H2S Loss through Nalophan™ Bags: Contributions of Adsorption and Diffusion

Hydrogen-sulfide (H2S) is a molecule of small dimensions typically present in the odor emissions from different plants. The European Standard EN 13725:2003 set a maximum storage time allowed of 30 hours, during which the sampling bag has to maintain the mixture of odorants with minimal changes. This study investigates the H2S losses through Nalophan bags and it shows that nonnegligible losses of H2S can be observed. The percent H2S loss after 30 hrs with respect to the initial concentration is equal to 33%  ± 3% at a relative humidity of 20% and equal to 22%  ± 1% at a relative humidity of 60%. The average quantity of adsorbed H2S at 30 h is equal to 2.17 105 gH2S/gNalophan at a storage humidity of 20% and equal to 1.79 105 gH2S/gNalophan at a storage humidity of 60%. The diffusion coefficients of H2S through Nalophan, for these two humidity conditions tested, are comparable (i.e., 7.5 10−12 m2/sec at 20% humidity and 6.6 10−12 m2/sec at 60% humidity).


Introduction
Although odors do not have a direct effect on human health, they are considered one of the main causes of discomfort for the population living in areas impacted by odor emissions. Nowadays, olfactory pollution has become a serious environmental concern because it may be the cause of physiological stress to the population [1]. Concerning olfactory nuisance, different European countries have recently adopted specific regulations. The standard methodology for odor concentration measurement is a sensorial technique, that is, dynamic olfactometry [2], which is commonly applied for testing odors for environmental management purposes [3]. This technique is based on the sensation caused by an odorous sample directly on a panel of human assessors [4].
Performing olfactometric analyses on site presents some difficulties. To overcome these problems, the odorous pollutants are collected and stored in appropriate containers until they are analyzed in an olfactometric laboratory [4][5][6]. In order to regulate the quality of the olfactometric analysis, the European Standard on dynamic olfactometry [2] defines the requirements for the materials used for sampling equipment. The requirements determined by the EN13725 for the olfactometry materials are as follows: being odorless and being able to minimize the physical or chemical interaction between sample components and sampling materials and having low permeability in order to minimize sample losses caused by diffusion and smooth surface.
The materials allowed by EN13725 for sample containers (i.e., bags) are as follows: tetrafluoroethylene hexafluoropropylene copolymer (FEP); Tedlar6 (polyvinyl fluoride, PVF), and Nalophan (polyethylene terephthalate, PET). Moreover, European Standard set a maximum storage time allowed, during which the sampling bag has to maintain the mixture of odorants with minimal changes.
Since the publication of the Standard in 2003, several studies have been carried out in order to test the characteristics of the materials listed in the EN 13725 [2] and to verify their suitability for olfactometric measurements. In Table 1 literature studies are reported investigating losses of odorous molecules through sampling bags [1,.
More in detail, in Table 1, beside the author and year, the polymer film studied, the thickness of the film, the pollutant taken into account and the detection system adopted are reported. 2 The Scientific World Journal The Scientific World Journal  The Scientific World Journal The results of the studies reported in Table 1 underline that the chemical pollutants diffused through the polymeric film are mainly small molecules, like ammonia (NH 3 ) and H 2 S. Nalophan is generally the most used material for the manufacturing of sampling bags for olfactometric analyses, due to its inert properties and cost-effectiveness. Despite these advantages, it is known in literature that Nalophan allows the diffusion of specific compounds, such as water [15]. Water can diffuse quickly through the Nalophan polymeric film because of its structure [15]. The results of the studies reported in Table 1 showed that the chemical compounds that diffuse through the Nalophan film are water, NH 3 , and H 2 S [1,9,10,15,27]; the last two compounds diffuse easily because these molecules have dimensions similar to water [1,9,10,15,27].
H 2 S and NH 3 are typically odorous pollutants present in the emissions from several plants such as solid waste and waste water treatment.
