Environ. Sci. Technol. 1994,28, 1129-1133
Measurement of Polychlorinated Biphenyls and Polycyclic Aromatic Hydrocarbons in Air with a Diffusion Denuder Mark S. Krleger and Ronald A. Hltes’ School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A diffusion denuder using sections of capillary gas chromatographic columns as the sampling tubes was used to measure polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons in air. PCBs were sampled simultaneously with the denuders and with a filter/ polyurethane foam sampler. The total (vapor and particle) concentrations measured by the two different sampling techniques were not statistically different, indicating that accurate measurements were made with the denuder. Breakthrough experiments indicate that 80 % of the PCBs are collected in the first 25 % of the denuder, and 99 5% are collected in the first 75 % . Vapor-particle partitioning of 14 polycyclic aromatic hydrocarbons (PAH)was measured in Indianapolis, IN, using the denuder. Heats of desorption comparable to those of Yamasaki et al. were obtained.
Introduction
Semivolatile organic compounds (SOCs), such as polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB),and polychlorinated dibenzodioxins and dibenzofurans, are present in the atmosphere both in the vapor phase and associated with particles. The distribution of an SOC between the vapor phase and the particle phase is an important determinant of that compound’s environmental fate and transport. Currently, the most common sampling method for SOCs is the high-volume (hi-vol) air sampler. It consists of a filter to collect particles, followed by a sorbent to collect vapor-phase material, followed by a blower motor to move the air through the filter and sorbent ( I ) . Glass fiber, quartz fiber, or Teflon membrane filters have been used, and polyurethane foam (PUF) is the most common sorbent (2,3). Hi-vol samplers have been used for over 10 years to determine vapor-particle partitioning ratios of SOCs (47). Their high flow rates allow a large volume of air to be collected in a reasonable time (typically 1500 m3 in 24 h). However, vapor-particle partitioning ratios from these experiments must be operationally defined as sorbentfilter ratios because of artifacts inherent in the experiment. This ratio can be positively biased relative to the true vapor-particle partitioning ratio if adsorption of vaporphase molecules onto the filter or onto particles collected on the filter is the dominant artifact (8), or it can be negatively biased by volatilization of sorbed compounds off the particles (9). Adsorption of vapor-phase molecules onto the filter matrix and the particles trapped on it can be partially compensated by using two filters, one in back of the other; the amount collected on the back filter is subtracted from the amount collected on the front filter to correct for the filter’s vapor-phase sorption. This may still underestimate the significance of the adsorption artifact if adsorption onto the particles on the filter is a more important mechanism than adsorption onto the filter
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0 1994 American Chemical Society
medium itself, Also, the surface of the front filter may be modified by the atmospheric sample and actually enhance adsorption of vapor-phase molecules, again leading to an underestimation of the adsorption artifact (IO). Furthermore, volatilization of sorbed SOCs off the particles cannot be experimentally compensated for. Reactions can also occur between molecules on the filter and gaseous components of the atmospheric sample ( I I , 1 2 ) . This results in an apparent decrease in the atmospheric concentration of the SOC. Thus, there are three competing and variable artifacts to consider when trying to determine the true vapor-particle partitioning of SOCs with hi-vol samplers, and there is no clear consensus which artifact dominates. Obviously, an alternate sampling technique is needed to determine vapor-particle partitioning of SOCs in the ambient atmosphere. For this reason, we have investigated diffusion denuders. These devices are tubes or sets of tubes through which an atmospheric sample is passed. Under laminar flow conditions, vapor-phase molecules diffuse to the walls of the tube where they are collected, and particles pass through the tube where they are collected on a filter. Collection of SOCs with diffusion denuders is not susceptible to the same artifacts seen in hi-vol samplers because the vapor-phase molecules do not have a chance to sorb to the collected particles or to the filter medium. Volatilization of SOCs off of particles collected on the filter can still occur (13),but these desorbed molecules can be collected