Anal. Chem. 1995, 67, 3541 -3549
Supercritical Fluid Extraction=Gas Chromatographic Analysis of Organic Compounds in Atmospheric Aerosols Kristen J. Hansen,tl* Brian N. Hansen,*ls Eric Cravens,ti* and Robert E. Sievers*its* Global Change and Environmental Quality Program, Cooperative Institute for Research in Environmental Science, Department of Chemistty and Biochemistry, University of Colorado, Boulder, Colorado 80309-0216
An integrated sampling and supercritical fluid extraction (SFE) cell has been designed for whole-sample analysis of organic compounds on tropospheric aerosol particles. The low-volume, polymer-free, temperature-controlled extraction cell has been interfaced with a sampling manifold for aerosol collection in the field. After sample collection, the entire SFE cell was removed, transferred, and coupled to a gas chromatograph; after on-line extraction, the cryogenically focused sample was separated and the volatile compounds were detected with either a mass spectrometer or a flame ionization detector. A 20-min extraction at 450 atm and 90 "C with pure supercritical COz is sufticientfor quantitative extractionof most volatile compounds in aerosol samples. A comparison between SFE and thermal desorption, the traditional whole-sample technique for analyses of this type, was performed using ambient aerosol samples, as well as samples containing known amounts of standard analytes. Organic compounds are emitted into the atmosphere as products of many biogenic and anthropogenic processes. A significant fraction of this organic material ends up as inhalable particulate matter, or aerosols, as a result of physical condensation processes, emission processes, or photochemical Determining the organic composition of tropospheric aerosols and monitoring how this composition changes on a time scale of meteorological changes is fundamental to understanding many chemical and physical processes in the atmosphere. In the past, studies designed to determine the role of organic aerosols in the atmosphere have utilized liquid solvent extraction or thermal desorption to separate the organic component from the aerosol for analysis. Liquid extractionstypically require large volumes of solvent (100-500 mL) and extended extraction times (8-48 h); these characteristics make the technique inconvenient for analyzing a large number of samples. Liquid extraction results in an aliquot of sample extract (-40 yL), only a fraction of which, typically 1 yL, can be utilized in a single gas chromatographic Due to this inherent dilution factor, liquid extraction Global Change and Environmental Quality Program. Department of Chemistry and Biochemistry. 5 Permanent address: Department of Chemistry, National Institute of Standards and Technology, Boulder, CO 80303. (1) Rogge, W.; Hildemann, L M.; Mazurek, M. A; Cass, G. R Enuiron. Sci. Technol. 1 9 9 4 , 2 8 , 1375-1388. (2) Yokouchi, Y.; Ambe, Y. Atmos. Enuiron. 1 9 8 5 , 19, 1271-1276. (3) Grosjean, D.; Seinfeld, J. H. A h o s . Enuiron. 1 9 8 9 , 23, 1733-1747. +
0003-2700/95/0367-3541$9.00/0 8 1995 American Chemical Society
techniques require 24-72 h sample collection periods4 Because the composition of an air mass can change on a time scale of minutes to hours, it has been difticult to elucidate chemical and physical relationships in aerosol composition using analysis techniques based on liquid extraction with long sample collection time^.^-^ Thermal desorption has been used to monitor short-term changes in the organic content of aerosols. In this method, whole aerosol samples are heated for a short period of time at 250-300 "C; the entire volatile component of the aerosol is swept into a cryogenically cooled gas chromatograph. Short sampling times result from the increased sensitivity of the wholesample desorp tion technique.8-10 However, thermal desorption is known to create some analysis artifacts; the high desorption temperatures required by the technique can cause some unstable organic compounds to polymerize, char, or pyrolyze.11J2 In addition, thermal desorption may not adequately remove extremely lowvolatility, high molecular weight compounds from the aerosol matrix. Supercritical fluid extraction (SFE), a wholesample analysis technique, provides a low-temperature alternative to liquid solvent extraction and thermal desorption. A supercritical fluid, often characterized as a dense gas, has a mass transfer coefficient similar to that of a liquid; for this reason, SFE efficiencies for many analytes are comparable with those achieved using liquid extraction.13 In addition, analytes in a supercriticalfluid have diffusivities characteristic of those in a gas; as a result, the time required for SFE is on the order of that required for thermal desorption (minutes).l4 The critical parameters of carbon dioxide, the solvent (4) Stephanou, E., G. Atmos. Enuiron. 1 9 9 2 , 26A, 2821-2829. (5) Parrish, D. D.; Hahn, C. H.; Fahey. D. W.; Williams, E. J.; Bollinger, M. J.; Hubler, G.; Buhr, M. P.; Murphy, P. C.; Trainer, M.; Hsie, E. Y.; Liu, S. C.; Fehsenfeld, F. C. J. Geophys. Res. 1 9 9 0 , 95, 1817-1836. (6) Fehsenfeld, F. C.; Bollinger, M. J.; Liu, S. C.; Parrish, D. D.; McFarland, M.; Trainer, M.; Kley, D.; Murphy, P. C.; Albritton, D. L. J. Atmos. Chem. 1 9 8 3 , I, 87-105. (7) Jacob, D. J.; Wofsy, S. C.1. Geophys. Res. 1988, 93, 1477-1486. (8) Greaves, R C.; Barkley, R M.; Severs, R E. Anal. Chem. 1985,57,28072815. (9) Greaves, R C.; Barkley, R M.; Severs, R E.; Meglen, R R Atmos. Enuiron. 1 9 8 7 , 21, 2549-2561. (10) Veltkamp, P. R; Hansen, K.; Barkley, R M.; Severs, R E.J Geophys. Res., in press. (11) Cachier, H.; Bremond, M.-P.; Buat-Menard, P. Aerosol Sci. Technol. 1 9 8 9 , 10,358-364. (12) Liousse, C.; Cachier, H.; Jennings, S. G. A h o s . Enuiron. 1993,27A, 12031211. (13) Hawthorne, S. B.; Miller, D. J. 1.Chromufogr. 1 9 8 6 , 24, 258-264. (14) bhleit, M.; Bachmann, K. J. Chmmafogr. 1 9 9 0 , 505, 227-235.
