A method for nitrate collection for delta N-15 and delta O-18 analysis from waters with low nitrate concentrations

Recently, methods have been developed to analyze NO3- for delta(15)N and de lta(18)O, improving our ability to identify NO3- sources and transformation s. However, none of the existing methods are suited for waters with low NO3 - concentrations (0.7-10 mu M). We describe an improved method for collecti ng and recovering NO3- on exchange columns. To overcome the lengthy collect ion loading times imposed by the large sample volumes (7-70 L), the sample was prefiltered (0.45 mu m) with a large surface area filter. Switching to AG2X anion resin and using a coarser mesh size (100-200) than previous meth ods also enhanced sample flow. Placement of a cation column in front of the anion column minimized clogging of the anion column by dissolved organic c arbon (DOC) accumulation. This also served to minimize transfer of unwanted oxygen atoms from DOC to the O-18 portion of the NO3- sample, thereby cont aminating the sample and shifting delta(18)O. The cat-AG2X method is suited for on-site sample collection, making it possible to collect and recover N O3- from low ionic strength waters with modest DOC concentrations (80-800 m u M), relieves the investigator of transporting large volumes of water back to the laboratory, and offers a means of sampling rain, snow, snowmelt, an d stream samples from access-limited sites.