A Chemical Ionization Mass Spectrometry Method for the Online

Anal. Chem. 2004, 76, 2820-2826

A Chemical Ionization Mass Spectrometry Method for the Online Analysis of Organic Aerosols John D. Hearn and Geoffrey D. Smith*

Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556

A new technique employing chemical ionization mass spectrometry (CIMS) is described that allows the composition of organic particles to be determined on the time scale of seconds. With this Aerosol CIMS technique, particles are vaporized thermally at temperatures up to 480 °C, and the resulting vapor is chemically ionized and detected with a quadrupole mass spectrometer. The separation of the vaporization and ionization steps allows greater control and more flexibility for the detection of condensed phases than with other chemical ionization methods. Consequently, composition can be correlated to volatility, providing an additional dimension of information. The use of a variety of positive and negative reagent ions, such as H+(H2O)2, H+(CH3OH)2, NO+, O2+, O2-, F-, and SF6-, offers flexibility in the detection sensitivity and specificity. Furthermore, the degree of fragmentation of the resulting ion can be controlled, providing more straightforward identification and quantification than with other commonly used methods, such as electron impact ionization. Examples are given of the detection of aerosols consisting of organics with various functionalities, including alkanes, alkenes, alcohols, aldehydes, ketones, and carboxylic acids. Applications of this technique to laboratory studies of atmospherically relevant aerosol reactions are discussed. Aerosols are found throughout the atmosphere with diverse chemical compositions and physical properties affecting the chemistry of the atmosphere and the climate of the earth. The particles comprising these aerosols are composed of a wide range of species, including water, acids, sulfates, nitrates, and mineral oxides, among others.1 Recent measurements have indicated that a large fraction of tropospheric particles also contains significant quantities of organic species2-7 originating from a variety of * Corresponding author. E-mail: [email protected]. (1) Finlayson-Pitts, B. J.; Pitts, J., J. N. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications; Academic Press: San Diego, 2000. (2) Middlebrook, A. M.; Murphy, D. M.; Thomson, D. S. J. Geophys. Res., [Atmos.] 1998, 103, 16475-16483. (3) Murphy, D. M.; Thomson, D. S.; Mahoney, M. J. Science (Washington, D. C.) 1998, 282, 1664-1669. (4) Jacobson, M. C.; Hansson, H. C.; Noone, K. J.; Charlson, R. J. Rev. Geophys. 2000, 38, 267-294. (5) Kawamura, K.; Semere, R.; Imai, Y.; Fujii, Y.; Hayashi, M. J. Geophys. Res., [Atmos.] 1996, 101, 18721-18728. (6) Ketseridis, G.; Hahn, J.; Jaenicke, R.; Junge, C. Atmos. Environ. (19671989) 1976, 10, 603-10.

