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Phase Ratio Variation approach for the study of partitioning behavior of volatile organic compounds in
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polymer sample bags: Nalophan case study
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Jim Van Durme*, Bas Werbrouck
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Research Group Molecular Odor Chemistry, KU Leuven Campus Ghent, Gebroeders De Smetstraat 1, B-9000
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Ghent, Belgium
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* Corresponding author: Tel.: +32 (0)9 265 86 39; Fax: +32 (0)9 265 86 38; E-mail address:
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jim.vandurme@kuleuven.be
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Abstract
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Sorption of volatile organic compounds on the inner surface of polymer sampling bags leads to important
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underestimations of the real headspace concentration. Introducing a wide range of volatiles in a two-phase
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system containing Nalophan, revealed that recoveries decreased down to 57% in a period of 22 hours. In this
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work a Phase Ratio Variation approach is investigated to quantify the degree of scalping, and thus enabling to
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compensate for sorption phenomena. This method requires limited measurements, without the need for time-
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consuming calibrations. By spiking identical amounts of volatiles in three two-phase systems, each having
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unique polymer volume/mass ratios , individual partitioning coefficients could be
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experimentally determined for a wide range of compounds. Additionally, a correlation was found between these
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partitioning coefficients and the liquid molar volume for a number of aliphatic, aromatic and oxygenated
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compounds.
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Keywords: scalping, sampling, polymer, partitioning, volatile organic compound, environmental analysis
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1.
Introduction
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In the field of environmental analysis, researchers rely on the accurate qualitative and quantitative assessment of
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odorous compounds in the gas phase. In recent years, attention has been mainly given to advanced hyphenated
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analytical techniques and the development of new detectors (e.g. e-noses, SIFT-MS, etc.). However, the sample
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collection step is equally important and strongly influences reproducibility and accuracy. Despite the availability
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of multiple sorbent media (Solid Phase MicroExtraction, Stirbar Sorptive Extraction, sorbent tubes) and
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cryocondensation techniques, whole air sampling using polymeric bags is still one of the most frequently used
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sample collection methods in the field, due to the ease by which they can be manipulated, their reduced cost and
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the possibility of reuse (Alonso and Sanchez, 2013). In accordance with the European Standard “Air quality –
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Determination of odor concentration by dynamic olfactometry” (EN 13725; CEN, 2003), Tedlar (polyvinyl
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fluoride) and Nalophan (polyethylene terephthalate) are frequently used for whole air sampling (Del Pulgar,
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Carrapiso, Reina, Biasioli, & García, 2013; Heynderickx, Huffel, Dewulf, & Langenhove, 2013, Hansen et al.,
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2011). Other polymeric materials used for gas sampling are Kynar, Flexfilm, Teflon (Mochalski et al 2013).
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The degree of scalping, which is defined as sorption of the volatiles on the inner surface of polymeric sampling
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bag, is often underestimated, in particular in the field of environmental sampling. Today, no simple method is
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available to estimate this degree of scalping for individual compounds on polymeric materials. Once volatile
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organic compounds (VOC) are collected inside gas sampling bags, a number of other phenomena can influence
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the final concentration of volatile compounds (Alonso and Sanchez 2013). Mochalski et al. (2009) reported how
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Tedlar polymer materials emit residual compounds (e.g. N,N-dimethylacetamide, phenol, carbonyl sulfide,
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carbon disulfide), making it obligatory to sufficiently rinse the sampling bags with clean air prior to headspace
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storage. Next, chemical reactions can cause biases such as oxidative reactions initiated by ambient ozone or
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photochemical conversions when exposed to ambient or artificial light. Also physical phenomena such as
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leakages or the transfer of volatiles to condensed water vapor in sampling bags strongly influence the final
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headspace concentration (Groves and Zellers 1996). Most of the abovementioned phenomena can be resolved by
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adapting the sample collection protocol. An excess of water in the sampled air can for example be reduced by
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condensation over dry ice, or by forcing it through a packed tube filled with hygroscopic salts (K 2CO3, MgCO3,
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Mg(ClO4)2) or water-sorbing polymers (e.g. Nafion) (Dewulf et al 1999).
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As mentioned earlier, the most critical parameter, however, is the tendency of certain polymer materials to sorb
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volatile compounds. This phenomenon is well described in the food packaging industry (Nielsen and Jägerstad
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1994). Next to migration of residual volatiles (Helmroth et al 2002) and permeation of gases through the
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packaging, sorption of aroma compounds may explain the change in aroma intensity or the development of an
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unbalanced flavor profile of packaged foods (Nielsen and Jägerstad 1994; Mentana et al 2009; Pati et al 2010).