on a sorbent located downstream from the filter. Although diffusion denuders have been widely used for the collection of volatile inorganic compounds (I4-16), their use for the collection of SOCs has been limited (I719). This is because they have several inherent drawbacks that make their application for the collection of SOCs difficult. First, denuders are low flow-rate samplers (on the order of 1-20 L/min, compared to approximately 1000 L/min for a hi-vol sampler). This low flow is required to maintain both laminar flow and a suitable residence time in the denuder tube@) to allow diffusion of vapor-phase molecules to the walls. Consequently, long sampling times must be used to compensate for the trace concentrations of SOCs in ambient air. Second, many SOCs of environmental interest, such as PCBs, PAH, and many organochlorine pesticides, are relatively nonpolar. This has made the collection of these compounds more difficult than for compounds that have acidic or basic functional groups. We have previously demonstrated a diffusion denuder that uses short sections of capillary gas chromatographic columns as the sampling tubes (20). In this paper, we will extend the application of this type of sampler to quantitative measurements of SOCs and for the determination of heats of desorption of PAH using the Junge-Pankow vapor-particle partitioning model. Envlron. Sci. Technol., Vol. 28, No. 6, 1994
1129
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Table 1. List of Denuders Used in This Study, Their Significant Design Parameters, and Important Flow Variables at Sampling Flow Rate of 2 L/min
sampler ID
internal diameter (rm)
D1 D2 D3 D4 D5 D6 D7 D8 D9 DU
320 320 320 530 530 530 530 530 530 320
film thickness (rm) 5 5 5 8 8 8 8 8 8
tube length (cm) 25 25 50 25 50 15 15 15 15 25
phase ratio (P) 16 16 16 17
no. of tubes 120 120 60 120
60 17 50 17 50 17 50 17 50 none NAa 120 The uncoated denuder was constructed from deactivated fused silica tubing. phase. 17
Experimental Section
Denuders. Nine different denuders were used in this study. Details of the construction process have been given earlier (20, 21). All denuders were constructed from commercially available capillary gas chromatographic (GC) columns. Denuders with 530-pm i.d. sampling tubes were constructed from 30 m X 530 pm i.d. columns with an 8-pm methylsilicone stationary-phase thickness (007-1, Quadrex, New Haven, CT). Denuders with 320-pm i.d. sampling tubes were constructed from 30 m X 320 pm i.d. columns with a 5-pm methylsilicone stationary-phase thickness (DB-1, J&W Scientific, Folsom, CA). A list of denuders used in this study and their design characteristics is given in Table 1. Analysis. Gas chromatographic mass spectrometry (GC/MS) analyses were performed on a Hewlett-Packard 5989 instrument in electron impact mode using selected ion monitoring. The analytical gas chromatographic column was a 30-m DB-5 capillary column, 250-pm i.d., with a 0.25-pm film thickness (J&W Scientific). The carrier gas was helium at a linear velocity of 45 cm/s. Other analyses were performed on a Hewlett-Packard 5890 GC with electron capture detection. The analytical column was the same as for GCIMS analysis. The carrier gas was hydrogen at a linear velocity of 50 cm/s. The detector makeup gas was nitrogen at a flow rate of 20 mL/min. Sampling. Air samples were collected using a denuder with two filters and two PUF plugs located after it and with a conventional filter/PUF system (20). Backup filters were used in both samplers to partially account for adsorption of vapor-phase molecules onto the filter media and onto particles sorbed to the filter. Backup PUF plugs were used to check for breakthrough. Air was pulled through the denuders using either a DuPont (Wilmington, DE) PM4000 personal sampling pump or an SKC (Eighty Four, PA) Aircheck Model 224 personal sampling pump. Typical sampling flow rates were from 1 to 3 L/min for both the denuders and the filter/PUF samplers. See Table 1 for important sampler parameters. Filters were Whatman QM-A (Hillsboro, OR) quartz fiber filters cut to a 1.1-cmdiameter, resulting in an exposed surface area of 0.95 cm2. Face velocities through the filter were approximately 35 cm/s (at a flow of 2 L/min). Filters were cleaned prior to use by baking them at 450 "C for at least 12 h. PUF plugs were 1cm in diameter by 2 cm in length. PUF plugs were cut from 25-cm-thick sheets of polyurethane foam (Olympic Products, Greensborough, NC) with a hole saw and plywood form and cleaned prior to use by extraction with supercritical NzO for 30 min at 450 atm and 50 "C at a flow rate of 750 pLImin. 1130
Envlron. Sci. Technol., VoI. 28, No. 6, 1994
internal vol (mL)
flow per tube (mL/s)
Reynolds no.
