Environ. Sci. Techno/. 1995, 29, 2099-2106
Computer-Controlled Low-Volume Air Sampler for the Measurement of Semivolatile Organic Compounds JEFFREY C. WALLACE AND RONALD A. HITES* School of Public and Environmental Affairs and Department of Chernistiy, Indiana University, Bloornington, Indiana 4 7405
We have developed an automated, low-volume air sampler, capable of collecting multiple samples for semivolatile organic compound analysis in a short time (several hours). The new sampler uses small polyurethane foam plugs as the sampling media and a sampling carousel that has a 20 plug capacity. A computer controls the sampling protocol, and after the sampler is loaded with plugs, a sampling program is carried out with no operator intervention. The plugs are removed, and the analyte is extracted by supercritical fluid extraction. A series of quality control experiments were undertaken to demonstrate that the new sampler performed properly. Air volumes in the range of 100 m3can be collected without exceeding the breakthrough volume of the foam plugs. Passive sampling and volatilization artifacts are minimal. Concurrent air measurements using our new sampler and a conventional high-volume air sampler over a week-long period gave statistically identical (at the 95% significance level) air concentrations for polychlorinated biphenyls and endosulfan.
Introduction Semivolatile organic compounds (SOCs) are those compounds with vapor pressures in the range of 10-1-10-6 Pa at ambient temperatures (1). These compounds include such well-known pollutants as polychlorinated dibenzop-dioxinsand dibenzofurans (PCDDIF),polycyclic aromatic hydrocarbons (PAHs),and polychlorinated biphenyls (PCBs). Atmospheric concentrations of these compounds range from less than a picogram to hundreds of nanograms per cubic meter of air (2). Conventional samplingof SOCs requires drawing a large volume of air (about 1000 m3) through a filter to collect particle-associated SOCs and then through a sorbent medium to collect gas-phase SOCs. This technique is known as high-volume air sampling, and it has been used widely and successfully since first described by Bidleman and Onley over 20 years ago (3).Bidleman used glass fiber sheets as the filter and polyurethane foam (PUF) cylinders as the sorbent media. While this is the most common * E-mail address:
[email protected].
0013-936W95/0929-2099$09.00/0
0 1995 American Chemical Society
combination of filter and sorbent, others have been used. and These include the sorbents XAD (4) and Tenax (3, Teflon membrane filters (6')have also been used. PUF is an attractive sorbent because it is inexpensive and easy to work with, but XAD and Tenax sorb larger amounts of analyte. Teflon membrane filters absorb less gas-phase components than glass fiber filters, leading to fewer sampling artifacts (7). Since the introduction of high-volume sampling, the technique has remained relatively unchanged, although there have been some small improvements. Hart etal. (8) modified a conventional high-volume air sampler to accommodate thin polyurethane foam sheets instead of PUF cylinders. The thin foam sheets allowed for higher flow rates with lower pressure drops across the sampling head. This gave faster sampling rates than conventional high-volume sampling, m i n i m i i g the possibility of volatilization losses from or adsorptive gains to particles on the filter. Zaranslci etal. (9)used polyurethanefoam-granular adsorbent sandwich cartridges to improve collection efficiencies for more volatile compounds. XAD sorbent casings have been modified to fit directly into a Soxhlet extractor,minimizing samplehandling (10). High-volume samplers have been insulated and acoustically damped to allow for quiet sampling of indoor air (11). There are a few drawbacks to high-volume sampling. First, it is a time consumingprocess. It typicallytakes about 24 h to draw sufficient air through the sorbent. Backup glass filters may be needed to correct for adsorptive artifacts (12). Also, Soxhlet extraction, which is used to remove the analyte from the filter or adsorbent, takes an additional 12-48 h. High-volume sampling is difficult to automate. A sample cartridge must be loaded, and the samplingpump has to be turned on by an operator, and at the end of the sampling period, he or she must remove the plug before the process can be repeated. All of these factors tend to limit the sampling frequency to 1-2 samples/week. Hence, most studies examining atmospheric SOCs using highvolume methodologies have been on long time scales (months to years) and have been based on weekly samples (13-16'). This low sampling frequency tends to limit our knowledge of how SOCs behave in the atmosphereon short time scales. We have developed an automated, low-volume air sampler, capable of collecting multiple samples in a short time (severalhours), coupled with a rapid sample preparation scheme. The low-volume sampler uses small PUF plugs as the sampling media and a sampling carousel that has a 20 plug capacity. A computer controls the sampling protocol, and after the sampler is loaded with plugs, a sampling program is carried out with no operator intervention. The plugs are removed, and the analyte is extracted by supercritical fluid extraction (SFE), which has been shown to be a rapid and efficient method for removing SOCs from PUF (17). While Soxhlet extraction takes approximately 1day to complete, a SFE experimentcan be completed in less than 1 h.
