Environ. Sci. Technol. 2002, 36, 85-91
Development of the Permeability/ Performance Reference Compound Approach for In Situ Calibration of Semipermeable Membrane Devices
generally not an issue for these applications because health concerns of VOCs are generally triggered at milligrams per cubic meter to grams per cubic meter levels (2). However, PSD detection limits are often a concern when sampling semivolatile organic compounds (SVOCs) in environmental media. This is because trace levels of SVOCs (, milligrams per cubic meter) may be bioconcentrated in organism tissues to potentially toxic levels.
J A M E S N . H U C K I N S , * ,† JIMMIE D. PETTY,† JON A. LEBO,† FERNANDA V. ALMEIDA,‡ KEES BOOIJ,§ DAVID A. ALVAREZ,† WALTER L. CRANOR,† RANDAL C. CLARK,† AND BETTY B. MOGENSEN| Columbia Environmental Research Center, U.S. Geological Survey, 4200 New Haven Road, Columbia, Missouri, 65201, Instituto de Quimica, State University of Campinas, 13081-970 Campinas, Sao Paulo, Brazil, Netherlands Institute for Sea Research, 1790 AB Den Burg, The Netherlands, and National Environmental Research Institute, Frederiksborgvej 339, DK-4000 Roskilde, Denmark
It is instructive to note that the maximum PSD uptake rates are achieved when the rate-limiting barrier to chemical vapor or solute transport is the external boundary layer (i.e., a thin air or water layer between the sampler exterior and the bulk environmental medium). In other words, the rate of mass transfer or PSD uptake across a series of barriers or layers can be no greater than the rate of supply. However, changes in the flow velocity-turbulence of the exposure medium affect the effective thickness of the external boundary layer of a PSD. Since mass-transfer resistance is directly proportional to boundary layer thickness, the sampling rates of analytes will vary with the hydrodynamics/aerodynamics of the deployment site. Under boundary layer control, PSD design features other than the external surface area for chemical exchange will have little or no effect on linear uptake rates.
Permeability/performance reference compounds (PRCs) are analytically noninterfering organic compounds with moderate to high fugacity from semipermeable membrane devices (SPMDs) that are added to the lipid prior to membrane enclosure. Assuming that isotropic exchange kinetics (IEK) apply and that SPMD-water partition coefficients are known, measurement of PRC dissipation rate constants during SPMD field exposures and laboratory calibration studies permits the calculation of an exposure adjustment factor (EAF). In theory, PRC-derived EAF ratios reflect changes in SPMD sampling rates (relative to laboratory data) due to differences in exposure temperature, membrane biofouling, and flow velocity-turbulence at the membrane surface. Thus, the PRC approach should allow for more accurate estimates of target solute/vapor concentrations in an exposure medium. Under some exposure conditions, the impact of environmental variables on SPMD sampling rates may approach an order of magnitude. The results of this study suggest that most of the effects of temperature, facial velocity-turbulence, and biofouling on the uptake rates of analytes with a wide range of hydrophobicities can be deduced from PRCs with a much narrower range of hydrophobicities. Finally, our findings indicate that the use of PRCs permits prediction of in situ SPMD sampling rates within 2-fold of directly measured values.
Unlike most PSDs (3), lipid-containing semipermeable membrane devices (SPMDs) are designed for high sampling rates of hydrophobic SVOCs in environmental media (410). In light of the above sampling rate discussion, it is not surprising that work by Booij et al. (11) and Huckins et al. (12) indicates that the SPMD uptake rates of SVOCs with log octanol-water partition coefficients (Kow’s) g4.4 are generally under aqueous boundary layer control (assuming water velocities at the SPMD membrane surface of 4.4 include nearly all polychlorinated biphenyl (PCB) congeners and chlorinated dioxins and furans and most organochlorine pesticides (OCs) and polycyclic aromatic hydrocarbons (PAHs). SPMD sampling rates are also affected by temperature and biofouling (12, 13). Because of the large range of potential environmental effects on the sampling rates of high Kow SVOCs (11-17), it appears impractical to conduct calibration studies for all exposure scenarios. Thus, SPMD environmental exposures requiring more quantitative measures or media concentrations would be limited to a narrow range of exposure conditions that can be directly related to calibration data.
