A simple, safe method for preserving dissolved methane in freshwater samples using benzalkonium chloride

Methane (CH₄) dissolved in water is readily consumed by CH₄-oxidizing bacteria, so the possibility of the dissolved CH₄ concentration (dCH₄) in sampled water changing before analysis is a concern. To determine the accurate in situ dCH₄ level, mercury chloride (HgCl₂) or sodium azide (NaN₃) is traditionally used for sample preservation. However, these preservatives are very toxic and great care must be taken when adding them to samples. Benzalkonium chloride (BAC), a quaternary ammonium salt cationic surfactant, is a readily available disinfectant that is less harmful to the human body than HgCl₂, NaN₃, and other preservatives. In this study, we investigated the usefulness of BAC in preserving dCH₄ in swamp water, which is a critical terrestrial source of CH₄. The dCH₄ in samples without BAC decreased immediately after sample collection, whereas the dCH₄ in the samples with added BAC did not change significantly for at least 15 d. In addition, when BAC was added to 18 water samples with different water chemistries, the dCH₄ did not change significantly from immediately after sampling to 1 week after sampling (average difference: 3%). Thus, in the water samples used in this study, BAC effectively preserved dCH₄ in the samples for at least 1 week. Further testing of the effect of BAC on the preservation of dCH₄ in different types of water samples worldwide will help to establish a more complete, simple, and safe method.