Anal. Chem. 1985, 57,2807-2815
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Rapid Sampling and Analysis of Volatile Constituents of Airborne Particulate Matter Randall C. Greaves, Robert M. Barkley, a n d Robert E. Sievers* Department of Chemistry and the Cooperative Institute for Research in Environmental Sciences, Campus Box 215, University of Colorado, Boulder, Colorado 80309
Low-volume sampling coupled wlth dlrect thermal desorptlon was used to lnvestlgate the composltlon of the volatlle organlc fraction of urban airborne partlculate matter. Samples of suspended particles are collected for 7 to 60 min by flltratlon of between 40 and 2000 L of alr through a glass tube contalnlng a small quartz fiber fllter. Thls same collectlon tube is then connected dlrectly to a cryogenlcally cooled fused silica capillary chromatography column into which the volatile organlc compounds are rapldly thermally desorbed, at a maximum temperature of 254 OC for 15 mln. As the column temperature Is subsequently raked, chromatography proceeds, wlth detectlon of the desorbed organlc compounds by flame bnlzatlon or mass spectrometry. The entlre procedure, from sample collectlon to chromatographic analysis, can be performed in less then 1.5 h wlth minimal sample handllng.
The importance of investigating the organic composition of airborne particulate matter is well documented. Some of the polycyclic aromatic hydrocarbons (PAHs) found on particulate matter are known carcinogens and, therefore, pose a risk to human health ( I ) . Specific classes of organic compounds are indicators of the source of the atmospheric particulate matter (2, 3). Several interesting and important chemical reactions are known to occur on the surface of particles ( 4 ) . The procedure most often applied in the organic analysis of atmospheric suspehded particles involves the filtration of hundreds of cubic meters of air, removal of the organic compounds on particles by solvent extraction, concentration of this extract, and finally fractionation of organic compounds by liquid chromatography (2,5). Specific organic constituents are then identified and quantitated by use of gas chromatography with flame ionization or mass spectrometric detection. The method described in this article for the organic analysis of airborne particulate matter is based on low-volume air sampling coupled with direct thermal desorption of volatile organic compounds into a cooled gas chromatographic column. Particles were collected by passing between 40 and 2000 L of ambient air through a small glass tube containing a quartz fiber filter (Figure 1) for time intervals between 7 min and 9 h (typically 60 min). The objective of this study was to determine whether lowvolume sampling coupled with direct thermal desorption is a viable method for identification and quantitation of the organic compounds found on urban airborne particulate matter. Results from a quantitative comparison between low-volume sampling with thermal desorption and high-volume sampling with solvent extraction/fractionation will be presented. Results of a precision study of the entire low volume sampling thermal desorption procedure, for normal saturated hydrocarbons and PAHs, will be described. Quantitative results of PAHs in the Standard Reference Material (SRM) 1649, as determined using direct thermal desorption of 11 samples, are reported.
EXPERIMENTAL SECTION Materials. All solvents were used as received from Burdick and Jackson, Muskegon, MI. Purity checks were performed on the normal saturated alkanes and PAHs by GC/FID, and the quality of these standards was found to be acceptable for quantitative work. Alkanes were obtained from PolyScience Corp., Niles, IL. Pyrene, phenanthrene, benzo[blfluoranthene, and indeno[1,2,3-c,d]pyrenewere purchased from Chem Service, Inc., West Chester, PA. Fluoranthene, chrysene, benz[a]anthracene, benzo[a]pyrene, and perylene were obtained from Aldrich Chemical Co., Milwaukee, WI. Benzo[ghi]perylene and benzo[elpyrene were obtained from Sigma Chemical Co., St. Louis, MO. Most of the other standards, used for the identification of unknown compounds, were from Chem Service, Inc. The SRM 1649 was from the National Bureau of Standards, Washington, DC. Vespel ferrules, Supeltex M-2, and Thermogreen injection port septa, LB-1, were obtained from Supelco, Inc., Bellefonte, PA. Silicic acid (J. T. Baker Chemical Co., Phillipsburg, NJ) was preconditioned for 3 h at 300 "C and stored under hexane. Quartz fiber filters, used for collection of high volume air samples, were obtained from Gelman Instrument Co., Ann Arbor, MI. Quartz fiber filters used in the construction of the thermal desorption tubes and in high-volume sampling were from Pallflex Products Corp., Putnam, CT. Corning Pyrex screw cap culture tubes, 25 mm X 200 mm, were used for the storage of the thermal desorption tubes. Reynolds Wrap aluminum foil was used to cover the tops of the screw cap culture tubes. Thermal desorption tubes were constructed from 40-60 pm fritted disk straight sealing tubes, lOC, 202 mm X 14 mm (V.W.R. Scientific, Brisbane, CA). Equipment. Gas chromatography with flame ionization detection (GC/FID) was performed with a Hewlett-Packard Model 5830 instrument. The chromatography column was a 30 m X 0.35 mm i.d. (1 pm film thickness) DB-5 manufactured by J & W Scientific, Inc., Rancho Cordova, CA. The linear flow rate through the column, as measured using methane injection, was 45 cm/s at 300 "C. The injection port temperature was 250 "C and the detector temperature was 300 "C. Quantitation and compound identification were performed with a Hewlett-Packard Model 5892A gas chromatograph/mass spectrometer with data system (GC/MS/DS) that was modified such that the end of the chromatographic column extends into the ion source of the mass spectrometer. The GC/MS system used a 25 m X 0.32 mm i.d. (0.52 pm film thickness) Ultra Performance fused silica capillary column, manufactured by Hewlett-Packard. The linear flow rate through the column, as measured by methane injection, was 40 cm/s at 300 "C. The mass spectrometer electron impact voltage was 70 eV. Helium carrier gas was used for both the GC/FID and GC/MS. High-volume air samples were collected with a HI-Volume sampler (GCA/Precision Scientific, Chicago, IL) enclosed in a standard ASTM weather shelter. The high volume sampler flow rate was calibrated with a top loading orifice calibrator with a Model 333 kit (Sierra Instrument, Inc., Carmel Valley, CA). Low-volume air samples were collected with a Nutech Model 221-1A gas sampling pump (Nutech Corp., Durham, NC) that was modified to allow collection of either one sample or two samples in parallel. The thermal desorption apparatus was constructed from a cylindrical aluminum block (7 cm diameter X 8 cm long with a 2.54-cm hole in the middle) and was fitted with a 165-W heating cartridge connected to an Omega 6000 thermal controller (Omega Engineering, Stamford, CT). Calibration of the flow rate through the low-volume pump was performed, prior to sample collection, with a 1-L soap bubble flowmeter. Microgram quan-
0003-2700/85/0357-2807$0 1.50/0 0 1985 American Chemical Societv
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Particle Inlet 0.251no.d.X4rnrni.d. Glass Tube
Outlet Side 6rnrnod.X Zrnrnid. Glass Tube
a
of^^^
collection tube I
~a;ollection shut,of
tubh2.
