R. C.; Seinfeld, J. H. J. Phys. Chem. A 2004, 108, 10147–10164. (47) Gao, S.; Ng, N. L.; Keywood, M.; Varutbangkul, V.; Bahreini, R.; Nenes, A.; He, J. W.; Yoo, K. Y.; Beauchamp, J. L.; Hodyss, R. P.; Flagan, R. C.; Seinfeld, J. H. Environ. Sci. Technol. 2004, 38, 6582–6589. (48) Kalberer, M.; Paulsen, D.; Sax, M.; Steinbacher, M.; Dommen, J.; Prevot, A. S. H.; Fisseha, R.; Weingartner, E.; Frankevich, V.; Zenobi, R.; Baltensperger, U. Science 2004, 303, 1659–1662. (49) Tolocka, M. P.; Jang, M.; Ginter, J. M.; Cox, F. J.; Kamens, R. M.; Johnston, M. V. Environ. Sci. Technol. 2004, 38, 1428–1434. (50) Baltensperger, U.; Kalberer, M.; Dommen, J.; Paulsen, D.; Alfarra, M. R.; Coe, H.; Fisseha, R.; Gascho, A.; Gysel, M.; Nyeki, S.; Sax, M.; Steinbacher, M.; Prevot, A. S. H.; Sjogren, S.; Weingartner, E.; Zenobi, R. Faraday Discuss. 2005, 130, 265–278. (51) Gross, D. S.; Galli, M. E.; Kalberer, M.; Prevot, A. S. H.; Dommen, J.; Alfarra, M. R.; Duplissy, J.; Gaeggeler, K.; Gascho, A.; Metzger, A.; Baltensperger, U. Anal. Chem. 2006, 78, 2130–2137. (52) Kalberer, M.; Sax, M.; Samburova, V. Environ. Sci. Technol. 2006, 40, 5917–5922. (53) Denkenberger, K. A.; Moffet, R. C.; Holecek, J. C.; Rebotier, T. P.; Prather, K. A. Environ. Sci. Technol. 2007, 41, 5439–5446. (54) Reinhardt, A.; Emmenegger, C.; Gerrits, B.; Panse, C.; Dommen, J.; Baltensperger, U.; Zenobi, R.; Kalberer, M. Anal. Chem. 2007, 79, 4074– 4082. (55) Surratt, J. D.; Murphy, S. M.; Kroll, J. H.; Ng, N. L.; Hildebrandt, L.; Sorooshian, A.; Szmigielski, R.; Vermeylen, R.; Maenhaut, W.; Claeys, M.; Flagan, R. C.; Seinfeld, J. H. J. Phys. Chem. A 2006, 110, 9665–9690. (56) Iinuma, Y.; Boge, O.; Gnauk, T.; Herrmann, H. Atmos. Environ. 2004, 38, 761–773. (57) Dommen, J.; Metzger, A.; Duplissy, J.; Kalberer, M.; Alfarra, M. R.; Gascho, A.; Weingartner, E.; Prevot, A. S. H.; Verheggen, B.; Baltensperger, U. Geophys. Res. Lett. 2006, 33, DOI: 10.1029/2006GL026523. (58) Kroll, J. H.; Ng, N. L.; Murphy, S. M.; Flagan, R. C.; Seinfeld, J. H. Environ. Sci. Technol. 2006, 40, 1869–1877.

