RATIONALE: Stable isotopes of nitrogen in nitrate (NO_-3) are frequently used to identify nitrate sources and to study nitrogen (N) transformation processes, but the measurement methods available are generally rather labor intensive and/or costly, and dissolved organic matter (DOM) can interfere with the δ¹⁵N signature of nitrate. We therefore have developed a simple cleanup procedure for freshwater samples with low nitrate and high DOM concentrations.
METHODS: Nitrate and DOM are extracted from a freeze-dried water sample by using a concentrated sodium hydroxide solution. By the subsequent addition of acetone, two liquid layers are formed, and nitrate migrates into the acetone while DOM remains in the concentrated NaOH solution, thus separating the nitrate from the DOM. For nitrogen isotope analysis, purified nitrate salts are combusted at 1030 °C to produce N₂ gas in an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS).
RESULTS: With this novel technique up to 99%n of DOM could be removed from river water and soil solutions. The method has been tested for sample amounts as small as 4 μmol NO3 with a precision of <0.1‰ (1SD). Nitrate standards are reproduced accurately without any blank correction.
CONCLUSIONS: The benefits of this method are the lack of interferences derived from DOM on the δ¹⁵N signature and the ease of sample preparation.