valve
6.5n -rc Figure 1. Aerosol collection/thermal desorption tube. t&ee w a y valve
tities of Standard Reference Material 1649 were weighed out with a Cahn model RG electrobalance (Cahn Instrument Co., Paramount, CA). Solvent extraction of organic compounds was performed with a 125-W, 50/60 Hz ultrasonic cleaning bath (Branson Cleaning Equipment Company, Shelton, CT). Collection/Desorption Tube Fabrication and Handling. The dimensions and shape of the sample collection thermal desorption tube are shown in Figure l. This tube was constructed by using the following procedure. With a no. 7 cork borer, a 1.3 cm diameter piece of quartz fiber filter was cut from a standard Pallflex high-volume filter. The filter was placed on top of the glass frit of a fritted glass sealing tube and suction was applied to the tube, with the gas sampling pump, while the sides of the filter were gently packed against the sides of the tube using the flat end of a glass stirring rod. All but approximately 1 cm of both ends of the sealing tube was removed and replaced by a 0.25 in. 0.d. X 4 mm i.d. X 1.5 cm glass tube on the filter side of the sealing tube and a 6 mm 0.d. X 2 mm i.d. X 1.5 cm glass tube on the opposite end of the sample tube. Sample tube construction could be completed in approximately 15 min by an experienced glass blower. Care was exercised to avoid excessive heating of the glass frit and to make sure that the sample inlets of the tubes were flat, even, and uniform, so that the quartz fiber filter fits well, and sample collection is not susceptible to flow variations. Organic contamination was removed by placing the collection/desorption tube inside a screw cap culture tube (uncapped) and heating it in a glass annealing oven at 600 "C for 5 h. Power was turned off and when the oven had cooled to approximately 100 "C the tubes were removed. The tops were immediately covered with aluminum foil and then capped, taking care not to touch the tube except with woven glass fiber gloves. In some comparative experiments, attempts were made to remove organic contaminants by heating the collection tube in an oven, at 300 "C, under a pure nitrogen or helium flow of 30 mL/min, for 4 to 5 h. Once the collection/desorption tubes were conditioned and sealed, they could be stored in laboratory air for up to 3 months without becoming appreciably contaminated. During the connection of the tubes to the sampling pump and the gas chromatograph, they were handled only with cotton gloves. After samples were collected, the tubes were again placed inside the same culture tube, covered on the top with aluminum foil, sealed with a screw cap, and stored in a freezer at -15 "C until just prior to thermal desorption. In circumstances in which the samples could not be immediately analyzed, the culture tubes were completely wrapped in aluminum foil to avoid exposure of the particulates to light. Procedure for Low-Volume Particulate Collection. Particulate collection time, volume of filtered air, and the flow rate were all dependent on the experiment performed, but the same basic apparatus was used in all of the low-volume sampling experiments. The pump was connected either to a 12-Vautomobile battery or to a standard 110-V power supply. Two 0.25-in. copper or stainless steel extension tubes were connected to the flow matching device on the sample pump and they extended about 1.5 m from the body of the pump. Sample collection tubes were connected to the ends of these extension tubes by 0.25411. stainless steel fittings using Teflon ferrules. A sample tube was operated in the vertical position with a 15-mL beaker suspended face down over the top of the tube such that the lip of the beaker was about 7 mm lower than the top of the tube. Two parallel samples were usually collected in each experiment, with a combined total flow rate of between 1and 10 L/min. When
Figure 2. Schematic of the device used for matching air flow between
two particle collection tubes collecting aerosols in parallel.
r" divert line
heating cartridge \
1
w \
Injection port
Figure 3. Particle collection/thermal desorption tube inserted In the chromatographicflow system for thermal desorption.
only one sample was collected, flow was shut off in the other extension tube. Only one sample pump was used, so if two samples were collected in parallel, the flows through the two sample tubes were matched by the following procedure (refer to Figure 2). Flow through one of the collection tubes was shut off, using one of the shut-off valves at either B or C, and the pumped air was diverted through a ball flowmeter by using the three-way valve at A. Adjustments to the flow were made by needle valves at D and E. This procedure was repeated in an iterative fashion, by switching flow between tube number 1 and tube number 2, until the ball flowmeter gave similar flow readings for both collection tubes. Flow was restored to both collection tubes by the three-way valve at A and opening both shut-off valves. The total flow matching process takes 1-3 min depending on the adjustments that are necessary. When samples were collected in parallel, checks were made periodically during sampling to ensure that flows remained matched. On only one occasion did the flows, once adjusted, drift. Procedure for Particle Thermal Desorption. The same basic procedure was used for thermal desorption of samples, standards, the Standard Reference Material, and spiked samples. The thermal desorption apparatus is illustrated in Figure 3. Organic contamination was removed from ferrules and fittings, prior to thermal desorption, by connecting them to a previously cleaned desorption tube and heating them in an oven, at 300 "C under helium or nitrogen flow of 30 mL/min, for 4 to 5 h. The filter side of the collection/desorption tube was connected to a carrier gas divert line on a gas chromatograph through the use of a 0.25-in. stainless steel fitting and a Vespel ferrule. The other side of the tube was connected, also using a Vespel ferrule, to a 0.25-in. to 0.0625-in. stainless steel reducing union fitted with a chromatography injection needle. An insulated cylindrical aluminum block was preheated to the desired temperature, usually 300 "C, and positioned over the gas chromatographic injection port. The carrier gas was diverted from the injection port through the collection/desorption tube, and the carrier flow out the injection needle was checked by placing the tip of the injection needle into water. The needle was wiped dry, with a lint-free
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
disposable tissue, and the desorption tube assembly was inserted through the hole in the aluminum block and the needle was inserted through the septum of the injection port. The aluminum block was positioned such that the entire assembly was heated, including the stainless steel fittings. All of the connections were checked for leaks, with a thermal conductivity leak detector, glass wool insulation was placed around the top and bottom of the desorption tube, and thermal desorption of the sample proceeded for 15 min. The flow through the desorption tube was 5 mL/min and the pressure at the outlet of the tube was 18 psig. Temperature in the chromatographic oven, during thermal desorption, was kept low enough (usually -60 "C, vide infra) to prevent significant movement of the compounds of interest through the chromatography column. After thermal desorption, the desorption tube was removed from the septum of the injection port and simultaneously removed from the heating block; the aluminum heating block was removed from the top of the injection port, carrier gas flow was restored to the injection port, and the chromatography oven temperature program (vide infra) was initiated. Unless otherwise specified, the column temperature was raised from -60 to 0 "C at 20 "C/min and then from 0 to 300 "C at 8 "C/min. Fittings were removed from the desorption tube, immediately after desorption, and placed inside a clean conditioned glass screw-cap culture tube. The top was immediately covered with aluminum foil and then capped. During the thermal desorption experiment, the temperature of the desorption tube is continually approaching thermal equilibrium but this process could require a long period of time and, therefore, it is important to measure the actual temperature inside the desorption tube as a function of time. One set of experiments was conducted to determine the actual temperature inside the desorption tube instead of on its surface. The temperature change was measured by placing a thermocouple on the surface of the quartz fiber filter and recording the temperature over time, using typical thermal desorption experimental conditions. Procedure for Thermal Desorption of SRM 1649. A portion, between 34 pg and 386 pg, of SRM 1649 was weighed on a tared aluminum foil electrobalance pan to the nearest microgram. The pan was transferred with tweezers to dump its contents through the inlet side of the thermal desorption