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sources, including primary emissions, the photooxidation of hydrocarbons, the partitioning of gas-phase organics to existing aerosols, and homogeneous nucleation.7-9 Several classes of organics are known to exist in particles, including alkanes, alkenes, alcohols, aromatics, and unsaturated carboxylic acids.10-13 Trace gases in the atmosphere, such as O3, OH, HO2, NO, and NO2, may react with these organic aerosol components and transform the particles. Such reactions can alter the particle’s ability to take up water, grow, reflect sunlight, and react further with other trace gases. Assessment of the significance of these transforming reactions requires a laboratory technique with (1) the ability to detect a wide range of organic species, which are typically present in particles, and (2) a sufficiently fast time response to observe changes in the particle composition due to reaction. Many existing methods for particle analysis suffer from a variety of limitations, including an insensitivity to organic species, excessive fragmentation of organics, and slow response time. Though several researchers have begun to explore the use of chemical ionization mass spectrometry (CIMS) for the analysis of condensed-phase species,14-21 only limited work has used this technique for detecting organic aerosols.15, 22-26 (7) Rudich, Y. Chem. Rev. 2003, 103, 5097-5124. (8) Bertram, A. K.; Ivanov, A. V.; Hunter, M.; Molina, L. T.; Molina, M. J. J. Phys. Chem. A 2001, 105, 9415-9421. (9) Ziemann, P. J. J. Phys. Chem. A 2002, 106, 4390-4402. (10) Slusher, D. L.; Pitteri, S. J.; Haman, B. J.; Tanner, D. J.; Huey, L. G. Geophys. Res. Lett. 2001, 28, 3875-3878. (11) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; John-Wiley & Sons: New York, 1998. (12) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci. Technol. 1991, 25, 1112-1125. (13) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A.; Cass, G. R.; Simoneit, B. R. T. Environ. Sci. Technol. 1993, 27, 636-651. (14) Huder, K. J.; DeMore, W. B. J. Phys. Chem. 1995, 99, 3905-3908. (15) Hoffmann, T.; Bandur, R.; Marggraf, U.; Linscheid, M. J. Geophys. Res., [Atmos.] 1998, 103, 25569-25578. (16) Curtius, J.; Arnold, F. J. Geophys. Res., [Atmos.] 2001, 106, 31965-31974. (17) Arnold, F.; Curtius, J.; Spreng, S.; Deshler, T. J. Atmos. Chem. 1998, 30, 3-10. (18) Voisin, D.; Smith, J. N.; Sakurai, H.; McMurry, P. H.; Eisele, F. L. Aerosol Sci. Technol. 2003, 37, 471-475. (19) Vincenti, M. In. J. Mass Spectrom. 2001, 212, 505-518. (20) Coon, J. J.; McHale, K. J.; Harrison, W. W. Rapid Commun. Mass Spectrom. 2002, 16, 681-685. (21) Lazar, A. C.; Reilly, P. T. A.; Whitten, W. B.; Ramsey, J. M. Anal. Chem. 2000, 72, 2142-2147. (22) Warscheid, B.; Hoffmann, T. Rapid Commun. Mass Spectrom. 2002, 16, 496-504. (23) Warscheid, B.; Kueckelmann, U.; Hoffmann, T. Anal. Chem. 2003, 75, 1410-1417. (24) Hoffmann, T.; Bandur, R.; Hoffmann, S.; Warscheid, B. Spectrochim. Acta, Part B: At. Spectrosc. 2002, 57, 1635-1647. 10.1021/ac049948s CCC: $27.50

© 2004 American Chemical Society Published on Web 04/24/2004

Figure 1. Schematic of aerosol CIMS apparatus. Aerosols generated using a nebulizer are vaporized in a heated inlet tube (up to 480 °C) followed by chemical ionization of the gas. Ions are detected with a quadrupole mass spectrometer.

In the present work, a new technique, called Aerosol CIMS, is presented for the analysis of organic particles based on chemical ionization mass spectrometry. Chemical ionization offers significantly reduced fragmentation of the organic ions as compared with other commonly used methods, such as electron impact ionization (EI). Reagent ions of both positive and negative polarities are used to detect many classes of organic species representative of those found in atmospheric particles, including alkanes, alkenes, alcohols, aldehydes, ketones, and carboxylic acids. In addition, the ability to correlate volatility with composition is demonstrated as an additional piece of information with which particle species can be identified. Finally, particles containing an unsaturated carboxylic acid, oleic acid, that have reacted with O3 are analyzed, demonstrating the usefulness of this technique in monitoring the reactive transformation of organic particles. EXPERIMENT Aerosol Mass Spectrometer. A schematic of the Aerosol CIMS instrument is shown in Figure 1. Particles are sampled into the mass spectrometer through a flow-limiting orifice (0.5-mm i.d.) placed at the inlet of a 7-in.-long (1/2-in. o.d., 9-mm i.d.) stainless steel tube. The flow through this orifice is measured to be 1.5 SLPM (standard liters per minute), and the pressure inside the tube is ∼25 Torr. The tube is wrapped with heating tape and heated to 480 °C. The temperature profile within the tube is not measured so it is not clear whether the heating is uniform, but particles vaporize when they impact on the walls of the heated tube as well as when they mix with the hot carrier gas. Alternatively, the flow-limiting orifice can be replaced by a needle valve wrapped with heating tape providing a variable aerosol flow (25) Kuckelmann, U.; Warscheid, B.; Hoffmann, T. Anal. Chem. 2000, 72, 19051912. (26) Warscheid, B.; Hoffmann, T. Rapid Commun. Mass Spectrom. 2001, 15, 2259-2272.

