A method, which is faster and more convenient than previous techniques, is described for converting ammonium sulphate into nitrogen gas for isotopic analysis on a mass spectrometer. The dry ammonium sulphate sample is contained in a small screw-topped vial, in which it can be stored pending analysis. This vial can be coupled with a neoprene O-ring to a special assembly, evacuated, and the sample oxidised with air-free hypobromite solution from a reservoir in the assembly. By using this procedure the usual inconvenience of de-greasing ground-glass joints on sample tubes is avoided, the difficulty of storing samples prior to analysis is overcome and the de-gassing of solutions prior to the conversion of each sample into nitrogen gas is eliminated. The procedure is particularly suitable when many hundreds of samples are to be analysed.