tube, taking care that the contents were delivered onto the filter. The pan was reweighed to determine the sample size quantitatively delivered. The organic compounds in the SRM were then directly thermally desorbed, at a heating block temperature of 300 "C for 15 min, into the GC/MS. The gas chromatographic oven was held at 20 "C during the thermal desorption, then heated at 20 "C/min to 180 "C, and then heated at 8 OC/min to 300 OC. PAHs were identified by comparison of their mass spectra to mass spectra reported in the literature and a comparison of their mass spectra and chromatographic retention times to those of authentic standards measured in our laboratory. Quantitation of the PAHs in SRM 1649 was performed, with the mass spectrometer operating in the selected ion monitoring (SIM) mode, by integrating the molecular ion of the PAH of interest. Two individual masses (the molecular ion and a second abundant characteristicion) were scanned, during the SIM experiment, for each PAH eluting. Standard working curves were obtained for each compound by injection of standard mixtures onto the filter of a collection/ desorption tube and then thermal desorption of the PAHs into the GC/MS. The average concentrations of the PAHs were determined from 11 individual portions of the SRM 1649. Procedure Comparing Low-Volume/Thermal Desorption to High-Volume/Solvent Extraction/Fractionation. Air particulate samples were collected from a site at the University of Colorado, Boulder. A total of six replicate comparison studies were conducted between September 1984 and January 1985. Low-volume samples and high-volume samples were collected in parallel for intervals between 5 and 9 h. The high-volume apparatus was calibrated, prior to operation, to a flow rate of approximately 1500 standard L/min. The low-volume sampling pump was calibrated, prior to operation, to a total flow rate of 5 to 6 standard L/min (Le., 2.5 to 3 L/min through each sample tube). Two low-volume samples were also collected simultaneously, as previously described, and the flow match between these two filter tubes was checked before, during, and at the end of
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sample collection. The total volume collected by the high-volume apparatus and the low-volume pump was between 500 m3 and 800 m3and 1.6 m3 per sample and 3.0 m3per sample, respectively. Sampling inlets were 5 m apart and about 1m from ground level. After collection, the high volume filter was placed in a desiccator for 2 h, weighed, and then stored in a refrigerator at 2 "C until extraction. The quantitative procedure for the low-volume sampling was identical with the one used in the thermal desorption of SRM 1649,except that the chromatographic oven temperature program was changed. In these experiments the gas chromatographic oven was held at 20 "C during thermal desorption and then temperature programmed at 20 "C/min to 120 "C and finally at 4 "C/min to 300 "C. This allowed simultaneous determination of the normal alkanes, as well as the PAHs. The solvent extraction/fractionation method has been described in detail (5) but was used with some minor modifications. Organic compounds were extracted from particles by ultrasonic agitation, for 2 h using 400 mL of methylene chloride, and the acidic and hydrophilic compounds were removed from the extract by partitioning with 1N NaOH. The methylene chloride extract was dried with anhydrous Na2S04, concentrated with a KudernaDanish concentrator, and finally concentrated to almost dryness under a flowing stream of nitrogen. This remaining extract was quantitatively delivered, in a stepwise fashion (adding small amounts of CH2C1, to dissolve the residue), to the top of a silicic acid column, and the residual solvent was evaporated from the top of the column at each transfer step with a stream of nitrogen. Organic compounds were then fractionated by sequential elution with hexane, 50:50 hexane/ toluene, toluene, 50:50 toluene/ methanol, methanol, and finally methylene chloride. Fractions were again concentrated to 1 mL and stored in a freezer at -20 "C prior to quantitative analysis. Saturated n-alkanes and the PAHs were quantitated by splitless liquid injection of the hexane and the hexane/toluene fractions into a GC/FID. External standards of authentic compounds were used to prepare calibration curves. For the analysis of all liquid samples, the gas chromatographic temperature program was 80 "C to 300 "C at 4 OC/min. Procedure for Determination of Recovery of Compounds from Spiked Samples. Several low-volume particle samples were collected and subsequently thermally desorbed according to the standard procedure. The mass of particles collected by low-volume sampling was calculated by using the particle concentrations as measured by the high-volume procedure and the volume of air sampled. Original weights of particles in each sample tube ranged from 160 pg to 250 pg. After thermal desorption of these particles, normal saturated alkanes (1 ng to 12 ng of each alkane) and PAHs (0.5 ng to 2 ng of each PAH) were delivered, by syringe injection of standard solutions, onto the surface of the exposed sample tubes and thermal desorption and compound quantitation proceeded in the usual fashion.
RESULTS AND DISCUSSION Several past investigations have been conducted to determine whether organic compounds can be thermally desorbed, from atmospheric and workplace particulate matter, directly into a gas chromatograph and detected with various selective and nonselective detectors (6-15). The basic principle of thermal desorption has been demonstrated, but a great deal of uncertainty exists about the efficacy of using this procedure for quantitative measurements and how thermal desorption compares with other techniques. Uncertainty also exists about the optimal design of the thermal desorption apparatus and operating conditions. A thermal desorption apparatus should have a minimum of heated transfer lines, active metal surfaces, dead volume between the sample and the analytical column, sample passage through heated switching valves, and modifications to the gas chromatographic injection port. The thermal desorption procedure should be as simple as possible with a minimum of direct sample handling. Multiple transfers of the sample should be eliminated and the desorption temperature should be the lowest which will quantitatively desorb the compounds of interest.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table I. Compounds Identified in Airborne Particulate Matter Using Low-Volume Sampling with Thermal Desorption"** carboxylic acids acetic acid (3)c 1-propanoic acid 1-butanoic acid 1-pentanoic acid 1-hexanoic acid 1-heptanoic acid I-octanoic acid 1-nonanoic acid ( 5 ) 1-decanoic acid (6) 1-undecanoic acid 1-dodecanoicacid 1-tridecanoic acid 1-tetradecanoic acid (9) 1-pentadecanoicacid 1-hexadecanoicacid hexadecenoic acidd I-heptadecanoic acid 1-octadecanoicacid octadecenoic acidd benzoic acid (4) furans 2-methylfuran (2) 3-methylfuran (1) 2,3-dihydro-5-methylfurand 2-dihydro-3-methylfurand 2,5-dihydro-3,4-dimethyl-
furand 2-furaldehyde chlorinated aromatics dimethyl 2,3,5,64etrachloroterephthalate (ll)d
alkanes
aromatic hydrocarbons
n-tridecane n-tetradecane n-pentadecane n-hexadecane n-heptadecane (8) n-octadecane n-nonadecane n-eicosane n-heneicosane (12) n-docosane (13) n-tricosane (14) n-tetracosane (15) n-pentacosane (16) n-hexacosane (17) n-heptacosane (18) n-octacosane (19) n-nonacosane (20) n-triacontane n-hentriacontane (21) n-dotriacontane n-tritriacontane n-tetratriacontane pristane phytane phthalates n-dimethyl phthalate n-diethyl phthalate (7) n-dibutyl phthalate (10) diisobutyl phthalate n-dioctyl phthalate di-2-ethylhexyl phthalate phthalic anhydride phthalide
naphthalene 1-methylnaphthalene 2-methylnaphthalene dimethylnaphthalene isomersd phenanthrene Cl-alkylphenanthrene1 anthracene isomersd C2-alkylphenanthrene/ anthracene isomersd anthracene fluoranthene pyrene C1-alkylpyrene/ fluoranthene isomersd C2-alkylpyrene/ fluoranthrene isomersd benz[a]anthracene chrysene benzo[b]fluoranthene benzo[klfluoranthene benzo [e]pyrene benzo[a]pyrene perylene C1-alkylbenzopyrene/ benzofluoranthrene isomersd dibenz[a,h]anthracene indeno[1,2,3-c,d]pyrene benzo[g,h,i]perylene ketones and aldehydes 9-fluorenone xanthoned methylacetophenoned benzaldehyde
'These are the total number of different compounds that were identified from 12 different samples taken from Feb 1984, until Jan 1985 in Boulder, CO. Air sample volumes ranged between 50 and 320 L, and sampling times ranged from 7 to 100 min. *Compounds were identified by comparison of their mass spectra to mass spectra reported in the literature, and by comparison of their mass spectra and retention times to those of authentic standards. CThisnumber refers to the chromatographic peak shown in Figure 4. dNo authentic standards were available, so compounds were only tentativelv identified bv comparison of their mass sDectra to literature mass spectra.
