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

Methane (CH 4 ) dissolved in water is readily consumed by CH 4 ‐oxidizing bacteria, so the possibility of the dissolved CH 4 concentration (dCH 4 ) in sampled water changing before analysis is a concern. To determine the accurate in situ dCH 4 level, mercury chloride (HgCl 2 ) or sodium azide (NaN 3 ) 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 HgCl 2 , NaN 3 , and other preservatives. In this study, we investigated the usefulness of BAC in preserving dCH 4 in swamp water, which is a critical terrestrial source of CH 4 . The dCH 4 in samples without BAC decreased immediately after sample collection, whereas the dCH 4 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 dCH 4 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 dCH 4 in the samples for at least 1 week. Further testing of the effect of BAC on the preservation of dCH 4 in different types of water samples worldwide will help to establish a more complete, simple, and safe method.