In this paper, the attention was focused on H 2 S, a malodorous compound with smell similar to rotten eggs. H 2 S is detected by human olfaction at very low concentrationsabout 1 ppb [37][38][39]-and it is typically found in the emissions from different plants, like industry [30], agriculture [16,31], waste water treatment [7], and waste treatment [21].
Generally, the articles present in literature (Table 1) focus the attention mainly on the H 2 S loss by determining the H 2 S recovery in the sampling bag.
The study of the contribution of pollutant losses, such as diffusion and adsorption, is not easy because the diffusion through the polymeric film is influenced by the nature of the polymer as well as by the nature of the diffusing pollutant [1,40].
More in detail, the polymer characteristics that influence the diffusion processes are as follows: the chemical nature of the polymer, its crystalline structure and orientation, the free volume, the molecular cohesion, the relative humidity, temperature, hydrogen bonding, polarity, solubility parameter, and solvent size and shape [40].
As reported by Klopffer and Flaconneche in 2001 [41], the polymer structure plays an important role in the determination of the transport phenomena through the polymeric film.
It is well known in literature that transport phenomena of small molecules through an amorphous polymer are governed by mechanisms of adsorption and diffusion [40]. Transport phenomena can be decomposed into five successive stages ( Figure 3) [40,41]: (i) the diffusion through the boundary layer of the side corresponding to the higher partial pressure (upstream side); (ii) the adsorption of the gas (by chemical affinity or by solubility) on the polymer; (iii) the diffusion of the gas inside the polymer's membrane; (iv) the desorption of the gas at the side of lower partial pressure; and (v) the diffusion through the limit layer of the downstream side.
Only few studies in literature [1,14,27,28] have faced the problem of diffusion through the sampling bags by calculating the diffusion coefficient of the inspected chemical compound. Moreover, in most studies, the amount of chemical compound lost due to adsorption on the polymeric  [1,27,28]), whereas for medium-low concentrations (e.g., in the range of ppb to few ppm) the effect of adsorption becomes significant. In this study, both the effects of diffusion through the polyethylene terephthalate (PET, Nalophan) film and the adsorption on the film are investigated. The experiments described in this paper aim to investigate the relative contributions of the two phenomena causing H 2 S loss in Nalophan bags, that is, adsorption and diffusion. The evaluations were carried out by calculating the amount of H 2 S adsorbed in the Nalophan film and the diffusion coefficient relevant to this material. Finally, the influence of physical parameter like relative humidity (RH) on both the diffusion coefficient and the adsorption was evaluated.

Materials.
The sampling bags studied with capacity of 6 liters are prepared from a tubular film of Nalophan supplied by Tilmmanns S.p.A. and shown in Figure 1. The polymer film consists of a 20-m thick one-layer foil.
The H 2 S decay over time was evaluated by measuring the H 2 S concentration inside the bag over time by means of a high performance miniature sensor able to detect H 2 S at ppb level. More in detail, the sensor used for the H 2 S concentration measurement is a CairClip apparatus, developed by Cairpol, a French start-up (Alès Engineer School of Mines), which consists in amperometric detection with a dynamic air sampling system, a special filter, and a high sensitive electronic circuit containing a data logger [42]. The instrument was calibrated by the manufacturer and it has a life-cycle of one year. The accuracy of this instrument declared by the manufacturer is 10 ppb, in a range between 30 and 1000 ppb of H 2 S and mercaptans.
All the test samples were prepared by filling the Nalophan bags with a gaseous mixture of 800 ppb of H 2 S in air, defined as the "test mixture" in the paper. The samples were obtained by withdrawing the H 2 S from a certified H 2 S gas cylinder (SAPIO technical gas, Milano, Italy) into Nalophan bags with a volume of 6 liters and a surface of 2580 cm 2 .