Removing Contamination and Artifacts. One of the most important steps in obtaining meaningful information by the thermal desorption technique is the elimination of organic contamination from the desorption apparatus by the proper conditioning, handling, and storage of the thermal desorption equipment. This step becomes even more critical as the sample size becomes smaller, and the reldive importance of artifacts increases. Initial investigations showed severe contamination on virtually every material used in this procedure, with the largest contribution coming from the low-volume collection tube and Vespel ferrules. High-temperature annealing at 600 "C cleaned organic contamination from the collection tube substantially better than passing nitrogen or helium through the tube at 300 "C, so this was adopted as the standard treatment. The procedure described in the Experimental Section for cleaning the ferrules and fittings was performed initially but was later determined to be unnecessary on a routine basis when these parts of the apparatus were properly stored. When ferrules and fittings were allowed to stand exposed to laboratory air overnight, they adsorbed significant amounts of organic contamination and had to be thermally reconditioned prior to use. Because of the ubiquitous nature of the organic contamination, empty tube chromatographic blanks were routinely measured on all sample tubes, fittings, and ferrules, even after contamination removal procedures were well es-' tablished. Organic Compounds Observed in Urban Airborne Particulate Matter. In general, when particulate sources
and atmospheric conditions are similar at two different locations, the organic constituents of particulate matter from these locations are similar. This similarity is important when investigating a new analytical technique because it provides a background for determining whether analytical results are reasonable. A list of compounds which have been identified in Boulder, CO, airborne particulate matter by low-volume sampling with thermal desorption is presented in Table I. The saturated hydrocarbons, PAHs, and carboxylic acids were always present in the highest concentrations and were the most ubiquitous volatile organic constituents in Boulder airborne particulate matter. Other investigators have also found these compounds to be the most predominant in urban aerosols (2, 16-18). Phthalates, furans, ketones, and benzaldehyde were also observed, and, except for the furans, these compounds have all been reported as constituents of urban airborne particles. Dimethyl 2,3,5,6-tetrachloroterephthalatewas observed in two samples; one taken on June 24, 1984 (Figure 4, another taken on June 19, 1984. This compound is a preemergent herbicide (Dacthal or DCPA) which is widely distributed and used in the Boulder area during the spring and early summer. Most airborne particulate matter from urban areas is related to the combustion of fossil fuels, but a small and sometimes very significant contribution can arise from natural sources (2, 3). Particulate matter which is dominated by natural sources does not contain substantial quantities of PAHs but contains saturated n-alkanes. Alkanes of recent natural origin show a significant preference of the odd carbon numbered
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Table 11. Low-Volume/Thermal Desorption and High-Volume/Solvent Extraction/Fractionation for the Quantitation of Hydrocarbons Associated with Airborne Particulate Matter, (ng/m3 of air, STP) January 12,1985d January 10,1985c January 9,1985' LVno. 1 LVno. 2 HV LVno. 1 LVno. 2 HV LV'no. 1 LV no. 2 HV' compound" phenanthrene fluoranthene pyrene benz[a]anthracene chrysene benzo[b] and [kIfluorantheneB benzo [e]pyrene benzo [a]pyrene perylene indeno[l,2,3-cd]pyrene benzo[ghi]perylene n-pentadecane n-hexadecane n-heptadecane n-octadecane n-nonadecane n-eicosane 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
0.8 1.2 1.2 0.4 0.7 0.8 0.4 0.2 0.09 0.6 0.7 0.4 1.0 2.1 2.8 3.8 4.8 3.8 3.4 2.6 1.2 2.8 1.5 1.4 1.2 1.9 1.0 1.7 0.5 0.6 0.5
0.7 1.1 1.1 0.3 0.6 0.7 0.3 0.1 0.06 0.4 0.5 0.3 0.9 1.8 2.6 3.6 4.8 3.6 3.5 2.4 1.3 2.6 1.3 1.3 1.2 1.8 0.8 1.6 0.5 0.5 0.5
1.9 1.6 2.4 0.4 0.4 NDh ND ND ND ND 1.9 0.6 1.1
2.1 3.2 4.1 4.0 3.5 3.1 2.2 2.1 2.6 1.9 2.6 1.8 4.0 1.9 3.0 1.4 1.5 0.9
3.3 5.9 5.4 2.1 3.4 3.1 2.0 1.0 0.3 1.7 2.2 0.7 1.8 3.9 6.4 9.0 14 13 11 7.0 5.3 7.1 4.1 3.0 3.2 4.0 2.8 3.6 1.5 1.6 1.8
4.3 8.6 7.4 1.4 3.8 2.7 2.8 1.7 0.5 3.5 4.8 0.7 0.8 1.4 2.1 3.8 6.7 8.3 7.2 4.4 4.4 4.0 3.4 3.5 3.3 4.1 3.2 3.8 2.3 2.2 1.6
3.0 5.6 5.1 1.8 3.3 3.0 1.7 0.8 0.2 1.5 2.2 0.6 1.5 3.0 5.2 8.4 15 13 11 6.9 5.1
7.2 3.8 3.2 2.9 3.7 2.7 3.6 1.6 1.6 1.6
3.6 7.8 7.4 2.8 3.1 3.4 2.1 2.2 0.5 2.3 3.9 1.1 2.6 4.9 6.6 7.9 8.4 7.9 7.3 5.8 4.5 7.1 3.7 3.1 3.0 3.5 2.5 3.7 1.8 1.6 1.5
3.8 7.6 7.4 2.5 3.5 3.5 2.1 2.3 0.5 2.7 4.8 0.9 2.8 4.9 6.4 7.6 8.6 7.8 7.5 6.2 4.8 7.3 4.1 3.4 3.0 3.5 2.6 3.2 1.6 1.5 1.7
6.5 8.7 7.3 1.9 2.6 1.9 4.2 2.2 0.7 1.9 3.9 1.3 1.6 2.9 4.3 6.2 7.5 8.5 6.7 5.2 5.7 6.3 3.1 4.5 3.6 4.3 4.3 3.6 2.8 2.4 1.5
"Results are from two low-volume samples and one high-volume sample collected in Boulder, in parallel during the same period of time, on the University of Colorado campus. 'Samples were collected for 9.5h. Samples were collected for 7.5h. dSamples were collected for 6.5 h. 'LV no. 1 is notation for low-volume sampling with thermal desorption for sample tube eo. 1. fHV is notation for high-volume sampling with solvent extraction/fractionation. 8 Benzo[b]fluoranthene and benzo[k]fluoranthene could not be chromatographically resolved, so the result reported is the concentration of both compounds assuming the same mass spectrometry response for each. hND refers to compounds that could not be detected. compounds over the even numbered compounds (19) and the alkanes n-C2,Hb6,n-CzgHso,and n-CS1HG,are usually present in the highest concentrations in aerosols. Fossil fuel combustion produces particulate matter which contains relatively large concentrations of PAHs and saturated alkanes. The n-alkanes from combustion of fossil fuels do not have a significant predominance of the odd carbon numbered compounds over the even carbon numbered compounds. The alkanes present in the highest concentrations in aerosols will usually be n-C1gH40,n-CZoH42, and n-CZ1H4& Particulate samples which showed varying degrees of contributions from natural and anthropogenic sources were observed in the Boulder area. Occasionally samples were observed that appeared to be substantially from natural sources, while in other cases pollution from anthropogenic sources so dominate the samples that compounds predominantly from natural sources were not discernible in the chromatograms. The data in Table I1 taken on January 9th indicate that this sample contains appreciable natural source contributions to the aerosol. It is important to note that the air was relatively clean on that day, as reflected in low absolute concentrations of all particulate volatile organic compounds, and, in garticular,the compounds stemming principally from anthropogenic sources. The odd carbon numbered n-alkanes were more abundant than the even carbon numbered n-alkanes, and the PAHs were present in lower concentrations than usual. A chromatogram of a summertime aerosol sample collected on June 24, 1984, when biological productivity is high, shows an even more pronounced contribution of natural sources to the airborne particulate matter (Figure 4).
