Fundamentals of the Use of Performance Reference Compounds (PRCs) in Passive Samplers

1. ABSTRACT Examples of PRC Compounds. The following perdeuterated PAHs are often used in environmental studies: naphthalene-d8, acenaphthene-d10, fluorene-d10, phenanthrene-d10, anthracene-d10 and pyrene-d10 (Caution: exposure of PAHs to solar radiation results in rapid photolysis [8]). PCB congeners commonly used as PRCs are 2,2’-dichlorobiphenyl (IUPAC no. 4), 3,5-dichlorobipheny (IUPAC no. 14) 2, 4, 5-trichlorobiphenyl (IUPAC no. 29) and 2,2’,4,6-tetrachlorobiphenyl (IUPAC no. 50). Also, 1,2,4-tribromobenzene and 1,2,4,5-tetrabromobenzene appear to hold promise as PRCs. Finally, many native compounds have been used as PRCs, but they must conform to the constraints listed above. Standard SPMDs. The “standard design” consists of a specified length (e.g., 91.4 cm between the inner welds in the low density polyethylene [LDPE] for 1 mL of triolein) of additive free, 2.5 cm wide layflat LDPE tubing. The LDPE wall thickness ranges between 70-95 m and the triolein used is  95% purity. Note that in the USA, all commercially available SPMDs are fabricated with  99% purity triolein. The AV1 ratio is about 90 cm2 cm-3 (lipid plus membrane), or about 460 cm2 mL1 of triolein. The standard SPMD thereby consists of approximately 20% triolein. For the 1 mL triolein configuration, the whole device typically weighs about 4.4 to 4.6 g. However, any length of SPMD with an AV1 ratio of about 460 cm2 mL1 of  95% triolein, having an approximate 0.25 lipid-to-membrane-mass ratio (i.e., 20% lipid) and a 70-95 m wall thickness is considered a standard SPMD. Method Used to Determine kes of Uptake (Eq. 1) and Loss (Eq. 2). SPMDs containing deuterated PAHs as PRCs were exposed in a flow-through system to a variety of vapor phase chemicals including native priority pollutant PAHs as described by Cranor et al. (7). This study is the first to determine the Rs of native PAH vapors while simultaneously determining the ke of their deuterated analogues. Air flow rate through each SPMD exposure chamber and the control system was maintained at 4.2 liters per minute. At the exit of each SPMD exposure chamber and the control system, the air stream was scrubbed of vapors using polyurethane funnel chromatography (i.e., flow through polyurethane foam (PUF) plugs and charcoal impregnated PUF plugs). Both types of PUF plugs were renewed every three days (72 hour exposure) and were subsequently analyzed to determine the time weighted average (TWA) vapor phase concentrations of the test chemicals. SPMDs were exposed for 3, 6, 9, 12, 15, 18, 24, and 30 days. Replicate SPMDs (n = 3) were collected at each sampling point and analyzed for sequestered chemicals and PRCs. Estimation of Ksa/w, a Key Coefficient. Examination of Eqs. 1-4 show that in all cases the Ksa/w value must be known or determined for analytes of interest. In other words, estimation of the ambient concentration of a HOC from its concentrations in a passive sampler generally requires knowledge of the Ksa/w value. Fortunately, the equilibrium Ksa/w value of an analyte is independent of flow dynamics, biofouling and, in the case of SPMDs, temperature between 2 °C and 30 °C (5). When Ksw is not directly measured, the characteristics above have facilitated the development of a simple regression model (1) for estimating the Ksw values of a wide range of chemicals. log Ksw = –0.1618(log Kow)2 + 2.321 log Kow + ao (7) where Kow is the octanol-water partition coefficient, ao is –2.61 for nonpolar compounds and –3.20 for moderately polar pesticides. The S.D. of the fit is 0.25 and the correlation coefficient r2 = 0.94. This quadratic equation does account for a small decline in Ksw values for analytes with Log Kow values > 6.0. To determine Ksa values needed for vapor phase concentrations, the following relationship can be applied Ksa = KswRT / H (8) where R is the gas law constant, T is the absolute temperature and H is the Henry's law constant. Henry's law constants (Pa m3 mol1) are available for a wide range of contaminants at various temperatures (9-11). An analogous approach (i.e., KowRT / H) is often used and has been shown to be a good method for estimating Koas (12). Shoeib and Harner (13) developed the following regression equation for the determination of polychlorinated