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Pau Balaguer et al (2012) determined the sorption and transport properties in gliadin and chitosan films with
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respect to four representative food aroma compounds (ethyl caproate, 1-hexanol, 2-nonanone and α-pinene).
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Laor et al (2010) reported how the average odor concentration of coffee originated volatiles stored both in Tedlar
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and Nalophan bags, decreased by a factor 4-5 after 24 hours storage.
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In recent years, growing attention has been given to the abovementioned sorptive and diffusive losses during
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environmental sampling. Harreveld (2003) determined that the odor concentration of samples in Nalophan bags
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decreased to about half the initial concentration after 30 hours of storage. Sironi et al (2014) studied the diffusion
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rate of ammonia through the Nalophan film, considering storage times ranging from 1 to 26 h. Kim & Kim
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(2013) determined sorptive losses of five volatile fatty acids (acetic, propionic, n-butyric, i-valeric, and n-valeric
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acid) on the surface of both stainless steel and quartz tubes. Hansen et al (2011) collected humid and dried air
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samples from a pig production facility in both Tedlar as Nalophan sampling bags. Chemical measurements
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revealed that concentrations of carboxylic acids, phenols, and indoles decreased by 50 to >99% during the 24 h
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of storage in both Tedlar and Nalophan bags. The concentration of hydrogen sulfide decreased by approximately
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30% during the 24 h of storage in Nalophan bags, whereas in Tedlar bags the concentration of sulfur compounds
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decreased by <5% (Hansen et al 2011).
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Mochalski, King, Unterkofler, & Amann (2013) evaluated the stability of 41 selected breath constituents in three
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types of polymer sampling bags; Tedlar, Kynar, and Flexfilm. Findings yielded evidence of better performances
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of Tedlar bags over the remaining polymers in terms of background emission, species stability (up to 7 days for
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dry samples), and reusability. From the abovementioned studies, it was concluded that the scalping effects
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resulted in systematic biases leading to an underestimation of the true concentration of the targeted pollutants.
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The aim of this study is to introduce the Phase Ratio Variation (PRV) method as a fast and efficient manner for
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predicting the degree of scalping for individual compounds in a complex gas mixture. Reported methods to
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quantify the scalping effect are inverse gas chromatography (Boutboul et al 2002) or by quantitative structure–
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property relationships (QSPR) (Tehrany and Fournier 2006). These methods are however technologically
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complex and labor-intensive. The proposed PRV method is based on the relationship between the partitioning
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coefficient and the phase ratio (ratio gas and polymer phase volumes) (Jouquand et al 2004). In food research,
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the PRV method has been used to determine partitioning coefficients of six migrants (ethyl acetate,
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acetaldehyde, acetonitrile, methyl ethyl ketone, isopropyl acetate and butyraldehyde) between four food
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simulants (water, 10% ethanol, 3% acetic acid and 95% ethanol) and two polymers (polyamide and polyethylene
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terephthalate) (Tehrany and Mouawad 2007). In the field of environmental research, this technique has
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successfully been used to determine Henry’s constants of other volatiles (Jouquand et al 2004). To the best of
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our knowledge, applications of the PRV method to understand and predict polymer/volatile interactions in
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environmental sampling have not yet been described.
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2.
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2.1. Nalophan
Materials and methods
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Nalophan with 20 µm thickness was purchased from Kalle Gmbh, Wiesbaden, Germany. Small pieces (1 cm²)
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were cut and cleaned by flushing for 8 hours with pure inert nitrogen gas at 40°C to remove the adsorbed
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compounds. The cleaned polymer material was stored in a closed vial which was kept in the dark and under
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atmospheric conditions (1 atm, 25°C).
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2.2. Standards: Volatiles Organic Compounds
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The Japanese Indoor Air Standard mix (Sigma Aldrich, Belgium) was used for both calibration and sorption
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experiments. This standard consisted of 50 volatile organic compounds in methanol at certified concentrations
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(100 μg/mL each component). To create a stock solution, the reference solution was diluted 1:40 in methanol
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(99%, Sigma Aldrich, Belgium) resulting in individual solute concentrations of 2.5 µg/mL.
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2.3. Preparation of two-phase systems
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From this stock solution 4.5 µL was injected in a preconditioned 20 mL glass vial in which polymeric material
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was added. In this way 11.25 ng of each compound was introduced in the system, resulting in individual
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headspace concentrations of 560 µg/m³.
