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Anal. Chem. 1982, 54, 354-357
Chemical Characterization of Organic Adsorbates on Diesel Particulate Matter James A. Yergey and Terence H. Rlsby" Dlvision of Envlronmental Chemistry, Department of Envlronmental Health Sciences, The Johns Hopkins University School of Hygiene and Pubilc Health, Baltimore, Maryland 2 1205
Samuel S. Lestz Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
The organlc adsorbates on dlesel particulate matter are characterlzed by a cmbinatlon of chromatographlc and mass spectrometric techniques. These data are obtained for partlcuiate matter generated under constant engine operating condltlons uslng a model fuel and a synthetic lubricant, wlth air or argon/oxygen as the oxidant systems. The significance of these results is that the poiycycllc aromatlc compounds whlch were produced by these experiments are the Inherent products from the diffusion controlled combustion In a dlesel engine. Also, these results suggest that there are common intermediates which produce the polycyclic aromatlc compounds that are Independent of the structure and chain length of the hydrocarbonfuel. Preliminary results also lndlcate that the nitro derlvatlves of polycyclic aromatlc compounds are produced by secondary reaction of the oxides of nitrogen whlch are formed from the nltrogen in the oxidant.
It has been generally accepted that the polycyclic aromatic compounds found in full-boiling range diesel fuels and lubricating oils survive the combustion process and account for part of the polycyclic aromatic compounds adsorbed on the emitted particles. The identification of these compounds has resulted in concern regarding the environmental implications of an increase in diesel engine utilization, since these engines have been shown to emit 50-80 times more particles than a catalyst-equipped spark-ignition engine (1). The potential health risks associated with the diesel particles stem from their small sizes and sorptive surface properties. The mass-median-diameters of the agglomerated particles are typically less than 1pm, which causes the particles to have atmospheric residence times on the order of days or weeks. The sizes of the particles are especially important for health studies, since particles less than 1pm in diameter can be respired by humans, with a significant portion depositing in the deep lung ( 2 , 3 ) . Recent studies by Ross and co-workers ( 4 , 5 )have demonstrated relatively large heats of adsorption for various organic compounds on diesel particles and shown that surface areas for the particles can exceed 100 m2/g. In addition, other studies have shown that compounds extracted from diesel particles by organic solvents exhibit mutagenic and/or carcinogenic activity in bacterial assays (6, 7) and animal studies (49). These resulta suggest that a better understanding of the formation and chemical composition of the soluble organic fraction of diesel particulate matter is a requisite for a complete understanding of the potential health risk which could be associated with an increase in diesel utilization. A number of investigators have attempted to analyze the soluble organic fraction of diesel particles generated from typical, full-boiling range diesel fuels ( 1 0 , I I ) . However, the complex and variable composition of full-boiling range diesel 0003-2700/82/0354-0354$01.25/0
fuels, magnified by the combustion reactions, makes it very difficult to gain information which would enable the fuel to be related to the emission products. This study also attempts to corroborate the classic studies of Badger and others (12-17) who have shown that polycyclic aromatic compounds could be produced in flames or by high-temperature pyrolysis of various hydrocarbons. The objective of this research program was to simplify the combustion process, by using a model diesel fuel, in order to provide a better understanding of the combustion process leading to the formation of the particle-adsorbed polycyclic aromatic compounds. During the c o m e of these investigations it became apparent to our group and others that certain nitro derivatives of the polycyclic aromatic compounds were contained in the extract from diesel particulate matter (11,18, 19). There has been considerable concern surrounding this discovery, since these compounds exhibit mutagenic activity, according to bacterial assays, which is far greater than that for benzo[a]pyrene (12). The fuel and lubricating oil used in this investigation do not contain nitrogen, and, therefore, if any nitro polycyclic aromatic compounds are found on the particles they must be a product of reactions with oxides of nitrogen formed from the intake air. In order to assess this possibility, we collected a number of samples using an argon/oxygen mixture for the oxidant system in place of laboratory air.