THERMAL DESORPTION OF AIRBORNE PARTICULATE MATTER
'P
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EMPTY COLLECTION TUBE BLANK
20 0
40 60 TIME (MINUTES AFTER DESORPTION INITIATION )
7'5
CHROMATOGRAPHIC
Ix) 225 COLUMN TEMPERATURE ('C)
360
Flgure 4. Particulate sample was collected in Boulder, from 322 L of air, for 100 min. The numbered chromatographic peaks were identified compounds with corresponding numbers in Table I . The ion source of the mass spectrometer was not 'turned on until the chromatographic oven temperature reached 0 O C .
The data in Table I1 for samples taken on January 10, 1985, and January 12,1985, are more typical of urban aerosols and
2812
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
indicate a much larger contribution from combustion sources. The PAHs are present in much higher concentrations than on January 9, there is only a slight odd carbon number predominance, and n-C21H44,n-C22H46,and n-C23H48are the hydrocarbons which are present in the highest concentrations. All of the chromatograms obtained by using low-volume sampling with thermal desorption showed a surprisingly large number of early eluting volatile organic compounds (Figure 4). These may be sorbed on carbonaceous particles, or they may arise from thermal decomposition of less volatile compounds. Investigations indicated that these volatile but polar compounds were, in general, not the same vapor phase compounds which are typically found in urban air, even though some of the former have lower boiling points than compounds found principally in the vapor state. Bertsch et al. (12) observed similar results when airborne particulate matter from a high-volume sampler was thermally desorbed. Our GC/MS studies showed that several of these compounds were low molecular weight monocarboxylic acids and alkyl furans. While it is possible that some of these compounds could be thermal decomposition products of higher molecular weight compounds, low molecular weight carboxylic acids, 2methylfuran, and 3-methylfuran have all been proposed as secondary reaction byproducts from the atmospheric oxidation of alkenes ( 2 0 , Z I ) . The ability to detect these low molecular weight compounds could offer significant advantages over traditional analytical methods, in which volatile compounds are lost or masked by solvent extraction coupled with evaporative concentration of the extract. However, it will be important to investigate the origin of each individual compound to ensure that they are not pyrolysis products, before atmospheric significance is inferred. Currently used high-volume methods for collecting organic constituents in airborne particulate matter exhibit experimental bias against the lower molecdar weight compounds. Other investigations have demonstrated that long sampling times, involving the passage of large volumes of air through the sample, cause a substantial sample volatilization, even for compounds with molecular weights as high as benzo[a]pyrene (17,22,23). In solvent extraction procedures, in which extracts are concentrated to dryness at room temperature or above, the detection and measurement of these early eluting species are difficult and in most instances impossible. The solvent peak also masks the presence of some low molecular weight compounds by coelution with these species. In the low volume sampling thermal desorption procedure, the volume of filtered air is small, preconcentrationsteps are eliminated,and solvents are not used, so the loss of low molecular weight compounds should be less extensive. Precision of Low-Volume Sampling Coupled with Thermal Desorption. The precision of low-volume sampling coupled with thermal desorption was determined by the parallel collection of aerosol samples in small volumes of ambient air. Replicate pairs were collected on six different days and 11 PAHs and 20 aliphatic hydrocarbons were quantitatively determined from each sample tube. These paired observations were statistically evaluated, using a two sided t test at a 95% confidence level, and the results showed that there is no significant difference between the sample pairs for all 31 of the hydrocarbons investigated. The statistical method was a standard treatment for comparing average performance when samples have been collected as pairs by two separate treatments (24). The method involves finding the difference in concentration between each hydrocarbon measured and then pooling these differences for all of the pairs which were analyzed and determining whether the differences are significant. The results from three sets of these paired samples are presented in Table 11, and a visual inspection of
the data reinforces the statistical conclusion. The precision of low-volume sampling coupled with direct thermal desorption demonstrates that the method can be relied upon to give reproducible quantitative results for the hydrocarbons listed in Table 11. Comparison of Low-Volume/Thermal Desorption with the High-Volume/Solvent Extraction Technique. Lowvolume sampling coupled with direct thermal desorption could have several advantages over the more traditional high-volume solvent extraction procedure. Large amounts of information can be obtained with a minimum of time and sample handling. A typical extraction/fractionation scheme requires 8-12 h, but in the thermal desorption procedure these steps are unnecessary. In the thermal desorption procedure, no solvents, containers, or fractionation columns are used, so contamination of the sample is minimized (25, 26). If filters with similar particle loadings are analyzed, a related thermal desorption technique has been stated to be 100 