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It is noteworthy to mention that such spiking method results in methanol (solvent) background headspace
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concentrations that are much higher than those of the targeted analytes. This is not a problem, since this specific
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gas matrix is only used as case study for the applicability of PRV to quantify scalping effects in complex gas
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mixtures. Nevertheless, some additional experiments were performed to verify to which degree the elevated
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methanol headspace concentrations influenced partitioning of higher molecular weight volatiles on both the
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SPME fiber material, as well as on the Nalophan polymer (competitive adsorption). As expected from literature,
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it was observed that SPME extraction efficiencies decreased with increasing methanol headspace concentration
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(results not shown). However, since the amount of injected methanol was identical throughout the experiments,
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interaction phenomena during extraction are similar and results can be compared with each other.
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Limam et al (2005) studied sorption and diffusion of organic solvents in polyethylene terephthalate. Although
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some swelling phenomena due to methanol sorption occurs, it was concluded that sorption dramatically depends
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on the chemical microstructure of the polymer (amorphous or oriented). It was concluded that highly crystalline
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polyethylene terephthalate polymers (e.g. Nalophan) show lower sorption of organic solvent and are generally a
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good barrier to alcohols. Nevertheless, an experiment was conducted to verify the impact of methanol vapor on
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the partitioning coefficient of volatiles on the used Nalophan material. Both ‘high methanol’ and ‘low methanol’
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20 mL systems were prepared, each being spiked with 560 µg/m³ of the individual volatiles. The first series of
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systems was made as abovementioned (injection of 4.5 µL 1:40 standard methanol solution), while the second
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series of systems was prepared by injecting 1.1 µL 1:10 standard methanol solution. Each series consisted of
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polymer-free vials (n=3) and Nalophan containing vials (0.025 g, n=3). After a 15 hour incubation at 25°C, peak
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ratios were measured for a selection of volatiles in both systems. As will be discussed in more detail further in
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this work, significant scalping was measured (§3.1.) in both series. However, no significant differences were
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observed in the measured scalping degree between the ‘low methanol’ and ‘high methanol’ systems (p > 0.05). It
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can be therefore concluded that competitive sorption between methanol and volatiles on the polymer surface are
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of less importance.
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Similar as described for methanol, humidity could have an influence on the investigated sorption phenomena.
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Ajhar et al (2010) found that the headspace concentration of siloxanes in sampling bags were only 4% higher
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when humidified gas (90% RH at 37°C) was used, compared to experiments in a dry nitrogen atmosphere.
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Performing the same experiments (90% RH) at 20°C had no significant impact on the total signal compared with
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experiments performed in dry air. For this study the partitioning coefficients under normal atmospheric
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conditions (70% RH at 25°C) were determined.
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2.4. Chemical analytical methodology
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Pre-concentration of the introduced volatiles was performed by using a Gerstel MPS2 autosampler, equipped
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with a headspace-solid phase microextraction unit. The conditions were as follow: the required amount of
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polymer material was hermetically sealed in 20-mL vials and incubated for 30 minutes at 25°C in a thermostated
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agitator. After equilibration for 30 min, the headspace was extracted on a well-conditioned CAR/PDMS SPME
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fibre for another 30 minutes.
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The chemical-analytical setup used in this work consisted of a fully automated sample preparation unit (multi-
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PurposeSampler® or MPS®, Gerstel®, Mülheim an der Rur, Germany) mounted on a 6890/5973 GC-MS
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system (Agilent Technologies®, palo Alto, CA). Helium was used as carrier gas (1 mL/min). Injector and
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transfer lines were maintained at 250°C and 280°C, respectively. The total ion current (70 eV) was recorded in
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the mass range from 40-230 amu (scan mode) using a solvent delay of 2 min and a run time of 5 min. For GC-
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MS profiling, the cross-linked methyl silicone column (HP-PONA), 50 m x 0,20 mm I.D., 0,5 µm film thickness,
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Agilent Technology®) was programmed: 40°C (5 min) to 160°C at 3°C/min, from 160°C to 220°C at 5°C/min,
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held 3 minutes. Identification of volatile organic compounds in the vegetable oil headspace was performed by
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comparison of the mass spectra with the Wiley ® 275 library. Since analytical standards are used no additional
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measurements are necessary to confirm the compound identification.
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Although 50 volatiles were injected in the two phase systems, results are discussed for a selection of 36
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compounds. The reason for this is that a number of the volatiles were too volatile to be sufficiently extracted
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using the described SPME method (e.g. ethanol, methylene chloride,…), while others had a low volatility and
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have to be classified as semi-volatiles (e.g. tridecane, tetradecane, pentadecane, …). Using the chemical–
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analytical data, paired comparison tests (t-test) were performed using SPSS Statistics 21 software to evaluate if
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the observed differences in headspace concentrations were significant. Significances for the differences were
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established at an alpha risk of 5%.
