The use of HPTLC and Direct Analysis in Real Time-Of-Flight Mass Spectrometry (DART-TOF-MS) for rapid analysis of degradation by oxidation and sonication of an azo dye

Advanced oxidation processes are efficient for the removal of recalcitrant compounds, like azo-dyes. However, the intermediates produced during their degradation can be more toxic than the parent compounds. Improving the knowledge concerning the degradation pathways may be therefore helpful to optimize the process. In this aim, HPTLC and Direct Analysis in Real Time-Of-Flight Mass Spectrometry (DART-TOF-MS) were considered and applied to analyze the sono-oxidation of an azo dye, methyl red sodium salt (MRSS) as a model compound. Initial and final UV-vis spectra showed a clear disappearance of the maximum absorption peak, but shows limit since it cannot allow by-products' identification. MRSS degradation was confirmed by HPTLC, which also confirmed that MRSS degradation was mainly due to oxidation, while in the considered experimental conditions the sonication effect appeared negligible. The analysis by DART-TOF-MS of the MRSS is characterized by a peak at m/z  = 279.137. Three major peaks were observed after the MRSS oxidation: m/z = 139.002, m/z = 223.073 and m/z = 279.137. Relative abundance of m/z = 139.002, which was much higher after oxidation, tends to prove that a large proportion of initial oxydized MRSS was fragmented. The MRSS m/z = 270.078. The coupling of HPTLC and DART-TOF-MS may be subsequently considered to identify the oxidation reaction products.