One aspect that had to be considered for the design of the experiment is that the CairClip has steel parts that may interact with the H 2 S and reduce its concentration, thereby affecting the measurements of the H 2 S concentration decay through the Nalophan, which is the aim of this paper. Therefore, in order to avoid undesired interactions of the  CairClip sensor with the H 2 S during the sample storage period, the concentration measurements were carried out by moving the gaseous mixture contained in the storage bag into another identical empty bag containing the CairClip sensor ( Figure 2). Because of the short time of the measurement, the adsorption/diffusion effect in this bag is assumed to be negligible. In order to evaluate the H 2 S concentration decay over time, this procedure had to be repeated for different time intervals. A new bag had to be prepared for each tested interval and then its contents transferred to the bag containing the measurement apparatus after the desired time interval (Figure 2). The H 2 S concentration after each tested time interval was then compared to the initial H 2 S concentration in the test mixture (800 ppb) in order to evaluate the H 2 S loss over time.

Methods.
In order to evaluate the contribution of adsorption and the diffusion phenomena into the Nalophan bags, several tests had to be performed, and three replications of each condition and time were tested, following the scheme in Figure 2.
After a first test using a bag with a volume of 6 liters and a surface of 2580 cm 2 (in the following defined as "Bno film"), other tests were repeated using bags with the same geometrical characteristics (i.e., volume of 6 liters and a surface of 2580 cm 2 ), in which sheet of film of the same material (i.e., a 20 m thick Nalophan sheet) was inserted. Three different tests were performed by changing the dimensions of the sheet of film inserted inside the bag. This way, besides the "B-no film" with no film in it, three different types of bags were prepared: (i) Nalophan bag with volume of 6 L and surface of 2580 cm 2 containing a sheet of film of 1900 cm 2 (in the following defined as "B-film 1900"). (ii) Nalophan bag with volume of 6 L and surface of 2580 cm 2 containing a sheet of film of 2580 cm 2 (in the following defined as "B-film 2580"). (iii) Nalophan bag with volume of 6 L and surface of 2580 cm 2 containing a sheet of film of 3520 cm 2 (in the following defined as "B-film 3520").
The idea of inserting the sheets of Nalophan of different dimensions inside identical bags had the aim of evaluating the contribution of adsorption of the H 2 S in the Nalophan film, which is expected to increase with the surface of the Nalophan film the H 2 S is in contact with.    Table 2, were repeated three times each.
The test temperature of the samples was fixed at 23 ∘ C. The role of humidity on the H 2 S concentration decay inside the bag was evaluated by storing the bags at different external humidity values, of 20% and 60%, respectively.
A suitable procedure had to be adopted in order to normalize the Nalophan films tests in terms of initial conditions of water absorbed. In fact, Nalophan is proven to be water permeable [15], and thus the water adsorption in the film is connected to the external environmental conditions. For this reason, in order to normalize the water content of the tested Nalophan films, all bags were stored for 12 hours at the test conditions using a climatic chamber before the beginning of the tests.
This procedure allows obtaining repeatable results by reducing the measurement errors related to the state of swelling of the polymer matrix.
The comparison of the H 2 S residual concentration inside the bag after the tested storage time with the initial H 2 S concentration in the test mixture allowed the evaluation of the H 2 S loss over time. As already mentioned, the aim of this paper was not only the quantification of the H 2 S loss over time but also the evaluation of the relative contribution of adsorption and diffusion to this loss. H 2 S adsorption was evaluated using (12) to (14) (see § Calculations), whereas diffusion was calculated based on Fick's law. To calculate The Scientific World Journal 9 the diffusion coefficient of H 2 S through Nalophan, (15) to (17) were used (see § Calculations). The measurements were performed at different times and the diffusion coefficient was averaged over 30 hours.

Calculations.
The model used to determine the H 2 S loss, due to both adsorption and diffusion, starts from the method developed in Sironi et al. 2014 [1] by adapting this for H 2 S. More in detail, the novelty of this work is to separate the two contributions on pollutants loss from the sampling bag: adsorption on polymeric matrix and diffusion through the film.