EXPERIMENTAL SECTION Engine. The model fuel used in this study consists of a 1:l volume ratio of n-tetradecane and 2,2,4-trimethylpentane. This fuel was selected because it has ignition properties which are similar to full-boilingrange diesel fuels. Also, a synthetic, ashless lubricating oil (polyalkylene glycol, Ucon LB-525, Union Carbide) was used in order to minimize any contribution of the lubricating oil to the exhaust chemistry. The engine utilized throughout this study is a single-cylinder,Avco-Lycoming Bernard W-51 industrial diesel engine, which was operated under constant conditions (2400 rpm, 3/4 rack). Particle samples were collected in an isokinetidy drawn sample line, maintained at 52 "C, on 142-mm Teflon filters (Pallflex Products Corp.). The filters were desiccated, weighed, and Soxhlet extracted for 24 h with dichloromethane. The soluble organic fraction was blown to dryness under nitrogen and weighed. A more detailed presentation of the engine operating conditions, sampling train, and typical gas-phase and particle emission results have been presented elsewhere (20,21). Biological Characterization. The biological characterization of the particulate matter generated in this study was performed by the Ames Salmonella typhimurium and the Bacillus subtilis comptest bacterial assays. A complete description of these methodologies and the results have been presented elsewhere (22, 23). Chemical Characterization. The complexity of the soluble organic fraction of diesel particulate matter generally requires more than one separation step prior to analysis of the individual components (11,18),but as a result of the simplifications introduced in this study the separation and identification can be made in a single experiment. Capillary gas chromatography and 0 1982 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
high-performance liquid chromatography were used as complementary separation techniques in this study. A Varian 3740 gas chromatograph equipped with splitless injection was used for all capillary gas chromatogtaphicseparations with a 15-m wall-coated open-tubular glass capillary column, coated with SE-54 (Supelcio,Inc.). Helium was used as the carrier gas with a linear flow velocity of 40 cm/s. The column over was temperature programmed from 100 to 300 O C at a rate of 5 OC/min, and the samples were introduced as 1-pL aliquots. The column effluent was monitored with either a flame ionization or a thermionic specific detector. Signals were collected and displayed by computer data acquisition which is described in detail elsewhere (24). A BIOSPECT chemical ionization mass spectrometer (Scientific Research Instruments Corp.) was used to monitor gas cbromatographic effluents1 using a heated 50-cm fused-silica interface and either the positive or negative ion detection modes. 14 Channeltron electron multiplier, Model 4770 (Galileo ElectroOptics Corp.), wai3 modified to allow negative ion detection, by grounding the entrmce plate, and biasing the ion beam deflector so that it operated as a conversion dynode. The construction and operation of the computer interface and software package have been reported in detail elsewhere (25). Mass spectra were collected in the range from nz/z 110 to m/z 310 at a rate of 75 scans/min for the duration of the chromatographic run. Methane reagent gas was maintained at a source pressure of 0.5 torr. Gas chromatography/electron impact mass spectrometry d a h were obtained with either a Finnigan Model OWA-20/30B GC/MS system or ;a HewlettPackard Model 5985 GC/MS system. Both gas chromatographic systems were equipped with splitless injectors and fused-silica linterfaces and used 30-m fused-silica columns coated with SE-54. A Varian Model 5000 liquid chromatograph, with a 4.6 mm >( 25 cm Excalibar stainlesEi steel column, packed with Spherisorb-S5W 5-pm silica (Applied Science, Inc.), preceded by n 4 mm X 5 cm guard column, packed with HC Pellosil (Whatman, Inc.), was used for the liquid chromatographic separations. All liquid chromatographic mobile phases were "distilled in glass" (Burdett and Jackson). The gradient program was initiated from the microproceasor following injiection of 25-pL aliquota of the sample. Solvent flow rates were maintained at 1.0 mL/min. An initial mobile phase of 100% n-hiexane was held for 10 min, followed by a linear programmed introduction of dichloromethane over the next 10 min. The mobile phase was held for 10 min at 100% dichloromethane. Acetonitrile was then introduced over the next 5 min and held at 1 0 % for the remaining 5 min of the separation run. This 40-min iaolvent program was followed by a reverse gradient to 100% n-hexane, which was held for 30 min in order to condition the column before the next sample. Sample effluenta were monitored with a 254-nm, fixed-wavelength UV absorption detector, and the signal was recorded on a strip-chart recorder Sixteen fractions were collected manually and allowed to evaporate overnight at room temperiiture in the dark. Either positive or negative ion, chemical ionization mass spectrometry was w3ed to identify the collected fractions, using a temperature-programmaldeplatinum-wire probe. This probe was developed for the rapid evolution of involatile and thermally labile samples and will be described elsewhere (26). The evaporated liquid chromatographic fractions were dissolved in 20-pL of dichloromethano, and a fj-pL aliquot was placed on the probe filament. The sample was allowed to air-dry for 2 min before inserting the probe into the m w spectrometer inlet. Seventy-five mass spectra were collected from m/z 110 to m/z 310 over a period of 1min. The probe filament was maintained at a low voltage for the f i t 10 scans in order to minimize evolution of the sample before the probe was seated in the source block. The temperature of the probe was then h e a d y increased from the source block temperature (150 "C)to approximately 600"C. Thii ramp caused the highly polar and involatile components of the samples to be vaporized rapidly, and intact molecular ions were observed for most solutes. Methane (0.5 torr) was used as the reagent gas for these experiments,
RESULTS AND DISCUSSION The soluble organic fraction from four air oxidant and four argon/oxygen oxidmt collection runs were analyzed in these
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10
20
30
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60
50
80
90
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RETEhTION T;IECY;N>
Flgure 1. Capillary gas chromatogram of air oxidant sample using
flame ionlzatlon detection. Peaks correspond to Table I.
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Flgure 2. Capillary gas chromatogram of argon/oxygen/carbon dioxide oxidant sample using flame ionization detection. Peaks correspond to Table I.
experiments. The gas chromatographic results, which wene found to be the most sensitive means of fingerprinting thle samples, demonstrated that there were excellent intrasamplle repeatability (correlation >0.95) and intersample similarity for the two sets of samples. These results show the reproducibility of the engine and fuel system used in this study. Gas chromatograplhic retention data for the air oxidant samples, based upon flame ionization detection results (Figure 11,and chromatographic data for standard compounds generated under the same conditions were extremely useful for differentiating between various isomers of the polycyclic aromatic compounds, Retention indexes presented in Table I were calculated by using the system developed by Lee and co-workers (27). Peak areas were integrated after computer-assisted base line subtraction had been performed. Average emissions presented in Table I in terms of pg/g particle and mg/kg fuel were calculated based upon a response factor for multiple injection of a standard solution of phenanthrene, calculated