Escher, Beate

Gaus, Caroline

Grant, Sharon

Hawker, Darryl W.

Schacht, Veronika J.

University of Queensland eSpace

Development and validation of a new method to upload polymers with super-
hydrophobic contaminants for passive dosing approaches 
Veronika J. Schacht
1*
, Sharon Grant
1
, Beate Escher
2
 Darryl W. Hawker
3 
and Caroline Gaus
1 
1
 The University of Queensland, Entox, 39 Kessels Road, Coopers Plains, QLD, Australia 
2 
Helmholtz Centre for Environmental Research, Permoserstr. 15, Leipzig, Germany 
3 
Griffith University, School of Environment, 170 Kessels Road, Nathan, QLD, Australia 
Introduction 
Contaminant uploaded polymers are increasingly used as delivery mechanisms for analytes into experimental 
systems (passive dosing). The concept of passive dosing was initially developed for the purpose of toxicity 
testing, where it allowed the delivery of a defined and constant dose to test organisms
1, 2
. The advantages have 
also been recognised for other applications, such as the determination of equilibrium sorption and speciation of 
hydrophobic contaminants in systems involving water, dissolved and solid hydrophobic phases
3-6
, as well as an 
alternative spiking approach for poorly soluble contaminants to aqueous test systems
5
. Passive dosing 
approaches, however, rely on precise and reproducible methods to load contaminants to the polymer. This is 
particularly important when working with superhydrophobic contaminants (SHOCs; log KOW >7) since 
measuring water phase concentrations is rarely feasible for such compounds, and aqueous concentrations are 
thus typically calculated under mass balance assumptions using measured concentrations in the polymer at 
equilibrium and known polymer-water partition coefficients. 
 
To date, contaminants are typically loaded to passive dosing polymers via direct partitioning from methanol or a 
methanol-water mix. While this partitioning driven upload method has been performed with highly 
reproducibility for a wide range of contaminants, variability in the mass loaded has been reported to increase 
with increasing contaminant hydrophobicity. For example Endo et al. (2013)
10
 reported increasing variability of 
PBDEs loaded to poly(dimethylsiloxane) (PDMS), with up to 40% (relative standard deviation (RSD)) for BDE 209 
(log KOW >9). A pilot study performed in our laboratory demonstrated similar variability for a range of PCDDs 
(log KOW 6.9-8.3) loaded to PDMS, with up to 47% RSD for the most hydrophobic, OCDD. The high variability 
in the loaded masses contribute undesirably high uncertainties to the experimental data. In addition, slow 
partitioning kinetics result in inconvenient waiting times before the polymers are ready for deployment in the 
experimental system.  Here we present a new polymer loading approach that overcomes these limitations, and 
achieves precise, rapid and reproducible SHOC concentrations in PDMS to provide a robust basis for passive 
dosing approaches. 
 
Theory 
Solvents like toluene and hexane have been reported to swell PDMS by a factor of 1.3 in any one dimension 
(which equates to an increase in volume of 2.2)
7
. During the swelling process, analytes dissolved in the solvent 
are transported into the PDMS where they remain after the solvent is evaporated
2
. Swelling can therefore 
significantly increase the mass of analyte loaded in a polymer compared to partitioning alone. A simplified 
model of the combined swelling and partitioning loading process predicts that the majority of SHOC mass in 
PDMS is achieved via swelling (e.g. range 82-90% of total mass for five PCDDs with log KOW ranging from 6.9 
to 8.3). The swelling method has several potential advantages, including 1) loading can be achieved rapidly since 
the process does not rely on partitioning kinetics which are typically very slow for SHOCs, and 2) the mass of 
analyte loaded is largely independent of its physico-chemical properties, being primarily driven by the 
concentration in the loading solvent, and the mass can thereby be easily controlled. Therefore, loading extremely 
hydrophobic organic contaminants (extreme SHOCs, e.g. OCDD, PCB 209) via swelling into PDMS may be a 
faster and more reliable loading method.  
 
Two potential issues were identified that could arise when loading passive dosing polymers via the swelling 
approach, and were investigated in this study: 
1. Potential trapping of analytes: SHOCs, and in particular extreme SHOCs, are often large molecules which, 
when loaded via swelling, may potentially be trapped in the PDMS after the solvent has evaporated (de-
swelling). Consequently, large molecules such as OCDD might not be available for subsequent dosing and 
partitioning processes.  
Organohalogen Compounds Vol. 76, 1468-1472 (2014) 1468 
2. Exceeding solubility: The maximum solubility of SHOCs in the PDMS may be exceeded when loading via 
the swelling approach. For some applications of passive dosing it is essential to stay below saturation limits 
in the PDMS to ensure that equilibrium concentrations in the dosing polymer and a depletion phase are below 
maximum solubility, or to, for example, avoid mixtures effects close to solubility. Therefore, it is important 
to know the maximum solubility of SHOCs in PDMS.  
 
