Environ. Sci. Technol. 2005, 39, 7616-7624
Simultaneous Determination of Mono- and Dicarboxylic Acids, ω-Oxo-carboxylic Acids, Midchain Ketocarboxylic Acids, and Aldehydes in Atmospheric Aerosol Samples YUN-CHUN LI AND JIAN ZHEN YU* Department of Chemistry, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong
This paper describes a method for the simultaneous determination of monocarboxylic acids (C6-C34), dicarboxylic acids (C2-C24), ω-oxo-carboxylic acids (C2-C9), ketocarboxylic acids (pyruvic and pinonic acid), and select aldehydes (glyoxal, methylglyoxal, and nonanal) in atmospheric particles. Quantification of these compounds gives information on important chemical characteristics of aerosols for source apportioning of aerosol organics and for studying atmospheric processes leading to secondary organic aerosol formation. These target analytes were determined as their butyl ester or butyl acetal derivatives using gaschromatography mass spectrometry. The method is modified from a method described by Kawamura. Kawamura’s original method involved a water extraction step, which practically restricted the method to the determination of only those compounds that are water-soluble. Our method eliminates the water extraction step and combines extraction and derivatization of the target compounds in one step. A mixture of hexane/butanol/BF3 simultaneously derivatizes the polar function groups (i.e., -COOH, -CdO) and extracts the target analytes from the aerosol filter substrates. A prominent advantage of our method is improved recoveries for the more volatile analytes in the target compound classes as a result of eliminating the water evaporation step. Recoveries better than 66% were obtained for the target analytes, including the relatively volatile ones. This improvement for the light species has allowed detection of a new midchain ketocarboxylic acid, 4-oxopentanoic acid, which would have escaped detection by the Kawamura method because of its high susceptibility to evaporative loss. Examples are presented to demonstrate the use of this method in analysis of ambient aerosol samples.
1. Introduction Kawamura and co-workers (1, 2) were the first to apply butyl esterification derivatization using BF3/butanol (abbreviated as BF3/BuOH hereafter) to determine low-molecular-weight (LMW) oxygenated species, including dicarboxylic acids (C2C10), ketocarboxylic acids (C2-C9), and dicarbonyls (C2-C3), by gas chromatography-mass spectrometry (GC-MS) methods. These LMW oxygenated species are ubiquitous components in tropospheric aerosols. They have been detected * Corresponding author phone: 852-2358-7389; fax: 852-23587389; e-mail:
[email protected]. 7616
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in aerosol samples in various ambient environments ranging from polluted (3, 4) and clean continental (5-7) to remote marine atmospheres (8-10). These compounds are highly water-soluble and hydrophilic because of the presence of polar functional groups in combination with their small molecular sizes. As a result of their high affinity for water, they play an active role in aerosol-cloud interactions (11, 12) and wet scavenging of atmospheric particles and formation of haze (13). These compounds are also known oxidation products of many common volatile organic compounds (1416). Reliable quantification of these LWM compounds can help us to evaluate the importance of secondary photochemical reactions. The BF3/BuOH reagent not only converts carboxyl groups into butyl esters but also converts aldehyde groups into dibutyl acetals (17, 18). This conveniently enables the simultaneous GC-MS analysis of a few classes of compounds containing carboxyl (-COOH) or aldehyde (-C(O)H) groups or both. In comparison with another commonly used BF3/ methanol methylation scheme, the BF3/BuOH butylation scheme has distinct advantages for quantifying the LMW compounds because the resulting butyl derivatives are less volatile and more resistant to evaporative losses. Several C2C4 oxygenated compounds, including oxalic acid, C2-C4 ω-oxocarboxylic acids, pyruvic acid, glyoxal, and methylglyoxal, were not detected using the BF3/methanol derivatization methods (9, 19), but they were detected when using the BF3/BuOH method (1, 2). Because of the abovementioned advantages, the BF3/BuOH method has become the most utilized method for the determination of these LMW oxygenates in various types of atmospheric samples (3, 6, 20). For the convenience of the ensuing discussion, we refer to the analytical protocol established by Kawamura and coworkers as the Kawamura method. In the Kawamura method, the oxygenated compounds are first extracted with water from their filter collection substrates. Water must be evaporated before the BF3/BuOH derivatization step because water facilitates the reverse of the derivatization reaction (17, 18). The water evaporation step is time-consuming and causes significant evaporative losses of the smaller and more volatile target compounds (e.g., glyoxal, methylglyoxal, oxalic acid). In addition, the water extraction step practically limits the measurable species to those that are water-soluble. The higher molecular weight oxygenates (e.g., >C10 dicarboxylic acids), monocarboxylic acids, and a few aldehyde species cannot be measured because of their low water solubility. Poore (21) simplified the Kawamura method by dropping the water extraction and evaporation steps and mixing the BF3/BuOH reagent directly with filter substrates for derivatization. He applied the modified method to the quantification of oxalic acid in aerosols and reported a recovery of 93.4%. This recovery was expectedly better than the value (70%) reported by Sempere and Kawamura (3) as a result of eliminating the evaporative loss associated with the water evaporation step. In Poore’s method, 1 mL of 10% BF3/BuOH was used to extract a 37-mm diameter filter. As a result, Poore’s method is practically limited to the analysis of oxalic acid because other LMW oxygenated species typically exist at concentration levels one magnitude or more lower than that of oxalic acid (3, 4, 6, 7, 22) and consequently require more filter materials for their detection. Although it is possible to increase the volume of the BF3/BuOH solution to accommodate a larger amount of filter materials, such an option is not economically viable because of the high cost of purified BF3/BuOH reagent. 10.1021/es050896d CCC: $30.25
2005 American Chemical Society Published on Web 08/19/2005
We here describe and characterize a method that expands the idea embodied in Poore’s method to other oxygenated species that can be derivatized by the BF3/BuOH reagent. The compounds in addition to those measured by the Kawamura method include C6-C34 monocarboxylic acids, pinonic acid, C11-C24 dicarboxylic acids, and aldehydes (e.g., nonanal). Among them, a few compounds, such as nonanal and pinonic acid, are unique tracers for certain aerosol sources. Pinonic acid is a known major oxidation product of R-pinene (14) and its presence has been documented in forest aerosols (19, 23-25). Nonanal is identified to be abundant in cooking exhaust (26, 27) and has been proposed to be used as a tracer for cooking emissions (28). Monocarboxylic acids (i.e., fatty acids) contribute large fractions to fine aerosols as a result of multiple common emission sources of both anthropogenic and biogenic origins (28 and refs therein, 29).