particle emission rates, and fuel consumption rates Some of the minor peaks, which were not observed by gao chromatography electron impact mass spectrometry, could not be identified on the basis of their retention indexes and their chemical ionization mass spectra. However, for completeness these compounds have been included in Table I with their nominal molecular ions. The results for gas chromatographic separation of the argon/oxygen oxidant samples, using flame ionization detection, are presented in Figure 2. The striking difference between these samples and the air oxidant samples is the presence of relatively broad peaks throughout the chromatograms which were obtained for the extracts from the argon/oxygen oxidant. These peaks are attributed to an increased level of lubricating oil in the combustion chamber, which was a result of the start-up procedure for the modified oxidant experiments. However, the major peaks in the argon/oxygen samples were the polycyclic aromatic compounds. These identifications were subsequently confirmed by gas chromatography/chemical
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
Table I. Chemical Characterization of Organic Adsorbates on Diesel Particulate Matter
ref no 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
35 36 37 38 39 40 41 42 43 44 45
compd name unknown naphthalene benzofuran, 7-methylinden-1-one, 2,3-dihydromethylnaphthalene methylnaphthalene phthalate anhydride unknown unknown biphenyl n-tetradecane unknown unknown 1-benzopyran-2-one biphenylene, or acenaphthylene acenaphthene unknown dibenzofuran unknown fluorene unknown 9-fluorenone phenanthrene anthracene methyl-9-fluorenone unknown methyl-9-fluorenone unknown methyl-9-fluorenone benzo[c]cinnoline fluorene quinone phenanthrene quinone cyclopentaphenanthrene-5-one naphtho[l,8-cd]pyran-1,3-dione-, or fluoranthrene pyrene methylpyrene methylpyrene methylpyrene benzo [ghilfluoranthene cyclopenteno [ cd Ipyrene chrysene benzofluoranthene benzofluoranthene benzo [ghilperylene nitrop yrene
retention indexes M, std dev
av emissions wglg particle
mg/kg fuel
198.0, 0.44 200.0, 0.10 204.3, 0.37 212.3, 0.19 214.0, 0.32 216.6, 0.51 217.9, 0.45 222.0, 0.56 224.8, 0.75 227.7, 0.76 230.1, 0.64 232.1, 0.87 235.8, 0.84 238.7, 0.91 239.9, 0.18 248.4, 0.93 250.0, 0.94 251.4, 0.97 253.4, 0.99 266.6, 0.55 271.8, 0.56 292.6, 0.21 300.0, 0.11 301.6, 0.19 306.7, 0.16 308.6, 0.98 309.7, 0.24 311.0, 0.26 312.8, 0.10 319.8, 0.54 327.3, 0.33 330.3, 0.61 341.7, 0.82 343.7, 0.69
187.22 329.10 69.49 22.41 9.89 11.12 42.39 12.66 21.76 76.38 596.78 83.04 47.71 42.77 30.29 16.97 27.70 90.03 20.20 21.46 39.30 232.03 582.44
1.361 2.394 0.505 0.163 0.007 0.081 0.308 0.092 0.158 0.555 4.340 0.604 0.347 0.311 0.220 0.123 0.201 0.655 0.147 0.156 0.286 1.688 4.236
29.58 21.08 21.29 15.13 20.35 209.31 35.18 181.74 501.67 345.57
0.215 0.153 0.155 0.110 0.148 1.552 0.256 1.322 3.649 2.513
351.4, 0.97 365.9, 0.21 372.9, 0.25 375.8, 0.92 389.4, 1.13 394.6, 0.17 400.3, 1.80 436.1, 3.55 443.4, 0.25
309.75 8.82 6.04 22.21 64.08 74.32 35.75 16.75 19.09
2.253 0.064 0.044 0.162 0.466 0.541 0.260 0.122 0.138
method of ID 123-PCI d b, d b, d a, b, d a, b,
a, b
b, c
147-PCI 1 35-pc1 a, b, d a, b, d 147-PCI 133-PCI b, c, d a, b, d a, b
197-PCI d 157-PCI
a, b, a, b
171-PCI, 170-NCI c, d, e d, e
a, b, a, b, a, b C
198-NCI C
198-NCI C
a, b,
d, e
b, e b, c, e b, c, e a, b, e
d, e e e b, e b, d, e b, e b, e b, e b, e
a, b, a, b, a, b,
a, a, a, a,
a, a,
e e
a Capillary gas chromatographic retention index. Gas chromatography/positive chemical ionization mass spectrometry. Gas chromatography/negative chemical ionization mass spectrometry. Gas chromatography/electron impact mass spectrometry. e High-performance liquid chromatography-temperature-programmed chemical ionization mass spectrometry. Unknowns are identified by molecular ion observed by either positive (PCI) or negative (NCI) chemical ionization mass spectrometry.