that is sampled into the mass spectrometer. The gas from the vaporized particles enters the stainless steel ion tube (1/4-in. o.d., 4-mm i.d., 12 in. long), which is also wrapped with heating tape and is typically maintained at ∼400 °C. Here, the particle vapor mixes with the reagent ions generated from the radioactive ion source (see Ion Generation). The vapor is then chemically ionized through a variety of ion-molecule reactions in the ion tube where the pressure is typically 15 Torr and the residence time is ∼1 ms (see Results and Discussion section). Ions are drawn into the front chamber by a mechanical pump (Varian DS-400) and then sampled through a 100-µm-i.d. orifice into the vacuum chamber, which houses the quadrupole mass spectrometer (ABB Extrel, Inc.). This orifice is biased by ∼10 V relative to the ion tube to enhance transmission of the ions, and the ions are further focused by ion optics inside the vacuum chamber. The chamber houses the mass spectrometer and is differentially pumped by two turbomolecular drag pumps (Pfeiffer TMU 520), both of which are backed by a single diaphragm pump (Pfeiffer MVP 055-3). The ions are detected with a Channeltron electron multiplier (Burle) once they are filtered by the mass spectrometer. Particle Generation. Particles are created using a glass nebulizer (Meinhard Glass Products), which creates an aerosol of droplets by aspirating a liquid sample through a capillary. In most experiments, pure samples of the liquid to be aerosolized are used. However, in the experiments using the protonated methanol cluster (H+(CH3OH)2) as the reagent ion, mixtures of methanol and the organic liquid are nebulized. Particles produced by the nebulizer are polydisperse and are typically 1-2 µm in size, as sized with an aerodynamic particle sizer (TSI, Inc.). The exact range of sizes produced by the nebulizer depends on the pressure of the gas used for aspiration, the viscosity of the liquid, and the inner diameter of the capillary. The aerosols are collected in a 500-mL filtration flask, carried out by a 100 sccm (standard cubic centimeters per minute) flow of N2, and diluted by a larger N2 flow (2-3 SLPM), after which they are introduced into the aerosol mass spectrometer. Ion Generation. Protonated water cluster ions (H+(H2O)2) are generated by flowing 3 SLPM of N2 through a radioactive polonium (210Po) source (NRD, Inc.). With residual water from the N2 the dominant ion at room temperature is H+(H2O)4 (m/z ) 73), but with the ion tube heated (∼400 °C) nearly all of the ion signal appears as H+(H2O)2 (m/z ) 37). Protonated methanol cluster ions are produced by adding methanol vapor to the ion tube after the polonium. The extent of methanol clustering can be controlled by adjusting the magnitude of the methanol flow. In practice, this flow is not controlled and the methanol solvent from particle generation is sufficient to produce H+(CH3OH)2 (m/z ) 65) as the dominant ion in the spectrum. Oxygen ions (O2+) are created by flowing a trace (10 sccm) of O2 through the polonium accompanied by 3 SLPM of N2. To eliminate O2+‚H2O clusters, the gas is passed through a glass trap containing silica gel, which is immersed in liquid N2 (77 K). The NO+ ion is created by either flowing a trace amount of NO (0.2 sccm) or NF3 (2 sccm) with 3 SLPM of N2 through the polonium ion source. Both NO+ and H+(H2O)2 are created, but the protonated water clusters can be eliminated by passing the NO/N2 or NF3/N2 mixture through a liquid N2 trap before it enters Analytical Chemistry, Vol. 76, No. 10, May 15, 2004

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Table 1. Observed Chemical Ionization Pathways of Organic Aerosols Using Various Reagent Ionsa

oleic acid linoleic acid octyl aldehyde 1-octadecene hexanal oleyl alcohol decyl alcohol 2-decanone hexadecane

mass (amu)