2. Experimental Methods 2.1. Reagents and Chemicals. Commercially available BF3/ BuOH is sold in 1-mL ampule packages, suitable for use in the Kawamura method, which requires 0.2 mL reagent per sample. High cost aside, the commercial reagent is not tailored to be used for an amount exceeding 1 mL per sample. We, therefore, prepared in our laboratory larger quantities (∼100 mL at a time) of the reagent mixture of BF3, butanol, and diethyl ether in the proportion of 15%:68%:17% (w/w). The reagent mixture was prepared by mixing purified and redistilled BF3 diethyl etherate (BF3-etherate, (C2H5)2O‚BF3) (Aldrich, Milwaukee, WI) and laboratory-purified n-butanol in a 1:3 ratio by volume. Preliminary experiments indicated that numerous impurities in as-received AR-grade butanol led to an unacceptably elevated baseline. The AR-grade n-butanol (BDH, Poole, England) was therefore redistilled in our laboratory using an all-glass apparatus. Before distillation, butanol was first mixed with 2,4-dinitrophenylhydrazine (DNPH) (Sigma, St. Louis, MO) dissolved in HCl overnight followed by drying over anhydrous Na2SO4. The addition of DNPH facilitated removal of aldehyde and ketone compounds, which would react with butanol in the presence of BF3 and constitute interferences in the analysis of the target analytes. Hexane and acetonitrile (AR grade, Mallinckrodt, Baker, Phillipsburg, NJ) were used as purchased without further purification. The oxygenated standard compounds used in this work are listed in Table 1, along with their molecular weights before and after derivatization. They were purchased from several sources and were used as purchased without further purification. Oxalic acid and fumaric acid were from Merck (Schuchardt, Hohenbrunn, Germany); methylglyoxal was from Sigma; glyoxal, malonic acid, methylmaonic acid, maleic acid, glyoxylic acid, glutaric acid, and sebacic acid were from Acros (Geel, Belgium); succinic acid, decanoic acid-D19, methylsuccinic acid, pyruvic acid, malic acid, pimelic acid, phthalic acid, suberic acid, and azelic acid were from Aldrich; 3-methylglutaric acid and adipic acid were from International Laboratory (Las Vegas, NV); and phthalic acid-D4 and heptadecanoic acid-D3 (98%) were from Cambridge Isotope Laboratories (Andover, MA). A mixture of eight deuterated standards (IS#4) was provided by Professor James J. Schauer at the University of Wisconsin-Madison. This mixture includes succinic acid-D4, octanedioic acid-D4, phthalic acidD4, decanoic acid-D19, tetradecanoic acid-D27, heptadecanoic acid-D33, eicosanoic acid-D39, and tetracosanoic acid-D47. The deuterated acids were included for assessment of their suitability in acting as internal standards (IS). Multicomponent standard solutions were prepared in purified nbutanol and with their relative concentrations resembling those found in aerosol samples. Tetracosane-D50 was added
into samples and calibration standards just before injection to serve as an injection IS to account for variation in injection volume. Glyoxal and methylglyoxal were purchased as 40% solution in water. The free form of glyoxal coexists in equilibrium with various forms of hydrated oligomers, and the dominant form varies with the concentration (30) and solution pH. Methylglyoxal is also expected to have multiple forms existing in equilibrium. The derivatization reaction of BF3/BuOH with glyoxal and methylglyoxal shifts the equilibrium toward their free forms. As a result, the current analytical scheme quantifies the total glyoxal and methylglyoxal concentration present in a calibration standard or in a sample. 