times more sensitive than most solvent extraction methods (6). Thermal desorption is more sensitive because the entire sample is used for analysis, not just a small portion of an extract, and artifacts are reduced by elimination of extensive sample handling. The solvent extraction technique provides adequate sample volumes for replicate chromatographicmeasurements, but the accuracy of determinations is limited by the effectiveness of extraction and the introduction of artifacts in the prefractionation steps (25,27-29). The entire sample is used in the thermal desorption technique, but it is a simple matter to collect replicate samples which can be independently analyzed or used to assess precision. An estimation of the accuracy of the low volume sampling thermal desorption technique was obtained by comparing the technique with the well-established high-volume sampling solvent extraction/fractionation procedure. The comparison involved the parallel collection of airborne particulate matter by both the low-volume and high-volume procedures and the subsequent quantitative analysis of selected organic constituents in the samples. The results from the investigations are reported in Table 11. The two procedures agree within a factor of 2.5 for 98% of the 87 individual measurements shown in Table I1 and agree by a factor of 1.25 for half of the compounds quantitated. The agreement between these two procedures is similar to results obtained by other investigators when different sampling and analysis schemes, performed side by side, were compared (30, 31). To determine whether the conventional solvent extraction technique removes all of the organic compounds from a high-volume filter, sections of an exposed/solvent extracted filter were subsequently thermally desorbed. On the average, an additional 20% of each of the n-alkanes from n-pentadecane through n-pentacosane was measured upon thermal desorption, beyond that originally determined by the solvent extraction procedure. The PAHs from phenanthrene to chrysene, and the other normal alkanes were also detected, but only at levels which were about 5% to 10% of the values measured by the solvent extraction procedure. The inefficiency of solvent extraction for the quantitative removal of the organic constituents in airborne particulate matter has been well documented (27-29). Thermal Desorption of SRM 1649. The concentrations of PAHs in the SRM 1649, which consists of particles collected from the ambient air of a Washington, DC, suburb, were quantitatively determined by thermal desorption of these compounds into a GC/MS/DS operating in the selected ion monitoring mode, Results from averages of 11determinations of 11 compounds are given in Table 111. A two sided t test was conducted, using a 95% confidence level, to determine
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table 111. Concentrations of Polycyclic Aromatic Hydrocarbons in the National Bureau of Standards Standard Reference Material 1649 Measured by Thermal Desorption, pg/g
compound
measured using NBS thermal determined desorptionu values
phenanthrene fluoranthene pyrene benz[a]anthracene chrysene benzo[b] and [klfl~oranthenes~ benzo[e]pyrene benzo[a]pyrene perylene
indeno[1,2,3-cd]pyrene benzo[ghi]perylene
4.9 f 1.3 7.3 f 2.7 6.0 f 2.1 2.8 f 1.1 3.8 f 1.1 7.1 f 1.9 3.1 f 1.8 2.2 f 1.4 0.9 f 1 4 f 9
519
4.5 f 0.3* 7.1 f 0 3
7.2 f 0.2d 2.6 f 0.3c 3.6 f 0.2b 8.2 f 0.4 3.3 f 0.2d 2.9 f 0.5c 0.84 f O.Ogd 3.3 f 0.5c 4.5 f 1.1c
aValues are averages of analysis of 11 samples, and the uncertainties are 95% confidence intervals of the mean. bValue reported by NBS using a liquid chromatography method, not a certified value. NBS certified value determined by several methods. Value reported by NBS using a gas chromatography method, not a certified value. e Benzo[b]fluoranthene and benzo[k]fluoranthene could not be chromatographically resolved, BO the result reported is the sum of the congentration of both compounds assuming the same mass spectrometry response. !This is the sum of two values reported by NBS using a liquid chromatography method. whether the average concentrations measured by thermal desorption and the average values determined by the National Bureau of Standards (NBS) were different. The results showed that there is no statistical difference between the thermal desorption values and the NBS-determined values for all 11 of the PAHs quantitated. A two-sided f statistic was calculated, using a 95% confidence level, to determine whether the thermal desorption values and the NBS-determined values had a different variability. The results indicated that there was a significant difference in the variability of the two procedures for all of the PAHs measured. The precision of the concentration measurements in the low-volume sampling thermal desorption procedure was determined to be much better than the precision indicated by the thermal desorption of microgram-size samples of the SRM 1649. The poorer precision of measuring the PAHs in the SRM 1649 is, therefore, thought to result from the relative inhomogeneity of the SRM at the microgram level. NBS certifies the homogeneity of SRM 1649 only if samples of 1 g or larger in size are taken for analysis. Even though the thermal desorption concentration values have a greater uncertainty than those determined by NBS, this uncertainty was still relatively low considering that less than 1 mg of SRM 1649 was taken for analysis by thermal desorption. The precision of thermal desorption of SRM 1649 is graphically illustrated in Figure 5. It is seen that the measurement of benzo[e]pyrene in different amounts by thermal desorption results in reasonable precision. The two points that lie off of the best fit line may indicate expected inhomogeneities in SRM 1649. When a similar linear regression analysis was performed for each of the 11 PAHs determined, the correlation coefficient of these plots ranged from 0.96 for benzo[elpyrene to 0.70 for benzo[ghi]perylene, with an average correlation coefficient of 0.89. The lowest correlation coefficients were observed for the six-ring PAHs; this may be partly due to an insufficiently high temperature (254 "C)at which the thermal desorption of these less volatile compounds was effected. Efficacy of T h e r m a l Desorption. The temperature of thermal desorption was always lower than the temperature setting of the heated aluminum block because of the time
3500'
2813
Benzo[e]pyrena o[e]pyrena from the Thermal Desorption of SRM 1649 Correlation Coefficient
t
0.96
h
s! I! 11750-
b
w-
0
e!