Material and Methods 
 
Materials: Coated fibres were obtained from Fibreguide Industries Inc (Stirling, USA) and consisted of a 50 µm 
layer of PDMS coated on a glass fibre of diameter 100 µm. PDMS sheeting, thickness 7.5 µm, was obtained 
from Specialty Silicone Products, Inc (New York, USA). A PCDD mix of 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 
1,2,3,4,7,8-HxCDD, 1,2,3,4,6,7,8-HpCDD and OCDD was purchased from AccuStandard (New Haven, USA), 
and 1,2,3,4,6,7,8-HpCDF (used as an internal standard) from Cambridge Isotopes Laboratory (Andover, USA). 
Standard solutions for PCBs 136, 182, 204, 206 and 209 as well as solid PCB 209 and PCB 118 (used as internal 
standard) were obtained from AccuStandard (New Haven, USA). All solvents used were of analytical grade.  
 
Swelling load method: The swelling load method was tested for consistency and reliability using a mixture of 
the five PCDD congeners (in toluene) and five PCBs (in isooctane) listed above, which differ in their physico-
chemical properties. To perform the experiment, PDMS coated fibres were cut into 2 cm pieces, washed in 
MilliQ water and dried for 24 h. PDMS fibres were then transferred into a 20 mL vial and swelled in hexane for 
24 h to clean and remove unpolymerised PDMS oligomers
7
. After all solvent was evaporated, PDMS fibres were 
weighed using a Mettler microbalance and subsequently stored in methanol. The variability (relative standard 
deviation, %RSD) in PDMS weight between fibres was less than 2%.  
For the swelling load, PDMS fibres were transferred to 2 mL vials containing 1.8 mL of 0.7 µg mL
-1
 PCDD 
standard in toluene, or 1.8 mL of 3.5 µg mL
-1
 PCB standard in isoctane. The PCDD/PCB concentrations in the 
loading solvent were calculated (based on known swelling ratios for the loading solvent) to ensure that the 
loaded concentration in the PDMS after swelling was below maximum solubility of the most hydrophobic 
congener (see method below to determine maximum solubilities). To ensure an even swelling, fibres were 
completely submerged in the loading solution. The loading vial was placed onto an orbital shaker in an incubator 
(23°C, 100 rpm) for 24 h. Loaded PDMS fibres were transferred onto dry lint-free tissue and immediately 
separated. To establish the concentration in the PDMS after loading and quantify the variability between fibres, 
individual PDMS fibres were transferred to inserts and extracted in 270 µL of hexane for 24 h on an orbital 
shaker (23°C, 100 rpm). Based on previous tests, one extraction step was found to be exhaustive (>99% of mass 
recovered). The hexane extracts were evaporated under a N2 stream to just dryness. 20 µL of the injection 
standard 1,2,3,4,6,7,8-HpCDF or PCB 118 of known concentration was added for PCDDs and PCBs, 
respectively. Extracts were analysed as described below.  
 
Potential trapping of analytes: To ensure free availability of the total mass of analytes loaded, swelling loaded 
PDMS fibres were depleted using a swelling and a non-swelling solvent. If analytes are trapped in the polymer 
post-loading, the non-swelling solvent would be expected to recover less mass than the swelling solvent. To test 
for potential trapping of analytes, 2 cm PDMS fibres (n=20) were loaded in 2.5 µg mL
-1
 PCDD standard mix 
using the swelling load (as described above) and two fibres each were subsequently transferred into 2 mL glass 
vials. Two mL of a non-swelling solvent (methanol) was added and each of the 5 fibre pairs was depleted for 
24 h on an orbital shaker (23 °C, 100 rpm). Each fibre pair was extracted a further four times. The extracts were 
combined and analysed as described for the swelling load. The remaining 5 PDMS fibre pairs were extracted in a 
swelling solvent (hexane) using the same method as for methanol except only one extract was required. 
 