The diffusion phenomenon through a polymeric film can be described by Fick's law. Accordingly the specific molar flow is defined as (iv) is the differential thickness of the polymeric film of the bag.
The thickness of polymeric film of the bag can therefore be expressed as where is the thickness (m) of the polymeric film of the bag. Referring to Figure 3, which schematizes the diffusion phenomenon through the thin film that constitutes the sampling bag, it is possible to define the following: (i) is the surface of the polymeric film of the bag (m 2 ).

(ii)
is the thickness of the polymeric film of the bag (m).

(iii)
is the concentration in the inside volume (mol/ m 3 ).
(iv) 0 is the concentration outside the film (mol/m 3 ), and for a single bag it is generally considered negligible ( 0 = 0).
(v) is the specific molar flow through the polymeric film of the bag (mol/m 2 /sec), assuming in first approximation constant along the film ( ).
By integrating (1) in between 0 and , the specific molar flow can be expressed as where is relevant to an infinitesimal portion of the exchange surface .
Assuming that the internal molar concentration is homogeneous inside the whole internal volume and also the external concentration 0 is constant inside the external volume, then the global flow through the exchange surface can be calculated by integrating as follows: = . ( Combining (3) with (5), the molar flow through the surface can be expressed as If the external concentration 0 is assumed to be equal to zero ( 0 = 0), which is the case if the bag is placed in a neutral environment (where the presence of H 2 S may be considered negligible), (6) can be rewritten as According to this model, the concentration decay over time turns out to be a function of the surface area ( ), the volume of the sampled gas , the film thickness ( ), the time ( ), the diffusion coefficient ( ) that depends on the characteristics of the material, and the concentration gradient through the polymeric barrier (Δ ). The boundary conditions considered for the integration of (7) are = for = * , = in for = 0.
The integration of (7) allows computing the concentration trend over time: The H 2 S loss (percent) through the bag over time can be expressed as where is the concentration measured at time and in is the initial concentration.
The loss of H 2 S is due both to adsorption in the Nalophan and to diffusion through the bag walls.
The H 2 S loss due to these phenomena can be calculated as the difference between the initial amount of H 2 S (H 2 S in ) and the amount measured at the time (H 2 S ): The Scientific World Journal In order to evaluate the relative contributions of the two phenomena (adsorption and diffusion) to the H 2 S loss inside the Nalophan bag, the following system has to be solved: where (i) H 2 S loss_1 is the amount of H 2 S loss at time ( g) measured for the simple Nalophan bag, (ii) H 2 S loss_2 is the amount of H 2 S loss at time ( g) measured for the Nalophan bag with the Nalophan sheets inserted, (iii) is the contribution of the adsorbed H 2 S ( g/m 2 ), (iv) is the contribution of the diffused H 2 S ( g), (v) is surface area of the bag (m 2 ), (vi) film is surface area of the sheet of film inserted in the bag (m 2 ).