2.2. Sample Preparation and Analysis. Twenty-seven ambient aerosol samples at seven locations in Hong Kong were collected onto a precombusted quartz fiber filter (20 × 25 cm) using a high volume sampler (GT22001; Andersen Instruments, Smyrna, GA) at a flow rate of 1.13 m3/min for 24 h. The collection period spans from October to December 2003. One-fourth of the filter was cut into pieces and mixed with 1.6 mL 15% BF3/BuOH and 20 mL hexane in a conical flask capped with a ground-glass stopper and clamp-sealed. The filter was extracted in an ultrasonication bath for 30 min and then was placed in a water bath at 70 °C for 1 h. After cooling, the reaction mixture was transferred to a vial and was extracted with a mixture of 2 mL hexane, 6 mL water, and 0.5 mL acetonitrile. Acetonitrile allowed more effective transfer of butanol into the aqueous phase. The extraction step was repeated twice and the resulting extracts were combined. The hexane layer (∼25 mL) was washed with four portions of 5 mL water and then was filtered through a PTFE syringe filter into a 50-mL round-bottom flask. The filtrate volume was reduced to almost dryness through rotary evaporation under vacuum. Trace amounts of water were removed by adding ∼500 mg anhydrous Na2SO4 (prebaked at 550 °C for 8 h). The filtrate was then transferred to a 2.0mL graduated conical vial with three portions of hexane and was further concentrated to 200 µL under an ultrahigh purity nitrogen stream. Finally, 50 µL of tetracosane-D50 was added as an injection IS and another ∼100 mg anhydrous Na2SO4 was added to eliminate trace amounts of water. Calibration standards in butanol were derivatized in a mixture of 1.6 mL 15% BF3/68% butanol and 20 mL hexane at 70 °C for 1 h. The reaction mixture was then mixed with 0.5 mL acetonitrile, washed with four portions of 5 mL water, and dried over Na2SO4. Finally, the hexane extract was concentrated to ∼200 µL, and 50 µL of tetracosane-D50 was added. The resulting derivatives were injected into a GC-MS system (Agilent 6890 GC and 5793 MS) for analysis. The GCMS system was equipped with a split/splitless injector and a DB-5 fused capillary column (0.25 mm × 60 m × 0.25 µm). The column oven temperature program was as follows: initially 50 °C for 2 min, increased to 250 °C at 5 °C/min, then to 310 °C at 15 °C/min, and finally maintained at 310 °C for 20 min. The injector and the detector were maintained at 275 °C and 280 °C, respectively. Selected-ion monitoring (SIM) was used for quantification of the target analytes (Table 1). The SIM ion was chosen to be either the base peak ion or one of the more abundant characteristic fragment ions if the base peak ion had interference from coeluting peaks (e.g., fumaric acid and decanoic acid) or had high baselines (e.g., m/z 56 for oleic acid and C6-C28 monoacids). Calibration curves were established by plotting peak area ratios between the SIM ions of an analyte and the injection IS C24D50 versus the amounts of the analyte.
3. Results and Discussion 3.1. Choice of Derivatization Agent Amount and Reaction Temperature. The derivatization reaction was initially carried out in 20 mL 15% BF3/BuOH. A large peak was found to elute VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. List of Target Analytes, Major Fragment Ions of the Derivatives, Recoveries, Method Detection Limits, and Their Concentrations in Aerosol Samples compounds
parent derivative MW MW
RT (min)
Dicarboxylic Acids 57 57 105 105 101 101 99 99 101 101 115 115 155 117 115 115 129 129 129 129 143 143 149 149 157 157 227 227 241 241
oxalic (ethanedioic) malonic (propanedioic) methylmalonic maleic (cis butenedioic) succinic (butanedioic) methylsuccinic fumaric (trans butenedioic) glutaric (pentanedioic) 3-methylglutaric adipic (hexanedioic) pimelic (heptanedioic) phthalic suberic (octanedioic) azelic (nonanedioic) sebacic (decanedioic)
90 104 118 116 118 132 116 132 146 146 160 166 174 188 172
202 216 230 228 230 244 228 244 258 258 272 278 286 300 314
19.3 5 21.2 5 21.6 1 23.5 4 24.0 8 24.4 7 24.5 8 26.3 2 26.7 6 28.6 4 30.7 2 32.6 5 32.7 5 34.7 0 36.4 6
4-oxopentanoic pyruvic pinonic
116 88 184
172 274 240
Midchain Ketocarboxylic Acids 16.4 7 99 99 24.8 9 61 61 26.3 3 83 83
2-oxoethanoic (glyoxylic) 3-oxopropanoic 4-oxobutanoic 5-oxopentanoic 6-oxohexanoic 7-oxoheptanoic 8-oxooctanoic 9-oxononanoic