Analytical Chemistry, Vol. 67, No. 19, October 1, 1995 3541
used in this work, are 78 atm and 31 "C.I5 As SFE does not require high temperatures, thermally labile compounds are less likely to be pyrolyzed during SFE than they are during thermal desorption analysis. A method and apparatus for removing organic compounds from tropospheric aerosols using supercritical fluid extraction is described here. In this method low-volume air sampling is used, followed by SFE of the aerosol with pure COz with the solution formed being transferred directly into a cryogenically cooled gas chromatographic column. The C 0 2vaporizes as the pressure is lowered, leaving the analytes focused at the head of the capillary column. Aerosols are collected by passing 200-2000 L of air through a quartz-fiber filter positioned on a support screen. Following collection, the sample and support are placed in the SFE cell. The SFE cell has a low volume and contains no polymer seals; both of these characteristics minimize the possibility of system contamination. The SFE cell is designed to be easily adapted to a sampling manifold for aerosol collection in the field. During extraction, the SFE cell is interfaced with a cryogenically cooled GC column through the oncolumn injection port The sample extraction, carried out at 450 atm and 90 "C,is completed in 20 min. After extraction, the GC oven is temperatureprogrammed and organic compounds are separated and detected with either a mass spectrometer (MS) or with a flame ionization detector (FID). Previous studies on the effectivenessof SFE have discussed extraction efficiencies for a variety of compounds; this work provides an apparatus and a method optimized specifically for routine whole-sample analysis of ambient aero~ols.'~J~ The purpose of this study was to determine whether SFE is a viable alternative to liquid extraction and thermal desorption for removing organic compounds from tropospheric aerosols for subsequent separation and detection. Standard solutions of some representative compounds found on atmospheric aerosols were prepared and spiked onto quartz-fiber filters. Recovery of these compounds using SFE and thermal desorption was compared. Chromatograms obtained by SFE and by thermal desorption of parallel aerosol samples collected in an urban environment are presented. EXPERIMENTAL SECTION
Chemicals and Inslmmentation. All solvents were purchased from Mallinckrodt SpecialtyChemical Co. (Paris, KY) and were used as received. The ncarboxylic acids and the 1-alcohols were obtained from Sigma Chemical Co. (St Louis, MO); guaiacol, nicotine, and vanillin were purchased from Aldrich Chemical Co. (Milwaukee. WI). Carbon dioxide, research gradeFID certified, was purchased from Scott Specialty Gases (Plumsteadville. PA). Quartz-fiberfilters used for spike recovery studies and collection of ambient samples were from Pallflex Products Corp. (put", CT). Pyrex 5mL weighing vials were used for holding the SFE sample support containing the sample after aerosol particle collection, while Pyrex screwcap culture tubes were used for holding the thermal desorption sampling tubes with aerosol samples. Both types of samples were desiccated over Drierite anhydrous calcium sulfate from W. A Hammond Drierite Co. (Xenia, OH). The thermal desorption tubes were constructed from 40-6Cbp fritted straight sealing tubes, 202 mm x 14 mm, (15) Vannoon. R W.; Chewel. J. P.; Lingeman. G. L.; DeJong, G. I.;Brinkman. U. A. J. Chromotogr 1990,505.45-77. (16) Hawthome. S. B.: Miller. D.J. Anal. Chrm. 1987. 59. 1705-1708.
3542 Analflical
Chemistry, Vol. 67, No.
19, October 1, 1995
1116" o.d
rmin1ens steel tranSrerilnc : a r h n dioxide
IN
25 pm fused silica resfkclor-carbon dimidc
0111
&
GC ""-column injecliw po"
Supercritical fluid extraction cell for SFE-GC analysis of tropospheric aerosols. Parts machined specifically for the cell are marked with an asterisk. All others are commercially available. Figure 1.
from VWR Scientific (Brisbane. CA). Further details concerning construction of thermal desorption tubes are available elsewhere? Parts specially designed and constructed for the SFE cell are indicated with an asterisk in Figure 1. The extraction cell was machined from 316 stainless steel; the sample support (external diameter 2 cm; internal diameter 1.4 cm; height 0.9 cm) was constructed from brass. The high-pressure seal between sample support and extraction cell is achieved when the stainless steel cell is tightened firmly against the more malleable copper or brass sample support. The screen in the sample support is stainless steel. The internal volume of the extraction cell, measured with COz at 100 atm, is 1.95 mL. The SFE cell restrictor is -7 cm long, with an internal diameter of 25 pm; this internal diameter provides good chromatographic peak shape while maintaining acceptable extraction flow rates.)' The outside diameter of the restrictor is 150pm, small enough to easily pass through the oncolumn injection port on the gas chromatograph into the capillary GC column. The fused silica restrictor was purchased from Polymicro Technologies Inc. (Phoenix, AZ). A high-pressure syringe pump, manufactured by Isco (lincoln, NE) was used for pressurizing and delivering COz. Gas chromatography with flame ionization detection was accomplished with a Hewlett-Packard Model 5890 gas chromato graph Wilmington. DE). Separation of aerosols and standards was achieved using a 25 m x 0.32 mm internal diameter column packed with 5% phenylmethylsiloxane (phase thickness 0.52 pm) from Hewlett-Packard. A 5 m x 0.32 mm internal diameter, uncoated, deactivated fused-silica capillary (Hewlett-Packard) preceded the analytical column as a retention gap (Hewlett(17) Hawthome. S. B.: Miller. D.
