Nalophan bags are commonly used for air sampling and especially for odour analysis. Even if olfactometric measurement must be carried out within maximum 30 hours after sampling, the question of potential sample evolution is always present. This study illustrates the behaviour of selected sulphur compounds in Nalophan bags from filling to analysis (over a period up to 100 hours).
Select compounds were hydrogen sulphide, carbon disulphide, methyl mercaptan, ethyl mercaptan, dimethyl sulphide, diethyl sulphide and dimethyl disulphide and tested at high concentration level (in a range of 3900 to 1800 ppb each) to facilitate their direct and quick measurement by gas chromatography with flame photometric detector. The chemical analysis shows losses by adsorption and by diffusion depending on time and other conditions. Even if the variation seems limited during the first hours, the evolution shows that the need for a better film is real.
Jean-Michel Guillota*, Manuel Toledo Padronb
aIMT Mines Alès, 30319 Alès cedex, France. * firstname.lastname@example.org
bUniversity of Cordoba, 14071 Cordoba, Spain.
Competing interests: The author has declared that no competing interests exist.
Academic editor: Carlos N. Díaz.
Content quality: This paper has been peer-reviewed by at least two reviewers. See the scientific committee here.
Citation: Jean-Michel Guillot, Manuel Toledo Padrón, Main losses of sampled volatile compounds in nalophan bags: focus on sulphur compounds, 9th IWA Odour& VOC/Air Emission Conference, Bilbao, Spain, www.olores.org.
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Keywords: Adsorption, Diffusion, Hydrogen sulphide, Sampling, Storage losses.
Nalophan bags are commonly used for air sampling and especially for odour analysis. Even if olfactometric measurement must be carried out within maximum 30 hours after sampling, the question of potential sample evolution is always present. This study illustrates the behaviour of selected sulphur compounds in Nalophan bags from filling to analysis (over a period up to 100 hours). Select compounds were hydrogen sulphide, carbon disulphide, methyl mercaptan, ethyl mercaptan, dimethyl sulphide, diethyl sulphide and dimethyl disulphide and tested at high concentration level (in a range of 3900 to 1800 ppb each) to facilitate their direct and quick measurement by gas chromatography with flame photometric detector. The chemical analysis shows losses by adsorption and by diffusion depending on time and other conditions. Even if the variation seems limited during the first hours, the evolution shows that the need for a better film is real. Nalophan is a very nice material, stable polymer without VOC emissions itself, cheap and easy to find but it’s not perfectly gastight so it presents limitations. Losses are observed just few hours after filling the bag and can reach 10% after 30 hours for the smaller compounds such as hydrogen sulphide, methyl mercaptan or even carbon disulphide. This limited variation combined to olfactometry uncertainty cannot reject the sampling protocol but for a precise chemical analysis and mainly, if the sample must be stored for few days, it is important to consider that the result can be very different comparatively to collected atmosphere.
Because sampling is an essential step in environmental analysis, it is essential to keep sample integrity between sampling (on a site) and measurement (in a laboratory). Bags can be considered highly convenient equipment to collect an atmosphere based on cost, easiness to use and lightness. But of course the sampled atmosphere is in contact with the material inside the bag and then, questions about potential diffusion or adsorption are in mind of all analysts. In order to simply verify if such phenomena exist or not, double layer bags were used. This concept was firstly used to show the fast diffusion of humidity through the film (Cariou and Guillot, 2006) and to how how a can be dried. This diffusion was increased creating a strong gradient between inside and outside bag (Beghi and Guillot, 2006, 2008). Even if bigger molecules than H2O present lower diffusion, the diffusion must be studied to verify if analysed samples are representative of collected samples depending on time between these two steps.
If sampling bags are convenient for both chemical and sensorial analysis, it is typically the way to collect an atmosphere for odour concentration determination (EN 13725, 2003). So if the content can evaluate by different processes (adsorption, diffusion, reaction), the chemical composition will change. And of course, for an odorous atmosphere, it is obvious that the odour concentration can be affected too. Many studies were carried out to show such evolution. As an example, an odour from a tobacco processing plant collected in Nalophan bags leads to relative stable odour concentration from 4 h to 12 h of storage when after 30 h, the resulting concentration is half of the initial one (Van Harreveld, 2003).
2. Materials and methods
2.1. Odorous sulphur compounds and their analysis
Seven volatile sulphur compounds (VSCs) were selected: hydrogen sulphide (H2S), methyl mercaptan, ethyl mercaptan, dimethyl sulphide (DMS), diethyl sulphide (DES), dimethyl disulphide (DMDS) and carbon disulphide (CS2). The mixture of these VSCs was at a global level of 700 ppm in the cylinder and then diluted with pure air to start experiment in a range 3900–1800 ppb for analytical reasons. So, four initial concentrations were tested 3900, 3250, 2600 and 1800 pbb.
These compounds were analysed by gas chromatography (GC) coupled to a flame photometric detector (FPD) equipped with a 250 µL sampling loop for gas injection. This GC/PFD is a Chroma S model from Chromatotec, France. The detection limits of this analytical device are in a range of 4 to 7 ppb for studied compounds so it was necessary to fill internal bags with concentrations relatively high (3900–1800 ppb) to be able to detect the diffusion. Because the gas mixture is from a certified cylinder in terms of concentration, calibration of the analytical device was carried out based on these data.
2.2. Home-made Nalophan bags
With a Nalophan roll (400 mm diameter, 20 µm thickness) manufactured by Kalle (Germany), bags were prepared clamping sides and adding a polypropylene valve. Nalophan bags of different size were prepared and pre-flushed with filtered air for 24 h. Such a pre-flush is carried out, firstly to verify if a bag is leakless and secondly to clean the material in case of low background pollution emitted by the material.