74 88 102 116 130 144 158 172
260 274 288 302 316 330 344 358
ω-Oxocarboxylic Acidsb 25.2 1 57 57 26.9 6 89 89 28.2 0 57 215 NDd 57 229 33.1 0 57 113 34.0 0 57 127 36.0 0 57 141 37.8 8 57 155
glyoxal methylglyoxal nonanal
58 72 142
318 202 272
29.0 8 16.9 5 27.7 1
hexanoic heptanoic octanoic nonanoic decanoic undecanoic dodecanoic tridecanoic tetradecanoic pentadecanoic hexadecanoic heptadecanoic oleic octadecanoic nonadecanoic eicosanoic henicosanoic docosanoic tricosanoic tetracosanoic hexacosanoic octacosanoic
116 130 144 158 172 186 200 214 228 242 256 270 282 284 298 312 326 340 354 368 396 424
172 186 200 214 228 242 256 270 284 298 312 326 338 340 354 368 382 396 410 424 452 480
Monocarboxylic Acids 14.44 56 99 17.20 56 113 19.88 56 127 22.30 56 159 24.63 56 129 26.86 56 187 29.04 56 201 31.06 56 215 33.05 56 229 34.90 56 243 36.86 56 257 38.40 56 271 39.67 55 264 40.15 56 285 41.65 56 299 43.04 56 313 44.0 8 56 327 44.92 56 341 45.64 56 355 46.28 56 369 47.64 56 397 49.36 56 425
Aldehydes 57 57 57
57 57 159
air concentrations (ng/m3) mean range
recovery (%)a
MDL (ng/µL)
81.1 ( 4.1 77.9 ( 4.4 83.0 ( 9.3 93.8 ( 1.9 74.2 ( 5.7 88.5 ( 4.5 65.8 ( 5.1 92.2 ( 8.2 89.0 ( 3.5 89.8 ( 0.3 80.2 ( 1.4 50.8 ( 5.0 86.6 ( 8.2 94.7 ( 6.3 88.7 ( 0.4
0.45 0.24 0.13 0.13 0.26 0.14 0.10 0.17 0.17 0.25 0.16 1.06 0.10 0.14 0.15
1084 142 2.4 10.0 118 7.2 9.9 33.8 2.8 27.3 23.7 129 37.3 148 8.6
179-2372 38-324 1.1-4.2 4.3-29.0 35-297 2.4-18.3 2.9-34.7 7.9-119.1 0.6-8.8 6.9-85.7 4.7-70.0 34-267 6.4-97.6 54-311 3.2-19.5
106 ( 20 53.5 ( 1.3 102 ( 13
0.21 0.06 0.23
14.9 111 4.3
3.9-47.2 21-370 1.5-11.5
65.9 ( 6.0
0.21
c
c
c
c
c
c
c
c
c
c
c
c
c
c
123 3.4 1.5 0.66 0.28 14.5 20.7 1.8
11-366 1.2-6.1 0.3-4.8 0.0-3.1 0.0-1.2 5.1-45.4 6.3-60.7 0.3-4.2
83.4 ( 2.4 87.5 ( 2.8 89.2 ( 3.9
0.08 0.13 0.11
23.7 63.6 2.8
6.4-56.0 0.0-218. 4 0.6-9.2
74.5 ( 6.1 94.0 ( 3.3 120 ( 11 84.3 ( 5.2 118.6 ( 3.7 89.7 ( 4.2 94.9 ( 3.3 96.7 ( 4.8 108.1 ( 5.4 96.9 ( 3.0 108 ( 13 95.2 ( 2.0 93.8 ( 2.1 92.9 ( 1.4 93.4 ( 2.3 96.7 ( 4.0 94.2 ( 1.2 96.0 ( 1.8 95.1 ( 2.0 92.9 ( 7.9 92.3 ( 7.6 87.2 ( 7.2
0.14 0.26 0.16 0.27 0.16 0.18 0.12 0.10 0.09 0.08 0.06 0.06 0.07 0.07 0.10 0.24 0.38 0.61 1.16 1.80 NDd NDd
13.1 12.2 15.0 11.7 4.4 2.4 14.4 2.1 15.2 3.9 310 66.5 6.2 115 2.7 17.5 6.5 43.5 22.6 71.4 46.2 92.9
4.5-24.0 0.0-34.0 4.4-31.0 5.7-22.2 1.6-13.1 1.0-4.5 5.4-30.8 1.0-3.4 8.1-20.5 2.2-5.6 123-424 3.8-252.1 3.3-9.4 51-181 1.2-6.5 6.4-39.3 2.7-19.6 14.1-122.8 6.0-58.2 18.1-200.6 17.6-166.2 28-344
base peak ion for ion quantification
a Recovery is calculated as the ratio between the injection IS-normalized responses of the spiked blank filter and a calibration standard mixture of the same concentration. The standard deviations were obtained from three replicate experiments. b All compounds in this group except glyoxylic acid do not have authentic standards. Their concentrations were estimated assuming the same response factor per mole as that for glyoxylic acid. c No measurements owing to no available standards. d ND means not detected at the lowest calibration concentration level.
between the derivatives of methylglyoxal and oxalic acid, making the two target analytes appear as shoulder peaks. As a result, the quantification of methylglyoxal and oxalic acid became inaccurate and unreliable. This interfering peak was found to increase with the volume of butanol used. It was identified from the NIST-library spectra to be B(OC4H9)3, a byproduct of the derivatization reaction. For the determi7618
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nation of the optimal amount of BF3/BuOH, seven parallel multicomponent standard solutions were derivatized with 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, and 20.0 mL BF3/BuOH. The B(OC4H9)3 peak became unacceptably high when the amount of BF3/BuOH exceeded 2.0 mL. We further compared recoveries of individual oxygenated species that were spiked on blank filters using 1.0, 1.2, 1.4, 1.6, and 2.0 mL BF3/BuOH.