J.J.
Chmmofop. 1987. 403. a-76.
Packard) and was joined to the analytical column by a HewlettPackard Chromfit glass connector. The oncolumn injection port was unheated during SFE; for thermal desorption analyses, the desorption port was thermostated at 250 "C. The detector temperature was 285" C for both analysis methods. Identification of compounds in ambient samples was carried out with a Model 589OA GUMS from Hewlett-Packard. The same capillary column was used in GC/MS analysis as that used in GC/FID. The GC/ MS analyses were performed by electron impact ionization with an electron energy of 70 eV. Both chromatographic systems used helium as the carrier gas; the linear flow velocity in the GC/FID system was 1.45 mL/min at 80 "C and in the GC/MS system was 2.41 mL/min at 80 "C, as measured by the retention time of an unretained compound (butane). Ambient aerosol samples were collected using a manifold sampling system designed to ensure isokinetic sample flows. Flow into the manifold was maintained with a brushless blower from Ametek (New York, MI). Aerosols were sampled under isokinetic conditions from the manifold using an oil-free pump manufactured by GAST Manufacturing (Benton Harbor, MI). Flow rates through matched, duplicate samples were adjusted and monitored with a flowmeter and flow controller from Omega Technologies Co. (Stamford, CT). Procedure for Spike Recovery Studies by SFE and Thermal Desorption. Aliquots (1 pL) of standard solutions were injected onto a clean quartafiber filter positioned in the thermal desorption tube or in the SFE extraction cell. The solutions were desorbed or extracted immediately after spiking under conditions identical to those used in ambient aerosol analysis. Compound responses were compared to those from 1-pLoncolumn injections of each standard. Procedure for AmbientAerosol Collection. Aerosol samples were collected at a sampling site maintained by the Colorado Department of Health on the University of Colorado campus in Boulder. Simultaneous aerosol samples were collected from within a manifold under isokinetic sampling conditions. Sample supports for SFE and thermal desorption sample collection tubes were each connected to the manifold by a 5-cm section of l/&.0.d. stainless steel tubing. The thermal desorption tube was connected to the tubing with I/&. Teflon ferrules and fittings while the SFE sampling system was connected with I/&. stainless steel fittings. Samples for both SFE and thermal desorption were positioned vertically withii the manifold and collected at flow rates of 5-8 Wmin. To collect duplicate samples, the sample collection flow rate through each sample filter was matched with a mass flow controller and a series of needle and shutoff valves (see F i r e 2). Once collected, the samples were removed from the manifold, sealed in aluminum foil-wrapped glass containers, and stored in a desiccator at 3 "C until analysis. The samples were analyzed within 48 h of collection to m i n i i e potential sample degradation. Field blanks were collected by this same method, except after flow matching, the shutoff valve to the blank thermal desorption tube or the blank SFE sample support was closed. The SFE sample support and thermal desorption tubes were handled with tweezers or cotton gloves to minimize the possibility of contamination. Procedure for SFE of Aerosols. The sample collection support was removed from the glass storage container with tweezers and placed into the SFE cell. The SFE cell was sealed and then connected via l/l&.-o.d. stainless steel transfer line to
oil-free pump
Mass flow controller
114"Tdlon tubing
SFE sample support
+--
Shut afi valve
1
Needle valve
Thermal desorption sampling tube
Figure 2. (A) and (B), duplicate sampling procedure detailed in text. To collect duplicate samples, the sample collection flow rate through each sample filter was matched using a mass flow controller and a series of needle and shutoff valves, depicted above. Flow through one of the sample collection lines, marked A, was interrupted by closing valve 1. Total flow through the remaining sample, 6, was recorded on the digital readout of the mass flowmeter. Subsequently, sample line B was closed (valve 2 shut) and sample line A opened. Flow through line A was matched to line 6 by adjusting a needle valve (valve 3). Flow through each sample line was monitored and adjusted, as necessary, with the corresponding needle valve. When both flow rates were identical, the sample lines were opened to begin aerosol collection. Flow rates through each line was checked at the end of the sampling period to ensure that no significant drift occurred during collection.
the high-pressuresyringe pump. Immediately prior to extraction, the SFE cell was placed in the heating block and the exit restrictor threaded through the oncolumn injection port -6 cm into the retention gap (refer to F i r e 1). The SFE cell was then pressurized, and the aerosol sample was extracted for 20 min by passing the dissolved solutes through the pressure restridor directly into the cryogenically cooled GC column. The gas chromatograph oven was cooled to -30 "C to sharply focus extracted compounds onto a narrow band at the head of the column as the COZvaporized at the reduced pressure of the restrictor outlet. Others have determined that this temperature is sufficient for trapping even low molecular weight compounds such as a-pinene and camphene.18 In addition, the use of the low temperature prolongs column lifetime as interaction between the stationary phase in the column and the Cot is minimized. Although the FID is extinguished by the COZalmost immediately, reignition is possible moments after removing the restrictor from the injection port after extraction. A 5-m retention gap precedes the analytical column in order to trap nonvolatile compounds that may be extractable but not volatile enough to be eluted. In addition, the retention gap protects the analytical column from the COZwhich is violently expelled from the end of the restrictor. Incorporation of uncoated column to serve as a retention gap is the only instrumental modification to the GC required for the SFEGC/FID analysis of aerosols. The GC was held at -30 "C for 3-5 min after extraction to ensure that all of the COZhad been flushed from the column. (18) Hawthorne, S. B.; Krieger, M. S.; Miller, D. J. Anal. Chem. 1988,60,472477.
