A method for nitrate collection for δ¹⁵N and δ¹⁸O analysis from waters with low nitrate concentrations

Abstract

Recently, methods have been developed to analyze NO 3 - for δ 15 N and δ 18 O, improving our ability to identify NO 3 - sources and transformations. However, none of the existing methods are suited for waters with low NO 3 - concentrations (0.7-10 µM). We describe an improved method for collecting and recovering NO 3 - on exchange columns. To overcome the lengthy collection loading times imposed by the large sample volumes (7-70 L), the sample was prefiltered (0.45 µm) with a large surface area filter. Switching to AG2X anion resin and using a coarser mesh size (100-200) than previous methods also enhanced sample flow. Placement of a cation column in front of the anion column minimized clogging of the anion column by dissolved organic carbon (DOC) accumulation. This also served to minimize transfer of unwanted oxygen atoms from DOC to the 18 O portion of the NO 3 - sample, thereby contaminating the sample and shifting δ 18 O. The cat-AG2X method is suited for on-site sample collection, making it possible to collect and recover NO 3 - from low ionic strength waters with modest DOC concentrations (80-800 µM), relieves the investigator of transporting large volumes of water back to the laboratory, and offers a means of sampling rain, snow, snowmelt, and stream samples from access-limited sites.