Environ. Sci. Technol. 2004, 38, 4828-4834
Passive Air Sampling Using Semipermeable Membrane Devices at Different Wind-Speeds in Situ Calibrated by Performance Reference Compounds HANNA S. SO ¨ DERSTRO ¨ M* AND PER-ANDERS BERGQVIST Environmental Chemistry, Department of Chemistry, Umea˚ University, SE-901 87 Umea˚, Sweden
Semipermeable membrane devices (SPMDs) are passive samplers used to measure the vapor phase of organic pollutants in air. This study tested whether extremely high wind-speeds during a 21-day sampling increased the sampling rates of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), and whether the release of performance reference compounds (PRCs) was related to the uptakes at different wind-speeds. Five samplers were deployed in an indoor, unheated, and dark wind tunnel with different wind-speeds at each site (650 m s-1). In addition, one sampler was deployed outside the wind tunnel and one outside the building. To test whether a sampler, designed to reduce the wind-speeds, decreased the uptake and release rates, each sampler in the wind tunnel included two SPMDs positioned inside a protective device and one unprotected SPMD outside the device. The highest amounts of PAHs and PCBs were found in the SPMDs exposed to the assumed highest windspeeds. Thus, the SPMD sampling rates increased with increasing wind-speeds, indicating that the uptake was largely controlled by the boundary layer at the membrane-air interface. The coefficient of variance (introduced by the 21-day sampling and the chemical analysis) for the air concentrations of three PAHs and three PCBs, calculated using the PRC data, was 28-46%. Thus, the PRCs had a high ability to predict site effects of wind and assess the actual sampling situation. Comparison between protected and unprotected SPMDs showed that the sampler design reduced the wind-speed inside the devices and thereby the uptake and release rates.
Introduction Passive samplers have been used for integrative measurements of several air pollutants such as radioactive compounds and volatile organic compounds in indoor environments as well as ambient, outdoor environments (e.g., refs 1-3). Passive air sampling of a chemical is achieved by free flow of chemicals from the air to a collecting medium due to differences in chemical potentials (fugacity capacities) and has the advantages of not requiring electricity or maintenance, thus being user-friendly and cost-effective. In addition, passive samplers often can be exposed for a long period of * Corresponding author phone: +46-(0)90-786 9339; fax: +46(0)90-12 81 33; e-mail:
[email protected]. 4828
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time, and time-weighted average (TWA) concentrations in air can be determined. Semipermeable membrane devices (SPMDs) are passive samplers originally used to measure dissolved fractions of persistent organic pollutants (POPs) in water (4). Since the development of SPMD sampling in water, SPMD technology has been used for passive sampling of POPs in air by, for example, Petty et al. (5) and So¨derstro¨m and Bergqvist (6). In a SPMD, passive sampling is achieved by physical absorption in the membrane, or diffusion through the membrane, and absorption into triolein (a neutral lipid found in most aquatic organisms). There are three possible barriers that can restrict the uptake in SPMDs: a thin boundary or diffusion layer at the membrane-exposure medium interface, biofouling aqueous phase at the exterior membrane surface, and the membrane. The high concentration capacities of SPMD for POPs and the rate-limiting barriers result in a linear and integrative sampling for a relatively long time, from days up to years depending on the compound sampled and the barrier situation. The sampling rate of SPMDs is affected by the physicochemical properties of the compound sampled, the design of sampling devices, and the environmental conditions. In water, the uptake of compounds with log octanol-water partition coefficients (log KOW) > 4.5, such as polycyclic aromatic hydrocarbons (PAHs) and nearly all polychlorinated biphenyls (PCBs), is controlled by the boundary layer (7, 8). The water flow/turbulence influence the thickness of the boundary layer and thereby affect the SPMD sampling rate in water (7, 8). Temperature (7-10) and biofouling (4) have an effect on the sampling rate in water as well. Both temperature and wind-speed are expected to affect the SPMD sampling rate in air in a similar way as in water. However, the effect of biofouling is expected to be negligible, whereas chemicals bound to the particles or aerosols can be trapped on the membrane surfaces and influence the amount sequestered by the SPMD in air. Another aspect to consider is that environmental conditions such as temperature, UVradiation, and wind-speed are much more variable in air at a certain site as compared to water. One way to reduce the effects of the environmental conditions during SPMD sampling in air is to protect the SPMDs from direct