biphenyl congener Ksa values. log Ksa = 0.8113log Koa 4.8367 (9) where r2 = 0.83 and Ksa values have units of m3 g-1. Therefore, to express Ksas as unitless coefficients similar to Ksws one must multiply antilog of values derived from Eq. 9 by 1.1 x 106 g-1. CONCLUSIONS MP • ◘ PRCs are now widely used in passive samplers to reduce the error associated with environmental concentration estimates. • ◘ A number of models and the sequence of steps necessary for the use PRCs in estimates of ambient environmental concentrations have been presented. • ◘ A large set of SPMD calibration data will soon be available, which will facilitate the estimation of the concentrations of an expanded range of HOCs. • ◘ The results of this work strongly support the isotropic exchange of PAH PRCs with the atmosphere. • ◘ Measurements of bulk flows are shown not to correlate with SPMD sampling rates supporting the need for PRCs in passive samplers. • ◘ Criteria for selecting PRC data to calculate in situ sampling rates are presented. • We propose the use of surrogate samplers (e.g., SPMDs or silicone sheets) as PRC-based sensors for assessing the effects of flow/turbulence on boundary layer controlled infinite sink samplers such as POCIS. • ◘ There is a strong need for further improvements in protective deployment chambers and the use of a photolysis standard. PRCs are analytically non-interfering compounds with moderate- to relatively high-fugacities, which are added to passive samplers (e.g., the lipid of SPMDs) prior to deployment. The rate of PRC loss during an exposure can be used to estimate in situ sampling rates of the analytes (Rs) of interest. Estimates of Rs are possible because the PRC release rate constant (ke) is equal to the sampling rate of the PRC (Rs; mL d-1) divided by the sampler’s clearance capacity, where the clearance capacity is equal to the sampler’s equilibrium partition coefficient for the PRC (Ksa/w) times the sampler’s volume (Vs; mL). Although PRCs have been used for over a decade, there is very little information available about PRCs relative to: the appropriate numbers of chemicals and the Kow range of candidate PRCs to employ, the applicability of the approach to field situations and different samplers, the effects of sampling media on the selection of PRCs, the analytical methods required for their quantification, the approaches for calculating sampling rates using PRCs, and the limitations of the technique. In this work, these issues are discussed in detail and general guidelines are recommended for using the PRC approach. Also, the types of calibration data (e.g., Ksa/w) needed for the extrapolation of ambient concentrations of target compounds from their concentrations in a sampler are elucidated. Finally, a method is proposed to extend the PRC approach to integrative samplers such as the polar organic chemical integrative sampler, where uptake and release curves are likely anisotropic. BURST EFFECT LAG EFFECT tL TIME tB = 2tL INTRODUCTION Figure 3. Illustration of potential burst and lag effects of PRC release rates, where Mp represents the mass of PRC released, tB and tL represent the burst and lag times, respectively. The relationship of 2tL = tB is valid only under membrane control. REFERENCES In recent years, there has been a rapid growth in the development and application of passive environmental samplers. Because environmental conditions can affect the performance of passive samplers, a number of protective deployment chambers and performance reference compounds (PRCs) have been applied to moderate large differences in environmental conditions and to determine in situ sampling rates of analytes. The focus of this report is PRCs, which are analytically non-interfering chemicals with moderate- to relatively high-fugacities. PRCs are added to a passive sampler (e.g., the lipid of SPMDs) prior to deployment. The rate of PRC loss (ke) during an exposure can be used to estimate in situ sampling rates of analytes of interest. A wide variety of labeled and native (non-labeled) compounds have been used for PRCs. The use of PRCs to determine the effects of biofouling of semipermeable membrane devices (SPMDs) on the uptake rates of hydrophobic organic compounds (HOCs) was proposed in 1991 (1). However, Prest et al. (2) first used phenanthrene as a PRC in a field exposure of