H+(H2O)2

H+(CH3OH)2

NO+

O2+

O2-

282 280 128 252 100 268 158 156 226

PT PT PT PT x PT PT, -OH PT none

PT, IS PT, IS IS none IS PT, IS IS x x

CT, A, HA, -H2O CT, A, HA HA, A A HA, A A, HA, [M-3H]+, [M-2H+NO]+ HA, [M-3H]+, [M-2H+NO]+ A HA

CT CT x CT x -H2O -H2O x x

PA, A PA, A A none x A A none none

a PT, proton transfer ([M + H]+); IS, ion solvation (e.g., H+(CH OH)(oleic acid)); CT, charge transfer (M+); -OH, hydroxyl loss ([M - OH]+), 3 -H2O, water loss ([M - H2O]+); A, association (e.g., [M + NO]+); HA, hydride abstraction ([M - H]+); PA, proton abstraction ([M - H]-); none, no ions observed; x, not obtained.

the polonium. The mechanism for the creation of NO+ from NF3 has not been determined, though it may require the presence of a trace amount of O2 or H2O. Alternatively, NO+ may be created from a small NO impurity that was identified in EI mass spectra taken of the NF3 gas. Interestingly, a small amount of NO+ is present even in the absence of NO and NF3, though it is generally 5-10 times less abundant than the primary ion. It is possible that this NO+ comes from HNO3 used in the production of the polonium source, and in fact, we have observed evidence of HNO3 in some negative ion spectra. Either the NO or NF3 source may provide advantages over more conventional methods for creating NO+, such as those that use NO with a filament to initiate ionization. That method is limited in its usefulness because of the potentially short lifetime of the filament in the presence of high concentrations of NO, with some authors reporting lifetimes as short as 6 h.27 Nitrous oxide (N2O) can also be used as the reagent gas,28 but both NO+ and N2O+ are created with that source leading to a more complicated chemical ionization spectrum. Negative ions are also generated with the polonium ion source, primarily through electron capture by the reagent gas. Both O2and SF6- are created in this manner by flowing 10 sccm of O2 or 1 sccm of SF6 with 3 SLPM of N2 through the polonium. F- ions are made by using a trace amount (2 sccm) of NF3 with 3 SLPM of N2 through the polonium. Both F- and F-‚H2O ions are created, but the water clusters can be reduced by flowing the NF3/N2 mixture through a liquid N2 trap before it enters the ion source. Gases and Chemicals. All gases used are commercially available and all were purchased from National Welders Supply: N2 (99.99%), O2 (99.5%), SF6 (99.9%), NF3 (CP grade), and NO (99.5%). Aerosols are created using the following organic liquids, which were all purchased from Aldrich unless otherwise indicated: oleic acid (90%, Mallinckrodt), linoleic acid (95%), 1-octadecene (97%), oleyl alcohol (85%), decyl alcohol (99%), 2-decanone (98%), hexadecane (99%), 1-hexanal (98%), and 1-octanal (99%). Methanol (HPLC grade) was used as a solvent and as a reagent gas for experiments involving the protonated methanol ion. RESULTS AND DISCUSSION Chemical Ionization Reactions. One of the advantages of using chemical ionization for the analysis of particle composition is the ability to select the reagent ion used for ionization. The (27) Hunt, D. F.; Harvey, T. M. Anal. Chem. 1975, 47, 2136-2141. (28) Polley, C. W., Jr.; Munson, B. Anal. Chem. 1983, 55, 754-757.

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reagent ion can be either positively or negatively charged and can react with neutral molecules through multiple channels yielding different product ions, including

charge transfer:

X + + Y f Y+ + X -

proton transfer:

X +YfY +X

(2)

H+‚Xn + Y f H+‚Y + n X

(3)

proton abstraction: ion solvation: ion association:

(1)

-

X- + Y f [Y-H]- + XH

+

(4)

+

H ‚Xn + Y f H ‚X‚Y + (n - 1)X (5) X+ + Y f X+‚Y

(6)

X- + Y f X-‚Y hydride abstraction:

+

(7) +

X + Y f [Y-H] + XH

(8)