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2.5. PRV-method
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The sorptive uptake of a volatile by a polymer can be described by sorption isotherms. In the most simple model
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the equilibrium headspace concentration (Cg) is directly proportional to the equilibrium concentration on the
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polymer surface (CA). In this case a partitioning coefficient K can be derived by the ratio between C g and CA. If
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sorption isotherms are nonlinear, partitioning coefficients depend on the initial concentration (C g, CA). Langmuir
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sorption isotherm are applicable in the case of a logarithmic decrease of the adsorption enthalpy with increasing
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sorption on the polymer surface. The Langmuir model describes adsorption on a surface where the monolayer
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coverage represents the maximum adsorbed concentration on the surface CA,max as;
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C A,t eq 
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Where KL the Langmuir sorption coefficient
K L C A,max C g ,t eq
1  K L C g ,t eq
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Cg,t=eq the gas headspace concentration after equilibrium (in mol/m³)
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CA,t=eq the concentration of the volatiles on the surface after equilibrium (expressed in mol/m²)
[1]
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CA,t=max the maximum concentration of sorbed volatiles on the surface after equilibrium (in mol/m²)
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For very low concentrations and KL.Ct=eq <<1 equation [1] predicts a linear relationship and a partitioning
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coefficient equals K = Cg,t=eq/CA,t=eq can be defined. Assuming that during whole air sampling a linear sorption
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model can be assumed, the PRV-method can be applied which is based on the following mass balance equation:
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C g ,t 0  Vg  C A,t eq  S A  C g ,t eq  Vg
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Where Cg,t=0 the initial gas headspace concentration (expressed in mol/m³)
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Vg the gas volume (expressed in m³)
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SA the surface area of the adsorbent (expressed in m²)
[2]
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Assuming a volume ratio factor  = SA/Vg (m²/m³) and the partition coefficient K = Cg,t=eq/CA,t=eq
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((mol/m³)/(mol/m³)) and after dividing by Vg, equation [2] can be rewritten as:
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C g ,t 0  C A,t eq    C A,t eq  K  C A,t eq  (   K )
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or
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Taking the reciprocals of both sides, and taken into account that the area of a gas-chromatograph peak
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(=PeakArea) is proportional to the equilibrium headspace concentration (Peak Areat=eq=fi x Cg,t=eq with fi =
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proportional factor) equation [4] can be rewritten as (Tehrany et al 2007):
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C A,t eq 
Cg ,t eq
K

[3]
Cg ,t 0
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 K
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1
1



PeakAreat eq f i  Cg ,t 0 f i  K  Cg ,t 0
[5]
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As will be shown further in this manuscript experiments were only done in a concentration range in which a
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linear decrease of concentration was observed in function of increased amount of introduced polymer surface.
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Hence, it can be assumed that the partitioning coefficient K is constant in this range. Thus, equation [5] can be
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corresponds to a linear equation of the following type: 1/PeakArea = a + b . 
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Where a = 1/(fi.Cg) and b = 1/(K.fi.Cg) and thus K = a/b.
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Using the above equations K can be calculated from the values of a and b obtained by plotting 1/PeakArea t=eq
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against different corresponding  values (Jouquand et al 2004).
[6]
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3.
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3.1. Sorption in function of contact time and Nalophan mass
Results and discussion
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Figure 1a presents the average relative headspace concentrations in the two-phase system (n=3) as a function of
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contact time with 0.025g Nalophan for some randomly selected volatiles from different VOC classes. The closed
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two-phase systems were simultaneously spiked with a wide range of volatiles, each at an identical initial
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headspace concentration of 560 µg/m³. For n-heptane (aliphatic compounds), toluene (aromatic compounds),
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limonene (terpenoids), trichloroethylene (chlorinated compounds), and ethyl acetate (oxygenated compounds),
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the concentration compared to the initial headspace concentration proved to significantly decrease (p < 0.05)
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over a period of 22 hours incubation at dark and ambient conditions (298.15K, 1 atm, 70% relative humidity). It
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was chosen only to study the scalping effect at 298.15K since this is a realistic storage temperature of sampled
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bags. It is noticeable that the degree of scalping after 22 hours differs depending on type of molecule; a recovery
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of 91% was found for n-heptane and limonene, while only 77% recovery was measured for trichloroethylene and
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ethylacetate. Also for the other spiked volatiles the raw average peak areas (n=3) obtained at 10 different
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sampling times over a period of 22 hours were evaluated by one-way ANOVA. In agreement with the results
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presented in Figure 1a scalping was also measured for the other volatiles (results not shown) with scalping
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degrees varying between 57% and 102%. These differences in sorption behaviour are expected and can be
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explained by unique adsorption behavior of a compound depending on electrostatic forces which are influenced
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by specific hydrogen bonding, polarity, and van der Waal’s interactions (McGarvey and Shorten 2000; Pau
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Balaguer et al 2012).