The first equation of the system refers to the test condition in which the bag has no additional film inserted in it. On the contrary, the second equation refers to the bags containing the sheets of Nalophan film. Moreover, it is important to notice that using the same thickness of the film (i.e., 20 m) the data are expressed in terms of surface unit. Therefore, the data obtained are directly correlated to the data expressed in terms of mass unit. The adsorbed amount per unit of surface (H 2 S adsorbed /m 2 ) can be obtained by subtracting the contribution of the diffusion (i.e., ) from the amount of H 2 S losses at time (i.e., H 2 S loss ( g)), according to (12): The adsorbed amount (H 2 S adsorbed ) related to the considered surface can be obtained by multiplying H 2 S adsorbed /m 2 by the inner film surface (i.e., film ): The diffused amount (i.e., H 2 S diff ) was calculated as the difference between the H 2 S amount losses (H 2 S loss ) at time and the adsorbed amount: The diffusion coefficient for each time interval was calculated according to the following equation: where is the time interval and H 2 S diff is the concentration diffused at time . The diffusion coefficient of H 2 S through Nalophan was finally calculated as the average of the different values of weighted on the corresponding storage time :

Results and Discussion
As previously mentioned, the main objective of this study was the estimation of the relative contribution of the two phenomena (i.e., adsorption and diffusion) that are responsible for the H 2 S concentration decay inside Nalophan bags used for olfactometric sampling. Table 3 shows the ratio / in , where is the H 2 S concentration measured at different time intervals ( ) normalized to the initial concentration ( in ), and the percent loss of H 2 S (%) with respect to the initial concentration. The storage temperature was fixed at 23 ∘ C and the relative humidity was 20% and 60%, respectively. Table 3 reports the results obtained for the simple Nalophan bag ("B-no film") and the other three bags prepared by inserting sheets of Nalophan of different dimensions inside the bags, that is, 1900 cm 2 ("B-film 1900"), 2580 cm 2 ("B-film 2580"), and 3520 cm 2 ("B-film 3520"), respectively, as described in the Methods.
The percent loss of H 2 S (%) ( Table 3) inside the bag with respect to the initial concentration over time was calculated according to (10). The H 2 S concentration decay is due to both the adsorption into the Nalophan (i.e., both the bag itself and the inserted film sheet) and the diffusion through the bag walls.
The percent loss of H 2 S (%) from the simple bag that does not contain the extra Nalophan film sheet in it ("B-no film") after 30 hr turns out to be equal to about 33% ± 3% at a storage humidity of 20% and equal to 22% ± 1% at a storage humidity of 60%. This trend is coherent with other data reported in the scientific literature dealing with the same subject. As an example, a study by Akdeniz et al. (2011) [7], also dealing with H 2 S losses through polymeric films (Tedlar and Flex Foil), reports losses of about 20% after 36 hours.
Moreover, it is possible to observe for the single bag how the data show that the trends of the H 2 S losses (%) are little bit higher decreasing the storage relative humidity. This is due to the presence of water caused by the humidity gradient, as already observed in Sironi et al. (2014a, b) [1,27].
The data reported in Table 3 show also an increase of the H 2 S losses (%) increasing the surface of the polymeric film sheet inserted in the bag. The H 2 S percent loss (%), at a storage humidity of 20%, after 30 hr turns out to be equal to 47% for the bag containing the film sheet with a surface of 1900 cm 2 , increasing up to 71% for the bag containing the film sheet with a surface of 3520 cm 2 . The same trend is observed at a storage humidity of 60%: the H 2 S percent loss (%) after 30 hr turns out to be equal to 46% for the bag containing the film sheet with a surface of 1900 cm 2 , increasing up to 63% for the bag containing the film sheet with a surface of 3520 cm 2 .
As said above, the H 2 S losses (%) inside the bag with respect to the initial concentration are affected by two contributions: adsorption into the Nalophan and diffusion through the Nalophan bag walls. In order to evaluate these two contributions separately, the H 2 S ratio adsorbed into the Nalophan film was evaluated as the ratio between H 2 S adsorbed (estimated according to (14)) and the initial concentration (H 2 S in ). Figures 4 and 5 report the adsorbed H 2 S (%) at Table 3: Experimental data relevant to the H 2 S loss over time in a Nalophan bag stored at temperature of 23 ∘ C and humidity of 20% and 60%. The bag tested was without any film inside (B-no film) and with the film inside. The surface of the internal film sheet was equal to 1900 cm 2 (B-film 1900), 2580 cm 2 (B-film 2580), and 3520 cm 2 (B-film 3520), respectively. The data reported are the average of the results from three different tests performed at the same conditions.   specific time intervals at a storage temperature of 23 ∘ C and a humidity of 20% and 60%, respectively.