Analytical Chemistty, Vol. 67, No. 19,October 7, 1995
3543
After extraction, the GC was temperature-programmed from -30 to 60 "C at 30 "C/min; the temperature program was continued at 4 "C/min to 300 "C. The column remained at 300 "C for 20 min. In order to ensure that the SFE cell stayed clean, after aerosol extraction the quartz-fiber filter was removed from the SFE sample support. The SFE cell, with an empty support screen, was extracted for 20 min at 475 atm and 90 "C. RESULTS AND DISCUSSION The results of this study have shown that SFE of atmospheric aerosols allows quantitative measurement of compounds from several classes. Furthermore, information that is complementary to that gained by the thermal desorption technique can be obtained by SFE. These conclusions are based upon the following comparative analyses of spiked samples and of ambient aerosol Figure 3. First SFE-GCIFID of an atmospheric aerosol sample (a, particle samples between thermal desorption and SFE. The top), followed by a second extraction of the same sample (b, bottom). results also indicate that SFE with COZcan be validated as an Both 20-min extractions were carried out at 450 atm of pure CO2 at 90 "C; the vertical scale is the same in both chromatograms. alternativeto thermal desorption for quantitation of several organic Chromatographic conditions are reported in the text. compounds. A variety of anthropogenic and biogenic processes contribute particulate phase organic compounds to the lower atmosphere. detection, a characteristic that makes SFE ideal for whole-sample Gasoline and diesel-powered vehicles, residential wood burning, measurement of trace organic species.I7 Although a variety of surface abrasion of plant leaves, meatcooking operations, natural compounds and mixtures of compounds, for example, COZ,C02/ gas appliances, and cigarette smoke are some sources that methanol, N20, and ethane, make acceptable supercritical solcontribute to the organic content of ambient aerosol ~ a m p l e s . ~ ~ - ~ l vents, pure COZwas used in this work, for safety and simplicity. Each of these sources emits a complex mixture of organic Pure C02 has easily achievable supercritical parameters (78 atm, compounds; as a result, ambient aerosols typically contain several 31 'C); it is inexpensive, environmentally sound, and relatively hundred compounds. Some classes of compounds found in n o n r e a c t i ~ e . ~ ~SFE b ~ ~can * ~usually ~ also be effected much more aerosol samples include alkanes, carboxylic acids, alkanols, rapidly than solvent extraction using organic liquid solvents polycyclic aromatic hydrocarbons (€'AH$, monoterpenes, furans, (minutes vs hours). phenols, and alkylated benzenes, as well as p e s t i c i d e ~ . ~It* is ~,~~ Optimal extraction conditions were determined from literature the complex nature of the organic composition of the aerosols reviews of extraction efficiencies, results of standard extraction which complicates wholesample analysis. recoveries, and qualitative tests performed on aerosol samples. The 250-300 "C temperature range utilized in thermal desorp The solvating strength of a supercritical fluid is related to its tion analysis is optimized to maximize volatility and minimize density, a parameter primarily dependent upon pressure.18~28 A pyrolysis of organic compounds?J Nonetheless, at these temliterature review of extraction efficiencies from a variety of peratures, not all compounds are fully volatilized and some matrices indicates that the recoveries for some compounds, such compounds may be pyrolyzed.11J2 Other references contain as PAH, increase as extraction pressure increases up to 450 details about the optimization and full characterization of the atm.24~25~283 Extraction efficiency also increases with temperathermal desorption technique.8J1 ture;25328 however, temperatures in this study were kept below 90 Supercritical fluid extraction provides an alternative means of "C to minimize degradation of thermally labile compounds. This removing organic compounds from aerosol particles. SFE has temperature is well above the critical temperature of COZ(31 "C). been used to remove organic compounds from a variety of Figure 3a shows a chromatogram of an aerosol sample extracted environmental matrices, some with near 100%efficiency with according to the conditions described above. A second extraction respect to liquid solvent In some instances, of the same sample (Figure 3b) shows that these extraction SFE extracts more of some compounds from NIST standard conditions provide nearly quantitative removal of organic comreference materials than organic solvents can by Soxhlet extraction pounds from the aerosol sample. techniques requiring more time.24-26SFE is easily coupled to a Spike Recovery Studies of Standards by SFE and Thermal GC equipped with either mass spectrometric or flame ionization Desorption. A comparison between SFE and thermal desorption (19)Simoneit, B.R T.; Mazurek, M. A CRC Cn't. Rev. Enuiron. Control 1981, 11, 219-276. (20) Simoneit, B. R T. Ah" Enuiron. 1984, 18, 51-67. (21)Hildemann, L.M.; Mazurek, M. A; Cass, G. R Environ. Sci. Technol. 1991, 25,1311-1325. (22) Rogge, W.F.; Hildemann, L.M.; Mazurek, M. A; Cass, G. R; Simoneit, B. R. T. Enuiron. Sci. Technol. 1991, 25, 1112-1125. (23)Hawthome. S. B.; Miller, D. J.; Krieger, M. S. J, High Resol. Chromatog. 1989, 12,714-720. (24)Hawthome. S. B.;Miller, D. J. Anal. Chem. 1994, 66,4005-4012. (25)Burford, M. D.; Hawthome, S. B.; Miller, D. J. Anal. Chem. 1993,65,14971505. (26) Langenfeld, J. J.; Hawthome, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1993, 65,338-344. 3544 Analytical Chemistty, Vol. 67, No. 19, October 1, 1995
was conducted using standard solutions of compounds previously shown to be present on atmospheric aerosol^.^-^^ The recoveries (27) Hawthome, S. B.; Krieger, M. S.; Miller, D. J. Anal. Chem. 1989,61,736-
740. (28) Levy, J. M.; Dolata, L. A; Ravey, R M. J. Chromatogr. Sci. 1993,31,349-
352. (29)Rein, J.; Cork, C. M.; Furton, IC G.Jountal ofChromatographic Science 1991, 545, 149-160. (30)Graedel, T. E.;Hawkins, D. T.; Claxton, L. D. Atmospheric Chemical Compounds: Sources, Occurence, and Bioassay; Academic Press: Orlando, FL, 1986. (31) Rogge, W.F.; Hildemann, L. M.; Mazurek, M. A,; Cass, G. R Enuiron. Sci. Technol. 1993, 27,636-651.