ionization mass spectrometry. When the flame ionization detector was replaced by a thermionic specific detector, which is a nitrogen-selective detector, and the same samples were analyzed, a lower number of nitrogen-containing compounds were observed for the argon/oxygen samples as compared to the air oxidant samples. This result is consistent with the reduction of the oxides of nitrogen in the exhaust, from an average of 470 ppm for the air oxidant to less than 1ppm for the argon/oxygen oxidant system. This indicates that the latter oxidant system was successful in removing nitrogen from the combustion chamber. The low levels of nitrogen-containing compounds in all samples, in relation to the major components of the soluble organic fraction, precluded any positive identifications based on mass spectrometry. Positive chemical ionization mass spectrometric total ion chromatograms showed a good correlation with the flame ionization results and allowed the assignment of molecular
weights for the many of the solutes. With the exception of n-tetradecane, which showed a typical aliphatic hydrocarbon base peak at (M - l)+,each peak exhibited a mass spectral base peak at (M 1)+ due to proton transfer. Extracted ion profiling was used to deconvolute many of the minor peaks from the total ion chromatogram. Negative chemical ionization mass spectrometry was especially useful as a selective tool for substantiating the presence of oxygenated derivatives of the polycyclic aromatic compounds, since the major nonoxygenated constituents of the samples did not produce a signal. Base peaks in the mass spectra were observed at M-, corresponding to resonance electron capture by the compounds containing the electronegative oxygen substituents. Electron-impact mass spectra were used to confirm many of the identifications using library searches which were based on the National Bureau of Standards library of 25 000 mass spectra.
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of polycyclic aromatic compounds which are contained in tihe diesel fuel persisting unchanged during combustion. In addition, the similarities between the compounds identified in this study and other investigations, where fuels included aromatic compounds, indicates that the polycyclic aromatic compounds must be formed from similar intermediates. Future research concerning the health effects of an increased use of diesel engines should employ a similar model fuel system and analytical methodology. The reactions of oxides of nitrogen with the polycyclic aromatic compounds are presently being studied in the continuation of this research.
ACKNOWLEDGMENT 1 4 r - I
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Figure 3. H w o r m a n c e liquld chromatogram of alr oxldant sample.
Liquid chromatographic separations of the same samples provided unique information, which was useful for the identifications, and Figure 3 shows a typical chromatogram. Fluoranthene, cycclopenitaphenanthrene-5-one,and naptho[1,8-c,d]pyran-19&dioneeluted in fractions 3, 8, and 14, respectively, while ithe gas chromatographic separation of thle same compoundn yielded retention indexes which differed only by a single unit. The temperature programmable solids probe developed in this study proved to be extremely useful for characterizing the involatile components of the soluble organic fraction. Many higher molecular weight polycyclic aromatic compounh, including isomers of ben zofluoranthene and benzoperylene, were identified by liquid chromatographic separation, followed by direct-probe chemical ionization mass spectrometry. These higher molecular weight compounds were not eluted from the gas chromatograplhic column. Nitropyrene was isolated and identified in fraction 6 of eaclh of the air oxidant samples and one of the argon/oxygen sariples, using the temperature-programmableprobe coupled witih negative chemical ionization mass spectrometric detection. Concentrations were too low to quantify this compound in these experiments. The !presence of this compound in one of the argon/oxygm samples is explained by the introductioin of nitrogen into the combustion chamber, as a reriult of cracking a piston ring during the experimental run. With this exception, the presence of nitropyrene only in the air oxidant samples indicates that its formation must occur as a secondary process and is not dependent upon fuel-bound or lubricantbound nitrogen. The major compounds identified in the soluble organic fraction of diesel particulate matter, generated from a twocomponent, aliphatic hydrocarbon fuel, are polycyclic aromatic compounds. The compounds identified are quite similar to those observed by invesitigators who employed full-boiling range diesel fuels (10-12). These results indicate that the fuel used in this study is a good model of a full-boiling range fuel. The findings also imply that polycyclic aromatic compounds found on diesel particles are inherent products of the diffusion-controlled combustion process and are not only the result,
We thank S. R. Prescott and V. Laverty for allowing us to use their gas chromatograph electron impact mass spectroroeter systems. Also, J. D. Herr is acknowledged for obtaining the diesel particulate samples.