Positive reagent ions, H+(H2O)2, H+(CH3OH)2, NO+, and O2+, and negative reagent ions, O2-, F-, and SF6-, are created and used to detect vaporized particles. Particles containing organic molecules with various functional groups are analyzed with these ions to determine which ions can be used to detect different classes of molecules representative of those found in atmospheric particles. The results of these studies are summarized in Table 1. Additionally, spectra of one organic molecule in particular, linoleic acid (CH3(CH2)4CHdCHCH2CHdCH(CH2)7COOH), obtained with five different reagent ions are presented in Figures 2 and 3 demonstrating the flexibility of this technique. Positive-Ion Chemical Ionization. Protonated Water Cluster. The protonated water cluster ion, H+(H2O)2, is one of the least specific reagent ions used in this study, ionizing most of the different organic molecules. The majority of molecules are detected simply as the protonated molecular ion, [M + H]+, as has been demonstrated previously in many studies in which proton-transfer mass spectrometry has been used for the detection of volatile organics.29-32 The low degree of fragmentation in most of the spectra makes it straightforward to identify and quantify (29) Harrison, A. G. Chemical Ionization Mass Spectrometry, 2nd ed.; CRC Press: Boca Raton, FL, 1992. (30) Lindinger, W.; Hansel, A.; Jordan, A. Int. J. Mass Spectrom. Ion Processes 1998, 173, 191-241. (31) Taucher, J.; Hansel, A.; Jordan, A.; Lindinger, W. J. Agric. Food Chem. 1996, 44, 3778-3782. (32) Hewitt, C. N.; Hayward, S.; Tani, A. J. Environ. Monit. 2003, 5, 1-7.

Figure 3. Negative chemical ionization spectra of vaporized linoleic acid particles using O2- and F- reagent ions.

Figure 2. Positive chemical ionization spectra of vaporized linoleic acid particles using NO+, H+(H2O)n, and H+(CH3OH)n reagent ions. Note that nearly all of the water and methanol cluster ions exist as the n ) 2 clusters.

components of internally mixed particles especially when used in conjunction with complementary off-line techniques. This ability is particularly useful for the analysis of reacted particles in laboratory studies (discussed below) and perhaps for atmospheric particles, which often contain complex mixtures of organics. Protonated Methanol Cluster. The H+(CH3OH)2 ions react through ion solvation (reaction 5) primarily yielding protonated methanol cluster ions, e.g., H+(CH3OH)(oleic acid). However, some species, such as oleic acid, linoleic acid, and oleyl alcohol, are also ionized through proton transfer.

Nitric Oxide. The NO+ ion is the most general reagent ion used, ionizing every molecule in the study, and its use in the detection of a wide range of gas-phase organic species has already been established.27,29,33-36 In fact, NO+ ionizes hexadecane, a molecule that was not detected with any of the other ions used and that exhibits extensive fragmentation with other methods, such as EI,37 and even with other chemical ionization reagent ions (e.g., CH5+ 29). Many of the organics are ionized via hydride abstraction ([M - H]+) or ion association ([M + NO]+), common pathways with the NO+ ion.38,39 The carboxylic acids also exhibit molecular ion peaks resulting from charge transfer. The alcohols are also detected as the [M - 3H]+ and [M - 2H + NO]+ ions, which are known to result from chemical ionization of long-chain alkanes and alcohols using NO+.29 Though it is not demonstrated in the current study, the NO+ reagent ion has proven useful in the determination of isomers and in locating the positions of (33) Hunt, D. F.; Ryan, J. F. J. Chem. Soc., Chem. Commun. 1972, 620-621. (34) Hunt, D. F.; Harvey, T. M.; Brumley, W. C.; Ryan, J. F., III; Russell, J. W. Anal. Chem. 1982, 54, 492-496. (35) Hunt, D. F.; Harvey, T. M. Anal. Chem. 1975, 47, 1965-1969. (36) Daishima, S.; Iida, Y.; Kanda, F. Anal. Sci. 1991, 7, 203-208. (37) NIST Mass Spectrometry Data Center, NIST Chemistry WebBook, NIST Standard Reference Database Number 69, March 2003; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, 2003; http://webbook.nist.gov. (38) Chai, R.; Harrison, A. G. Anal. Chem. 1983, 55, 969-971. (39) Einolf, N.; Munson, B. Int. J. Mass Spectrom. Ion Phys. 1972, 9, 141-160.