The recoveries initially increased with the amount of BF3/ BuOH and reached a plateau when the amount was larger than 1.6 mL. On the basis of this experiment, 1.6 mL BF3/ BuOH was determined to be the optimal amount. On one hand, the desire for reliable quantification of methylglyoxal and oxalic acid limited the maximum amount of butanol to 1.6 mL. On the other hand, a certain minimal amount of extracting liquid was required to submerge the filter materials to ensure sufficient contact between the aerosol materials and the derivatizing agent. As a result, some inert solvent was needed to make up the liquid volume. Hexane became the most natural choice for this purpose, considering that hexane would be used to extract the derivatives at the end of the derivatization reaction. An experiment was designed to test whether hexane would affect the derivatization efficiency and introduce additional impurities. In this experiment, the same amount of the multicomponent standard solution was spiked onto two separate blank filters. Subsequently, one was mixed with 1.6 mL BF3/BuOH and 20 mL hexane and the reaction temperature was set at 70 °C to avoid excessive hexane (bp 69 °C) evaporation and the other was mixed with 20 mL BF3/BuOH and was allowed to react at 95 °C. The subsequent procedure was the same as that described in the Experimental Section. Comparison of the total ion chromatograms (TIC) of the two reaction mixtures reveals that the use of 20 mL BF3/BuOH resulted in a higher baseline and more impurities. The ratios between the responses of the derivatized products at 70 °C and 95 °C were found in the range of 0.87-1.21. The relative standard deviations of three replicate samples were better than 15%. In summary, 70 °C was an acceptable choice for the reaction temperature, and the presence of hexane did not affect derivatization efficiency or introduce interfering contaminants. 3.2. GC and MS Characteristics of the Butyl Ester and Butyl Acetal Derivatives. Most of the butyl ester and butyl acetal derivatives were well-resolved on a commonly used DB-5 GC capillary column. Figure 1 shows a TIC chromatogram of a derivatized standard mixture consisting of 21 monocarboxylic acids, 15 dicarboxylic acids, 3 midchain ketocarboxylic acid, 3 aldehydes, and 1 ω-oxocarboxylic acid. Among them, five compound pairs, fumaric acid and decanoic acid, 3-methylglutaric acid and undecanoic acid, glutaric acid and pinonic acid, glyoxal and dodecanoic acid, and phthalic acid and suberic acid, partially coeluted. However, their reliable quantification was not affected by using unique SIM ions. Figure 2 shows five example mass spectra of the butyl derivatives (i.e., the derivatives of nonanal, pinonic acid, hexadecanoic acid, oleic acid, and 4-oxopentanoic acid). Kawamura and co-workers have presented the mass spectra of the butyl derivatives of C2-C10 dicarboxylic acids (2), glyoxal, methylglyoxal, pyruvic acid, and C2-C9 ω-oxocarboxylic acid (1). Our observations of the mass spectra of these compounds are consistent with theirs. The common fragment ions arising from the -OC4H9 moiety include m/z 57 ([C4H9]+) and m/z 41 ([CH2CHdCH2]+) (Scheme 1 in Figure 3). The m/z 73 ([OC4H9]+) ion is abundant only for the butyl ester derivatives. Many derivatives have one of these three common ions (e.g., m/z 57 as the base peak ion for nonanal dibutyl acetal) as their base peak ions. The cleavage of the C-O bond adjacent to the butyl group to give rise to [M-73]+ is also common to the derivatives (Scheme 1 in Figure 3). For those derivatives with two or more butoxyl groups (e.g., dicarboxylic acids and aldehydes), a further loss of a C4H8 alkene fragment from [M-73]+ is possible to give rise to an [M-129]+ ion fragment (Scheme 2 in Figure 3) (31). This fragmentation pathway is especially prominent for the derivatives of C4-C8 dicarboxylic acids (except for fumaric acid), giving rise to the base peak ions. The dibutyl acetal