Sampler Design The sampling system has three main components; the sampler, the pumping system, and the control electronics.
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diameter) bisects the inner sleeve 0.3 cm from the bottom.
This rod prevents the plug from being sucked into the pump.
FIGURE 1. Schematic diagram of the low-volume air sampler. Inset shows detail of the sampling head.
The sampler, shown in Figure 1, weighs approximately 30 kg. A description of the individual components is given below. Unless otherwise specified, all mechanical components used in the sampler are made of anodized aluminum, and all screws and fasteners are stainless steel. Frame. The dimensions are 61 cm wide x 61 cm deep x 43 cm high. The frame of the sampler is covered on the sides and bottom with 1.6 mm thick aluminum sheeting. These sheets are connected to the frame at approximately 10-cm intervals by machine screws. A central aluminum beam (A) (10 cm x 63 cm x 1.3 cm) bolted to the frame is used to mount the sampling carousel. Lid. The lid of the sampler has two parts; an inner and an outer section. The inner section (B) (61 cm x 61 cm) is connected to the frame with two hinges. A hole (C) (5 cm diameter) bored through the inner lid allows for air flow. This hole is reinforced by a collar (2.5 cm high). The outer lid (D) (71 cm x 71 cm x 13 cm) connects to the inner lid by four aluminum rods (E). The upper lid serves as a cover to keep water and other debris from entering the air sampler. There is a clearance of 8 cm between the outer and inner lids. The inner lid, when closed, forms a tight seal with the frame. Carousel. The sampling carousel (F) is made of aluminum and has a radius of 25 cm. Twenty holes are bored equidistant along the perimeter at a radius of 21.5 cm. These holes are where the sample holders are placed. An aluminum pedestal (G)is connected to the carousel by six screws. Sampling Sleeves. There are two parts to the sampling sleeve assembly, an inner and an outer sleeve. The inner sleeve (H)has an inner diameter of 2.6 cm and an outer diameter of 3.2 cm and is 8.6 cm long. The exit hole at the bottom of the sleeve has a diameter of 2.5 cm. The inner sleeve holds a foam plug. An aluminum rod (0.3 cm
The inner sleeve is inserted into the outer sleeve (I) prior to sampling. Small screws attach each outer sleeve to the carousel. The outer sleeve has an inner diameter of 3.2 cm. The inner sleevesare designed to incorporateparticulate filters. The inside of the upper rim of each sleeveis recessed 0.2 cm wide by 0.2 cm deep. A glass fiber filter 3.2 cm in diameter can be placed over the top of the sleeve. An aluminum retaining ring (3.2 cm in diameter, 0.2 cm thick) fits tightly into the recess and holds the filter firmly in place. The retaining ring has a 0.2 cm notch cut into it to facilitate easy removal after sampling. DriveTrain. The drive train of the sampler is responsible for rotating the carousel. It consists of a stepper motor 0) (Superior Electric Co., Bristol, CN) connected to a gear reducer (K), which connects to the carousel. The stepper motor connects to the frame by an aluminum collar. The gear reducer slows the rotation of the carousel. The carousel pedestal and stepper motor connect to the gear reducer by aluminum tubes (L). The tubes are secured to the shafts by double set screws at the top and bottom of each tube. Home Indicator. An optical transistor (M)is attached to the frame by a stainless steel arm (N). This optical transistor serves as the home indicator for the sampling carousel. A small aluminum tooth (0)is connected to the carousel (17cmfrom center). The home indicator is aligned so that the tooth will pass through it as the carousel rotates. Sampling Head. The sampling head connects the sampling sleeve to the air pump. The sampling head connects to the frame by an aluminum block. Refer to the inset of Figure 1 for a side-on view of the sampling head. The top of the sampling head is a milled aluminum tube with an outer diameter of 5.1 cm. The inner diameter is 2.2 cm. A 0.6 cm wide groove at a radius of 3 cm houses a rubber O-ring. The O-ring provides a tight seal between the bottom of the outer sampling sleeve and the top of the samplinghead. An L-shaped aluminum tube (PIis welded to the bottom of the sampling head. This tube is connected to a 30 cm, 2.5 cm i.d. section of flexible stainless steel tubing (Q). The entire samplinghead is supported by two rails (see inset of Figure 1). These are stainless steel rods 15cm long. A spring and a stainless steel