On‐surface Fenton and Fenton‐like reactions appraised by paper spray ionization mass spectrometry

On‐surface degradation of sildenafil (an adequate substrate as it contains assorted functional groups in its structure) promoted by the Fenton (Fe 2+ /H 2 O 2 ) and Fenton‐like (M n+ /H 2 O 2 ; M n+  = Fe 3+ , Co 2+ , Cu 2+ , Mn 2+ ) systems was investigated by using paper spray ionization mass spectrometry (PS‐MS). The performance of each system was compared by measuring the ratio between the relative intensities of the ions of m/z 475 (protonated sildenafil) and m/z 235 (protonated lidocaine, used as a convenient internal standard and added to the paper just before the PS‐MS analyzes). The results indicated the following order in the rates of such reactions: Fe 2+ /H 2 O 2  ≫ H 2 O 2  ≫ Cu 2+ /H 2 O 2  > M n+ /H 2 O 2 (M n+  = Fe 3+ , Co 2+ , Mn 2+ ) ~ M n+ (M n+  = Fe 2+ , Fe 3+ , Co 2+ , Cu 2+ , Mn 2 ). The superior capability of Fe 2+ /H 2 O 2 in causing the degradation of sildenafil indicates that Fe 2+ efficiently decomposes H 2 O 2 to yield hydroxyl radicals, quite reactive species that cause the substrate oxidation. The results also indicate that H 2 O 2 can spontaneously decompose likely to yield hydroxyl radicals, although in a much smaller extension than the Fenton system. This effect, however, is strongly inhibited by the presence of the other cations, ie, Fe 3+ , Co 2+ , Cu 2+ , and Mn 2+ . A unique oxidation by‐product was detected in the reaction between Fe 2+ /H 2 O 2 with sildenafil, and a possible structure for it was proposed based on the MS/MS data. The on‐surface reaction of other substrates (trimethoprim and tamoxifen) with the Fenton system was also investigated. In conclusion, PS‐MS shows to be a convenient platform to promptly monitor on‐surface oxidation reactions.