Fig. 4. Liquid chromatography–mass spectrometry (LC-MS) chromatogram of a solution containing a mixture of 1,5-naphthalendisulfonic acid disodium salt (NDS) and persulfate (each 0.13 mM) recorded after 2 h and 20 min of the degradation process. Cromatographic conditions: see method I in LC-MS analysis (experimental session). Detector MS. Peak identification: at 7.03 min coelute NDS and intermediate II (Fig. 4a,b) and at 9.88 min elutes intermediate III. Figure 4d reportes the mass spectrum of NDS (287 m/z) and intermediate II (253 m/z); Fig. 4e shows the mass spectrum of intermediate III (257 m/z).