Introduction Traditionally, passive sampling devices (PSDs) have been used to monitor vapors of volatile organic chemicals (VOCs) in occupational environments (1). Detection limits are * Corresponding author phone: (573)876-1879; fax: (573)876-1896; e-mail:
[email protected]. † U.S. Geological Survey. ‡ State University of Campinas. § Netherlands Institute for Sea Research. | National Environmental Research Institute. 10.1021/es010991w CCC: $22.00 Published on Web 11/22/2001
2002 American Chemical Society
Huckins et al. (4, 5) proposed the use of permeability (membrane control of uptake rates)/performance (aqueous boundary layer control of uptake rates) reference compounds (PRCs) to address the issue of biofouling effects on SPMD sampling rates. PRCs are analytically noninterfering organic compounds with moderate to high Kow’s that are added to the SPMD lipid prior to membrane enclosure. The measured rate of PRC loss during environmental exposures is compared to similar values derived from laboratory calibration studies. This novel in situ calibration approach is based on theory and experimental evidence that PRC dissipation rate constants (kePRC-f’s) at sampling sites are related to the uptake rate constants (kua-f’s) of target compounds. In other words, isotropic exchange kinetics (IEK) govern the accumulation of hydrophobic SVOCs by SPMDs (4, 11, 13). Moreover, research (11, 12) has indicated that PRCs can be used to assess the effects of site hydrodynamics and temperature on SPMD sampling rates. VOL. 36, NO. 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Compounds commonly used as PRCs are perdeuterated priority pollutant (PP) PAHs (caution: photolysis may occur without shading) with no larger molecular weight than chrysene-d12, 2,2′-dichlorobiphenyl, and 2,4,5-trichlorobiphenyl. However, other compounds can be used as PRCs assuming they do not interfere with the analysis of target SVOCs and QC standards and that adequate losses can be measured. For example, if SVOCs are analyzed by mass spectrometry, then 13C stable isotopes can be used as PRCs. Ideally, each target SVOC would have a stable isotope PRC analogue, but this approach is limited by the cost and commercial availability of the appropriate compounds and the need for mass spectrometric analysis. The requirement for measurable losses of PRCs generally excludes the use of activated adsorbents as SPMD sequestration phases and compounds with log Kow’s > 5.5. In addition, the use of PRCs is invalid when the rate-limiting step in analyte uptake is desorption of residues from sediment particles (e.g., direct exposure of SPMDs to organic-rich benthic sediments containing high Kow contaminants). Huckins et al. (17) have given more detail on standard procedures for the use of PRCs in SPMD exposure studies. In this work, we briefly review passive sampling theory and elucidate the basis of the PRC approach for calibration of PSDs to site-specific conditions. Moreover, we present experimental data supporting the use of PRCs for determining the effects of environmental conditions on SPMD sampling and thereby improving the accuracy of water concentration estimates. Two types of PRC approaches can be used to estimate in situ sampling rates of hydrophobic SVOCs. One of these approaches, i.e., the use of PRC-derived exposure adjustment factors (EAFs), is examined in detail, whereas Booij et al. (11) have described the other approach, which is briefly discussed in this work. Models needed for the use of PRCs are presented, and the role of KSPMD’s (equilibrium SPMD-water partition coefficient) in SPMD exchange rates is delineated.
Theory and Modeling Before experimental results are discussed, it is instructive to examine some fundamentals of mass-transfer theory relevant to SPMD sampling and the application of PRCs. The time that it takes for an SPMD to extract detectable levels of a hydrophobic SVOC from an environmental medium is dependent on physicochemical properties of the compound, the area of the exchanging membrane or surface, the overall resistance encountered in solute or vapor mass transfer, the ambient chemical concentration, and the analytical considerations such as the level of interferences after cleanup, dilution factor, and instrumental detection limits. Accumulation of target SVOCs in SPMDs requires the movement of residues out of the bulk sample medium, across multiple layers or barriers (each layer has some resistance to solute mass transfer), and into the sampler matrix. Assuming steadystate flux of hydrophobic SVOCs and insignificant biofouling, the overall conductance (1/Qo) across multiple SPMD barriers is the mean of individual conductances:
1/Qo ) 1/(CwkuoMSPMD) ) 1/(kmKmwACw) + 1/(kwACw) (1) where 1/Qo is the time required to sample a unit mass (d or s/ng) of SVOC or the reciprocal of flux; Cw (ng/cm3) is SVOC concentration in water; kuo is the concentration-independent overall uptake rate constant (cm3/d‚g); MSPMD is the total mass (membrane plus lipid) of the SPMD (g); km and kw are the membrane and the aqueous boundary layer mass-transfer coefficients (cm/s), respectively; Kmw is the equilibrium membrane-water partition coefficient; and A is the area of the exchanging surface (cm2). When using SPMD with 95% 86
