(i) The ion-exchange method represents the earliest developed approach and employs an anionexchange column for the preconcentration of nitrate and nitrite. They are then removed from the column by hydrochloric acid and analyzed as silver nitrate and silver nitrite using an elemental analyzer. (ii) The denitrifier method uses bacterial strains with lacking nitrous oxide reductase activity to stop the denitrification of nitrate and nitrite at N2O, which is purified and trapped prior to IRMS analysis. (iii) The cadmium azide method uses a cadmium sponge to reduce nitrate to nitrite in a first step and to further reduce nitrite to nitrous oxide prior to IRMS analysis. (iv) Another method describes the measurement of N abundances of aqueous nitrate and nitrite by on-line reduction to NO with vanadium or titanium chloride and subsequent measurement with a membrane-inlet quadrupole MS (SPINS/MIMS) , based on a continuousflow mass spectrometry method developed in 1999. This method has been improved over the years and the latest iteration uses an IRMS system instead of a quadrupole MS to measure isotopic ratios directly. Samples with a nitrogen concentration of 35 µmol L -1 N for nitrate can be measured and δ N values from standards are reported with an accuracy of less than 0.9 ‰.