Maximum solubilities in PDMS: To date, no data on maximum solubilities of SHOCs in PDMS are available. 
The maximum solubility of analytes can, however, be measured via PDMS-PDMS partitioning of analytes from 
super-saturated PDMS sheets loaded via swelling (at concentrations orders of magnitude higher than the 
expected PDMS solubility) into clean PDMS fibres. The equilibrium concentration in the clean fibres provides 
the solubility limit in PDMS. To determine the maximum solubility in PDMS for the least soluble tested SHOCs 
(HpCDD, OCDD, PCB 209) PDMS sheets of dimensions 2.5 x 1.5 x 0.0075 cm were prepared as previously 
described for fibres. Two PDMS sheets were submerged for 24 h in 2 mL of 15 µg mL
-1
 PCDD standard mix in 
toluene. Predicted concentrations of each congener in the PDMS sheets were in excess of 25 ng µL
-1
. A further 
Organohalogen Compounds Vol. 76, 1468-1472 (2014) 1469 
Figure 1. Concentrations of PCDDs (left axis scale) and PCBs (right axis 
scale) in the PDMS fibre using the swelling load method with toluene and 
isooctane, respectively. Error bars represent the standard deviation. 
Values of the relative standard deviation % (RSD) are given for each 
congener. 
 
 
two PDMS sheets were submerged for 48 h in 2.2 mL of 940 µg mL
-1
 PCB 209-solution, premade from solid 
PCB 209 in toluene. Predicted concentration of PCB 209 in the PDMS sheets was approximately 1,460 ng µL
-1
. 
After loading, the PDMS sheets were placed on Teflon sheeting to allow the toluene to completely evaporate. 
Two cm clean PDMS fibres were tightly sandwiched between either the two PCDD loaded, or the two PCB 209 
loaded sheets. Maximum contact between loaded sheets and the fibres was ensured by weighing the top sheets 
down. Replicate PDMS fibres (5 replicates PCDDs, 3 replicates PCB 209) were taken out after different times 
(to establish when equilibrium was reached), wiped with a lint free tissue to remove surface adsorbed analyte, 
and extracted in 270 µL hexane for 24 h on an orbital shaker (23°C, 100 rpm). Based on previous tests, one 
extraction step was found to be exhaustive for PCDDs (>99% of mass recovered). A second 24 h extraction for 
PCB 209 was performed and the two extracts combined for analysis.  
 
Instrumental analysis: The concentration of PCDDs and PCBs in the PDMS fibre extracts were quantified on a 
Hewlett Packard 5890 Gas Chromatography-Electron Capture Detector (GC-ECD) Series II with a DB-5 fused 
silica capillary column (30 m × 0.25 mm i.d.) (Agilent Technologies, Santa Clara, USA).  
PCDD concentrations were quantified based on their peak area relative to the internal standard 1,2,3,4,7,8,9-
HpCDF, and using a 5-point calibration series consistent of 50-800 pg µL
-1
 PCDD mix and containing 300 pg 
µL
-1
 1,2,3,4,7,8,9-HpCDF. The PCB concentrations were quantified with PCB 118 as internal standard and the 
concentration for the calibration series was 100-1,600 pg µl
-1
, containing 200 pg µL
-1
 PCB 118. 
 
Results and discussion: 
Using a PCDD loading concentration of 0.7 µg mL
-1
 in 1.8 mL toluene (1.26 µg mass) the loaded PDMS 
contained approximately 1.5 µg mL
-1
 (range 1.3-1.6 µg mL
-1
, or 1.16  ng mass) of each congener after swelling 
(Figure 1). For the PCBs, a higher loading concentration of 3.5 µg mL
-1
 per congener (6.3 µg mass) in isooctane 
resulted in loaded PDMS concentration of approximately 8.9 µg mL
-1
 (range 8.7–9.2 µg mL-1, 6.95 ng mass, 
Figure 1). The consistent PDMS concentrations for all five PCDDs (log KOW = 6.9-8.3)
8
, and the five PCBs (log 
KOW = 6.22-8.2)
9
 indicate that loading via swelling is mostly independent of the analyte’s physico-chemical 
properties. By contrast, the methanol and methanol-water partitioning loading methods result in increasing 
loaded mass with increasing compound hydrophobicity
10
. 
 