processes such as wood burning, petroleum combustion in vehicles, and natural gas appliance^.^^^^^,^^ F'yrene is a PAH emitted to the atmosphere primarily as a result of petroleum % recovered combustion by vehicles.3l by SFEasb by TD'J compound The spiked primary alcohol standards were quantitatively 105 f 2 27 f 9 1-tridecanol recovered from the filter by SFE, with the exception of l-octal-octadecanol 100f.5 8 f 6 cosanol, which showed a 76% recovery. Significantly lower l-octacosanol 78 f 8 0 recoveries of the primary alcohols were achieved using thermal n-nonanoic acid 68f 14 30 f 13 desorption, in part due to thermal decomposition of these 71 i 13 18 f 5 ndodecanoic acid n-octadecanoicacid 105 f 11 16 f 6 compounds. A typical chromatogram of thermally desorbed 100 f 12 9 f 6 neicosanoic acid alcohol standards contains several peaks in addition to those of 17 f 5 44 f 12 guaiacol the spiked standards. Mass spectral analysis and retention time 15 f 13 55 f 3 nicotine matching have identified the decomposition products as alkenes 85 f 10 vanillin 29 i 13 produced by thermal dehydration of the parent alcohol. Neither Sfluorenone 77 f 12 81 f 5 87 f 13 75 f 3 pyrene 1-octacosanol, the highest molecular weight primary alcohol standard investigated, nor its corresponding alkene decomposition a Averages of four replicate analyses. Recoveries were normalized to the average of four replicate oncolumn injections of each compound. product was detected in the thermal desorption analysis. For the injected standards,the standard deviation was ~ 5 in%all cases. Supercritical fluid extraction was also more effective than thermal desorption in recovering the spiked n-carboxylic acids. Recovery efficiency by SFE increases as the polarity of the acid decreases (68% for n-nonanoic acid; 100%for n-eicosanoic acid). of these spiked compounds from filters by SFE and by thermal The n-carboxylic acids with molecular weights higher than that desorption are shown in Table 1; recoveries are normalized to of eicosanoic acid, although expected to be soluble in supercritical peak areas resulting from on-column injections of the standard COz, are not reproducibly analyzed by the GC temperature solutions of these compounds. The comparison was performed program utilized in this study. The low volatility of the acids is in order to elucidate general differences between the two apparent in the low recoveries achieved using thermal desorption techniques with respect to atmospheric aerosol analysis. analysis. It should be noted that the matrix of the filter is significantly ExtractiodDesorption of Ambient Aerosols. Table 2 d ~ e r e nfrom t that of atmospheric aerosol particles. Others have contains a list of compounds identified in the ambient air in reported that methods of sample spiking are rarely indicators of Boulder during the autumn of 1994. Duplicate samples were extraction efficiencies from environmental samples in which one collected and analyzed by either SFE-GC/MS or by thermal can place much c o n i i d e n ~ e .The ~ ~ inorganic compounds and desorption-GC/MS. Organic compounds identified in the samples elemental and organic carbon composition of the aerosol matrix subjected to SFE analysis and not in the duplicate samples vary greatly depending upon emission source, aerosol age, local analyzed by thermal desorption are indicated with an asterisk. meteorology, and transport conditions of the airmass.11Jz~33~34 For Similarly, there were some compounds identified in the thermal this reason, it is difficult to create in the laboratory samples that desorption analysis that were not found in the SFE analysis that mimic actual atmospheric particles. Nevertheless, taking into are marked by a dagger. account the difficulties inherent in extraction of organic comSeveral possibilities may be considered to account for the pounds and the character of the constantly changing aerosol apparent d ~ e r e n c e in s the duplicate samples analyzed by the two matrix, a general comparison of recoveries for spiked standards techniques. The Iirt possibility is that the compound identified can provide some indication of the effectiveness of each technique by one technique and not by the other may have been selectively for the removal of these compounds from atmospheric aerosol removed from the sample matrix only by one removal technique. samples. For this to be a valid explanation, there must be reason to expect Recoveries of two series of standards, primary alcohols, from that the compound in question was actually present in the aerosol 1-tridecanol to 1-octacosanol, and n-carboxylic acids, from nsample and not an artifact, such as a thermolysis product arising nonanoic acid to n-eicosanoic acid, were measured; compounds from a step in the analytical procedure. In addition, there should in both classes are present in atmospheric a e r o s 0 1 s . ~The ~~~~~~~ also be a chemical or physical reason for why the compound was remaining compounds included in the comparison, nicotine, not removed from the sample matrix by one, but only one, of the guaiacol, vanillin, Sfluorenone, and pyrene, were chosen to reflect two techniques. For example, a compound with extremely low a variety of compounds known to be emitted from various sources vapor pressure may not be removed by thermal desorption; into the atmosphere. Nicotine and guaiacol have been identified similarly, a very polar molecule might not be effectively removed as unique source tracers for cigarette smoke and wood smoke, from an aerosol particle upon which it is strongly adsorbed by respectively. Vanillin has been identified in wood smoke as well SFE using only COz, a nonpolar solvent. as in particle samples collected in forested area^;^,^^ Sfluorenone Under the conditions studied, SFE and thermal desorption are is emitted into the atmosphere by a number of combustion similar in their effectiveness for removing n-alkanes, carboxylic acids, furanones, and several PAHs from atmospheric aerosol (32) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A; Cass, G. R Environ. Sci. Technol. 1993,27,2700-2711. particles. Methyl esters of tetradecanoic and hexadecanoic acid, (33) Hildemann, L. M.; Klinedinst, D. B.; Klouda, G. A; Currie, L. A; Cass, G. as well as some molecular markers of softwood combustion such R Environ. Sci. Technol. 1994,28,1565-1576.
