Water Nitrate Stable Isotope Analysis Method

The primary objective of this study was to develope a method forδ¹⁵N analysis of dissolved nitrate-nitrogen through laboratory test.The method include three major steps: The anion exchange column technique for concentrating nitrate from fresh waters,the Kjeldahl distillation method for converting NO₃^--N to NH₄⁺-N and the isotope ratios measuring using a EA-MS system.The factors which can strongly interfereδ¹⁵N analysis have been discussed.In order to assess anion exchange column technique,the adsorption-desorption experiments have been carried out using standard nitrate solution with the concentration of 100μmol/L.The results indicated that using KCl solution to drip through the column before adsorbing NO₃^- has a better result than using NaOH solution.Given the low NO₃^-concentrations and the relatively high DOC concentrations for surface waters,the suitable resin volume is 5.0 ml,and the result indicated that the appropriate flow rate is about 15 ml/min during the adsorption process.The most suitable flow rate is 0.5 ml/min when eluting NO₃^- from resins using 2 mol/L KCI solution with the least volume of 40 ml.In addition,high concentrations of anions in solution can interfere with nitrate adsorption on the anion exchange resins,which may result in isotope fractionation of nitrogen.The chloride and sulfate was added to standard KNO₃ solutions to test the interference,and an evident decrease of nitrate adsorption efficiency can be caused when the chloride concentration is up to 10 mmol/L or the sulfate concentration up to 3 mmol/L.The results for Kjeldahl distillation method are as follows:the N blank is about 0.6μmol N during every distillation,which will not cause significant isotope fractionation of the NO₃^- concentrated through anion exchange column.The reduction ability of Devarda's Alloy has also been tested,and the results showed that the fully reduction of NO₃^- of at least 100μmol can be reached with every 0.3 g.We also test the adsorption efficiency of Molecular Sieves for NH₄⁺,the result indicated that for every 50 mg,it can adsorb at least 30μmol N with the adsorption efficiency up to 90%.The measurement of isotope ratios for nitrate-nitrogen indicated that the lower of the nitrate-nitrogen concentration,the more significant of isotope fractionation because of the influence of the N blank as well as the insufficiency of NO₃^- reduction.Moreover,the isotope fractionation can be notably inhibited when adding certain amount of KCI solid to KNO₃ stand solutions to increase ionic strength,this is probably because of the large amount of the ionic charges which may enhance the reaction for converting NO₃^--N to NH₄⁺-N to take more thoroughly,but the mechanism needs a further study.This study showed that the accurate measurement of nitrogen isotope could be resulted by using the method of nitrate preconcentration.