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To verify whether these decreasing concentrations are not attributed to diffusive losses or chemical-physical
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conversions (e.g. ozone reactions, photolysis) identical experiments were done on similar systems in the absence
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of polymeric material, showing no significant decrease in headspace concentration (p>0.05). Therefore, it can be
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assumed that during our experiments, observed effects are mainly related to sorption.
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Figure 1b represents the relative headspace concentration of the selected volatiles in function of the introduced
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mass of Nalophan measured after 15 hours incubation. As it was already reported earlier that the ratio of the
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sample volume to the bag film area is an important factor influencing scalping on polymers (Beghi and Guillot
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2008). Again for each individual volatile the initial headspace concentration was 560 µg/Nm³. Triplicate two-
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phase systems were prepared by introducing respectively 0.000g, 0.025g, 0.050g, 0.075g and 0.100g of
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Nalophan in 20 mL glass vials. Taking into account a surface/mass ratio of 668.9 cm²/g for the used Nalophan
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material (experimentally determined), calculated gas/polymer volume ratios  for the different two-phase
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systems are respectively 80.9 m²/m³; 161.8 m²/m³; 242.8 m²/m³ and 323.7 m²/m³. Results as represented in Table
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1 and Figure 1b indicated that the equilibrium headspace concentration for the different volatiles significantly
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decreases with increasing amount of Nalophan mass. This trend was expected, as Ajhar et al (2010) measured
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increasing amount of siloxane sorption on Tedlar material with higher surface-to-volume ratios. In agreement
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with previous results, clear differences in recovery were noticed among the spiked volatiles.
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Experimental results illustrated in Figure 1b can be explained by the Langmuir Adsorption Isotherm that
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correlates the partitioning coefficient with the fractional coverage A as:
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A 
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The fractional coverage A can be defined as the fraction of total surface adsorption sites that are occupied by the
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sorbed volatiles. Equation 7 explains that at low headspace concentration C g, the fractional surface coverage is
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proportional to Cg with a proportionality constant of K (partitioning coefficient), whereas at high headspace
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concentrations, the surface coverage goes to unity and becomes independent of Cg (i.e. saturation of the available
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sites). It is clear that sorption kinetics will be different depending on the degree of coverage A. This Langmuir
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sorption behavior is also reflected in Figure 1b in which initially a significant decrease in equilibrium headspace
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concentration with increasing amounts of Nalophan is observed. However in the case that more than 0.050 g ( =
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161,8 m²/m³) is introduced, a different sorption behavior is observed. Since in reality  values are typically
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below 100 m²/m³ (0.5 L sampling bag (sphere) = 0.03046m² /0.0005m³ = 61), it is chosen only to use data
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obtained from two-phase systems with the lowest  values, more specific those containing 0.000g ( = 0.0),
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0.025g ( = 80.9 m²/m³) and 0.050g ( = 161.8 m²/m³) of Nalophan.
K  Cg
1  K  Cg
[7]
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3.2. Experimental determination of partitioning coefficient using Phase Ratio Variation approach
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In Figure 2, the inverse peak areas (n=3) for a selection of volatiles are shown in function of varying  values,
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while other experimental conditions (25°C, 70% RH, initial headspace concentration 560 µg/m³) are maintained
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constant. Results are shown for a selection of chlorinated compounds (tetrachloroethylene, trichloroethylene,
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1,2-dichloropropane), aliphatic compounds (n-nonane, n-heptane, 2,4-dimethylpentane), aromatic compounds
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(1,2,3-trimethylbenzene, o-xylene, toluene) and oxygenated compounds (4-methyl-2-pentanone, 2-propanol,
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ethyl acetate).
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Figure 2 shows that within each VOC class linear correlations are observed between the equilibrium headspace
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concentrations and the corresponding  values. Similar, Table 1 represents average reciprocal peak areas (n=3)
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for all spiked volatiles in function of  value. Again, for the majority of compounds a linear regression was
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found based on the three data points with correlations R² varying between 1,00 and 0,76. Since for the majority
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of volatiles R² correlation above 0.95 are observed, it can be concluded that the theory of Phase Ratio Variation
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can indeed be applied to evaluate and predict sorption behavior of individual volatiles in sampling bag materials.