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tube and assuming that a signal corresponding to 3σ of the baseline noise could be detected, where the peak-to-peak baseline variation is taken as 5σ.59 This gives a detection limit of ∼3 ng. This is a conservative estimate since complete transfer of the sample by manual contact is not likely. At an airborne particle concentration of 10 µg m-3, in principle only 0.3 L of air would need to be sampled to have detectable signal. However, this assumes that all of the particles are captured on the melting point tube and that the particle size distribution/volume is similar to that for the laboratory samples studied here. In practice, larger volumes of air would be required to trap 3 ng. Given the dominance of NH4NO3 in the ambient infrared spectra, a question is whether the presence of this salt might alter the thermal desorption and ionization pathways. Mixtures of NH4NO3 and pinonic acid, a common component of laboratory and field samples, were analyzed using ASAP-MS. At a 1:1 molar ratio of salt to acid, the major peaks from pinonic acid remained at m/z 185 [M + H]+ and 167 [M + H - H2O]+ and the ratio of the 167 to the 185 peak was the same as in the absence of NH4NO3, suggesting that the ASAP-MS data of organics will not change significantly due to the presence of inorganics in complex airborne particles. Another concern is the presence of artifacts, particularly from gas-phase clustering, generated in the ion source.54,60,61 ASAP-MS data of an authentic pinonic acid sample show no evidence of dimers or other higher MW oligomerization products. Previous studies48–55,57 in which oligomers were directly detected in the R-pinene and isoprene oxidations using laser ablation techniques provided much more rapid heating. It is interesting that the present studies show that these oligomers can also be vaporized, rather than decomposing, when the temperature is increased relatively slowly. Lower molecular weight compounds of higher volatility desorbed first and as the temperature increased, the product distribution shifted to higher masses (data not shown). Although outside the scope of this letter, it may be possible to use the temperature dependent signal to investigate the thermal stability and vapor pressures of the oligomers.62 ASAP-MS does not have the high time differentiation of realtime instruments, such as ATOF-MS,51,53 HR-ToF-AMS19,58,63 or SPLAT II.21 However, unlike ASAP-MS, these techniques require specialized, dedicated instruments and do not allow for complementary analysis of the same sample, e.g., using spectroscopic techniques. As demonstrated here, FT-IR data can provide functional group and chemical bond information. However, not all individual products are readily seen by ASAPMS. For example, GC/MS analysis of filter samples from the forested site using (N,O-bis(trimethylsilyl)trifluoroacetamide) (BFSTA) derivatization64 showed varying amounts of levoglu(59) Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis; Thomson Brooks/Cole: Belmont, CA, 2007. (60) Ku ¨ ckelmann, U.; Warsheid, B.; Hoffmann, T. Anal. Chem. 2000, 72, 1905– 1912. (61) Hoffmann, T.; Bandur, R.; Hoffmann, S.; Warscheid, B. Spectrochim. Acta, Part B 2002, 57, 1635–1647. (62) Chattopadhyay, S.; Tobias, H. J.; Ziemann, P. J. Anal. Chem. 2001, 73, 3797–3803. (63) DeCarlo, P. F.; Kimmel, J. R.; Trimborn, A.; Northway, M. J.; Jayne, J. T.; Aiken, A. C.; Gonin, M.; Fuhrer, K.; Horvath, T.; Docherty, K. S.; Worsnop, D. R.; Jimenez, J. L. Anal. Chem. 2006, 78, 8281–8289. (64) Yu, J.; Flagan, R. C.; Seinfeld, J. H. Environ. Sci. Technol. 1998, 32, 2357– 2370.

cosan, a biomass burning tracer, but there was no evidence of this compound in the ASAP-MS data. In addition, smaller molecular weight compounds (MW < 100) and inorganics such as NH4NO3 cannot be identified using this method. CONCLUSIONS On the basis of this first application of ASAP-MS to SOA analysis, ASAP-MS is a promising technique for examining the signatures of organic components of airborne particles, including higher molecular weight products. A major advantage of ASAPMS is that it requires very little sample preparation and does not require a dedicated, specialized instrument. In addition, depending on the collection method, other analyses of a given sample can be carried out prior to ASAP-MS using nondestructive techniques such as FT-IR. This method should have wide applicability in both laboratory and field studies.

ACKNOWLEDGMENT We are grateful to the U.S. Department of Energy (Grant No. DE-FG02-05ER64000) for support of this work. E.A.B. thanks the National Science Foundation for a Graduate Research Fellowship. Additional support was provided by the AirUCI Environmental Molecular Sciences Institute (Grant No. CHE-0431312) funded by the National Science Foundation. We are also grateful to Dr. J. N. Pitts for manuscript comments and Jerry Holiday for technical assistance.

Received for review April 19, 2010. Accepted June 15, 2010. AC101028J

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