sleeve are placed over each rail. Tension from the springs keeps the top of the sampling head flush with the sampling sleeve. The rails connect to the bottom of the samplinghead by set screwsat the bottom of each rail. The rails go through the top of the sampling head collar through two holes. The sampler head collar is then free to move up and down on the two rails. A solenoid is connected to the sampling head collar by a small metal connector. The solenoid is mounted directly below the samplinghead. By energizing the solenoid, the sampling head retracts downward about 0.2 cm to allow for free carousel rotation. Check Valve. A 2.5 cm i.d. stainless steel check valve (R)is placed between the pump and the sampling head. This valve allows air to flow only from the head to the pump and not the other way. This eliminates any “chimneyeffect” that could result if air flows from the pump into the sampler. The check valve exits the sampler through a nozzle (2.5cm i.d.) that is secured to the side of the sampler. Baffle. The upper half of the sampler is separated from the lower half by an aluminum baffle (1.6 mm thick). For
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clarity, the baflle is not shown in Figure 1. The purpose of the bafne is to keep the carouselisolatedfrom the electronic and mechanical parts in the bottom of the sampler. Four holes are cut through the baffle; one for the carousel pedestal, one for the sampling head, and two for the rails that support the sampling head. Air is circulated through the bottom of the sampler to keep it cool by a small electric fan attached to the front wall of the lower portion of the sampler. Air is vented from the lower portion of the sampler through a hose (2.5cm i.d.1 connected to a nozzle mounted to the side of the sampler opposite the sampling head. Pumping System. There are three main components to the pumping system; the air pump, the butterfly valve, and the fldvmeter. Air flows from the sampling head, through the flowmeter,through the valve, and out the pump. The sampler is connected to the pumping system through a length of flexible plastic tubing. The pump is a ringcompressor type (Fuji Electric Corporation of America, Model VFC404P-5T, Lincoln Park, NJ); the flowmeter (American Meter Company, Indianapolis, IN) is a Model Al-425.The flowmeter has been modified with two optical interrupters to allow accumulation of gas flow measurement in units of 0.02 m3. The butterfly valve is used to regulate the flow of air through the foam plugs. All plumbing in the pumping system has a minimum internal diameter of 2.5 cm. Computer and Electronics. An IBM-XT personal computer controls the sampling procedure. The Quick-BASIC program that controls the sampling procedure is stored on the PC's hard drive. The system could be made less cumbersome by using a laptop computer. To understand the air sampler system, an example of a sampling sequence is given here: When a sample time programmed into the computer is reached, the following control sequence is initiated: The air pump is deenergized, stopping the air flow. The sampling head solenoid is energized, breaking the seal to the current sample tube. This must be done to allow the computer to advance the carousel. The carousel is rotated to the desired sample. The sampling solenoid is deenergized, allowing the spring-loaded seal to be reestablished. The gas flow butterfly valve is positioned for the desired gas flow rate, and the air pump is energized. The sample algorithm either initiatesthe collection of a specifiedvolume of air and time is recorded or the air flows for a specified time and the effective volume is recorded. At the completion of this sample period, the computer turns off the air Pump. Since this sampler is exposed to the elements in routine use, extensive optical isolation of electronic signals was necessary for lightningprotection. Backup power supplies running from marine batteries can also be employed to give a great degree of protection from power disturbances. An un-interruptable power supply (UPS) is routinely used to provide emergency power to the computer. If an outage occurs, the computer will continue operation for approximately 1 h before the UPS powers down. The other advantage of the UPS is its abilityto filter out power surges in line voltagesthat would otherwise reboot the computer. The entire sampling system could be battery powered; in this case, however, the pumping subsystem should be redesigned to limit the current draws and to maximize the sampling duration.
