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

Abstract

AbstractMethane (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.