FIGURE 1. Total SIM ion chromatogram of the butyl derivatives of standards. (Peak identification: (1) hexanoic acid; (2) 4-oxopentanoic acid; (3) methylglyoxal; (4) heptanoic acid; (5) oxalic acid; (6) octanoic acid; (7) malonic acid; (8) methylmalonic acid; (9) nonanoic acid; (10) maleic acid; (11) succinic acid; (12) decanoic acid-D19; (13) methylsuccinic acid; (14) fumaric acid; (15) decanoic acid; (16) pyruvic acid; (17) glyoxylic acid; (18) glutaric acid; (19) pinonic acid; (20) 3-methylglutaric acid; (21) undecanoic acid; (22) nonanal; (23) adipic acid; (24) dodecanoic acid; (25) glyoxal; (26) pimelic acid; (27) tridecanoic acid; (28) phthalic acid; (29) suberic acid; (30) tetradecanoic acid; (31) azelic acid; (32) pentadecanoic acid; (33) sebacic acid; (34) hexadecanoic acid; (35) heptadecanoic acid; (36) tetradecane-D50; (37) oleic acid; (38) octadecanoic acid; (39) nonadecanoic acid; (40) eicosanoic acid; (41) henicosanoic acid; (42) docosanoic acid; (43) tricosanoic acid; (44) tetracosanoic acid; and (45) hexacosanoic acid.) derivatives arising from the aldehydes (e.g., glyoxal, methylglyoxal, nonanal, ω-oxocarboxylic acids) have two common fragment ions at m/z 159 ([C4H9O)2CH+]) and m/z 103 ([HOCHOC4H9]+) (Scheme 3 in Figure 3). The m/z 159 ion is typically more abundant than the m/z 103 ion. These two ions could be used to differentiate the acetal derivatives from the ester derivatives. Common to the butyl esters of acid compounds are m/z 56, postulated to be [CH2CHCH2CH3]+‚ (Scheme 4 in Figure 3) and [M-55]+ ion fragments. The two ion fragments are derived from decomposition pathways following γ-hydrogen rearrangement of the butyl ester (31) (Scheme 4 in Figure 3). The m/z 56 ion is the base peak ion for the butyl esters of C6-C34 monocarboxylic acids. The butyl esters of alkanoic acids also show a periodicity in the C4H9OCO(CH2)n+ ion series, that is, m/z at 101, 115, 129, 143, 157, 171, 185, 199, 213, 227, 241, and so forth (Figure 2c). The abundance of the molecular ion M increases with the MW of the parent acid compound. 3.3. Recoveries and Effects of the Aerosol Matrix. Recovery experiments were carried out by spiking known amounts of standards onto one-quarter of a 20 × 25 cm blank quartz filter. The spiked filters were processed the same way as the real aerosol samples. The recoveries were obtained by comparing the injection IS-normalized responses of the spiked blank filter and a calibration standard mixture of the same concentration. The results are listed in Table 1. The recoveries of the target compounds were in the range of 66120%, except for phthalic acid (50.8%) and pyruvic acid (53.5%). The standard derivations of three replicates were better than 20%. The two exceptional compounds had a commonality in that steric hindrance was anticipated for the butylation of the last functional group. Absorbing by quartz filter fabric may have reduced the derivatization reaction efficiency. The two deuterated monocarboxylic acids had recoveries of 93.8% (decanoic acid-D19) and 93.5% VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Mass spectra of butyl ester or butyl acetal derivatives of (a) pinonic acid, (b) nonanal, (c) hexadecanoic acid, (d) oleic acid, and (e) 4-oxopentanoic acid. (heptadecanoic acid-D33). In comparison with the range of recoveries for the target oxygenates (66-120%), the deuterated monocarboxylic acids could be used as IS to track variations in the entire analytical procedure. The sample matrix simulated in the recovery samples (i.e., standards spiked onto blank filter substrates) was short of aerosol particles. A separate experiment was therefore carried out to observe if the presence of aerosol particles in the matrix would have an effect on the recoveries. In this experiment, a mixture of the eight deuterated acid standards was spiked 7620
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onto two filter quarters from the same 20 × 25 cm parent filter preloaded with ambient aerosols. The two filter quarters were processed using the optimized extraction and reaction conditions. The recoveries were computed by comparing the injection IS-normalized responses of the spiked aerosol samples with those of a standard mixture containing the same amount of deuterated standards but without going through the filter extraction step. Except for phthalic acidD4, the recoveries for the deuterated standards were 96.8105.3% (Table 2), demonstrating that the extraction/
FIGURE 4. Comparison of the relative responses of the target analytes in an ambient aerosol sample as obtained using the Kawamura method versus our method.
FIGURE 3. Important ion fragmentation pathways of the butyl derivatives.