linear velocity (cm/s)
2.41 0.28 73 345 2.41 0.28 73 345 2.41 0.56 145 691 6.62 0.28 44 126 6.62 0.56 88 252 1.65 0.67 105 302 1.65 0.67 302 105 1..65 0.67 105 302 1.65 0.67 105 302 2.41 0.28 73 345 The phase ratio is undefined for a tube with no stationary
Indoor Air. Indoor air samples were collected in our laboratory using denuders D2-D9 and DU (see Table 1) in parallel with an automated PUF sampler under development in our laboratory. Samples were collected over 24 h, with total volumes ranging from 1.5 to 2.5 m3. The denuders were extracted by liquid extraction (see below). P U F was extracted with supercritical COz, which was supercritical fluid extraction grade from either Scott Specialty Gases, Air Products, or Liquid Carbonics, delivered by an ISCO (Lincoln, NE) Model 260D syringe pump. Supercritical fluid extractions were performed at 450 atm and 100 "C for 30 min. Commercially available extraction cells (Keystone Scientific, Bellefonte, PA) with internal volumes of either 167 or 500 pL were used for all extractions. Flow rates (measured as liquid at the pump) were typically 600 pL/min. Pressure was maintained with a 25 pm X 15 cm fused silica outlet restrictor. The extracted analytes were depressurized into a small vial of hexane. The filters were not extracted. All extracts were spiked with 1300 pg of PCB congener 30 and 500 pg of PCB congener 204 as internal standards. Samples were evaporated to a volume of approximately 1 mL under a gentle stream of dry nitrogen prior to analysis. All analyses were done by gas chromatography with electron capture detection. Ambient Air. Ambient air samples were collected in Indianapolis, IN. Samples were collected with denuders D2-D5. The sampling site was on the roof of the School of Public and Environmental Affairs Building a t Indiana University-Purdue University in Indianapolis (IUPUII SPEA). Samples were approximately 24 h in duration, with total volumes ranging from 1to 4 m3. The denuders were extracted by liquid extraction (see below). P U F and filters were extracted with supercritical NzO, which was supercritical fluid extraction grade from Scott Specialty Gases, delivered by an ISCO Model 260D syringe pump. These extractions were performed at 450 atm and 100 "C for 30 min. Commercially available extraction cells with internal volumes of either 167 or 500 p L were used for all extractions. Flow rates (measured as liquid at the pump) were typically 600 pL/min. Pressure was maintained with a 25 pm x 15 cm fused silica outlet restrictor. The extracted analytes were depressurized into a small vial of methylene chloride. All extracts were spiked with known amounts of fluorene-dlo, phenanthrene-dlo, pyrene-dlo, chrysene-dlz, and perylene-dlz as internal standards. Samples were evaporated to a volume of approximately 50 pL under a gentle stream of dry nitrogen prior to analysis. Denuder extracts were cleaned by passing them through a microcolumn of MgS04 and alumina and eluting
Table 2. Concentrations (in ng/m3) of Total PCBs in Indoor Air Measured with Two Different Methods
day 1 2 3 4 5 6
filteriPUF samples sample sample 1 2 av 273 254 258 198 179 130
264 253 222 187 180 110
269 254 240 193 180 120
-
SD
D2
D3
D4
6.4 0.7 25 7.8 0.7 14
246
221
267
152
179
190
194
179
223
with CH2Clz. Liquid Extraction. The organic epoxies that we used to construct the denuders exhibited a lack of resilience to the stresses induced by thermal desorption. The temperature ramp from ambient to 220 "C over a few minutes often caused cracks to form in the epoxy body of the denuder. Consequently, the helium purge gas would be vented out the body of the denuder instead of passing through the sampling tubes to desorb the collected analytes. Cracking of the denuders was so severe that thermal desorption had to be abandoned as a method for removal of collected analytes from the denuder tubes. Thermal desorption was replaced by liquid solvent extraction, which did not have any deleterious effects on the epoxy. Liquid extraction was performed by simply passing CH2Clz through the denuders. Recovery experiments indicated that 300 mL of CHzClz quantitatively recovered all analytes from D2 to D5, while only 100mL was required for D6-D9. The liquid extraction flow rate was 3 mL/ min, and it did not effect