Release of dinitrogen from nitrite and sulphamic acid for isotope ratio analysis of soil extracts containing nitrogen-15 labelled nitrite and nitrate

Abstract

Two procedures for converting nitrite and nitrate to N₂ for isotope ratio analysis were compared. The classical procedure requires the reduction of nitrite and nitrate to ammonium with Devarda's alloy during alkaline steam distillation, followed by the alkaline hypobromite oxidation of ammonium to N₂. The alternative procedure involves the reduction of nitrate to nitrite on a copper-activated cadmium column, followed by the reaction of nitrite with sulphamic acid to release N₂. The same apparatus is used to prepare N₂ in the final stages of the analysis. The isotopic compositions of aqueous solutions and soil extracts containing ¹⁵N-enriched nitrite, nitrate and nitrite + nitrate determined by the two procedures were precise and were in close agreement. However, several advantages are offered by the sulphamic acid procedure. Contamination of samples with ammoniacal nitrogen from external sources does not affect the isotopic composition of N₂ prepared by the sulphamic acid procedure. In addition, problems encountered with the classical methodology such as incomplete removal of ammonium from extracts prior to reduction of nitrate and nitrite to ammonium with Devarda's alloy, ¹⁵N cross-contamination between distillates and interference from co-extracted, alkali-labile organic nitrogen compounds, do not occur with the reduction of nitrate to nitrite on the copper-activated cadmium column. An equimolar contribution of nitrogen from nitrite and sulphamic acid provides twice as much N₂ per unit of sample nitrogen for mass spectrometric analysis. The sulphamic acid procedure is a viable alternative in studies where the principal interest lies in the determination and isotope ratio analysis of nitrite and nitrate, such as those involving measurements of rates of nitrification and denitrification in soils and sediments.