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Table 1 represents the calculated partition coefficients K for a wide range of volatiles, ranging from 10,73 (n-
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undecane) to 0,14 (tetramethylbenzene). It is noteworthy that for a limited amount of volatiles (styrene,
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tetramethylbenzene, 1,4-dichlorobenzene, n-butanol and nonanal) experimentally determined partitioning
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coefficients are lower than one, indicating that concentrations are increasing with higher amount of introduced
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Nalophan. This phenomenon might be explained by impurities in the vials or on the fiber, or due to VOC
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emission from the material into the headspace despite the flushing of the Nalophan material during the sample
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preparation. Next, Tromelin et al. (2012) described that for some compounds the calculation of partitioning
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coefficients using the described first order PRV model can be disappointing, in those cases the application of a
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second-order PRV method could be an interesting alternative.
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Based on the theory of adsorption potential, Wang et al (2012) derived a correlation between partitioning
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coefficient, K, and the liquid molar volume, Vl, as: ln(K) = k Vl + b, where, k and b are constants for VOCs with
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similar function groups and chemical bonds for the same material. Wang et al. (2008) reviewed how for different
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building materials the natural logarithmic partition coefficients are linearly correlated against the molar volume
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for various categories of VOCs with more than 0.91 fitting degree. The results obtained in this study revealed a
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similar behavior between volatiles and polymeric material. In Figure 3 this linear correlation between ln K and
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the molar volume is shown for aliphatic, aromatic and oxygenated compounds.
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For the aliphatic (2,4-dimethylpentane; n-heptane, n-nonane, n-decane, n-undecane), aromatic (toluene,
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ethylbenzene, xylenes, 2- and 3-ethyltoluene, 1,2,4-trimethylbenzene, 4-ethyltoluene, 1,2,3-trimethylbenzene)
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and oxygenated (2-propanol, ethylacetate, 4-methyl-2-pentanone, n-butylacetate) compounds linear correlations
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were measured with correlation coefficients ranging between 0.9751 and 0.9195. In the case that linear
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correlations can be found for specific VOC classes, it is possible to correct measurement data for the degree of
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scalping after whole air sampling in polymeric bags. No general lineair correlations could be made for terpenes
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since results were only obtained for three compounds (-pinene, -pinene, limonene). Although not presented in
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Figure 3, a linear correlation was also found for the chlorinated compounds 1,2-dichloropropane,
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trichloroethylene, 1,2-dichloroethene, however no general trend was observed in our measurements.
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As confirmed by our measurements, deviations from the general correlation can be expected within each VOC
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class. Adsorption is the result of both Van der Waals interactions, as specific proton donor-acceptor interactions.
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The presence of certain functional groups may strongly effect the relative importance of each type of interaction
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in the overall sorption process, causing deviations from the general sorption behavior in a certain VOC class.
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Indeed, particular properties such as the presence of an extra double bound or substituded groups might
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significantly affect the interaction between volatiles and polymeric surface. Next, not all compounds can easily
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be attributed to a certain class, which is for example the case for 1,4-dichlorobenzene, being both an aromatic as
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chlorinated compound. Due to its mixed character, no fit was found for 1,4-dichlorobenzene in both classes. The
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same was observed in the class of chlorinated compounds in which deviating results were for measured for
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tetrachloroethene. This might be explained by a high density of chloro-groups, possible causing shielding and
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different intermolecular forces. Similar observations were made by Wang et al (2008) who described a poor
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correlation between the partition coefficient of seven benzene compounds and polyurethane, with a fitting degree
300
of only 0.67. It was seen that styrene and chlorobenzene deviated from the regression line due to the presence of
301
some unique characteristics such as double bond for styrene or a chloro-group for chlorobenzene.
302
303
4.
Conclusions
304
Measurements using a two-phase system containing polymer material used for whole air sampling, revealed
305
significant scalping for a wide range of volatiles. Results illustrated that scalping of volatiles on Nalophan leads
306
to significant underestimation of effective headspace concentrations. Therefore it is of great importance that the
307
degree of scalping is evaluated systematically to compensate analytical variability and biases during the
308
sampling stage.
309
A relatively easy methodology, the Phase Ratio Variation method, has been studied to quantify the degree of
310
sorption for individual volatiles in a complex gas mixture. It is shown that using the PRV approach, individual
311
partitioning coefficients could be determined. Moreover, for some important classes of volatiles a correlation
312
was illustrated between the individual partitioning coefficient, and the liquid molar volume. Depending on the
313
presence of particular functional groups deviations were observed from this general correlation. Such deviation
314
sorption behavior is the result of interactions between Van der Waals interactions and specific proton donor-
11
315
acceptor interactions. The availability of quantitative information on individual sorption behavior, enables to
316
compensate for the inevitable sorption losses during storage of loaded polymer sampling bags.