Experimental Section Sampling Location. The sampling site was on the roof of the Business School Buildingof Indiana University; the roof of this building is 20 m high. The low-volume sampler and two high-volume samplers (Sierra-Misco, Berkeley, CAI were placed on the western side of this building 2 m from the edge. A 0.7-m wooden platform elevated the samplers from the roof deck. The sampling heads were all 1.7 m above the roof deck. Air temperature was recorded using a circular chart recorder (Omega, Stamford, CT) at deck level. Sampling took place from September 6 through September 11,1994.Four low-volume samples were collected in series each day at a flow of approximately 0.2 m3/min; the sampling time was 6 h. Air volumes ranged from 60 to 70 m3. Two high-volume air samplers were used concurrently with the low-volume samplers. High-volume samples were collected over 24 h at a flow rate of approximately 1 m3/min. High-volume sample volumes ranged from 1100 to 1200 m3. Air was first drawn through a glass fiber filter to collect particles and then through a polyurethane foam (PUF)plug to collect vapor-phase PCBs. The high-volume PUF plugs were 10 cm in diameter and 10 cm long. Sampling started at 1 PM September 6 each high-volume samplers was loaded with a clean plug and filter; and four clean plugs were loaded into the low-volumesampler. Each subsequent day at 1 PM, the sampled plugs were removed, and the next day's plugs were loaded. The face velocities used in this study were about 500 cmls for the low-volume sampler and 200 cm/s for the high-volume sampler. These velocities are faster by at least a factor of 2 than those use in other studies (13-16). We do not recommend using velocities this high if one is trying to measure accurate vaporlparticle partitioning ratios, which we were not. Blanks. Both high- and low-volume blanks were collected. A high-volume blank consisted of a clean plug and filter loaded into the high-volume sampler and allowed to sit for 24 h with the pump turned off. A low-volume blank was collected by placing a clean plug in a sampling sleeve and letting it stand in an unoccupied outer sleeve in the sampler for 24 h. Low-volume blanks were collected concurrentlywith air samples. Both low- and high-volume blanks were prepared identically to real samples as described below. Sample Media Procedures. Prior to sampling,glass fiber filters (Gelman Scientific, Ann Arbor, MI) were heated to 450 "C for 12-24 h to remove organic contaminants. Both high- and low-volume PUF plugs (Olympia Products, Columbia, SC)were Soxhletextractedusing a 50:50mixture of acetone:hexane for 24-48 h. Plugs were then dried in glass jars. High-volume plugs were inserted in a glass cylinder, and low-volume plugs were placed in aluminum sampling sleeves immediately prior to sampling. After samples were collected, the plugs were stored at -5 "C until they were extracted and analyzed. Extraction. For analysis,high-volume PUF plugs were warmed to room temperature and injected with known amounts of PCB congeners 30 and 204 (2,4,6-triand 2,2',3,4,4',5,6,6'-octachlorobiphenyl,respectively) (Ultra Scientific, North Kingstown, FU) and endosulfan-& (MSD, St. Louis, MO) as internal standards. Congeners 30 and 204 were not present in commercial Aroclors; hence, they serve as ideal internal standards. Plugs were then Soxhlet extracted in a 50:50 mixture of acetone:hexane for 24 h. VOL. 29, NO. 8,1995
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The extract was reduced in volume to 1 mL by rotary evaporation. Silica chromatography was then performed as described below. Analytes were extracted from the low-volume plugs by supercritical fluid extraction (SFE). All extractions used SFE certified carbon dioxide (Liquid Carbonic Specialty Gases, Chicago, IL). The tank was equipped with a diptube and a helium head pad. An ISCO (Lincoln,NE) Model 260D syringe pump supplied carbon dioxide at 400 atm. The factory-installed graphite-filled Teflon piston and wiper seals of the pump were replaced with ultrahigh molecular weight polyethylene seals (ISCO,Lincoln, NE). Prior to the start of this experiment, the pump was dynamically extracted with supercritical CO:! as described previously (18).