SPMDs. The authors reported that study results appeared to be consistent with PRC theory. Several years later, the validity of the use of PRCs for biofouled SPMDs was substantiated in controlled laboratory studies (1). Booij et al. (3) first demonstrated the use of PRCs to assess the effects of different flow velocities. The use of PRCs for determining the effects of differences in temperature on SPMD sampling rates were evaluated by Huckins et al. (4) and demonstrated by Booij et al. (5). More recently, Bartkow et al. (6) has proposed the use of selected photolabile PAHs with very large octanol-water (Kow) or octanol-air (Koa) partition coefficients as photolysis reference compounds. In this work, we provide guidelines and report new developments in the use of the PRC approach for estimating in situ sampling rates. Furthermore, we review practical, theoretical, and modeling requirements of the method as well as its limitations. Finally, we present SPMD concentration data that indicates the exchange of PRC solutes and vapors is isotropic in atmospheric exposures. 1. Huckins, J.N., Petty, J.D. and K. Booij. 2006. Passive monitors of organic chemicals in the environment: semipermeable membrane devices. Springer, New York, NY, In Press. 2. Prest, H.F., W.M. Jarman, S.A. Burns, T.M. Weismuller, M. Martin and J.N. Huckins. 1992. Passive Water Sampling Via Semipermeable Membrane Devices (SPMDs) in Concert with Bivalves in the Sacramento/San Joachin River Delta. Chemosphere 25: 1811-1824. 3. Booij, K., H.M. Sleiderink and F. Smedes. 1998. Calibrating the Uptake Kinetics of Semipermeable Membrane Devices Using Exposure Standards. Environ. Toxicol. and Chem. 17: 1236-1245. 4. Huckins, J.N., Petty, J.D., Lebo, J.A., Almeida, F.V., Booij, K., Alvarez, D.A., Cranor, W.L., Clark, R.C. and B.B. Mogensen. 2002. Development of the permeability/performance reference compound approach for in situ calibration of semipermeable membrane devices. Environ. Sci. and Technol. 36: 85-91. 5. Booij, K., Hofmans, H.E., Fischer, C.V. and E.V. Van Weerlee. 2003. Temperature-dependent uptake rates of nonpolar organic compounds by semipermeable membrane devices and low-density polyethylene membranes. Environ. Sci. and Technol. 37: 361-366. 6. Bartkow, M. E., K. E. Kennedy, J. N. Huckins, N. Holling and J. F. Müller Accepted, August 15th 2005. Photodegradation of polyaromatic hydrocarbons in passive air samplers: Field testing different deployment chambers. Environmental Pollution Special Issue: Passive Air Sampling. 7. Cranor, W.L., Alvarez, D.A., Huckins, J.N., Petty, J.D., and G.L. Robertson. 2004. Calibration of semipermeable membrane devices(SPMDs) for vapor phase airborne contaminants. Fourth SETAC World Congress and 25th Annual Meeting, Portland, OR, USA, IP045. 8. Orazio, C. E., S. A. Haynes, J. A. Lebo, J. C. Meadows, J. N. Huckins and J. D. Petty. 2002. Potential for photodegradation of contaminants during SPMD sampling. 23rd Annual National Meeting of the Society of Environmental Toxicology and Chemistry, Salt Lake City, Utah, USA. P192. 9. Mackay, D., W.Y. Shiu and K.C. Ma. 1992a. Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals. Volume II. Lewis Publishers, Chelsea, MI. 10. Mackay, D., W-Y Shiu and K-C Ma. 1992b. Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals, Volume I. Lewis Publishers, Chelsea, MI. 11. Mackay, D., W-Y Shiu and K.C. Ma. 1997. Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals, Volume V. Lewis Publishers, Chelsea, MI. 12. Meylan, W.M. and P.H. Howard. 2005. Estimation octanol-air partition coefficients with octanol-water partition coefficients and Henry’s law constants. Chemosphere 61: 640-644. 13. Shoeib, M. and T. Harner. 2002. Characterization and comparison of three passive air samplers for persistent organic pollutants. Environ. Sci. Technol. 36: 4142-4151. 14. Luellen, D.A. and D. Shea. 2002. Calibration and Field Verification of Semipermeable Membrane Devices for Measuring Polycyclic Aromatic Hydrocarbons in Water. Environ. Sci. Technol. 36: 1791-1797. 15. Vrana, B. and G. Schüürmann. 2002. Calibrating the Uptake Kinetics of Semipermeable Membrane Devices in Water: Impact of Hydrodynamics.Environ. Sci. Technol. 36: 290-296. 16. Gale, R.W.1998. Three-Compartment Model for Contaminant Accumulation by Semipermeable Membrane Devices. Environ. Sci. Technol. 