As it is possible to observe in Figure 4 and in Figure 5   (ii) about 16% for the bag containing the film sheet with a surface of 2580 cm 2 ("B-film 2580"), (iii) about 24% for the bag containing the film sheet with a surface of 3520 cm 2 ("B-film 3520").
The data reported above show a weak increase of the ratio of adsorbed H 2 S (%) for the bag stored at low humidity (i.e., 20%). The Nalophan film is made with PET (polyethylene terephthalate) that is known from literature to be water permeable [15]. Therefore, when storing the bag at high humidity (i.e., 60%), the amount of water that can be adsorbed on the film is greater compared to the storage condition at low humidity (i.e., 20%). At a temperature of 23 ∘ C and relative humidity of 20% the partial pressure of water is equal to 4 mmHg, whereas at a temperature of 23 ∘ C and relative humidity of 60% the partial pressure of water is equal to 13 mmHg. Therefore, in this second condition, it is likely that the water is adsorbed on the polymer matrix instead of the H 2 S (competitive adsorption). Figures 6 and 7 illustrate the amount of H 2 S in terms of cumulative losses ( g) and the two contributions, that is, on one hand the H 2 S adsorbed on the polymeric film and on the other hand the H 2 S diffused trough the bag walls. The results are shown in function of the surface area of the Nalophan film sheet inserted inside the test bags at a storage humidity of 20% and 60%, respectively.
As expected, the amount of H 2 S that is adsorbed increases by increasing the surface of the Nalophan film sheet inserted inside the bag. Also, the contribution of diffusion remains almost constant for the two values of relative humidity tested (i.e., RH 20% and 60%, resp.). This aspect was expected because the film sheet inserted has no internal concentration gradient (Δ ) (see Fick law (7)).
Moreover, it is possible to observe that diffusion is predominant compared to adsorption, although the latter is not negligible. The only exceptions are observed at a temperature of 23 ∘ C and a relative humidity of 20% in the bag containing the Nalophan film sheet with a surface of 3520 cm 2 ("B-film 3520") ( Figure 6), since in these conditions the contribution of diffusion is comparable to that of adsorption.
The averaged data of the adsorbed amount per surface unit (H 2 S adsorbed /m 2 ) in g/m 2 (see (12)) at specific times (i.e., 3 hr, 24 hr, and 30 hr) are reported in Table 4.
It is possible to observe ( Table 4) that the results at 24 hours and 30 hours relevant to both the storage conditions tested present comparable values of H 2 S adsorbed /m 2 . At 3 hr, the value of H 2 S adsorbed /m 2 is lower. The averaged values relevant to 24 and 30 hr of H 2 S adsorbed /m 2 are equal to 5.8 g/m 2 (which corresponds to a ratio H 2 S adsorbed (g) /g Nalophan equal to 2.17 10 5 g H 2 S /g Nalophan ) at a relative humidity of 20% and to 4.8 g/m 2 at a relative humidity of 60% (which corresponds to a ratio H 2 S adsorbed (g) /g Nalophan equal to 1.79 10 5 g H 2 S / g Nalophan ), respectively. The value of H 2 S adsorbed /g was obtained by combining the value of H 2 S adsorbed /m 2 with the thickness of the film, which is equal to 20 m, and the density of amorphous PET, which is equal to 1.335 g/cm 3 [43].
As already observed, at a storage humidity of 20% the amount of adsorbed H 2 S is higher than the adsorbed amount    at the storage humidity of 60%. This may be due to the fact that to a relative humidity of 60% corresponds a higher amount of water, given that the water can compete with the H 2 S in the adsorption on the polymeric film. Therefore, it is possible to assert that the adsorption of H 2 S on the polymeric film is influenced by the storage humidity.