4
v +
s
2
-
"
9
0
00
160 240 pg of Urban Dust SRM 1649
320
Flgure 5. Linear regression line from the measurement of benzo[elpyrene in 11 different sample sizes of SRM 1649 by direct thermal desorption followed by GUMS. Mass spectrometer integration of the charge to mass ratio of 252 for benzo[e]pyrene plotted against the micrograms of SRM 1649 thermally desorbed.
required for heat transfer. For heating block temperature settings from 150 "C to 300 "C,the maximum temperature eached a t the filter of the desorption tube, for a 15-min desorption time, can be calculated from the following empirical equation: ?'filter
+
= 0.8T+,lock 14 "C
(1)
In most of the thermal desorption experiments, desorption was performed at a heating block temperature of 300 O C which resulted in a maximum desorption temperature of 254 "C at the filter of the desorption tube. For a heating block temperature of 300 "C, the temperature at times after initiation of heating is described by the following empirical formula (Tfilterin "C,t in min): Tfilter = 23.1 75.5t - 10.4t2 0.661t3 - (1.56 X 10-2)t4 (2) Early investigations showed that any "cold spots" in the thermal desorption apparatus resulted in incomplete transfer of less volatile compounds. To solve this problem, heat was applied from the top of the sample tube to the head of the chromatography column, including the ferrules, fittings, injection needle, and sample tube. When thermal desorption was performed a second time on a previously desorbed sample the chromatograms were indistinguishable from those obtained by analyzing a new clean filter tube. There was initial concern that some organic compounds might remain on the ferrules or in the injection needle, after sample desorption, but subsequent heating of the ferrules and needle showed no memory effect. A thermal desorption temperature study is presented in Table IV, and the results show that if quantitation of PAHs and hydrocarbons is desired, the thermal desorption experiment must be conducted at an appropriate temperature. At a lower temperature, only the more volatile aliphatic hydrocarbons and PAHs were quantitatively vaporized. The optimal thermal desorption temperature depends on the volatility and thermal stability of the compounds to be determined and the particle matrix, but discrepancies in the literature indicate that it is also dependent on other factors, such as the geometry of thermal desorption apparatus and the procedures used (8). Pyrolysis during Thermal Desorption. Articles dealing with thermal desorption of airborne particulate matter have discussed the possibility of thermal degradation of organic compounds during sample heating (6, 32). Given the vast number and complexity of the organic compounds in particulate matter, thermal degradation of some compounds is a reasonable possibility. Our experimental investigations in-
+
+
2814
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table IV. Effect of Temperature on Desorption Recoveries (Ratios of Quantity Measured to Quantity Added)" compound
300 O C
n-dodecane n-tridecane n-tetradecane n-pentadecane n-hexadecane n-heptadecane n-octadecanec n-nonadecane n-eicosane n-heneicosane n-docosane n-tricosane n-tetracosane n-pentacosane n-hexacosane n-octacosane n-triacontane n-dotriacontane phenanthrene fluoranthene pyrene benz[a]anthracene chrysene benzo[b] and [Fzlfluoranthene benzo [e]pyrene benzo [a]pyrene perylene indeno [1,2,3-cd]pyrene benzo[ghi]perylene
b 0.78 0.94 0.97 b 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.88 1.0 1.0 1.0 1.0 1.0 1.1 1.0 0.95 1.1 0.95 0.95
ratios at the following heating block temperatures 275 OC 250 " C 225 O C 190 o c 0.74 0.90 0.95 0.95 b 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.1 1.0 1.0 0.97 1.0 0.66 1.0 0.97 0.96 0.98 0.98 0.98 0.98 1.0 0.89 0.78 0.88
0.62 0.82 0.95 0.95 b 1.1 1.0 1.0 1.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.82 0.47 1.0 0.96 1.0 0.96 0.97 0.84 0.84 0.81 0.75 0.54 0.54
0.78 0.95 0.95 1.1 b 0.99 1.0 1.0
1.0 1.0 1.0 1.0 1.0 0.98 0.96 0.83 0.54 NDc 1.0 0.99 1.0 0.94 0.94 b b b
ND ND ND
b 0.97 0.97 0.98 b 1.0 1.0 1.0 1.0 1.0 0.98 0.91 0.80 0.64 0.47 0.20 0.05 ND 1.0 0.92 0.97 0.67 0.65 0.55 b 0.34 ND ND ND
150 OC 0.80 0.97 0.97 1.0 b 1.0
1.0 1.0 0.93 0.76 0.54 0.35 0.22 0.13 95%) upon thermal desorption. The quantitations of PAHs in the SRM 1649 also indicate that thermal desorption does not create or destroy significant quantities of these compounds (Table 111). It has been experimentally determined that a minimum amount of sample pyrolysis and a maximum amount of volatilization occurs when atmospheric particulate matter is heated as rapidly as possible (32). In our procedure samples were heated rapidly by inserting them into a preheated block, and heat transfer was facilitated by keeping the thermal mass of the desorption tube as small as possible. Furthermore, heating was accomplished while passing helium over the filter, so that vaporized compounds were removed from the hottest zone as quickly as they reached the minimum temperature a t which they became appreciably volatile.
CONCLUSION Low-volume sampling coupled with direct thermal desorption is a more sensitive and rapid technique than the traditional high-volume sampling solvent extraction procedure. The technique exhibits acceptable precision and shows reasonable quantitative agreement with the high-volume solvent extraction/fractionation method. The technique is simple, and, therefore, should also be useful in the routine determination of specific organic compounds in airborne particulate matter, such as benzo[a]pyrene. Low-volume sampling coupled with thermal desorption should also be more reliable than existing techniques for studying the more volatile organic
compounds associated with airborne particulate matter because these compounds can be lost by volatilization or masked by the solvent in the high-volume sampling solvent extraction procedure. Registry No. Phenanthrene, 85-01-8;fluoranthene, 206-44-0; pyrene, 129-00-0;benz[a]anthracene, 56-55-3;chrysene, 218-01-9; benzo[blfluoranthene, 205-99-2; benzo[k]fluoranthene, 207-08-9; benzo[e]pyrene, 192-97-2;benzo[a]pyrene, 50-32-8;perylene, 198-55-0; indeno[ 1,2,3,-c,d]pyrene,193-39-5;benzo[ghi]perylene, 191-24-2;n-pentadecane, 629-62-9; n-hexadecane, 544-76-3;nheptadecane, 629-78-7;n-octadecane, 593-45-3;n-nonadecane, 629-92-5; n-eicosane, 112-95-8; n-heneicosane, 629-94-7; n-docosane, 629-97-0;n-tricosane, 638-67-5;n-tetracosane, 646-31-1; n-pentacosane, 629-99-2; n-hexacosane, 630-01-3; n-heptacosane, 593-49-7;n-octacosane, 630-02-4;n-nonacosane, 630-03-5;ntriacontane, 638-68-6; n-hertriacontane, 630-04-6; n-dotriacontane, 544-85-4; n-tritriacontane,630-05-7; n-tetratriacontane,14167-59-0; n-dodecane, 112-40-3; n-tridecane, 629-50-5; n-tetradecane, 62959-4.
LITERATURE CITED (1) Josephson, J. Environ. Sci. Techno/. 1981, 15, 20-22. (2) Sirnoneit, E. R. T. Atmos. Environ. 1984, 18, 51-67. (3) Simoneit, E. R. T.; Mazurek, M. A. Atmos. Environ. 1982, 16, 2139-2159. (4) Kamens, R.; Bell, D.; Dietrich, A,; Perry, J.; Goodman, R.; Claxton, L.; Tejada, S. Environ. Sci. Techno/. 1985, 19, 63-69. (5) Yu, M.; Hites, R. A. Anal. Chem. 1981, 5 3 , 951-954. (6) Crouch, R. L.; Hawley-Fedder, R. A,; Parsons, M. L.; Karasek. F. W. J. Chromatogr. 1984, 303, 53-60. (7) Gab, E.; Grennfelt, P. J. Chromatogr. 1983, 279, 643-648. (8) Kopczynski, S.L. Anal. Lett. 1984, 17, 97-111. (9) Higgins, C. E.; Griest, W. H.; Caton, J. E.; Harmon, S. H. Anal. Chim. Acta 1983, 15, 173-180. (IO) Maiissa, H.; Puxbaum, H.; Zojer, K. Mikrochim. Acta 1981, 1, 1-8. (11) Wauters, E.; Sandra, P.; Verzele, M. J. Chromatogr. 1979, 170, 125-131. (12) Bertsch, W.; Chang, R. C.; Zlatkis, A. J. Chromatogr. Sci. 1974, 72, 175-182. (13) Burchfleld, H. P.; Green, E. E.; Wheeler, R. J.; Billedeau, S. M. J. Chromatogr. 1974, 9 9 , 697-708.