32: 2292-2300. 17. Booij, K., Smedes, F., E.M. van Weerlee. 2002. Chemosphere 46: 1157-1162. 18. Huckins, J.N., Petty, J.D., Prest, H.F., Clark, R.C., Alvarez, D.A., Orazio, C.E., Lebo, J.A., Cranor, W.L., B.T. Johnson. 2002. A Guide for the Use of Semipermeable Membrane Devices (SPMDs) as Samplers of Waterborne Hydrophobic Organic Contaminants; Publication No. 4690; American Petroleum Institute (API): Washington, DC. 19. Zander, M. 1983. Physical and chemical properties of polycyclic aromatic hydrocarbons, in Bjorseth, A. (Ed), Handbook of Polycyclic Aromatic Hydrocarbons. Marcel Dekker Inc., New York, NY, pp. 1-26. 20. Alvarez, D.A., Cranor, W.L., Huckins, J.N., Cicenaite, A., Schroder, V., and C.E. Orazio. 2005. Protective deployment systems for sampling airborne organic contaminants. 26th National Meeting of the Society for Environmental Toxicology and Chemistry, Baltimore, MD, RP018. • Figure 3 is based on the assumption of membrane control of release rates, which is generally not the case for SPMDs and most other passive samplers. However, both burst and lag effects are quite possible under boundary layer control because boundary layer control only implies that >50% of the overall resistance to mass transfer resides in the boundary layer. For low Kow PRCs, a significant portion of the overall resistance may still reside in the membrane, especially at higher flows. • When possible, we recommend the use of PRC Rss or kes that represent PRC losses > 50% but not more than the percentage of PRC that represents its quantitation limit. The percentage of PRC lost at its quantitation limit will depend on the initial concentration, the method and type instruments used for quantitation, and the physicochemical characteristics of the analyte. In some case, it is possible that the quantitation limit concentration represents only 1% of the initial PRC concentration in the sampler. • Applying the PRC Approach to Infinite-Sink Samplers. • Isotropic exchange is expected to be observed in the partitioning of residues between immiscible liquid phases and between certain non-polar polymeric films (those with rubbery or liquid-like regions) and water or air. This assumption may not be valid for solid phase extraction adsorbents (SPEAs) because of the fundamental differences between solute partitioning and adsorption phenomena. The adsorption of aqueous solutes on active surfaces results in a greater loss of kinetic energy than their partitioning into liquids or liquid-like polymers. However, the capacity of a unit mass of a SPEA for solutes or vapors is ultimately limited by the accessible surface area, and the related binding mechanisms and the strength of adsorption sites. • The SPEAs used in the polar organic chemical integrative sampler (POCIS) include Oasis HLB, Isolute ENV+ and carbonaceous Ambersorb 1500 or 572, which have very high porosities, pore sizes ranging from about 15 to 900 Å and high surface areas ranging from 800 to 1100 m2 g-1. Solute interaction with these SPEAs include - cloud, electrostatic, lone pair, and hydrogen bonding. POCIS SPEAs appear to act as infinite sinks during deployments. • Difficulties in Applying the “Classic” PRC Method to SPEA Based Samplers. •  To date, we have been unable to find PRCs with measurable fugacities from POCIS SPEAs. •  Even if PRC losses from SPEAs could be measured, anisotropic exchange is expected for many SPEAs. • Rate control during uptake is typically the boundary layer whereas the rate control during the release of chemicals is likely inter-particle diffusion. • The Case for Using Surrogate Samplers as PRC-Based Sensors. • An alternative to using PRCs incorporated into POCIS SPEAs is to use SPMDs or silicone films with PRCs under boundary layer control to sense flow effects on the uptake of residues by POCIS. Table 2 indicates that the rate-limiting step in the uptake of a variety of chemical by POCIS is the effective thickness of the water boundary layer, which facilitates the use of surrogate samplers such as SPMDs or silicone films. Because the kinetics characterizing the approach tosaturation or equilibrium of POCIS sorbents likely does not follow first order (Freundlich, Langmuir or other isotherms are more likely), application of PRC containing surrogates requires that uptake of POCIS remain in the zero order or linear region of