Sample preservation methods for nitrous oxide concentration and isotope ratio measurements in aquatic environments

Abstract

AbstractThe nitrogen (N) and oxygen (O) stable isotope analysis of dissolved nitrous oxide (N2O) can provide important constraints on the sources and cycling of N2O in aquatic environments. The isotopic composition of aqueous N2O, both in field (natural abundance) or experimental (15N‐labeling) samples, however, may be altered by abiotic reactions involving nitrite () or hydroxylamine (NH2OH) and microbial activity during sample storage, if samples are not adequately preserved. Here we tested five different preservatives, mercuric chloride (HgCl2), copper sulfate (CuSO4), zinc chloride (ZnCl2), hydrochloric acid (HCl) mixed with sulfamic acid (SFA), and sodium hydroxide (NaOH), for fixing natural water samples from an estuary and a lake with different concentrations over a range of different storage times for N2O analyses. ZnCl2 and CuSO4 decreased the pH, and led to abiotic N2O production from , shifting the N2O isotopic composition significantly. Removal of with a mixture of SFA and HCl did not always prevent the alteration of the original N and O isotope composition of N2O, confirming the requirement for complete removal, and underscoring the biasing effects of at very low pH, even at trace levels. At low background, HgCl2 preservation led to robust and accurate results. However, in light of its toxicity and bioaccumulation potential in food webs, this fixative should be avoided. The addition of NaOH reduced abiotic side reactions involving , and yielded the most reliable and reproducible results for N2O concentration and isotope ratio measurements across tested settings and storage times.