The swelling method resulted in low 
variability of PCDD concentrations 
between PDMS fibres for all 
congeners (%RSD = 6.0–7.1% n = 5 
(Figure 1)), indicating high 
reproducibility of the loading 
method. Similarly, highly 
reproducible results were observed 
for PCB concentrations in PDMS 
loaded using isooctane  
which has a similar swelling ratio to 
toluene (%RSD = 8.0–8.3% n = 5 
(Figure1)). Furthermore the 
variability was not observed to 
increase with increasing 
hydrophobicity of the congeners as 
was found for the methanol 
partitioning loading method for 
SHOCs
10
.  
In comparison to partitioning based 
loading methods which require several days to achieve equilibrium (e.g. up to 7 days)
10
, a major advantage of the 
swelling method is that equilibrium is not required and highly reproducible loading can be completed within 24 
hours (and possibly even less time since the swelling process was observed to be very rapid). A further 
advantage of the swelling method is that the mass of SHOCs loaded into the polymer is primarily driven by 
swelling and therefore the expected mass loaded from any solvent can be readily predicted based on the swelling 
Organohalogen Compounds Vol. 76, 1468-1472 (2014) 1470 
Figure 2. HpCDD and OCDD partitioning into clean 
PDMS fibres via PSMS-PSMS partitioning over time. 
Maximum solubility occurred after approximately 10 
days for HpCDD and 2 days for OCDD. 
ratio for the loading solvent. A comprehensive list of solvent swelling ratios for PDMS has been reported by Lee 
et al. (2003)
7
. 
The practical application of swelling-loaded polymers for passive dosing studies requires, however, that large 
molecules are not trapped in the polymer following de-swelling and that maximum solubilities in PDMS are not 
exceeded after loading. To investigate whether large compounds are trapped in the polymer after de-swelling, the 
recoveries of OCDD, as one of the largest model SHOCs used, were compared via extraction in a swelling and a 
non-swelling solvent. Almost identical masses of OCDD were depleted from the swelling loaded PDMS fibres 
(loaded concentration exceeded OCDD solubility) using a swelling solvent (hexane: extracted mass average 
3.4 ng, n=5, %RSD 6.2%), and a non-swelling solvent (methanol: extracted mass average 3.6 ng, n=5, %RSD 
8.2%). The OCDD trap-test validated the use of swelling as a feasible loading method as high molecular weight 
SHOCs loaded to the passive dosing polymer are freely available for partitioning depletion and passive dosing. 
 
The maximum solubilities of HpCDD and OCDD in 
PDMS were determined to be 11 and 1.9 µg mL
-1
, 
respectively. Due to the lower mass transfer of OCDD 
from the loaded PDMS sheets into the clean PDMS 
fibre (1.49 ng compared to 8.91 ng of HpCDD in the 
clean PDMS at equilibrium), equilibrium was reached 
within 2 days compared to approximately 10 days for 
HpCDD (Figure 2). By comparison to the PCDDs, the 
maximum solubility of PCB 209 was significantly 
higher at 530 µg mL
-1
 (equilibrium was achieved 
within 40 days). The high value was surprising as the 
three compounds have similar hydrophobicities (log 
KOW for HpCDD, OCDD and PCB 209 of 7.8, 8.3 and 
8.2, respectively) and similar water solubilities (SW of 
approximately 9.4, 1.2 and 6.2 ng L
-1
, respectively), 
indicating similar saturation concentrations in octanol. 
The difference in molecular configuration between the 
two planar PCDD congeners and PCB 209 (which occupies a larger 3 dimensional space) may confer differences 
in physico-chemical properties that explain the higher activity of PCB 209 in PDMS. 
Knowing the maximum solubilities in PDMS for the least soluble SHOCs means that, for the swelling load, the 
concentration in the loading solution can be selected to achieve very specific concentrations in the loaded 
polymer. As a consequence, experimental set-ups can be accurately adjusted to meet criteria regarding the 
maximum solubility as well as instrument detection limits. 
The results from these experiments suggest that the swelling load method represents a simple, rapid and robust 
approach to achieve accurate and reproducible loaded masses of SHOCs with log KOW up to 8.3 in polymers for 
passive dosing and other applications. The method has been shown to work with two loading solvents that have 
similar swelling ratios for PDMS, toluene and isooctane, and for two compound groups. Future work will focus 
on understanding the differences in maximum solubilities of PCDDs and PCBs in PDMS.  
 
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Organohalogen Compounds Vol. 76, 1468-1472 (2014) 1471 
  
Organohalogen Compounds Vol. 76, 1468-1472 (2014) 1472 


Development and validation of a new method to upload polymers with super- hydrophobic contaminants for passive dosing approaches

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Development and validation of a new method to upload polymers with super- hydrophobic contaminants for passive dosing approaches

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