uptake throughout an exposure. THEORY AND MODELING RESULTS AND OBSERVATIONS Basic Equations. Because the exchange of aqueous residues by SPMDs has been shown to be isotropic (2, 5), the following first-order equations can be used to describe the uptake of HOCs and the release of PRCs by SPMDs, and to derive ambient HOC concentrations (1). N = VsKsa/wCa/w (1exp[Rst / VsKsa/w]) (1) N = No exp(Rst / Ksa/wVs) (2) Ca/w = N / (VsKsa/w[1exp(Rst / VsKsa/w)]) (3) where N is the amount of a HOC sampled by (Eq. 1 & 3) or remaining in (Eq. 2) an SPMD at time t (d), No is the initial amount of the HOC (Eq. 2), Vs is the volume of an SPMD (mL or L), Ksa/w is the equilibrium SPMD-air or water partition coefficient (unitless), Ca/w is the concentration of the chemical in the air or water exposure medium, and Rs is the SPMD sampling rate in m3 or Ld-1 of a HOC or a PRC. Equations 1-3 are more familiar when the exponent is given as ket, where ke is the loss or dissipation rate constant (t-1). Thus, ke is Rs/Ksa/wVs. Equation 1 & 3 fit SPMD accumulation regardless of whether uptake is in the linear, curvilinear or equilibrium phase of sampling. Strictly speaking, residue exchange by a passive sampler is isotropic only when kes or Rss derived from Eqs. 1 & 2 are equivalent. Determination of PRC Based Exposure Adjustment Factors (EAFs). For samplers shown to exhibit isotropic exchange of analytes, only two points (i.e., initial and final concentrations) are used to determine PRC ke values and thereby enable derivation of the in situ sampling rate of a PRC (Rsp) for a particular exposure site. The solution of Eq. 2 for the in situ Rsp is Rsp = Ksa/wVs(ln [N/No])/t (4) After determining the in situ Rsp of the PRC, this value is compared to the sampling rate (Rspc) of the same chemical determined during calibration studies. The ratio of these two values is approximately equal to the EAF as given by EAF Rsp/Rspc (5) A key feature of the EAF is that it is relatively constant for all chemicals that have the same rate-limiting barrier to uptake (4). Thus, the EAF can be used to derive the in situRs values of analytes of interest from laboratory calibration data by RsEAFRsc (6) Extensive calibration data (Rs and Ksw values) will soon be available for standard SPMDs (1), which will facilitate determination of analyte in situRs values. Comments about Calibration Data. Calibration data may consist of: ▲ Sampling rates (Rss). ▲ Rate constants (kus and kes). ▲ Mass-transfer coefficients (e.g., kws). ▲ Partition coefficients (Ksas and Ksws). Generally, at least one of the first three constants/coefficients and one of the two partition coefficients must be measured or derived to estimate ambient environmental concentrations from sampler concentrations. Because the Rss, kus or kws of some complex mixtures can vary by about two to eight fold under a particular set of conditions, and Ksas and Ksws can vary by orders of magnitude, it is important to have at least one set of calibration data for each chemical class of interest. Not all chemicals in a class must be includes but several compounds should be included for each log unit change in class Ksas or Ksws. PRCs Are Sensitive to the Following Environmental Conditions.  Flow-turbulence.  Temperature.  Biofouling.  Photolysis of selected chemical classes. Definition and Illustration of Isotropic Exchange Kinetics. Generally, the standard PRC approach is only applicable to samplers that exhibit isotropic exchange of targeted chemicals. A sampler exhibits isotropic exchange kinetics when both the uptake and loss of chemicals obey first-order kinetics and the halflives (t1/2) or kes measured during uptake and loss are approximately identical (Fig. 1). In other words, resistance to mass transfer into and out of the sampler is the same and the loss rate constant (ke) is proportional to the uptake rate constant (ku). EXPERIMENTAL Most of this section has been published in recent reports (1, 4-7). However, we include a brief description of how PRCs are selected, and key materials and methods. Selection Criteria Used for PRCs. PRCs are selected from deuterated, 13C and non-labeled or native compounds. The obvious advantage of labeled compounds is that they don’t occur at significant levels in the environment. Typically, a minimum of three PRCs are recommended for a study. A