Phase ratio variation approach for the study of partitioning behavior of volatile organic compounds in polymer sample bags: Nalophan case study

Mechanisms of Loss of Agricultural Odorous Compounds in Sample Bags of Nalophan, Tedlar, and PTFE

Alteration of the chemical composition of odor samples during storage in polymer sample bags can significantly impair the accuracy of subsequent odor evaluations. To overcome or minimize this effect, the mechanisms determining compound loss must be more thoroughly understood. The present study examines the storage stability of a selection of key odorants from livestock production in polymer sample bags of Nalophan, Tedlar, and polytetrafluoroethylene (PTFE). The compounds included are acetic acid, butanoic acid, propanoic acid, 3-methylbutanoic acid, hydrogen sulfide, methanethiol, dimethyl sulfide, trimethylamine, and 4-methylphenol. The fate of the unrecovered compound fractions is clarified by means of thermal desorption and concentric double bags, allowing estimation of the magnitude of losses due to adsorption and diffusion, respectively. The degree of recovery was found to be PTFE > Tedlar > Nalophan, and smaller ratios of bag surface area to sample volume improved the recovery significantly. Furthermore, PTFE bags were found far superior for maintaining the original sample humidity and for storing 4-methylphenol. Analysis of sample humidity, partitioning coefficients, and thermal desorption suggested that the loss in PTFE bags was mainly controlled by adsorption, whereas for Nalophan and Tedlar, compound loss is a combined effect of adsorption and diffusion. It is suggested to heat the bags when evacuating the sample for analysis, as this was found to improve the recovery significantly. For a 5-L PTFE bag, all odorants could be found at concentration levels between 71.6 and 98.8% even after 48 h of storage when heated to 57°C prior to analysis.

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 g_H2S/g_Nalophan at a storage humidity of 20% and equal to 1.79 10⁵ g_H2S/g_Nalophan 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).