2.3. Storage during few days (until 95 hours)
As shown in figure 1, a double bag was built. A first bag (12 L) is used as external bag and a smaller one (4 L) is used as internal bag.
The VSCs mixture is placed into the internal bag when the external is filled with clean air. Because the volume of internal bag is limited, just four measurements were carried out at 0, 45, 69 and 95 hours to limit the influence of the collected part comparatively to the global volume and of course to verify the level decrease. External bag with a bigger volume was analysed more often at 7, 22, 30, 45, 57, 69, 77 and 95 hours. Bags are stored in the same condition: 25°C and darkness.
Figure 1: Set up of both internal and external Nalophan bags for diffusion tests
3. Results and discussion
In order to illustrate the diffusion process through a polymer film, the figure 2 shows the losses of H2O in both Tedlar and Nalophan bags. When a strong concentration gradient is present between the two faces of a film, the tendency to equilibrate concentrations is a driven force for diffusion. This example was obtained with dry external atmosphere comparatively to humid internal atmosphere. Of course, films are different regarding their polymeric structure: polyethylene terephtalate for Nalophan and polyvinyl fluoride for Tedlar but a key factor is also the thickness of the film because Nalophan is twice thicker than Tedlar. This property of quick water diffusion can be used in analytical procedure as a way to dry a sample because diffusion of VOCs is slower than H2O (Beghi and Guillot, 2006, 2008).
Figure 2: Fast diffusion of water through Tedlar and Nalophan films (from Beghi et Guillot, 2008)
The losses of sulphur compounds are well illustrated by the figure 3. During the duration of the test (95 h) losses are around 500 ppb independently to initial level (3900 or 1800 ppb). And the global detected level in external bag is quite the same at the end of the experiment but this value is the addition of detected compounds taking into account the detection limit of compounds by the GC/FPD. Because diffusion is different for each molecule and because of the dilution on the large volume of external bag, only 3 compounds of the mixture are well identified (Figure 4). This detection shows that these 3 compounds hydrogen sulphide, methyl mercaptan and carbon disulfide are the most diffusive compounds that seems logical taking into account the size of molecules. A close experiment (Kasper, 2018) using also double bags and a mixture of sulphur compounds and organic acids showed that methyl mercaptan and dimethyl sulphide were detected in the external bag when H2S cannot be detected even if losses were observed in the internal bag.
Figure 3: Examples of diffusion through Nalophan films: Initial mixture of VSCs at 3900 ppb on the left graph and at 1800ppb on the right, (extracted from Toledo et al., 2019)
Figure 4: Detected VSCs in the external bag with initial mixture at 3250 ppb in the internal bag, (extracted from Toledo et al., 2019)
The diffusion of H2S was also mentioned in different studies. For example Eusebio and al. (2016) mentioned H2S losses of 33% ± 3% and 22% ± 1% at a relative humidity of 20% and 60% respectively, for a 30 h storage time. These losses combine adsorption and diffusion phenomena with globally stronger diffusion. The proportion between phenomena is dependent of the surface of the film and the concentration inside the bag.
The methodology based on double bags is simple to use and efficient to study phenomena such as adsorption on the surface and diffusion through a film. Nalophan that is commonly used, because cheap and without strong background release, is one of the most permeable material used for bags. So this permeability must be known to be sure that global conditions to collected compounds are compatible with storage time before analysis. Of course, many key parameters influence the diffusion: the molecule (size and volatility), the film (polymeric structure, thickness, surface exposed to sample), the conditions (humidity level, temperature) and the storage duration. The study was focused on sulphur compounds for their interest in terms of odour impact even if the decay and losses were characterised only on chemical concentrations. The losses by adsorption and diffusion can be important depending on key parameters listed previously and analysts must be aware of this part of uncertainty in results obtained after analysing the content of a sample bag.
Beghi, S., Guillot, J. M. (2006). Sample water removal method in volatile organic compound analysis based on diffusion through poly (vinyl fluoride) film. Journal of chromatography A, 1127(1-2), 1-5.
Beghi, S., Guillot, J. M. (2008). Use of poly (ethylene terephtalate) film bag to sample and remove humidity from atmosphere containing volatile organic compounds. Journal of Chromatography A, 1183(1-2), 1-5.
Cariou, S., Guillot, J. M. (2006). Double-layer Tedlar bags: a means to limit humidity evolution of air samples and to dry humid air samples. Analytical and bioanalytical chemistry, 384(2), 468-474
EN 13725 (2013). Air quality – Determination of odour concentration by dynamic olfactometry. European Committee for Standardisation, Brussels.
Eusebio, L., Capelli, L., Sironi, S. (2017). H2S Loss through Nalophan™ Bags: Contributions of Adsorption and Diffusion. The Scientific World Journal, 2017.
Kasper, P. L., Oxbøl, A., Hansen, M. J., Feilberg, A. (2018). Mechanisms of loss of agricultural odorous compounds in sample bags of Nalophan, Tedlar, and PTFE. Journal of environmental quality, 47(2), 246-253.
Toledo, M., Guillot, J. M., Siles, J. A., Martín, M. A. (2019). Permeability and adsorption effects for volatile sulphur compounds in Nalophan sampling bags: Stability influenced by storage time. Biosystems Engineering, 188, 217-228.
Van Harreveld, A. (2003). Odor concentration decay and stability in gas sampling bags. Journal of the Air & Waste Management Association, 53(1), 51-60.
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