exposure to, for example, wind by using protective devices, as demonstrated by Ockenden et al. (11). These devices protect the SPMD from direct sunlight, rain, wind, and particle deposition. To use SPMDs for quantitative measurement of the air concentration, the sampling rate, RS, or the SPMD-air partition coefficient, KSA, has to be known. In water, the RS of SPMDs is generally precalibrated in the laboratory at given temperatures, as was made by Huckins et al. (8). However, the calibration data of SPMDs in air are limited to two studies by Ockenden et al. that reported the field-calibrated RS of PCBs (11, 12), and another study by Shoeib and Harner suggesting RS for a number of PCBs (2). Instead, SPMD air data have mostly been interpreted by comparing the difference between sites in the amounts sequestered by the SPMD (6, 11, 13). Calibrating the SPMDs prior to use requires tests of many different sampling conditions for a number of compounds. In addition, the field sampling conditions can be difficult to replicate and maintain over extended periods, making it complicated to generate precalibrated RS values that reflect the true sampling situation. Huckins et al. proposed the use of performance reference compounds, PRCs, to calibrate the SPMD sampling rate in situ and thereby assess the actual water sampling situation by predicting the site effects of 10.1021/es049637z CCC: $27.50
2004 American Chemical Society Published on Web 08/17/2004
hydrodynamics, temperature, and biofouling (14). PRCs are analytically noninterfering organic compounds which exhibit moderate to relatively high SPMD affinities. They are added to the SPMD’s lipid phase prior to membrane enclosure. The theory states that the release rates of these compounds are related to the uptake rates of the native compounds sampled. In water, PRCs appear to be able to assess the between site difference in temperature and hydrodynamics during a sampling period (7, 10, 14-16). In air, Ockenden et al. have used the 13C-labeled PCBs 28, 52, 101, 138, 153, and 180 as PRCs and found that uptake and release rates were related (11). To assess whether wind-speed affects the SPMD sampling rates in air, and whether PRCs can be used to predict the between site differences in environmental conditions, this study tested (i) whether extremely high wind-speeds (range 6-50 m s-1) increased the uptake of PAHs and PCBs in SPMDs and (ii) whether the release of PRCs was related to the uptakes at different wind-speeds. In addition, this study tested (iii) whether the sampler, designed to reduce the wind-speeds inside the devices and thereby reduce the differences between sites in environmental conditions, decreased the uptake and release rates. The SPMD sampling was performed using SPMDs spiked with four deuterated PAHs and four 13C-labeled PCBs, placed in an indoor, unheated, and dark wind tunnel with different wind-speeds at each site (6-50 m s-1). To test if the design of the sampler was able to reduce the site effect of wind, each sampler in the wind tunnel was equipped with two SPMDs protected from direct wind by being positioned inside a metal umbrella and one unprotected SPMD. Modeling of SPMD Sampling. The least complex approach to describe the SPMD sampling in water is to consider the SPMD as a single compartment (8). Assuming that this model can be used for SPMD sampling in air, and that the uptake is integrative and linear, the total amount, MSA (ng/ SPMD), taken up by a standard 1-mL triolein SPMD exposed to air at time t is given by
MSA ) CAkut ) CARStVSPMD-1
(1)
where CA is the air concentration, RS is the sampling rate (m3 d-1) at a given temperature, and ku is the uptake rate of a specific compound. The PRC Model. The exchange rate constant, ke, from the SPMDs into the sampling medium is calculated by
MPRC-A ) MPRC-A0 exp(-ket)
(2)
where MPRC-A is the amount of the PRC in the SPMD at the time t and MPRC-A0 is the initial amount of PRC in the SPMD. If the amount of PRC in the SPMD is measured at the beginning and the end of the sampling, as was done in this study, eq 3 is solved as a two-point derivation of ke (16):
ke ) ln(MPRC-A0/MPRC-A)t-1
(3)
When the uptake in SPMD is linear and integrative, the calculated ke value can be used in eq 1 to calculate the air concentration of the sampled compound:
CA ) MSAke-1KSA-1t-1
(4)
where KSA is the SPMD-air partition coefficient of a standard 1-mL triolein SPMD. When the released PRC amount is approximately g60% of the initial PRC amount in the SPMD, the uptake of compounds with similar or lower SPMD affinity is nonlinear or at equilibrium. In these cases, the use of eq
FIGURE 1. Design of sampling device used in this study including two PRC-spiked SPMDs deployed on separate 150 × 140 mm steel spiders horizontally placed on top of each other inside a metal umbrella (320/160 mm i.d.). The opening of the metal umbrella was covered by a metal net (340 mm o.d.). 4 will underestimate the CA, and instead an exponential or equilibrium model should be used.
