Assessment of methods for collection, preservation, and storage of water samples for dissolved methane and nitrous oxide concentration measurements

Anagha Payyambally, Josie L. Mottram, Julie Granger, Cara C M Manning 

Nitrous oxide (N2O) and methane (CH4) are potent greenhouse gases produced and consumed in freshwater and marine systems. Their measurement in water is challenging due to contamination risks at various sampling, storage, and analysis stages. Samples that are stored before analysis are commonly preserved with mercuric chloride (HgCl2), which is highly toxic. To date, no studies have conducted a thorough assessment or comparison among methods for sample collection, preservation and storage, for concurrent analysis of CH4 and N2O. This study systematically compares sampling and preservation methods in aqueous samples. Estuarine water samples at 4–11 oC were collected in 120 mL serum vials without a headspace, preserved, and stored at room temperature for up to 6 months before analysis. Preservation with hydroxide salts at pH 10–12 proved equally effective to preservation with HgCl2. Sample collection and septum insertion while the bottle is submerged minimized contamination with air, improving the accuracy of CH4 measurements. Depressurization of samples after collection proved necessary to prevent distention of septa (and associated gas transfer) in saltwater samples collected without a headspace, preserved with hydroxide, and stored at a significantly warmer temperature than the in situ water temperature. Two-leg and three-leg septa yielded the most stable concentrations. The number of piercings of the septa and storage orientation had no effects on gas concentrations. Air transportation of the samples did not significantly affect CH4 and N2O concentrations. This study will help to improve accuracy of CH4 and N2O measurements in water and reproducibility between research groups.