Isolation of NO3‐N as 1‐phenylazo‐2‐naphthol (Sudan‐1) for measurement of δ15N

Abstract

The conversion of nitrate (NO 3 À ) to 1-phenylazo-2-naphthol (Sudan-1) has been examined as a method for natural abundance measurement of d 15 N of NO 3 À . The reaction results in dilution of NO 3 À -N with only one reagent-derived N and the product is readily concentrated from dilute samples by reverse phase chromatography. There is systematic isotopic fractionation during the reaction, but this can be allowed for by analysing known NO 3 À standards along with each sample set. Sudan-1 prepared from surface water samples containing $50 mg NO 3 À -N can be analysed by automated continuous flow isotope ratio mass spectrometry with a precision of 0.2% (one standard deviation) and the accuracy is not affected by interference from other nitrogenous species in the sample or reagents.