Experimental Section Sampler Design and Sampling Handling. The study used standard 1-mL triolein SPMDs (U.S. patents 5,098,573 and 5,395,426) obtained from ExposMeter AB (Umeå, Sweden). Standard SPMDs, used for sampling, were 91.4 cm long and 2.5 cm wide tubes of low-density polyethylene (75-90 µm thickness, pore size ∼10 Å) filled with 1 mL (0.915 g) of >95% pure triolein. Before sampling, unexposed SPMDs were stored in sealed solvent-cleaned tin cans at -18 °C. During the sampling, each SPMD was mounted carefully between seven steel rods attached to a 150 × 140 mm steel disk; the steel devices were called spiders (Figure 1). Two SPMDs, mounted on separate spiders, were placed horizontally on top of each other inside a metal umbrella (Figure 1). The metal umbrella protected the SPMDs from direct sunlight, rain, wind, and particle deposition. The opening of the metal umbrella was covered by a metal net, allowing air to pass under and around the SPMDs. After the 21-day air sampling period, all SPMDs were retrieved and stored in separate solvent-cleaned tin cans at -18 °C until analyzed. To control the purity of used SPMDs and contribution of the sampling and the analytical procedures to the amounts found in the SPMDs, one extra SPMD (referred to as field control, FC) was exposed to air only during deployment and retrieval of the SPMDs and was analyzed with the same procedure as the others. Performance Reference Compounds. Deuterated PAHs with molecular weights no larger than that of deuterated chrysene (MW ) 240), together with 13C-labeled di- and trichlorinated biphenyls, are compounds commonly used as PRCs (7, 10, 16). When deciding which compounds to use as PRCs, the environmental conditions of the sampling sites and the sampling time should be considered to ensure that the PRC losses during the sampling are in an acceptable range. The changes in the initial PRC amounts should ideally meet the criteria of 1-(CSPMD/CSPMD0) × 100% > 3 × C.V. (%) when the PRC releases 3 × C.V. (%) when the PRC releases >50% (17). These criteria are generally met when the amount of PRC released is in the range between 20% and 80% of the initial amount in the SPMDs. For example, if the sampling time is about 2 weeks, deuterated naphthalene will be completely lost while no loss VOL. 38, NO. 18, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Performance Reference Compounds Used and Selected Physicochemical Properties of Their Analogous Native Compounds HLC (Pa m3 mol-1) log KOA log KOAg total Cl a b (ortho Cl ) log KOW (25 °C) (20 °C) (20 °C) 13C-PCBs
PCB#3 PCB#15 PCB#37 PCB#54
1 (0) 2 (0) 3 (0) 4 (4)
4.5 5.3 5.9 5.9
28c 23c 15c 58c
7.0e 7.9e
7.1 8.1 9.0 7.3
2H-PAHs
acenaphthene fluorene phenanthrene pyrene
3.9 4.2 4.5 5.3
24d 8.5d 4.0d 1.1d
7.1f 7.9f 9.2f
a Number of total Cl atoms; ortho-chlorine substitution in the brackets. Log octanol-waterpartition coefficient (log KOW) selected by Mackay et al. (21, 22). c Henry’s law constant (HLC) recalculated from values suggested by Dunnivant and Eizerman (23). d HLC recalculated from values recommended by Mackay et al. (22). e Log octanol-air partition coefficient (log KOA) measured by Harner and Bidleman (24). f Log KOA measured by Harner and Bidleman (25). g Log KOA predicted by Chen et al. (20). b
of 13C-PCB 180 will be detected. This study used four deuterated PAHs (2H-PAHs) and four 13C-labeled PCBs (13CPCBs) as PRCs (Table 1). These compounds were selected to cover as wide range of vapor pressure as possible with PRC releases that were suitable for different detectors and for both shorter and longer sampling periods, for example, 1 week to several months. To control the initial amount of PRC in the SPMD, one unexposed SPMD, referred to as laboratory control (LC), was analyzed with the same procedure as the other SPMD. The cases where the remaining amount of PRC was outside the 20-80% range of the initial amounts were identified in the results (Table 2). Wind-Speed Exposure Study. Five samplers were deployed simultaneously at different positions inside a wind tunnel (Figure 2), which was built in a large unheated and dark building. The air sampling was performed in an area with no point sources of PAHs and PCBs, which means that