Table I.Recoveries of Spiked Compounds Uslng SCE-GCICID and Thermal Desorption (TD)-GCICID
(34) Harrison, R M.; Pio, C. A Atmos. Environ. 1983,17,1733-1738. (35) Rogge, W. F.; Hildemann, L. M.; Mazurek, M. A; Cass, G. R Environ. Sci. Technol. 1993,27, 2736-2744.
(36) Hawthome, S. B.; Miller, D. J.; Langenfeld, J. J.; Krieger, M. S. Environ. sci. Technol. 1992,26, 2251-2262.
Analytical Chemistry, Vol. 67, No. 19, October I , 1995
3545
Table 2. Organic Compounds ldenltlfled In Urban Tropospheric Aerosols Using Low-Volume Sampllng with Either Thermal Derorptlon- or SFE-GC/FID.*c
ncarboxylic acids n-hexanoic acid n-heptanoic acid n-octanoic acid n-nonanoic acid n-decanoic acid
n-undecanoic acid n-dodecanoic acid n-tetradecanoic acid n-hexadecanoic acid n-octadecanoic acid neicosanoic acid* n-alkanes n-pentadecane
n-hexadecane n-heptadecane n-octadecane
n-nonadecane neicosane n-heneicosane n-docosane n-tricosane n-tetracosane n-pentacosane
n-hexacosane n-heptacosane
n-octacosane n-nonacosane n-triacontane
n-hentriacontane n-dotriacontane
n-tritriacontane n-tetratriacontane branched alkanes b-CZ2b b-CzSb
b-CZ4b
b-Czsb b-C28
b-CZ7'
PAHS fluoranthene pyrene
chrysene benz[alanthracene benzo(ghi1perylene ketones 6,10,14trimethyl-2-pentadecanone (9H)-fluorenone 2-heptadecanone* methyl esters of carboxylic acids 1-methylester of tetradecanoic acid 1-methylester of hexadecanoic acid alkylbenzenes 1,3-methyl-2ethylbennet l,Z,Strimethylbenzenet alcohols 2-(2-butoxyethoxy)ethanolb 2ethoxy-1-propanolb 1-pentacosanol* 1-hexacosanol*
l-octacosanol*
phthalates bis (1-methylethyl)phthalate bis(2-methyl propyl) phthalate bis(2ethyl hexyl) phthalate aldehydes l-nonanal benzaldehydet vanillin alkenes 1-nonacosenet miscellaneous 1,5diethyl-2,3dimethylcyclohexane*
Smethyl-2-butaneamineb guaiacol camphor retene PAHS
These are all compounds identified in at least one of three sets of duplicate samples collected between August 4 and December 2,1994 in Boulder, CO; all samples represent 600-2400 liters of air sam led over 2 to 12 h. Compounds were identified by comparison of &eir mass spectra to mass spectra reported in the literature, and by retention time matching that of an authentic standard. No authentic standards were available, so tentative identification was based on comparison of mass spectra. %sterisk indicates compound was removed and detected only in the aerosol sample analyzed by SFE with COz. Dagger indicates the compound was removed and detected only in the aerosol sample analyzed by thermal desorption.