variety of factors should be considered before selecting a PRC mixture which include: ■ The Kow or Koa (indicators of compound fugacity from samplers for HOC) of candidate compounds; note that PRCs are generally selected from compounds that have a range of Kows from 103-106 and a range of Koas from 106-109. ■ The range of PRC Kows or Koas; the minimal range of PRCs should span at least two orders of magnitude to ensure measurable ke values under a variety of site conditions. ■ The identities of deuterated or 13C labeled surrogates and internal standards used for laboratory QC of analytical methods. ■ Compatibility of PRCs with analytical instrumentation used for quantitation of targeted compounds. ■ Cost: native and deuterated compounds are generally much less expensive than 13C-labeled PRCs. ■ Site exposure conditions; e.g., if a site is highly turbulent and warm PRCs should include some compounds with abnormally high Kows or Koas. ■ The suite of contaminants present at an exposure site; this limitation only applies when native compounds are used for PRCs. Figure 2. Sampling rates of compounds in the log Kow range 6 to 7 as a function of water flow velocity. Connected data points represent measurements within single studies (dotted line: Luellen and Shea [13]; solid line: Vrana and Schüürmann [14]). 1 Analytes t1/2 Cs 0.5 PRCs 0 0 10 20 30 40 50 60 TIME (d) Figure 1. Illustration of isotropic exchange kinetics. Fundamentals of the Use of Performance Reference Compounds (PRCs) in Passive Samplers James N. Huckins1, Kees Booij2, Walter L. Cranor1, David A. Alvarez1, Robert W. Gale1, Michael E. Bartkow3, Gary L. Robertson4, Randal C. Clark1 and Roger E. Stewart5 1USGS, CERC, Columbia, MO, USA; 2Royal Netherlands Institute for Sea Research, Texel, The Netherlands; 3National Research Centre for Environmental Toxicology, Coopers Plains, Queensland, Australia; 4US EPA, NERL, Las Vegas, NV, USA; 5Virginia Department of Environmental Quality, Richmond, VA, USA Is the Exchange of PRCs Between SPMDs and the Atmosphere Isotropic? Huckins et al. (4) reported that exchange of selected chemicals (PRCs) between SPMDs and water was isotropic. However, comparable data have not been reported for SPMDs-atmospheric exposures. Table 1 summarizes data from PRC-containing SPMDs exposed to PAH vapors in a flow through atmospheric chamber. Table 2. Sampling rates (Rss) of POCIS (L d-1; A = 41 cm2) under quiescent (non-stirred) and turbulent (stirred) conditions. Values reported are means (n=3). • Knowledge of the water or air mass transfer coefficients (kw and ka, respectively) at the sampler surface is the key to understanding the effects of flow/turbulence on sampling rates under boundary layer control. To calculate kw and ka values the following relationship is used • ka/w = Da/w / a/w (10) • where Da/w is the diffusion coefficient of the HOC in air or water and a/w is the thickness of the boundary layer.Thus, it is actually the effective thickness of the boundary layer that is difficult to determine under different hydrodynamic regimes. • Chemical engineers have developed semi-empirical correlations to derive ka/ws, such as the following relationship for turbulent flow in water • Sh = kwdL / Dw = 0.036Re0.8Sc1/3 (11) • where Sh is the dimensionless Sherwood number, dL is a characteristic length, such as the diameter of the holes in the deployment chamber, that reflects the geometry of the system, Re is the dimensionless Reynolds number (udL/; u can be considered as the flow velocity remote from the sampler and  is the kinematic viscosity) and Sc is the dimensionless Schmidt number (/Dw). Because only directly measured PRC kes and Ksa/ws of analytes are needed to derive in situkwvalues of analytes (i.e., Rs = ka/wA = keKsa/w, where A is the sampler surface area), we hypothesize that the use of PRCs provides a more reliable measure of in situka/w values. However, we leave this analysis to future work. • PRC Assumptions and Methods. • Two assumptions are made when fitting a first-order model to the loss of PRCs from passive samplers. First, chemicals in all compartments are well mixed, and second, equilibrium exists at sampler interfaces. In regard to SPMDs, Gale (16) has shown that equilibrium does exist at the membrane-lipid interface. The validity of the first assumption is dependent on how the sampler