2815
Anal. Chem. 1985, 57,2815-2818
(26) Allen, 0.M.; Coleman, D. M. Anal. Chem. 1984, 56, 2984-2987. (27) Cautreeis. W.; Van Cauwenberghe, K. Water Air Soil follut. 1976, 6, 103-110. (28) Griest, W. H.; Yeatts, L. B., Jr.; Caton, J. E. Anal. Chem. 1980, 5 2 , 199. (29) Renkes, G. D.; Walters, S. N.; Woo, C. S.; Iles, M. K.; D’Silva, A. P.; Fassel, V. A. Anal. Chem. 1983, 55, 2229-2231. (30) Camp, D. C.; Van Lehn, A. L.; Loo, B. L. “Intercomparison of Sampiers Used in the Determination of Aerosol Composition”; Report No. EPA-600 17-78-1 18; U S . Environmental Protection Agency, Environmental Sclence Research Laboratory: Research Triangle Park, NC, July 1978. (31) Clements, H. A,: Mc Mullen, T. B.; Thompson, R. J.; Akland, G. G. J. Alr follut. Control Assoc. 1972, 22, 955-958. (32) Tanner, R. L.; Gaffney. J. S.; Phillips, M. F. Anal. Chem. 1982, 54, 1627-1 830.
(14) Lloyd, R. J. J. Chromatogr. 1984, 284, 357-371. (15) Cronn, D. R.; Charison, R. J.; Knights, R. L.; Crittenden, A. L.; Appel, B. R. Atmos. Envlron. 1977, 1 1 , 929-937. (16) Lamb, S. I.; Petrowski, C.; Kaplan, 1. R.; Simonelt, B. R. T. J. Alr follut. Control Assoc. 1980, 30, 1098-1115. (17) Cautreeis, W.; Van Cauwenberghe, K. Atmos. Environ. 1978, 12, 1133-1 141. (18) Berkenbus, B. D.; Mac Dougall, C. S.; Grlest, W. H.; Caton, J. E. Atmos. Environ. 1983, 17, 1537-1543. (19) Cooper, J. E.; Bray, E. E. Geochim. Cosmochlm. Acta 1983, 2 7 , 1113. (20) Grosjean, D.; Van Cauwenberghe, K.; Schmid, J. P.; Kelley, P. E.; Pitts, J. N., Jr. Environ. Sci. Technol. 1978, 72,313-317. (21) Gu, C.: Rynard, C. M.; Hendry, D. G.; Mill, T. Envlron. Scl. Technol. 1985, 19; 151-155. (22) Van Vaeck, L.; Van Cauwenberghe, K.; Jenssens, J. Atmos. Envlron 1984, 78,417-430. (23) Katz, M.; Chan, C. Envlron. Sci. Technol. 1980, 14, 838-843. (24) Natrella, M. G. “Experimental Statlstics”, National Bureau of Standards Handbook 91, August 1963; pp 3-31. (25) Hawley-Fedder, R. A,; Bowers, W. D.; Parsons, M. L.; Karasek, F. W. J. Chromatogr. 1983, 269, 161-170.
.
RECEIVED for review April 8, 1985. Accepted July 15, 1985. We are grateful to the National Science Foundation for support of this research under Grant ATM-8317948.
Concentration of Trace Elements in Water Samples by Reductive Precipitation R. K. Skogerboe and W. A. Hanagan’ Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
H. E. Taylor*
U S . Geological Survey, Box 25046, M S 407, Denver Federal Center, Denver, Colorado 80225
6-9), the degree of preconcentration usually is limited (8); a range of extraction conditions may be required to concentrate a diverse range of elements (1,3,7,8); interference effects are not uncommon (3,9-21); and many chelating agents are unstable and/or difficult to obtain in pure form (1, 9 ) . In addition, if PES is the determination method of choice, the use of organic solvents may adversely affect the plasma for the determination of selected elements. Since PES is enjoying increasing popularity, the development of other preconcentration procedures not subjected to the same problems is a worthwhile goal. The use of borohydride for elemental preconcentration of trace elements by coprecipitation was investigated for a variety of reasons listed below. The results summarized herein indicate that this approach offers several features that favor its use for a variety of applications. The general half-reaction for the reduction of metals to the elemental state can be specified as follows:
The use of borohydride reduction as a means of preconcentratlng elements by precipitation as the element or as a boride has been explored. It has been shown that the optimized procedure reproducibly effects the precipitation of all 18 elements studied; only four of these exhlblted recoveries less than 90%. The general ease of use, the demonstrated accuracy and precision, the high preconcentration factors available, the self-cleansing properties of the primary reagent, the granular character of the precipitate, and the possibility of direct analysis of the preclpltate are ail factors to recommend thls approach.
Concern about the chemistry and toxological effects Of nonmetals and transition metals in waters has enhanced needs for their determination at trace to ultratrace levels (1-4). Although many of the requirements are satisfied by atomic absorption (AA) or plasma emission spectrometry (PES) ( 1 , 5) concentrative pretreatment Of significant numbers Of samples is often required to achieve the requisite overall analvsis sensitivitv (1-3). In other cases. Dreconcentration
M,,
9
+ ne-
-
M(s)
(1)
When borohydride is used as the reducing agent, the following oxidation half-reaction may be written ~.
that cause interferences. -Among the characteristics that a preconcentration procedure should exhibit are high preconcentration factors, sufficient selectivity to allow exclusion of unwanted constituents, freedom from interference and contamination problems, to different types and analysis techniques, and ease of use. Solvent extraction commonlv is used for trace-element preconcentration. Although it offers several advantages ( 1 ,
Since the reduction potential of borohydride in alkaline solution is relatively high, reduction of Ag, As, Au, Bi, Cd, Ge, Hg, Ir, os,Pb, Re, Rh, Ru, Sb, Se, and T1 to the elemental state is thermodynamically favored (12-15). In addition, precipitation reactions occw by the formation of insoluble metal borides
Mn++ nB-
’
‘Present address: 17 Barclay Court, Blue Bell, PA 19422.
-
MB,
(3) Elements that can be precipitated by this mechanism include Co, Cu, Fe, Mn, and Ni (16). Preconcentration by reductive
This article not subject to US. Copyright. Published 1985 by the American Chemical Society
