Isolation of NO3--N as 1-phenylazo-2-naphthol (Sudan-1) for measurement ofdelta N-15

The conversion of nitrate (NO3-) to 1-phenylazo-2-naphthol (Sudan-l) has be en examined as a method for natural abundance measurement of delta(15)N of NO3-. The reaction results in dilution of NO3--N with only one reagent-deri ved N and the product is readily concentrated from dilute samples by revers e phase chromatography. There is systematic isotopic fractionation during t he reaction, but this can be allowed for by analysing known NO3- standards along with each sample set. Sudan-1 prepared from surface water samples con taining similar to 50 mu g NO3--N can be analysed by automated continuous f low isotope ratio mass spectrometry with a precision of 0.2 parts per thous and (one standard deviation) and the accuracy is not affected by interferen ce from other nitrogenous species in the sample or reagents. Copyright (C) 1999 John Wiley & Sons, Ltd.