TABLE 2. Recoveries of Deuterated Internal Standards Spiked into an Aerosol Loaded Filter Sample compound succinic acid-D4 octanedioic acid-D4 phthalic acid-D4 decanoic acid-D19 tetradecanoic acid-D27 heptadecanoic acid-D33 eicosanoic acid-D39 tetracosanoic acid-D47
parent derivative quantification recovery MW MW ion (%) 122 178 170 191 255 303 351 415
234 290 282 247 311 359 407 471
105 217 153 192 256 304 352 417
100.3 100.3 52.8 102.9 105.3 101.0 99.2 96.8
derivatization steps were insensitive to the presence of an aerosol matrix. Recoveries using the Kawamura method have been reported for only a small subset of the target compounds by previous researchers. Kawamura and co-workers reported recovery data for oxalic acid (71% and 81%), malonic acid (77% and >90%), succinic acid (86% and >90%), glutaric acid (>90%), and adipic acids (>90%) in some of their studies (1, 22). Recovery data for other analytes are not available in the published studies by Kawamura and co-workers. Wang et al. (4) reported recoveries for C2-C9 dicarboxylic acids and methylglyoxal in the range of 51-67%. The presence of glyoxal was not reported in Wang et al.’s study, although glyoxal and methylglyoxal are typically of similar abundance (32, 33). One possible explanation is that the evaporative loss of glyoxal with the Kawamura method was such that it escaped detection. 3.4. Method Detection Limits. The method detection limits (MDLs) of our improved method were calculated using the following equation: CL ) kSb1/S, where CL is the MDL, k is a constant related to the confidence level and is set to be 3, Sb1 is the standard deviation of seven replicate
measurements of the calibration standard mixture at its lowest concentration level (∼0.5 ng/µL), and S is the slope of the calibration curve (34). The target analytes had their MDLs (Table 1) in the range of 0.06-1.8 ng/µL. These MDLs in ng/µL translate to 0.04-1.12 ng/m3 in air concentrations if we assume that one-quarter of a high-volume sample collected over 24 h (1600 m3 air) was used for analysis. 3.5. Comparison with the Kawamura Method. The most important improvement of our method over the Kawamura method (1, 2) is the elimination of water extraction and the subsequent step of evaporation to dryness. In addition to simplifying the analytical protocol, another benefit of such an improvement is a reduced loss for the more volatile oxygenated species (i.e., glyoxal, methylglyoxal). In the Kawamura method (1, 2), the derivatization step takes place after the evaporation to dryness step. As a result, the added resistance to evaporative loss through derivatization is not available at the volume reduction step. The volatile species are susceptible to evaporative loss during rotary evaporation under vacuum. In our method, the target analytes are already converted to their less volatile derivatives (e.g., HCOCOH versus (C4H9O)2CHCH(OC4H9)2) before the rotary evaporation step, and the rotary evaporation is carried out in the volatile hexane system. As a result, a significant reduction in the evaporative loss can be predicted in our method. In a comparison experiment, two one-quarter filters from the same parent 20 × 25 cm filter preloaded with ambient aerosols were processed separately using our method and the Kawamura method. We adopted the earliest version of the Kawamura method, in which the aqueous extracts were pH adjusted to 8.0-9.0 before water evaporation (2). The basic pH condition made the acid species exist as in their ionic form, therefore reducing their evaporative loss during the subsequent water evaporation step. The final volumes before injection were adjusted to be the same value (i.e., 250 µL) in both methods. Figure 4 compares the relative responses from the individual oxygenated species detected in the sample. The results verified that the Kawamura method recovered less of the more volatile species (e.g., glyoxal, glyoxylic acid) because of higher evaporative loss. For example, the amounts of glyoxal, methylglyoxal, nonanal, glyoxylic, and pinonic acid recovered by the Kawamura method were, respectively, only 56, 49, 18, 42, and 84% of those by our method. We also observed that increasingly less of an analyte was recovered with increasing molecular size within the same homologue series of dicarboxylic acids, monocarboxylic acids, and ω-oxocarboxylic acids in the Kawamura method. Less than half was recovered for monoand dicarboxylic acids larger than C10. Such a trend is an anticipated result of their increasingly lower water solubility. 3.6. Application to Real Samples. Figure 5 shows three reconstructed ion chromatograms for the derivatives of mono- and dicarboxylic acids and aldehydes in a same aerosol sample. Ion [M-55]+ is an abundant fragment ion for the butyl ester derivatives of the monocarboxylic acids and therefore is used for the reconstruction of the GC chroVOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Reconstructed ion chromatograms of selected characteristic ions for the derivatives of (a) moncarboxylic acids, (b) dicarboxylic acids, and (c) aldehyde group containing compounds in an aerosol sample. (The characteristic ions selected are m/z 173 for pyruvic acid, m/z 83 for pinonic acid, [M-55]+ for other monocarboxylic acids and oxalic acid, [M-73]+ for all the dicarboxylic acids except for oxalic acid, and m/z 159 for all the aldehyde group containing compounds. The peak labels Cn-al in (c) denote simple aldehydes whereas Cnω denotes an ω-oxo-carboxylic acid of n carbon atoms (e.g., C3ω is 3-oxo-propanoic acid). Nonanoic acid appears in Figure 5c because of its [M-55]+ ion coinciding with m/z 159.) matogram (Figure 5a). Also included in Figure 5a are three midchain ketocarboxylic acids (4-oxopentanoic acid, pyruvic acid, and pinonic acid). Ion [M-73]+ is an abundant ion fragment for the butyl esters of the dicarboxylic acids and is used to reconstruct the GC chromatogram for this class of compounds (Figure 5b). The ion at m/z 159 is characteristic of parent compounds containing one or more C(O)H (aldehydes) groups. Figure 5c plots the reconstructed m/z 159 ion chromatogram, therefore showing the presence of simple aldehydes and ω-oxocarboxylic acids. Twenty-four monocarboxylic acids (C6-C28), 3 midchain ketocarboxylic acids (pyruvic, pinonic, and 4-oxopentanoic acid), 1 ω-oxocarboxylic acid (glyoxylic acid), 15 dicarboxylic acids (C2-C24), and 3 aldehydes (glyoxal, methylglyoxal, and nonanal) were positively identified by comparing retention times and mass spectra with those of the authentic standards. An additional five monocarboxylic acids (C29-C34 except C33), twelve dicarboxylic acids (C11-C20, C22 and C24), seven ω-oxocarboxylic acid (C3-C9) compounds and eight aldehydes (C1-C5, C7, C14, and C16 alkanals) were tentatively identified through searching the NIST mass spectra library. The presence of ω-oxocarboxylic acids was previously 7622