these compounds occurred in the area mainly due to longrange transport. Thus, the ambient air concentration was at background levels and was considered to be relatively stable during the sampling period. The wind tunnel was built in two compartments constructed to produce different windspeeds in different parts of the wind tunnel (Figure 2). An electrical fan, mounted in the wall outside the building and in the inlet of the wind tunnel, produced constant high wind flow during the 21-day sampling period. The wind-speeds could not be measured at each sampling site due to high turbulence in the wind tunnel, but the wind-speed in the inlet and the outlet of the wind tunnel was calculated to be 6 and 50 m s-1, respectively. Two additional samplers, one deployed outside the wind tunnel and one outside of the building, were used as controls of the SPMD sampling rates at the nearly stagnant air flow in the building (C1) and those at ambient air conditions (C2), respectively. Because the study was performed at ambient air conditions, the air temperatures varied during the 21-day sampling period. However, the temperature range during the 21-day sampling in the unheated building, in the wind tunnel, and outdoors was 1 to 19, -1 to 20, and -3 to 23 °C, respectively, with the average temperature being 9 °C at each site. Thus, the air temperature varied similarly for each site and at each sampling time, suggesting that the air temperature had a low effect on the results. Sampler Design Study. Each of the five samplers in the wind tunnel included two SPMDs, which were protected from 4830
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direct wind by being positioned inside a metal umbrella, and one unprotected SPMD mounted on a spider outside (on the top) the metal umbrella. Extraction, Cleanup, and Analysis. To clean the membrane surface from particles and lipids, the SPMDs were brushed and washed by hand in clean water for less than 5 min. They were then shaken in hexane followed by hydrochloric acid (1 M) for 10-20 s, respectively, and then dried with Kleenex tissues. The organic pollutants were extracted from the SPMD by dialysis in 50 mL of 95:5 (v:v) cyclopentane: dichloromethane for 24 h followed by dialysis for another 24 h in a fresh 90 mL batch of the same solvent mixture. The two fractions were pooled, and two mixtures of 13C-PCB and 2H-PAH standards, respectively, were added as internal standards after dialysis. Before the cleanup, the sample volume was reduced by rotary evaporation. The spiked extracts were cleaned with a gel permeation chromatography (GPC) system including a high-resolution (HR)-GPC column followed by a mixed silica gel column with deactivated silica and potassium silicate. The samples were analyzed for 14 EPA priority PAHs (excluding naphthalene and acenaphthylene) and benzo[e]pyrene, 21 PCBs, and eight PRCs by highresolution gas chromatography/low-resolution mass spectrometry (HRGC/LRMS). For more details on the materials, chemicals, and analytical methods used during the SPMD cleanup, see So¨derstro¨m and Bergqvist (6). Quality Assurance/Quality Control. A mixture of 10 13CPCB and four 2H-PAH standards, respectively, was used as internal standards of the chemical analysis. 13C-labeled PCB#80 and PCB#128 were used as recovery standards during the HRGC/LRMS-analysis. The quality of the materials used was controlled by analyzing the solvents and the chemicals with the same procedures as described above. This sample was referred to as the laboratory blank (LB). To control the reproducibility of the SPMDs, each SPMD was analyzed separately.
Results and Discussion Data Quality. After the chemical analysis, the recoveries of the PAH- and PCB-standards were in the ranges of 46-107% and 72-106%, respectively. All data were corrected for these recovery values. The levels of PCBs were below the detection limit ( 80% of the initial PRC amount. c One replicate sample lost during the cleanup. d Membrane was probably damaged, causing high ke values. e Average ke-value of samples with PRC amount released in the range between 20% and 80% of the initial amount of PRC. f I and II, protected replicate SPMDs; U, unprotected SPMD; n.d., not detected (levels