as retene, guaiacol, and ~anillin,3~S~ were identified in samples analyzed by both techniques. Several compounds identified in the sample analyzed by thermal desorption were not seen in the sample analyzed by SFE. These compounds included two alkylated benzene compounds, benzaldehyde, and 1-nonacosene. Alkylbenzene compounds and benzaldehyde are emitted to the atmosphere from a variety of sources, including petroleum combustion, tobacco smoking, and some growing vegetati0n;3~these low molecular weight compounds have been identiiied in aerosol samples in past ~ t u d i e s . ~ Hawthorne et al. have shown that a variety of terpenes, hydrocarbons with molecular weights and polarities similar to substituted benzene compounds, are soluble in supercritical COZand are effectively trapped in the cryogenically cooled analytical column under the conditions used in this a n a l y s i ~ . ~Given ~ . ~ the ~ ~
relatively small concentration of these compounds in ambient aerosols, however, it is likely that the total amount extracted was below the detection limit of the FID. Thermal desorption is probably more effective than SFE for removing the alkylated benzene compounds from the aerosol. 1-Nonacosene, another compound removed and identified by thermal desorption analysis but not seen in the SFE analysis, has not, to our knowledge been previously reported to be present in tropospheric samples. It is unlikely that this compound was actually present in our ambient aerosol particles, as it is expected to be quite soluble in supercritical COZunder the conditions utilized in this analysis. Alkenes have been shown to be generated as a result of alcohol dehydration during thermal desorption (see above). The presence of nonacosene in the sample analyzed by thermal desorption may be due to dehydration of 1-noncosanolduring desorption; l-nonacosanol is contributed to the atmosphere by vascular but it was not observed in the sample analyzed by SFE. Compounds identified in the sample treated by SFE, but not in the sample treated by thermal desorption, include several high molecular weight alcohols (e.g., 1-pentacosanol, 1-hexacosanol, and 1-octacosanol)and eicosanoic acid. These alcohols and this acid are present in suspended particulates formed by abrasion of plant leave^;^^^^ they are also found, to a lesser extent, in cigarette smoke.' These compounds all have fairly low vapor pressures relative to much of the organic material identified in aerosols. It is unlikely that the temperature used for thermal desorption in this study was sufficientlyhigh to volatilize the compounds from the aerosol. Again, analysis of alcohol standards indicates that some thermal degradation does occur at these temperatures. It is likely that the alcohols and acids were actually present on the aerosols, were removed by SFE, and either were not desorbed or were thermally degraded during the thermal desorption analysis. Qualitative Comparison 0fDesorbed and EjLbracted Ambient Aerosol Samples. A qualitative comparison of SFE vs thermal desorption was also conducted on duplicate samples by performing two analyses on each sample (SFEfollowed by thermal desorption on one sample and thermal desorption followed by SFE on the duplicate). Chromatographic separation and detection followed each step of the two analyses in the sequence in order to compare the peaks with those in the duplicate sample analyzed in the opposite sequence. A matched pair of ambient aerosol samples was collected simultaneously from an urban air mass and analyzed in the following manner. One sample was first thermally desorbed into the gas chromatograph for GC/FID analysis. After thermal analysis, this sample was extracted into the gas chromatograph using supercritical COZ. The other matched sample was first extracted with supercritical COz and the extract was analyzed online by GC/FID; then, in a second step, the sample was subjected to thermal desorption-GC/FID analysis. The desorption and extraction steps were performed under conditions earlier determined empirically to be optimal for the removal of most of the organic compounds from the aerosol. Therefore, any organic compounds detected in the second step of the analysis sequence must not have been quantitatively removed in the first analysis. Chromatograms of the two-step analysis of each matched sample are shown in Figure 4.
~~~~~~
(37) Ramdahl, T.Nature 1983,306,580-582.
3546 Analytical Chemistry, Vol. 67, No. 19, October 1, 1995
(38) Simoneit, B. R
T.;Mazurek, M. A. Atmos. Enuiron. 1982,16,2139-2159.
A:TD first treatment by thermal desorption
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Figure 4. Chromatographic traces of analyses of two duplicate ambient aerosol samples from 200 L of air. Chromatogram A:TD results from a thermal desorption-GC/FID analysis of one sample; chromatogram A:SFE is the result of a subsequent SFE-GCIFID analysis of the previously desorbed filter. Chromatogram B:SFE is the result of the first treatment of the duplicate sample, by SFE-GC/FID analysis. B:TD results from the second step, thermal desorption-GC/FID analysis of the previously extracted filter.
The results in Figure 4 show that most compounds are removed by either SFE or thermal desorption; the chromatograms for the first treatment steps are similar. More than 90% of the
volatile/extractable organic compounds are removed in one step by either technique. Sixteen peaks present in both ATD and B:SFE were selected at random from the middle portion of the Analytical Chemistty, Vol. 67,No. 79, October 7, 1995
3547
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Table 3. Recoverles of Randomly Selected Compounds from Duplicate Particle Samples Analyzed by Either Thermal Desorption (TD). or SFE-GC/FID
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SFE
13.7 17.4 12.4 4.8 6.0 8.8 7.6 10.6 14.2 25.7 14.3 6.1 16.7 14.2 10.0 19.1
19.2 10.6 10.2 3.2 11.3 8.7 5.6 6.1 14.2 22.1 5.2 2.5 10.4 18.6 5.9 9.5
16.4 14.0 11.3 4.0 8.6 8.8 6.6 8.4 14.2 18.9 9.8 4.3 13.6 16.4 7.9 14.3
83 124 110 120 69 100 115 127 100 83 147 141 124 87 126 134
117 76 90 80 130
mean compd
mass of
% of mean
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100
85 73 100 117 53 59 76 113 74 66
"Selected peaks are indicated, according to letter, in Figure 4 chromatogram ATD and chromatogram B:SFE. Mass is calculated by multiplying the integrated peak area for each compound by the FID response for 1 ng of n-docosane.