Moreover, the data in Table 4 show that after three hours of storage the polymeric film is not yet saturated. The steady state conditions, at which the polymer film is completely saturated, are reached at 24 hours. The steady state The Scientific World Journal 13 Table 4: Averaged data of the amount of H 2 S adsorbed per surface unit (H 2 S adsorbed /m 2 ). The bag tested was without any film inside (B-no film) and with the film inside. The surface of the internal film sheet was equal to 1900 cm 2 (B-film 1900), 2580 cm 2 (B-film 2580), and 3520 cm 2 (B-film 3520), respectively. The data reported are the average of the results from three different tests performed at the same conditions. RH% Table 5: Diffusion coefficient of H 2 S over time in a Nalophan bag stored at a temperature of 23 ∘ C and a humidity of 20% and 60%, respectively. The bag tested was without any film inside (B-no film) and with the film inside. The surface of the internal film sheet was equal to 1900 cm 2 (B-film 1900), 2580 cm 2 (B-film 2580), and 3520 cm 2 (B-film 3520), respectively. is considered reached when the sorption amount of H 2 S does not vary with time in analogies with Fick law [41]. Therefore, in order to calculate the diffusion coefficient ( ) only the data acquired at 24 hours and 30 hours were used. The diffusion coefficient was evaluated according to (16). Table 5 reports the diffusion coefficient for each time interval at a storage temperature of 23 ∘ C and a humidity of 20% and 60%, respectively.
The diffusion coefficient of H 2 S ( ) through Nalophan is finally calculated as the average of the different values of ( Table 5) weighted on the corresponding storage time according to (17).
The resulting value for , at a storage humidity of 20%, is equal to 7.5 10 −12 m 2 /sec with a standard deviation equal to 1.2 10 −14 m 2 /sec. The resulting value for , at a storage humidity of 60%, is equal to 6.6 10 −12 m 2 /sec with a standard deviation equal to 7.9 10 −15 m 2 /sec.
The resulting values for obtained at two different storage conditions (i.e., humidity of 20% and of 60%., resp.) present the same order of magnitude.

Conclusions
The H 2 S losses from the Nalophan bag always turned out to be significant. The H 2 S loss after 30 hr was equal to 33% at a relative humidity of 20% and equal to 22% at a relative humidity of 60%.
The average value of H 2 S adsorbed /m 2 turns out to be equal to 5.8 g/m 2 at a storage humidity of 20% and equal to 4.8 g/m 2 at a storage humidity of 60%.
The contribution of the adsorption phenomenon, under the test conditions evaluated, is less significant than the diffusion, though not negligible. When increasing the surface of the film sheet inserted in the bag (i.e., test with "B-film 3520" at a humidity of 20%) then the contribution of adsorption 14 The Scientific World Journal to the H 2 S loss inside the bag becomes comparable with the contribution of diffusion. Therefore, in the case of mediumlow concentrations as it happens for those tests (from few ppb to few ppm), an increase of the polymeric surface produces an increase in the H 2 S loss due to the adsorption on the polymeric film. As a consequence, in order to reduce the adsorption phenomena on the polymeric film when storing gases like H 2 S at medium-low concentrations (i.e., in a range of ppb to few ppm), it is better to reduce the contact surface exposed to the gas using small sampling bags and storing the bag at a high relative humidity (i.e., RH% equal to 60%). During sampling of H 2 S, in order to reduce the odor losses, special care should be taken when the expected H 2 S concentration is medium or low (e.g., in the range of ppb to few ppm) because the adsorption phenomena on the polymer film in this case are not negligible.
Evaluating the two contributions of H 2 S loss (i.e., adsorption and diffusion) is important to choose the best sampling strategy (i.e., the choice of the bag material), as well as the most proper storage time and conditions. In order to reduce the diffusion phenomena through the bag, it is possible to use polyethylene terephthalate (i.e., commercial named Nalophan) coupled with foils. Nevertheless, this choice does not solve the problems linked to the loss by adsorption of H 2 S on the polymeric matrix.