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triolein, methyl oleate and oleic acid impurities slowly diffuse through the membrane and sometimes form a very thin lipid film on the exterior surface of the SPMD (6; Robert Gale, personal communication, USGS, Columbia, MO). On the basis of the linearity of the uptake of SVOCs with high Kow’s (5, 6), this layer does not affect the rate of conductance. However, for completeness, the group 1/(kLfKLfwACw) (where Lf is the exterior lipid film) can be added to eq 1. On the other hand, when the time to transmit a unit mass of analyte across the aqueous boundary layer represents >95% of 1/Qo, other groups in eq 1 can be deleted. A decrease in the magnitude of terms in the denominators of eq 1 generally leads to a measurable increase in conductance time unless >95% of the magnitude of 1/Qo is associated with only one layer, as suggested above. Determination of the rate-limiting barrier in the movement of chemicals through a series of contiguous layers is normally accomplished by applying an additive resistance or impedance model. Assuming a steady-state flux, the relationship among layer and solute parameters that affects the overall impedance to mass transfer (Io) can be expressed as
Io ) 1/ko ) lw/Dw + lb/(DbKbw) + lm/(DmKmw)
(2)
where ko is the overall mass transfer coefficient, l is layer thickness (cm), D is the diffusion coefficient (cm2/s) within a layer, and the subscript b represents the biofilm or periphytic growths. Resistance (Io) has units of seconds per centimeter, and the rate-limiting step in mass transfer through multiple barriers is the step that requires the greatest time. Similar to conductance, when the rate-limiting step represents >95% of the total impedance, the resistance associated with other steps can be ignored. Clearly, as layer thickness decreases resistance falls. Also, decreases in the magnitude of partition coefficients (Kmw and Kbw) and diffusion coefficients (Dw, Db, and Dm) increase resistance. Models for estimating some of these parameters are available in the literature. Lefkovitz and Crecelius (18) and Hofmans (19) have shown that the Kmw’s of SVOC (log Kow’s e 6.0) are correlated to their Kow’s. Hofmans has also presented models for the estimation of SVOC Dw and Dm values. On the basis of evidence that SPMD uptake and clearance rates obey first-order IEK (4, 11, 13), then the clearance of a PRC is related to its uptake by
kePRC ) kuPRC/KSPMD-PRC
(3)
where kePRC is the clearance rate constant of the PRC (d-1), kuPRC is the linear uptake rate constant, and KSPMD is given in cm3/g in this case. Note that kua-cal, kea-cal, and KSPMD-cal are key parameters measured in laboratory calibration studies. The clearance and uptake rate constants of a PRC under aqueous boundary layer control can be written as
kePRC ) DwA/(lwKSPMDVSPMD)
(4)
kuPRC ) DwA/(lwVSPMD)
(5)
and
where VSPMD is the volume (cm3) of the whole SPMD. In the case of membrane or biofilm control of uptake, the numerators of eqs 4 and 5 would include Kmw or Kbw, respectively, and lw would be replaced by the thickness of the membrane or biofilm, respectively. The sole difference between eq 4 and eq 5 is the presence or absence of KSPMD. If temperature affects the magnitude of KSPMD, and KSPMD’s are derived from regression models (19) or measured at temperatures different
from actual exposures, the values used for KSPMD’s may not be acceptable for the derivation of in situ uptake rates of analytes. Methods for Estimation of kua-f. Two PRC-based approaches can be used to estimate in situ SPMD sampling rates (i.e., kua-f’s) of target compounds. Both methods require knowledge of analyte KSPMD, and for some chemicals, the KSPMD may be temperature dependent. See the subsequent discussion on the role of temperature in the magnitude of KSPMD. Regardless of the method used, PRCs must be chosen whose loss rates are governed by the same rate-controlling mechanisms as the target analytes. Method 1, or the EAF approach, is largely based on the assumption that the effects of environmental variables (e.g., facial velocity-turbulence and biofouling) on the uptake rates of chemicals under aqueous boundary layer control (generally those with log Kow’s in the range of about 4.4-8.0) can be closely approximated by the effects on the loss rates of PRCs, under the same environmental conditions. For boundary layer-controlled chemicals, the range of PRC log Kow’s is typically limited to 4.4-5.5, which is due to the opposing requirements of boundary layer control versus measurable residue (PRC) losses from SPMDs. Implicit in the PRC approach for chemicals under boundary layer control is that the effects of environmental variables on analyte exchange rates remain relatively constant across a wide range of log Kow’s. Method 2 is based on the use of PRCs that bracket the range of analyte Kow’s, as described by Booij et al. (11), and comes closer to approximating an isotopic dilution approach often used for the analysis of environmental samples. This method may require the use of PRCs with log Kow’s ranging from about 3.0 to 8.0. The approach is based on direct derivation of analyte in situ kea-f by regression analysis of data from multiple PRCs (i.e., PRC log Kow vs log kePRC-f). However, KSPMD’s still must be determined in the laboratory. After obtaining kea-f and KSPMD values, in situ kua-f values of analytes are readily derived by eq 3. The key assumption underlying this approach is that kePRC-f’s are measurable for compounds with log Kow’s greater than 5.5. In practice, detecting losses of PRCs with log Kow’s > 5.5 are often difficult unless exposures of extended duration are conducted in warm, highly turbulent environments. For example, earlier work (12) has shown that the ke for benzo[b]fluoranthene is 0.002 (d-1) at 26 °C and a flow rate of