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Figure 4. Detection of vaporized oleic acid particles using H+(H2O)2 and O2- reagent ions. Error bars represent 2 standard deviations but are too small to be seen for O2- ionization. Concentration of oleic acid is calculated from the particle size distribution as measured with an aerodynamic particle sizer. Detection limits for oleic acid are 1.0 × 1010 molecules/cm3 (4.7 µg/m3) with H+(H2O)2 and 4.5 × 1010 molecules/cm3 (21 µg/m3) with O2-.

double bonds in unsaturated molecules.40,41 It is likely that this ability will make NO+ chemical ionization a unique and useful technique for the analysis of complex organic aerosols. Oxygen. O2+ ionizes oleic acid, linoleic acid, and 1-octadecene through charge transfer. Oleyl alcohol and decyl alcohol are ionized through dissociative charge transfer resulting in [M H2O]+ peaks. Negative-Ion Chemical Ionization. Negative ions, such as O2-, F-, and SF6-, are also used to detect the organic particles, though in general these ions are more selective and less sensitive than the positive ions used. O2- ionizes the carboxylic acids, oleic and linoleic acid, through proton abstraction, as has been noted for the detection of dicarboxylic acids in aerosols by Hoffmann and co-workers.24 Oleic acid, linoleic acid, octyl aldehyde, oleyl alcohol, and decyl alcohol are all ionized through ion association. The F- and SF6- ions do not ionize organics efficiently, but small association peaks ([M + F]-) and proton-abstraction peaks ([M - H]-) are detected with these ions for oleic acid, linoleic acid, and octyl aldehyde. Linearity of Detection and Detection Limit. Quantitative determination of the kinetics and product yields of gas-particle reactions requires linear detection of the particle constituents with the Aerosol CIMS technique. Likewise, potential application to the measurement of atmospheric aerosols will require a linear relationship between concentration and signal. To confirm the linearity of detection, the concentration of oleic acid particles sampled into the heated tube is varied by using a variable N2 flow (0-10 sccm) combined with a fixed N2 dilution flow (2 SLPM). The integrated signal of the oleic acid peak (m/z ) 283 with H+(H2O)2, m/z ) 281 with O2-) is then measured as a function of the gas-phase concentration (Figure 4). Absolute concentrations of oleic acid are determined by measuring the particle size distribution with an aerodynamic particle sizer (APS, TSI, model (40) Budzikiewicz, H. Fresenius’ Z. Anal. Chem. 1985, 321, 150-158. (41) Budzikiewicz, H. In Studies in Natural Products Chemistry, Vol. 2: Structure Elucidation, Pt. A; Atta ur, R., Ed.; Elsevier: Amsterdam, 1988; Vol. 2, p 469.


