A new method for collection of nitrate from fresh water and the analysis of nitrogen and oxygen isotope ratios

A new method for concentrating nitrate from fresh waters for δ 15N and δ 18O analysis has been developed and field-tested for four years. The benefits of the method are: (1) elimination of the need to transport large volumes of water to the laboratory for processing; (2) elimination of the need for hazardous preservatives; and (3) the ability to concentrate nitrate from fresh waters. Nitrate is collected by, passing the water-sample through pre-filled, disposable, anion exchanging resin columns in the field. The columns are subsequently transported to the laboratory where the nitrate is extracted, converted to AgNO 3 and analyzed for its isotope composition. Nitrate is eluted from the anion exchange columns with 15 ml of 3 M HCl. The nitrate-bearing acid eluant is neutralized with Ag 2O, filtered to remove the AgCl precipitate, then freeze-dried to obtain solid AgNO 3, which is then combusted to N 2 in sealed quartz tubes for δ 15N analysis. For δ 18O analysis, aliquots of the neutralized eluant are processed further to remove non-nitrate oxygen-bearing anions and dissolved organic matter. Barium chloride is added to precipitate sulfate and phosphate; the solution is then filtered, passed through a cation exchange column to remove excess Ba 2+, re-neutralized with Ag 2O, filtered, agitated with activated carbon to remove dissolved organic matter and freeze-dried. The resulting AgNO 3 is combusted with graphite in a closed tube to produce CO 2, which is cryogenically purified and analyzed for its oxygen isotope composition. The 1 σ analytical precisions for δ 15N and δ 18O are ±0.05‰ and ±0.5‰, respectively, for solutions of KNO 3 standard processed through the entire column procedure. High concentrations of anions in solution can interfere with nitrate adsorption on the anion exchange resins, which may result in isotope fractionation of nitrogen and oxygen (fractionation experiments were conducted for nitrogen only; however, fractionation for oxygen is expected). Chloride, sulfate, and potassium biphthalate, an organic acid proxy for dissolved organic material, added to KNO 3 standard solutions caused no significant nitrogen fractionation for chloride concentrations below about 200 mg/l (5.6 meq/l) for 1000 ml samples, sulfate concentrations up to 2000 mg/l (41.7 meq/l) in 100 ml samples, and Potassium biphthalate for concentrations up to 200 mg/l carbon in 100 ml samples. Samples archived on the columns for up to two years show minimal nitrogen isotope fr