the cross-linked stationary phase, despite the large volumes of solvent used. On-Line Supercritical Fluid Extraction. This technique was used for the ambient air samples. It was performed by first extracting the filter and P U F samples using the supercritical fluid extraction techniques described above or by extracting the denuders using the liquid extraction technique described above. The extracts were then evaporated to avolume of approximately 50 pL under a gentle stream of dry nitrogen. A 167-pL extraction cell was prepared by packing it with two 0.5-cm diameter QM-A quartz fiber filters. Approximately 25 pL of the extract was then spiked onto the filters in the cell. The filters served to wick the solvent (CH2C12) into the cell and to prevent the analytes from being lost by passing completely through the cell. Using only half of the extract allowed a second analysis to be performed on the same extract, while minimizing the loss in sensitivity due to dilution by solvent. The solvent was allowed to air dry for 5 min, and the cell was reassembled. The cell was then purged with dry helium a t ambient temperature and a flow rate of 50 mL/min for 1 min. This purge removed any residual solvent from the supercritical fluid extraction cell before the extraction began. The sample was extracted with 450 atm of C02 at 50 "C for 5 min. Pressure was maintained with a 15pm X 15 cm fused silica outlet restrictor; the flow rate, measured as liquid a t the pump, was 180 pL/min. The extracted analytes were collected a t the head of the column which was held a t -30 "C.
Results and Discussion
Indoor Air. In order to determine if we could obtain accurate air concentration values with the denuders, we took a series of parallel samples in our laboratory with the denuders and with a prototype, automated, low-volume
D5
D6
denuder samDles in series D7 D8 D9
sum of D6-D9
DU
273
178
64
16
2
260
49
255
193
23
8
6
230
55
186
143
13
8
2
166
18
av
SD
245 267 174 243 199 176
23 9.2 20 18 22 14
filter/PUF sampler now under development. We chose to sample indoor laboratory air because PCBs are the primary SOC present, and they are 2 orders of magnitude higher in concentration in our laboratory than in the ambient air. PCBs are also found primarily (>go%) in the vapor phase, so we did not need to worry about particle transmission efficiencies. Samples were collected over 24-h time periods for 6 consecutive days. Denuder sample volumes were approximately 1.5 m3, while the filter/PUF sample volumes were 15 m3. Two filter/PUF samples and three denuder samples were obtained each day. Denuders 6-9 were used in series to determine breakthrough. The uncoated denuder (DU)was used to determine collection efficiencies of uncoated, deactivated fused silica tubing. The data are shown in Table 2. A paired t-test of the means of the concentrations measured (summing the values of D6-D9 and excluding DU) with the two different methods for the six sampling periods indicates that the means are not significantly different at the 95% confidence level. Paired t-tests between the three values obtained with each of the five denuders (summing the values of D6-D9 and excluding DU) paired in all cases with the mean of the values of the filter/PUF samples collected on the same day indicated that the concentrations measured with each individual denuder are not significantly different from those obtained with the filter/PUF sampler a t the 95 % confidence level. From these results, we conclude that we can accurately measure SOC air concentrations with the denuder. There are additional points of interest in this data set. One is the difference in the precision of the two methods, as shown by the relative standard deviations (RSD). The average RSD of the filter/PUF measurements was 4.9 % , while the average RSD of the denuder measurements was 8.5 % . It is reasonable that the filter/PUF system should be more precise since it collected a factor of 10 times more air (and PCBs) than the denuder. A second interesting result is the concentrations detected in the uncoated denuder (DU). One would expect that this denuder would collect no PCBs because it is not coated with any stationary phase. However, the concentrations found in denuder DU were substantially above blank levels (which were