is fortified with PRCs, the mass transfer coefficients of the PRCs in the sampler reservoir, and the time between PRC fortification and sampler exposure at a site. • Methods used to fortify passive samplers include: •  The “classic” method is based on spiking PRCs into triolein prior to its loading into the SPMD membrane tubing. •  Partitioning of PRCs into LDPE and silicone strips/sheets; the method (17) employs LDPE or silicone samplers submersed in an 80/20 (v/v, methanol-water) solution of PRCs. •  Pervaporative loading of PRCs into LDPE or silicone tubing; the method (7) involves loading a small amount (minimal amount required to wet all interior surfaces of the tubing) of a hexane solution containing PRCs in the the LDPE or silicone tubing, sealing the ends and allowing pervaporation of the hexane. Afterwards, the LDPE tubing can be cut along one edge to create strips. •  Soaking a polyurethane (PUF) disk (13) in 200 mL of a pentane solution of PRCs and allowing the pentane to evaporate (personal communication, Michael Bartkow, University of Queensland, Coopers Plains, Australia). •  Spiking 15 mL aliquots of a 30 mL petroleum ether PRC solution on opposite sides of a PUF disk in a glass dish and allowing the solvent to evaporate (personal communication, Tom Harner, ARQP, Aurora, Canada). • In all cases, PRC loaded samplers are stored for some time interval between spiking and deployment. There is little doubt about the homogeneity of PRC distribution when using the first and second methods (from the top). This is likely true for the third method but during the first few minutes slightly higher concentration of certain PRC may be present on the exterior surface of the tubing. Using the fourth and fifth methods the assumption of homogeneity may depend on the PRCs mass transfer coefficient in the PUF matrix, storage times and the distribution coefficient between the solvent and the PUF matrix. The potential problem with heterogeneous distribution of PRCs in a sampler is that the effective thickness of the boundary layer may differ across the external surface area of the sampler, resulting in localized difference in dissipation rates. • Steps and Data Required for Estimation of Ambient Concentrations. • Obtain the analyte’s Ksa/w from calibration or regression equations (Eqs. 7-9 or the simple equation Ksa/w = ku / ke may be used as well). • Determine Rsp or the PRC sampling rate from Eq. 4. (i.e., the PRC’s ke) • Go to calibration data tables given in reference 1 or other sources and find the Rspc or the sampling rate of the native PRC. • Calculate EAF from Eq. 5. • Apply Eq. 6 to determine the in situRss of analytes of interest. • Calculate the air or water concentration (Ca/w) of analytes with Eq. 3. • Not necessary in the rare event that field exposure conditions match conditions calibration data were generated under. Selecting PRC Data Best Suited for Derivation of Site EAF. • Earlier, we recommended that PRC losses of 20 to 80% be used as a criterion for the selection of acceptable PRC data (18). However, investigators have observed both a burst (i.e., a positive Y intercept) and lag in the uptake of chemicals by SPMDs. In controlled release systems these characteristics of chemical release rates are well documented (19). Fig. 3 illustrates this effect. Table 1. Comparison of kesa determined by the concomitant uptake and loss of native PAH vapors and PRC-d10 analogs, respectively. The flow rate was maintained at 3.2 cm sec-1 and the mean temperature was 22.4 °C  1.9. The differences between POCIS Rs values for nine analytes measured under quiescent and turbulent exposure conditions averaged 5.6 fold (n = 9) with a C.V. of 32% indicating WBL control. a. Values derived represent the mean of three replicate samples. b. Equilibrium attained to rapidly, insufficient points for regression analysis. c. Calculated from Eqs. 7 & 8. • To date, only small SPMDs containing PRCs have been used for field studies by either placing the SPMD inside the POCIS deployment canister or by attaching a small SPMD holder to the outside of the POCIS canister. However, by fixing PRC spiked SPMDs or silicone membranes between POCIS compression rings and mounting the resulting disk similarly to POCIS disks in deployment canisters, the turbulence regime experienced by the SPMD disk should more closely