317
The advantages of a PRV approach is that based on limited measurements, and without the need of time-
318
consuming calibrations, the partitioning coefficients of individual volatiles can be quantified in a complex gas
319
mixture. As many gas sampling bags are homemade, two-phase systems having different  values but identical
320
initial volatile concentrations can easily be made by enclosing additional masses of Nalophan in the sampling
321
bags prior to collecting equal gas volumes.
322
Our experiments were performed in controlled conditions, with relative humidity, temperature and light
323
intensities held constant throughout the experimental process. As it is accepted that fluctuations in temperature
324
may result in accelerated sample transformation (e.g. chemical reactivity), this is the focus of current ongoing
325
work. Next, it should also be further investigated whether the observed relationship between partitioning
326
coefficient and liquid molar volume can be generalized for specific polymer materials, or if this correlation is
327
dependent on the gas composition and concentration. Finally, it needs to be pointed that a sampling bags consist
328
of different surface materials (e.g. stainless steel) and/or contain chemical species (e.g. particulate matters,
329
reactive chemical compounds, etc.) that can both potentially influence the equilibrium headspace concentrations
330
during the sample post collection.
331
332
333
5.
Acknowledgements
The authors kindly acknowledge dr. Melissa Dunkle for the support of this work.
334
12
335
6.
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7.
415
Fig. 1 Relative average headspace concentration (n=3) in function of (a) incubation time
416
after introduction of 0.025g Nalophan polymeric material in 20 mL vial (darkness, T = 25°C;
417
relative humidity = 70%) and (b) introduced mass of Nalophan in 20 mL vial (darkness,
418
T=25°C, RH = 70%) measured after 15 hours of incubation (samples analysed in triplicates,
419
RSD <5%)
Figures and Tables
Relative headspace concentration (%)
120
100
80
n-heptane
Toluene
60
Limonene
40
20
0
0
420
5
10
15
20
Incubation time (hours)
8.
a
Relative headspace concentration (a.u.)
120
100
80
n-heptane
Toluene
60
Limonene
40
Trichloroethhylene
Ethyl acetate
20
0
0.00
0.05
0.10
0.15
mass Nalophan added in 20mL vial (g)
421
422
b
15
423
Fig. 2 Inverse average peak area values (n=3) in function of volume ratio factor  (darkness, T =
424
25°C; relative humidity = 70%, incubation time=48 hours)
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
16
441
Fig. 3 Natural logarithmic partition coefficient on Nalophan in relation the molar volume
442
(darkness, T = 25°C; relative humidity = 70%)
443
3
2.5
R² = 0.9329
2
ln K (-)
1.5
R² = 0.9751
aliphatic compounds
1
aromatic compounds
0.5
Oxygenated compounds
R² = 0.9195
0
0.0
50.0
100.0
150.0
200.0
250.0
-0.5
-1
Molar volume (ml/mol)
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
17
459
Table 1. Average repriprocal peak area values in function of volume ratio factor . Intercept,
460
rico and correlation R² after linear regression is represented, which was used to determine
461
individual partitioning coefficients K for each volatile (T = 25°C; relative humidity = 70%,
462
incubation time=48 hours)
Molar
volume
(mL/mol)
=0
 = 80,9
 = 161,9
rico (a.u.)
Intercept
(a.u.)
