On-line nitrate-delta(15)N extracted from groundwater determined by continuous-flow elemental analyzer/isotope ratio mass spectrometry

Nitrate-delta(15)N from groundwater samples is determined on an inorganic nitrate derivative using automated, continuous-flow elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). Nitrate is extracted and concentrated based on a recently published ion-exchange resin method. Freeze-dried AgNO(3) (0.5-1.5 mg) is packed in silver-foil cups and combusted within the reactor of an NC2500 elemental analyzer (CE Instruments, Milan, Italy) using its existing reaction scheme for nitrogen and carbon analysis. delta(15)N is determined using a Finnigan MAT DELTA(plus) isotope ratio mass spectrometer (Bremen, Germany). Results are drift-corrected to a AgNO(3) working standard that has been calibrated against known AgNO(3). Despite high concentrations of carbonate, the precision for all runs is better than 0.10 per thousand. The combination of this extraction procedure with commercially available delta(15)N analysis instrumentation offers a precise on-line alternative to existing methods, with considerable reduction in labor and analysis time.