LITERATURE CITED (1) Sprlnger, K. J.; Baines. T. M. Society of Automobile Engineers Paper No. 770818, MECCA, Milwaukee, WI, Sept 12, 1977. (2) . . Hatch. T. F.: Gross. P. "Pulmonarv Deoositlon and Retentlon of Inhaled .Aerosols"; At:ademlc Press: ' New York, 1964. (3) Task Group on Lung Dynamics Health Phys. 1966, 12, 173-2013, (4) Ross, M. M. Ph.D. Thesis, The Pennsyivania State Universlty. Unlverslty Park, PA, 1981. (5) Ross, M. M.; Risby, T. H.; Lestz, S. S.;Yasbin. R. E. Environ. Sci. Techno/.. In Dress. (6) McCann,'J.; im&, 6. N.; Chol, E.; Yanasaki, E., R o c . Natl. Acarl. Scl. U . S . A . 1975. 72. 5135-5139. (7) Dukovich, M.; YasbinrR. E.; Lestz, S. S.;Risby, T. H.; Zweidinger, IR. E. Envlron. Mutagen. 1981,3 , 253-284. (6) Kotin, P.; Falk, H. I..; Thomas, M. Arch. Environ. Health 1955, 1 1 ,
113-120. (9) Epstein, S.; Joshl, Si.; Andrea, J.; Mantel, N.; Sawicki, E.; Stanley, T.; Tabor, E. Nature (London) 1966,212, 1305-1307. (10) Funkenbusch, E. F.; Leddy. D. G.; Johnson, J. H. Society of Automobile Englneers Paper No. 79418, MECCA, Milwaukee, WI, Sept 12, 1977. (11) Schuetzle, D.; Lee, F. S.;Prater, T. J.; Tejeda, S . B. Int. J . Envlrori. Anal. Chem. 1981,9 . 93-144. Badger, 0. M.; Kimber, R. W. L.; Spotswood, T. M. Nature (London) 1960, 187, 663. Badger, 0. M. Natl. Cancer Inst. Monogr. 1962,No. 9 , 1 . Ray, S. K.; Long, R., Cornbust. Flame 1964,8 , 139. Thomas, A,, Combust. Flsme 1962,6 , 46. Porter, 0. Symp. ( h t.) Combust. Proc ., 4th 1952,538. LaHaye, J.; Prado, G. Water, Alr, Soil Pollut. 1974,3 , 473. . . Schuetzle. D.: Rilev. T.: Prater.. T. J.:. Harvev. ~. T. M.:, Hunt.. D. F. Anal. Chem. 1982,.54,-265.' (19) Pederson, T. C.; Slak, J., Second Chemlcai Congress of the Norlth Amerlcan Continent,, San Franclsco, CA, Aug 24-29, 1980, Abstract No. ENV. 188. (20) Yergey, J. A.; Lestz, S.S.;Risby, T. H.; Yasbin, R. E.; Prescott, S. R.; Dukovich, M.; Ryan, T. Technical Paper, CSSICI-79-08, Centrial States Section Combustion Institute, Columbus, IN, 1979. (21) Herr, J. A., M.S. Thnsls, The Pennsylvania State University, UniversHy Park, PA, 1981. (22) Yasbin, R. E. Mol. Cbn. Genet. 1977, 153, 211-218. (23) Dukovich. M. M.S. Thesis, The Pennsylvania State Unlverslty, Universlty Park, PA, 1981. (24) Yergey, J. A. Ph.D. Thesis, The Pennsylvanla State University, University Park, PA, 1981. (25) Campana, J. E.; RIsby. T. H.;Jurs, P. C., Anal. Chim. Acta 197I1, 112, 321-340. (26) Yergey, J. A.; Rlsby, T. H., unpublished results. (27) Lee, M. L.; Vassilaros, D. L.; White, C. M.; Novotny, M. Anal. Chem. 1979,51, 768-773.
RECEIVED for review October 16,1981. Accepted December 8,1981. This work was supported by a grant from the U.S. Environmental Protection Agency (R-806558) to T.H.R.