The four low-volume foam plugs (3 cm x 8 cm) were packed into four 3.47-mL extraction vessels (Keystone Scientific, Bellefonte, PA). The internal standards, PCB congeners 30 and 204 and endo~ulfan-d~, were spiked onto each plug, and the vessels were sealed. A 15-cm length of fused silica tubing (30 pm i.d., 375 pm o.d.1 was attached downstream of each extraction vessel by a Slip-Free connector (KeystoneScientific). The upstream end of each extraction vessel was connected to a four-port multiextractor, which consisted of five 1.6-mm stainless steel Swagelok tees interconnected with 1.6-mm stainless steel tubing. One tee was connected to the outlet of the pump, and the other four were connected to the SFE vessels. In this way, COz from the pump was split into four equal streams that entered the four SFE vessels. With this setup, four extractions (a day's worth of samples) could be carried out simultaneously. The four SFE vessels were loaded into a copper box heater (30 cm x 30 cm x 4 cm). The box was wrapped with heating tape and insulated; the box heater was thermostatically controlled at 60 "C. The SFE vessels were allowed to thermally equilibrate for 15 min before extraction. The C 0 2 at 400 atm was then admitted to the vessels, and the extracts were collected in four 12-mL culture tubes containing 5 mL of methylene chloride each. The culture tubes were partially submerged in water to minimize ice formation in the tips of the restrictors. Periodically, methylene chloride was added to the culture tubes to keep the solvent level at 5 mL. Extraction terminated after a total of 200 mL of C02 had passed through the vessels. Culture tubes were sealed with Teflon-lined caps and stored at -5 "C until solid-phase fractionation was performed. Solid-PhaseFractionation. Extracts generated by supercritical fluid extractionor Soxhlet extraction were solvent exchanged into hexane with three 1-mLaliquots of hexane and reduced in volume under a dry stream of nitrogen to 0.5 mL. Samples were transferred to a precleaned solidphase extraction (SPE)cartridge that contained 2 g of Si02 (Burdick & Jackson, Jackson, MS) and a 0.3 g anhydrous sodium sulfate cap. SPE cartridges were precleaned by sequentiallyrinsingthem with 10mL of hexane, 10%hexane in methylene chloride, and methylene chloride; they were then heated for 24 hat 165 "C. Prior to sample loading, the cartridgewas flushed with hexane until all air bubbles were removed. Once samples were loaded, 3 mL of hexane followed by 7 mL of 10%hexane in methylene chloride was eluted from the column and collected in a 12mL culture tube. This fraction contained the PCBs. A second fraction was eluted using 10mLof methylene chloride. This fraction contained endosulfan. Samples were then solvent ex2102
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changed into hexane with three 1-mL aliquots of hexane and reduced in volume under a dry stream of nitrogen to 100 pL. Gas Chromatography. AI PCB analyseswere done with a Hewlett-Packard5890 gas chromatograph equipped with an electron capture detector. A 30-m, DB-5, capillary column (250pm i.d.,0.25pm filmthickness,J&WScientific, Folsom, CA) was used with hydrogen as the carrier gas. The carrier gas linear velocity was 40 cm/s (measured at 200 "C). A standard was used to quantitate PCB concentrations; it consisted of 250 nglmLAroclor 1232,180ng/mLAroclor 1248, and 180 ng/mL Aroclor 1262 (U.S.Environmental Protection Agency, Cincinnati, OH) in hexane as well as internal standard congeners (PCB 30 and 204) at concentrations of 10.4and 6.00 ng/mL, respectively. This standard contained all congeners normally found in environmental samples, and it has been described in detail by Mullin (19). Sampleswere injected into the GC in the splitlessmode. The oven temperature was initially held at 40 "C for 1.0min with the split valve closed, the valve was then opened, and the oven temperature was ramped to 140 "C at 30 "Clmin, to 230 "C at 0.8 "Clmin, and then to 280 "C at 30 "C/min, with a 5-min hold at the end. Raw data from an HP3396 integrator were sent to a computer, where the PCBs were subsequently quantitated. Gas Chromatographic Mass Spectrometry. All endosulfan analyses were performed on a Hewlett-Packard 5985B GUMS system operated in the electron capture mode. A standard containing 53.6 pglpL endosulfan-d4 and 50.6 pglpL a-endosulfan (MSD, St. Louis, MO) was used for quantitation. The gas chromatograph on the system had been upgraded to a Hewlett-Packard 5890 Series 11. A 30-m, DB-5MS, capillary column (250 pm i.d., 0.25 pm film thickness, J&WScientific) was used with helium as the carrier gas at a linear velocity of 22 cm/s measured at 200 "C. Samples were injected into the column through a splitlsplitless injector operated in the splitlessmode. The following temperature program was used: The oven temperature was initially held at 40 "C for 1.Omin with the split valve closed, the valve was then opened, and the oven temperature was ramped to 140 "C at 30 "Clmin, to 230 "C at 3 "Clmin, and then to 300 "Cat 30 "Clmin,with a 2-min hold at the end. The transfer line between the GC and the MS was held at 300 "C, and the ion source temperature was held at 100 "C. The pressure of the reagent gas (methane) in the ion source was maintained at 0.43 Torr, as measured by a probe-mounted capacitance manometer. Data files were transferred to another computer, and the endosulfan was quantitated. Quantitation. PCBs and endosulfan were quantitated in the same way: Relative response factors (RRFs) were generated daily. These RRFs were used to calculate the analyte of interest. Blank correction was performed as follows. An average mass of each analyte was calculated for each set of high- and low-volume field blanks. This average mass was then subtracted from each sample mass for each analyte. For a sample's analyte concentration to have been included in the total concentration, the mass of analyte in that sample must have been at least three times larger than in the blank. Otherwise, it was considered a nondetect. Nondetects were limited primarily to the less volatile PCBs not found at high concentrations in the atmosphere.