3321) immediately before and after each measurement made with the aerosol mass spectrometer. Typical particle concentrations used were on the order of 50-500 µg/m3 corresponding to vapor concentrations of (1-10) × 1011 molecules/cm3. The detection limits for oleic acid using both reagent ions can also be determined from Figure 4. With an integration time of 5 s and a signal-to-noise ratio of 2, detection limits of 1.0 × 1010 and 4.5 × 1010 molecules/cm3 are obtained with the H+(H2O)2 and O2- reagent ions, respectively. The lower detection limit of 1.0 × 1010 molecules/cm3 corresponds to 10 particles/cm3 of 1-µmdiameter particles, or ∼5 µg/m3. This limit is similar to typical ambient particle densities (20-100 µg/m3 1) in urban areas, though ambient particles may contain many different types of molecules, and concentrations of individual organic components are often no more than a few nanograms per cubic meter.1 However, such detection is more than sufficient for laboratory studies in which densities as high as 106 particles/cm3 may be used. Though these limits are valid only for oleic acid detected with H+(H2O)2 or O2-, detection of other analytes with other reagent ions can be expected to be linear as well. Correlation of Volatility with Composition in Mixed Particles. Separation of the vaporization and ionization steps allows an additional dimension to be used in analyzing particles, especially those consisting of multiple components. By varying the temperature at which the particles are vaporized, the mass spectra can be correlated with volatility. This ability is demonstrated for particles consisting of a 1:1 mixture (by volume) of hexadecane (bp ) 281°C42) and oleic acid (bp ) 360 °C43) in Figure 5. Since the hexadecane cannot be detected with the protonated water clusters, the NO+ reagent ion is used, yielding the [M - H]+ ion. However, the oleic acid is detected more efficiently with the H+(H2O)2 reagent ion, so both ions are used simultaneously. The concurrent use of two reagent ions highlights the flexible detection possibilities for complex organic particles available with chemical ionization. The two molecules have different detection efficiencies, especially since they are ionized with different reagent ions. However, the relative intensity of the oleic acid peak (m/z ) 283) compared to the hexadecane peak (m/z ) 225) can yield information about the relative concentrations of these species in the particles. The volatility of the particles’ constituents can then be determined by monitoring the change in the relative intensities as the vaporization temperature is varied. In practice, the temperature is always lowered in these experiments to prevent the revaporization of low-volatility components, which could condense onto the walls of the heated tube at lower temperatures. Another possible complication, pyrolysis, was determined to be negligible as indicated by the minimal fragmentation even at the highest temperatures used. In Figure 5, it is clear that as the temperature rises the signal attributable to the less-volatile component (oleic acid) increases relative to the signal corresponding to the more-volatile component (hexadecane). At the highest temperatures, the ratio of the signals (42) NIST Mass Spectrometry Data Center. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69, March 2003; Brown, R. L., Stein, S. E., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, 2003; http://webbook.nist.gov. (43) Jordan, T. E. Vapor Pressure of Organic Compounds; Interscience Publishers: New York, 1954.

Figure 6. Mass spectra of pure oleic acid particles (lower figure) and oleic acid particles that have reacted with O3 offline in the flask (upper figure). The upper figure shows the depletion of the oleic acid peak (m/z ) 283) as well as the appearance of ozonolysis product peaks (nonanal, 9-oxononanoic acid, and azelaic acid).

Figure 5. Effect of vaporization temperature on mass spectra of particles containing both hexadecane and oleic acid. The upper figure shows the increase in the signal for the less volatile component, oleic acid, when the temperature is increased. The lower figure shows the oleic acid/hexadecane ratio as a function of vaporization temperature. Note that the asymptotic limit is normalized to represent the bulk liquid mole ratio of 0.8.

reaches a plateau, and this value has been set to 0.8, the known bulk mole ratio of the two compounds. The plateau indicates that the maximum extent of vaporization has been reached. Indeed, it is likely that this plateau corresponds to complete vaporization of the particles, though we do not have an independent means by which to verify the extent of vaporization. Nevertheless, it is possible to selectively distill the more volatile species from the particles at lower temperatures and identify them with the mass spectrum. The capability to correlate volatility with composition will aid in the identification of gas-particle reaction products in future laboratory studies. Such a correlation may also enhance the characterization of certain classes of ambient aerosols, though complex mixtures of organic components may complicate such a volatility analysis. Analysis of Processed Particles. In the atmosphere, reactions between organic aerosol constituents and reactive trace gases such as O3, OH, HO2, NO, and NO2 can substantially change the properties of the particles. Determination of the significance of such reactions requires an ability to detect the reaction products and measure the rates of these reactions occurring in the condensed phase. Thermal vaporization of the particles followed by “soft” chemical ionization makes it possible to sensitively detect many organic species typical of those found in such processed particles. This ability is demonstrated for the reaction of an unsaturated carboxylic acid, oleic acid, with O3. Shown in Figure 6 are the