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reported in aerosols, rainwater, and snow samples by Kawamura and co-workers (1, 3, 5, 20). The C1-C5 alkanals were also present in the field blank sample, with the levels of C1-C3 alkanals lower than those in the sample but the levels of C4 and C5 alkanals similar to those in the sample (Figure 5c). Some studies have suggested that retaining volatile carbonyls in the aerosol phase may be possible via hydration (35-37) or via forming reversible adducts with S(IV) species (38, 39). Liggio and McLaren (32) recently reported measurements of carbonyl species, including C1C3 alkanals, nonanal, glyoxal, methylglyoxal, nopinone, and pinonaldehyde, in Canadian aerosols. With our filter-only sample collection setup, we could not discern whether the aldehydes and other volatile oxygenates detected in our samples were truly part of the aerosol makeup or sampling artifacts arising from adsorbing their gaseous forms onto the sampling substrates (40). Nevertheless, the detection of these volatile species illustrates the unique advantage of our method. To our knowledge, 4-oxopentanoic acid is observed for the first time in aerosol samples here. The identity of this compound was initially established through interpreting the
FIGURE 6. Comparison of oxalate concentrations in aerosol samples determined by the GC/MS method and the IC method. mass spectrum of its butyl ester derivative (Figure 2e) and was subsequently confirmed with an authentic standard. 4-Oxopentanoic acid has a melting point of 33 °C, a boiling point of 245.5 °C, and a vapor pressure of 1 mmHg at 102 °C, therefore a fairly volatile compound. The successful identification of 4-oxopentanoic acid again demonstrates the advantage of our method. This advantage is especially useful in analyzing unknown compounds. As a midchain ketocarboxylic acid, 4-oxopentanoic acid is more volatile than other LMW oxygenated species in this study. This likely explains why it was not reported in previous studies using the Kawamura method. The detection of 4-oxopentanoic acid demonstrates that our method is more robust in detecting the LMW species. 4-Oxopentanoic acid could conceivably be an oxidation product of 4-oxopentanal, the presence of which has been observed in both gas and particulate phases in a forest atmosphere (41) and in Mediterranean ambient air (42). 4-Oxopentanal was postulated to be among oxidation products of many terpene compounds (e.g., squalene, a triterpene) (42). Table 1 lists the means and the ranges of concentrations for the target analytes in the 27 aerosol samples. The aerosol mass loadings of these samples ranged from 67.0 to 215.8 µg/m3 (average: 107.0 µg/m3) and the organic carbon ranged from 5.1 to 31.0 µgC/m3 (average: 14.0 µgC/m3). Among the target analytes, oxalic acid was by far the most abundant species, ranging from 179 to 2372 ng/m3. Species with their mean concentrations exceeding 100 ng/m3 included four dicarboxylic acids (C3, C4, C9, and phthalic acid), pyruvic acid, glyoxlic acid, and two monocarboxylic acids (C16 and C18). Others were less abundant, typically present in concentrations less than 100 ng/m3. The relative abundance of the measured species was consistent with similar measurements made elsewhere (e.g., 25, 43, 44). The sum of all the species quantified in this work accounted for an average of 10.5% of organic carbon on a carbon mass basis. Detailed data interpretation of the measurements of these individual compounds will be provided in a separate paper. Oxalic acid in this set of aerosol samples was also determined together with inorganic anions using ion chromatography (IC). The details of the IC method were available in our previous paper (45). The extent of agreement in the air concentrations in the 27 aerosol samples by the two methods was assessed as a simple linear fit (Figure 6). A reasonably good agreement was found, with a slope close to unity (1.03) and an r2 of 0.72. The IC method quantifies oxalic acid in its ionic form by direct injection of water extracts of the aerosol samples into the IC system. Neither solvent evaporation nor derivatization was involved in the IC method. As a result, the IC method is a simpler and more accurate method. The good agreement of the current BF3/BuOHGC/MS method with the IC method demonstrates the
reliability of the improved GC/MS method for the LMW aerosol species. In summary, we modified the Kawamura method by eliminating the water extraction and evaporation steps. Aerosol materials were directly mixed with the BF3/BuOH derivatization agent and the extracting solvent hexane. This modification leads to the following improvements: (1) a simplified and less time-consuming analytical protocol; (2) improved recoveries for both the more volatile species and the less water-soluble compounds; and (3) simultaneous determination of additional compounds that are either not water-soluble (e.g., larger mono- and dicarboxylic acids) or too volatile (e.g., nonanal) to be detected by the Kawamura method. Also, because of this modification, the improved method allowed detection of 4-oxopentanoic acid in aerosols for the first time. This compound would have escaped detection by the Kawamura method because of its high susceptibility to evaporative loss.
Acknowledgments This work was supported by the Research Grants Council of Hong Kong, China (604503 and 605404). We thank Prof. James Schauer for providing the deuterated standard mixture and the Hong Kong Environmental Protection Department for providing the aerosol samples.
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Received for review May 11, 2005. Revised manuscript received July 15, 2005. Accepted July 18, 2005. ES050896D