20, 18
-
16
-
14
-
I
B-
Congener Nurnbu
FIGURE 2. Average ratio of PCB masses on front and back sections
(F/B) of PUF plugs from breakthrough volume experiment, Only the
most volatile PCB Congeners are shown; the other 70 congeners had nondetectable masses on the back section of the plug.
Results and Discussion Quality Control. In an idealized sampling process, air would be drawn through a clean plug, all PCBs would be retained by the plug, and PCBs would not be lost from the plug while it is outside of the airstream. To ensure that our sampling system met this ideal, a series of quality control experiments was conducted. While a PUF plug is actively being used for sampling, it acts as a frontal gas-solid chromatography system (20). The analyte is constantly moved from the head of the PUF “column” to the end by the air flow. The breakthrough volume is the amount of air required to move 50% of the analyte completely through the plug. To assure that 130 m3 of air (twicethe typical amount sampled) does not exceed the breakthroughvolume of a plug, a “splitplug” experiment was performed as follows: Six plugs were loaded into the low-volume sampler, three samples and three blanks. Air was drawn serially through each of the three sample plugs in 10-minincrements until 130 m3 of air had been drawn through each plug. This incremental samplingintroduces equal amounts of PCBs into all plugs. The plugs (samples and blanks) were immediately removed from the sampler and stored at -5 “C until analysis. To determine if breakthrough occurred, each plug was cut into two pieces; the front two-thirds and the back one-third. Each section was analyzed for PCBs, and a front-to-back ratio (FIB)was calculated. A large F/B indicates that the bulk of the PCBs is on the front two-thirds of the plug, and breakthrough is not occurring. The blanks in this experimentwere also cut into two-thirds and one-third sections. Figure 2 shows F/B plotted against PCB congener number. Only the first 18 congeners are shown because the other 70 congeners had nondetectable PCB masses on the back third of the plug. The 18 congenersshown are the most volatile congeners measured, and the average F/B was 7. The average FIB for endosulfan was 70. An FIB of 4 indicates that 80% of the mass of a congener is on the front two-thirds,and onlythe fist five PCB congenersfailed to meet this criteria. These five congenershave only three chlorines, and they have vapor pressures (21)on the order of hexachlorobenzene (22) and lindane (23),which are poorly retained by PUF (24,25);thus, it is understandable that their FIB values would be low. The average air temperature over this experimentwas 26.5 “C. This is the
highest temperature at which our samples were collected. Since breakthrough volumes decrease with increasing temperature (267,we are confident that the low-volume sampler can sample and retain most SOCs using air volumes in the range of 100 m3. Volatilization occurs when a plug loses analyte during sampling or after it has been removed from the airstream. To determine if this occurred, the following experiment was performed: Six plugs were loaded into the sampler, and 65 m3 of air was drawn through each plug in the same manner as the breakthrough plugs. In this way, all the plugs should have the same amount of analyte on them. Three plugs were removed immediately upon sample collection and analyzed, while the other three plugs were left in the sampler for 7 days. After the week-long period, the remainingthree plugs were removed and analyzed. The immediately removed plugs served as the baseline to determine if volatilization had occurred. Passive sampling occurs when the PUF plugs absorb PCBs when they are not activelysampling air. To determine if this occurred, six clean plugs were placed in the sampler. Three were exposed to air,while the other three were sealed in their aluminum sampling sleeves with aluminum foil and plastic electrical tape. The plugs were allowed to stand in the sampler for a 1-week period. M e r this period, the plugs were removed and analyzed. The results of the passive sampling and volatilization experiments are shown in Table 1. For the sampled plugs (volatilization)left in the samplerfor 7 days, the mean mass of PCBs decreased by 7% relative to the baseline plugs (1670-1550 ng). For endosulfan, there was a 4% decrease (from 8.9 to 8.5 ng). Clearly,ifvolatilization occurredat all, it was only to a small extent (