reflect adjacent POCIS disks. With the exception of the differences in biofouling between the POCIS and SPMD or silicone membranes, environmentally induced changes in the rates of surrogates loaded with PRCs should reflect the changes in POCIS sampling rates. • Tracking the Potential for Analyte Photolysis During Exposures. • A number of HOCs are subject to photolysis such as PAHs and polybrominated diphenyl ethers. Deuterated PAHs are commonly used as PRCs in passive sampler environmental exposures, without sufficient attention to the potential problem of photolysis. Zander (19) proposed that PAH photolysis involved photooxidation mediated by the transfer of energy from the triplet state of the aromatic system to an oxygen molecule, producing singlet oxygen. Peroxides and quinones of PAHs would then be formed by subsequent reactions with singlet oxygen. In atmospheric exposures, oxygen is not a limiting factor in PAH photolysis because its concentration is about 105 higher than in water. However, it may be rate limiting in water for SPMDs because the permeability of oxygen through LDPE is very low. • Recently Bartkow et al. (6) and Alvarez et al. (20; see poster RP018) have shown that most deployment chamber designs are inadequate to prevent photolysis of PAHs in SPMDs, during atmospheric exposures. Bartkow et al. (6) tested four deployment chambers and found that only a double bowl design by Tom Harner fully protected deuterated anthracene and pyrene PRCs from photolysis. In the Alvarez et al. (20) atmospheric study, the double bowl design did not fully prevent photolysis of deuterated PAH PRCs but the gap between the two bowls may have been larger. Two types of double-can designs appeared to afford the most protection of PAHs from photolysis. Of the 16 deuterated priority pollutant PAHs tested by Alvarez et al. (20), benzo[a]pyrene and dibenz[a,h]anthracene appeared to be the most sensitive to photolysis. Because their fugacities are very low from SPMDs, they are logical candidates for use as a photolysis indicators. • Limitations of PRCs. • If the rate-limiting step in PRC exchange switches from boundary layer control to membrane control, while target compounds remain under boundary layer control (i.e., those with Kows or Koas higher than PRCs), due to an episodic high flow event or a permanent state of high flow velocity, PRC derived Rss will not fully reflect changes in analyte Rss. • There is some evidence that the impact and/or retention of particles on sampler surfaces may contribute significantly to the rate of chemical uptake; PRCs will sense any changes in mass-transfer coefficients but will not sense particle induced elevated concentrations at the membrane surface. • Because biofouling impedance is proportional to a compound’s Kow, PRCs do not fully reflect the biofouling induced reduction in Rs values of analytes with log Kows > 6. • If only one sampler in a deployment chamber contains PRCs, extrapolation of the PRC data to other samplers inside the chamber may increase errors in ambient concentration estimates. • As discussed earlier, most PAH PRCs are readily photolyzed, thus investigators must ensure that the dissipation of these PRC is not influenced by photolysis. The differences between kes given in Table 1 are not large in view of the fact that each ke value requires multiple analytical determinations. Fits of first-order uptake and loss equations to the data were excellent as indicated by the r2 values. This data suggest that the uptake and release of PAH vapors is indeed isotropic. Why Use PRCs? Huckins et al. (4) have suggested that in diverse aquatic systems SPMD sampling rates may vary as much as ten fold due to differences in facial velocity/turbulence at the membrane surface, about four fold due to differences in environmental exposure temperatures (i.e., for a range of 2 to 30 C) and 3 to 4 fold (compounds with log Kows > 6.0) for membrane biofouling. Even when SPMDs or most other samplers are exposed inside a protective deployment chamber, differences in facial velocity among sites will likely have the greatest effect on sampling. Unfortunately, at best flow meter measurements of linear flow rates provide only a rough indicator of changes in sampling rates. This fact is illustrated by the lack of correlation between flow rates and Rss in Fig. 2 (1).