proton-transfer spectra (using H+(H2O)2) of pure oleic acid particles (d ∼ 1-2 µm) and of particles that are reacted with O3 ([O3] ∼ 1015 molecules/cm3) offline in the flask for ∼1 min as analyzed with Aerosol CIMS. Comparison of the two spectra shows a marked decrease in the oleic acid signal (m/z ) 283) upon reaction and the simultaneous appearance of many peaks at lower masses. In particular, features at m/z ) 143, 173, and 189 are observed. The “soft” chemical ionization results in protonated molecular ions ([M + H]+) allowing these peaks to be assigned to expected ozonolysis products. Though these assignments are reasonable, more complete identification would require the use of other techniques such as collision-induced dissociation or GC/ MS. Indeed, previous studies of this particular reaction44-47 have separately indicated that nonanal (mass ) 142 amu), 9-oxononanoic acid (mass ) 172), and azelaic acid (mass ) 188) have been observed from this reaction. These studies have been limited by excessive ion fragmentation as well as the inability to detect reaction products in the gas phase and in the particle at the same time. In the present study, all three products are observed simultaneously and in real time as the reaction proceeds, thus eliminating complications that may arise from collection and extraction of liquid-phase samples. In addition, the relative volatilities of these products are determined, providing an additional means by which the products can be identified. Upon lowering the vaporization temperature, it is possible to selectively detect the most volatile particle species. Indeed, with no particle vaporization, only the peak at m/z ) 143 is still observed when particles are reacted with O3, suggesting that this product is significantly more volatile than the other (44) Morris, J. W.; Davidovits, P.; Jayne, J. T.; Jimenez, J. L.; Shi, Q.; Kolb, C. E.; Worsnop, D. R.; Barney, W. S.; Cass, G. Geophys. Res. Lett. 2002, 29, 71/ 1-71/4. (45) Smith, G. D.; Woods, E., III; DeForest, C. L.; Baer, T.; Miller, R. E. J. Phys. Chem. A 2002, 106, 8085-8095. (46) Moise, T.; Rudich, Y. J. Phys. Chem. A 2002, 106, 6469-6476. (47) Thornberry, T. D.; Abbatt, J. P. D. Phys. Chem. Chem. Phys. 2004, 6, 8493.


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measurement of uptake kinetics of online gas-particle reactions. The reaction kinetics of complex, internally mixed particles can be investigated by quantitatively measuring the loss of the particle constituent as a function of reaction time, as opposed to measuring the loss of the gas-phase species. Measurement of gas-particle reaction rates using the Aerosol CIMS technique will also appear in a forthcoming paper.

Figure 7. Time response of oleic acid signal (m/z ) 283) when the flow of particles is stopped. The ion signal decays exponentially with a decay constant of 5.5 s, only slightly longer than the 5-s signal averaging used. The fast decay confirms that the signal is from direct vaporization of the particles and not from vaporization of residual oleic acid, which might have condensed on the walls of the tube.

products. This finding is consistent with the m/z ) 143 product being an aldehyde since aldehydes are generally more volatile than carboxylic acids of similar chain length.1 The observed volatility of the nonanal also suggests a mechanism by which mass is removed from the particle, a finding that may have significant implications for similar particle reactions in the atmosphere. It is not possible to determine the extent to which such a semivolatile species evaporates from the particle since the reactions occur offline and the species may partition to the walls of the flask. Future work will address the measurement of gas-particle partitioning of semivolatile species. Finally, the fast time response (see Figure 7) will also make this technique useful for the


CONCLUSIONS Chemical ionization mass spectrometry has been coupled to thermal vaporization of aerosol particles, providing compositional analysis on the time scale of seconds. A variety of reagent ions have been compared for the detection of aerosols consisting of organic molecules, including alkanes, alkenes, alcohols, aldehydes, ketones, and carboxylic acids. The most general and most sensitive ions used are NO+ and the protonated water cluster, H+(H2O)2. These ions were used simultaneously to analyze internally mixed particles of hexadecane and oleic acid as a function of vaporization temperature, demonstrating the ability to use volatility in conjunction with the mass spectrum for obtaining compositional information. This capability was used in analyzing oleic acid particles that were exposed to O3, and multiple reaction products were identified both in the gas phase and in the particles. ACKNOWLEDGMENT This work was supported in part by the ACS Petroleum Research Fund and the University of Georgia Research Foundation. We also thank the reviewers for their helpful comments. Received for review January 8, 2004. Accepted March 18, 2004. AC049948S

A Chemical Ionization Mass Spectrometry Method for the Online

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