chromatograms, and the integrated peak areas of each were compared on a peak-by-peak basis. The results, presented in Table 3, show that many compounds are effectively removed from the aerosol particles by either SFE or thermal desorption. In most cases, there was reasonably good agreement between the two analytical methods. The most dramatic differences were by slightly greater than a factor of 2. There are some important differences in the early and late stages of the chromatogram that will be discussed in more detail. The trace ATD, in Figure 4, is the result of a 15min thermal desorption of one of the original matched aerosol samples. The large, off-scale, unresolved peak near the beginning of the chromatographic trace is characteristic of thermal desorption of an ambient aerosol sample and may arise in part from thermal degradation of some unstable compounds. The trace ASFE, in Figure 4,was obtained by SFE of the aerosol sample immediately after the thermal desorption-GC/FID analysis (ATD). Supercritical fluid analysis of the previously desorbed filter extracted additional organic compounds from the aerosol particles; most of the compounds present in kSFE were eluted in the later half of the chromatographic trace. Sample B:SFE resulted from the SFE-GC/FID analysis of the second original, matched aerosol analysis. Relatively few compounds are present in the first quarter of the chromatogram BSFE compared to the chromatogram from the matched sample k s t analyzed by thermal desorption (ATD). The later half of B:SFE contains a significantly more integrated area than the later half of ATD. Following the initial SFE, sample B was transferred to a desorption tube for subsequent analysis by thermal desorptionGC/FID; the results of this analysis are shown in trace B:TD in Figure 4. In B:TD, additional organic compounds have been desorbed from the previously extracted aerosol sample; most of the compounds desorbed were eluted early in the chromatogram. There are several possible explanations for the abundance of early-eluting compounds in the sample analyzed initially by thermal desorption (ATD) that are not present in the duplicate 3548 Analytical Chemistry, Vol. 67, No. 19, October 1 , 1995
sample analyzed initially by SFE (B:SFE) . These compounds give rise to the large unresolved peak that dominates the beginning of chromatogram ATD. The unresolved peak could arise from a group of volatile compounds that are poorly soluble in supercritical COZbut are effectively separated from the aerosol particles by thermal desorption. This conclusion seems unlikely, however, in light of the fact that the sample analyzed first by SFE and second by thermal desorption (B:TD) has a much smaller unresolved peak at the same retention time. If the compounds were truly insoluble in supercritical COz, they would remain on the aerosol particles until the subsequent thermal desorption analysis. It is also possible that during SFE some compounds were removed from the aerosol but not effectively trapped in the analytical column. Others have reported that the trapping temperature utilized in this study should be sufficiently low to trap most compounds;'* nevertheless, some of the most volatile compounds present in trace amounts on the particles may have been lost or incompletely transported to and collected in the capillary column. A third possibility is that the early-eluting compounds were absorbed to strong active sites on the aerosol particle. It is possible that the relatively low temperatures utilized in the SFE technique do not supply sdticient energy for certain analytes to overcome the "barrier of desorption" necessary to free the analyte from these active Langenfeld et al. hypothesized this to be one factor contributing to the increased recoveries of some compounds from environmental matrices observed in high-temperature SFE over recoveries achieved at lower temperatures more characteristic of those used in this ~tudy.*6J9 The unresolved peak in ATD may be thermal degradation products generated during the desorption. Thermally labile organic compounds on or in the aerosol particles are less likely to be degraded in the relatively low-temperature SFE analysis; this would explain the lack of a corresponding unresolved peak in B:SFE. The presence of some resolved peaks in the beginning of B:TD may indicate that the unresolved peak in ATD is a combination of both volatile organic compounds that are insoluble in COZand thermal degradation products. To further characterize the unresolved peak observed in the thermal desorption analysis, we performed a lower (130 "C) or a higher (300 "C) temperature thermal desorption analysis on two different fractions of the same aerosol particle sample and repeated this analysis with two different samples. Within every set analyzed, the first peak in the chromatogram resulting from the thermal desorption conducted at the lower temperature was dramaticallysmaller than the fist peak in the chromatogramfrom the higher temperature thermal desorption. Though this observation does not conclusively identify compounds within the first peak as thermal degradation products of the desorption analysis, it is consistent with that conclusion. Both chromatograms resulting from SFE analysis, ASFE and B:SFE, contain more late-eluting compounds than the corresponding thermal desorption analysis. Compounds eluted in this region may either be thermally labile, and therefore destroyed during thermal desorption, or have low volatilities, and thus are preferentially removed from the aerosol by SFE but not by thermal desorption. This is not unexpected as SFE with carbon dioxide is known to be an effective solvent for some high molecular weight (39)Langenfeld, J. J.; Hawthorne, S. B.; Miller, D.J.; Pawliszyn, J. Anal. Chem. 1994, 66,909-916.
compounds with low volatilities. Others have shown that SFE may extract some compounds, such as anthracene and phenanthrene, more efficiently from standard reference materials than NIST has certified to be present when a standard method such as Soxhlet extraction is sed.^^^^^*^^ This indicates that some fraction of certain compounds may be very difticultto remove from particular matrices. CONCLUSIONS
Supercritical fluid extraction with pure COZhas been shown to provide quantitative recovery of many compounds present in atmospheric aerosol samples. The SFE technique is a wholesample analysis that can be coupled directly to a gas chromatograph for separation and on-line detection of complex organic samples. Optimal extraction conditions have been determined and extraction can be completed in 20 min. The sensitivity afforded by the SFE technique is sufficient to accommodate lowvolume sample collection; the composition of organic compounds in atmospheric aerosol particles can be monitored on a shortterm (2 h rather than 24 h) basis. Recoveries of standards, spiked onto filters, by SFE and by thermal desorption, a wellcharacterized method of wholesample aerosol analysis, have been determined.
In addition, duplicate aerosol samples have been analyzed by SFE and thermal desorption in order to further demonstrate differences and similarities between the two techniques. The SFE technique uses relatively low temperatures; as a result, thermal degradation of labile compounds during analysis is minimized. Thermal desorption is a more effective technique for the removal of certain more volatile compounds and, possibly, those compounds that interact strongly with active sites from the aerosol particles. The two analytical methods offer complementary advantages and can also be used in combination. ACKNOWLEWMENT The authors thank the Colorado Department of Health and the City of Boulder for extending the use of their local air monitoring facility. Dr. Barbara Watkins, Dr. Tom Ryerson, and Dr. Craig Perman provided helpful comments on this work; the technical assistance of Paul Frazey is gratefully acknowledged. Received for review April 20, 1995. Accepted July 18, 1995.@ AC950393Y @Abstractpublished in Advance ACS Abstracts, September 1, 1995.
Analytical Chemishy, Vol. 67, No. 19, October 1, 1995
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