Transcript Document

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
ABSTRACT
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.
INTRODUCTION
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.
Examples of PRC Compounds.
Estimation of Ksa/w, a Key Coefficient.
Is the Exchange of PRCs Between SPMDs and the Atmosphere Isotropic?
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, 5trichlorobiphenyl (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.
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.
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 PRCcontaining SPMDs exposed to PAH vapors in a flow through atmospheric
chamber.
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 A V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 A V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.
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.
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
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.
RESULTS AND OBSERVATIONS
THEORY AND MODELING
▲ Sampling rates (Rss).
▲ Rate constants (kus and kes).
▲ Mass-transfer coefficients (e.g., kws).
▲ Partition coefficients (Ksas and Ksws).
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)
(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
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.
Uptake
Eq. 1
ke (r2)
PRC-d10-loss
Eq. 2
ke (r2)
Acenaphthene
5.9
0.43
0.430 (b)
0.527 (0.99)
Fluorene
6.2
0.53
0.411 (0.99)
0.301 (0.99)
Phenanthrene
7.2
0.97
0.079 (0.98)
0.069 (0.92)
Pyrene
8.0c
1.40
0.020 (0.96)
0.014 (0.97)
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.
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.
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).
Sh = kwdL / Dw = 0.036Re0.8Sc1/3
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 situ kw values 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 situ ka/w values. However, we leave this analysis to future
work.
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).
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 situ Rss of analytes of interest.
Calculate the air or water concentration (Ca/w) of analytes with Eq. 3.
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]).
0
0
10
20
30
40
50
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.
60
TIME (d)
Figure 1. Illustration of isotropic exchange kinetics.
LAG EFFECT
tB = 2tL
tL
TIME
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.
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
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 to
saturation 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.
◘ A number of models and the sequence of steps necessary for the use PRCs in
estimates of ambient environmental concentrations have been presented.
RS from Turbulent
Renewals (Ld-1)
◘ 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.
Diuron
0.011
0.100
Isoproturon
0.034
0.200
◘ The results of this work strongly support the isotropic exchange of PAH PRCs
with the atmosphere.
Atrazine
0.050
0.240
Diazinon
0.056
0.186
Azithromycin
0.048
0.270
Fluoxetine
0.027
0.200
Levothyroxine
0.021
0.120
Omeprazole
0.016
0.068
Ethynylestradiol
0.070
0.302
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.
PRC Assumptions and Methods.
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).
0.5
Prescription
Pharmaceuticals
(11)
Steps and Data Required for Estimation of Ambient Concentrations.
t1/2
Pesticides
BURST EFFECT
◘ PRCs are now widely used in passive samplers to reduce the error associated
with environmental concentration estimates.
RS from Quiescent
Renewals (Ld-1)
Analytes
Chemical engineers have developed semi-empirical correlations to derive ka/ws,
such as the following relationship for turbulent flow in water
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.
1
Cs
MP
(10)
 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).
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).
(6)
Extensive calibration data (Rs and Ksw values) will soon be available for standard
SPMDs (1), which will facilitate determination of analyte in situ Rs values.
ku
m3 d-1
Definition and Illustration of Isotropic Exchange Kinetics.
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 situ Rs values of analytes of interest from laboratory calibration data by
Rs  EAFRsc
Log
Ksa
 Flow-turbulence.
 Temperature.
 Biofouling.
 Photolysis of selected chemical classes.
(4)
(5)
Test
Compound
PRCs Are Sensitive to the Following Environmental Conditions.
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
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.
Why Use PRCs?
Calibration data may consist of:
ka/w = Da/w / a/w
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).
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.
(9)
Comments about Calibration Data.
Ca/w = N / (VsKsa/w[1exp(Rst / VsKsa/w)])
EXPERIMENTAL
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.
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
CONCLUSIONS
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.
◘ 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.
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 Because biofouling impedance is proportional to a compound’s Kow, PRCs do
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 If only one sampler in a deployment chamber contains PRCs, extrapolation of
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ambient concentration estimates.
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 As discussed earlier, most PAH PRCs are readily photolyzed, thus investigators
must ensure that the dissipation of these PRC is not influenced by photolysis.
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