R²
K
lnK
2,4-dimethylpentane
148,9
9,071E-06
1,017E-05
1,137E-05
1,422E-06
9,053E-06
1,00
6,36
1,85
n-heptane
146,5
4,167E-06
4,512E-06
5,184E-06
6,287E-07
4,112E-06
0,97
6,54
1,88
iso-octane
162,5
1,666E-06
1,435E-06
1,980E-06
1,940E-07
1,537E-06
0,33
7,92
2,07
n-nonane
178,6
5,535E-07
5,550E-07
6,460E-07
5,714E-08
5,386E-07
0,76
9,43
2,24
n-decane
194,9
2,461E-07
2,529E-07
2,845E-07
2,371E-08
2,420E-07
0,88
10,21
2,32
n-undecane
211,2
1,464E-07
1,489E-07
1,680E-07
1,338E-08
1,436E-07
0,84
10,73
2,37
Benzene
89,2
3,767E-06
4,292E-06
5,860E-06
1,293E-06
3,593E-06
0,92
2,78
1,02
Toluene
106,3
1,272E-06
1,566E-06
2,161E-06
5,489E-07
1,222E-06
0,96
2,23
0,80
Ethylbenzene
122,5
4,979E-07
6,416E-07
9,118E-07
2,556E-07
4,768E-07
0,97
1,87
0,62
o-xylene
120,6
4,840E-07
6,442E-07
8,695E-07
2,382E-07
4,731E-07
0,99
1,99
0,69
m-xylene
123,4
4,673E-07
6,476E-07
8,590E-07
2,420E-07
4,621E-07
1,00
1,91
0,65
4,766E-07
1,039E-06
1,911E-06
8,863E-07
4,249E-07
0,98
0,48
-0,74
Compound
Aliphatic Compounds
Aromatic Compounds
Styrene
p-xylene
123,4
3,933E-07
5,173E-07
7,320E-07
2,092E-07
3,782E-07
0,98
1,81
0,59
3-ethyltoluene
135,5
1,986E-07
2,575E-07
4,000E-07
1,244E-07
1,847E-07
0,95
1,48
0,39
2-ethyltoluene
135,5
2,164E-07
2,935E-07
4,490E-07
1,436E-07
2,034E-07
0,96
1,42
0,35
1,2,4-trimethylbenzene
136,6
1,943E-07
2,481E-07
3,891E-07
1,204E-07
1,798E-07
0,94
1,49
0,40
1,751E-07
2,215E-07
2,992E-07
7,667E-08
1,699E-07
0,98
2,22
0,80
1,726E-07
2,599E-07
3,749E-07
1,250E-07
1,680E-07
0,99
1,34
0,30
1,402E-07
2,051E-07
3,239E-07
1,135E-07
1,312E-07
0,97
1,16
0,15
7,449E-08
1,200E-07
1,905E-07
7,167E-08
7,032E-08
0,98
0,98
-0,02
1,013E-05
1,295E-05
1,457E-05
2,745E-06
1,033E-05
0,98
3,76
1,33
4-ethyltoluene
1,2,3-trimethylbenzene
1,3,5-trimethylbenzene
1,2,4,5tetramethylbenzene
136,6
154,6
Chlorinated Compounds
Chloroform
1,2-dichloroethane
79,2
1,852E-05
2,599E-05
4,024E-05
1,342E-05
1,740E-05
0,97
1,30
0,26
1,2-dichloropropane
97,4
6,234E-06
6,869E-06
9,138E-06
1,794E-06
5,962E-06
0,90
3,32
1,20
Trichloroethylene
90,0
4,294E-06
5,473E-06
8,129E-06
2,369E-06
4,048E-06
0,95
1,71
0,54
Tetrachloroethene
102,4
9,998E-07
1,362E-06
1,755E-06
4,667E-07
9,945E-07
1,00
2,13
0,76
2,194E-07
8,214E-07
1,887E-06
1,030E-06
1,422E-07
0,97
0,14
-1,98
1,4-dichlorobenzene
Oxygenated Compounds
Acetone
73,4
4,801E-06
5,798E-06
6,787E-06
1,227E-06
4,802E-06
1,00
3,91
1,36
2-propanol
76,5
1,703E-05
2,264E-05
2,469E-05
4,732E-06
1,762E-05
0,93
3,72
1,31
Ethyl acetate
98,2
n-butanol
4-methyl-2-pentanone
124,9
5,920E-06
7,603E-06
9,004E-06
1,905E-06
4,296E-06
7,882E-06
1,665E-05
7,630E-06
1,847E-06
2,338E-06
3,143E-06
8,003E-07
5,967E-06
3,433E-06
1,795E-06
1,00
0,94
0,98
3,13
1,14
0,45
-0,80
2,24
0,81
18
n-butylacetate
132,0
1,273E-06
1,640E-06
2,315E-06
6,440E-07
1,222E-06
0,97
1,90
0,64
Nonanal
172,0
2,080E-07
2,799E-07
5,154E-07
1,899E-07
1,807E-07
0,91
0,95
-0,05
(1S)-(-)-alpha-pinene
158,8
2,605E-07
2,613E-07
3,231E-07
3,872E-08
2,503E-07
0,76
6,46
1,87
beta-pinene
158,8
2,240E-07
2,275E-07
2,670E-07
2,653E-08
2,180E-07
0,81
8,22
2,11
(R)-(+)-Limonene
162,0
1,543E-07
1,748E-07
2,157E-07
3,792E-08
1,509E-07
0,96
3,98
1,38
Terpenes
463
464
19