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 (Fe²⁺ /H₂ O₂ ) and Fenton-like (Mⁿ⁺ /H₂ O₂ ; Mⁿ⁺  = Fe³⁺ , Co²⁺ , Cu²⁺ , Mn²⁺ ) 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: Fe²⁺ /H₂ O₂  ≫ H₂ O₂  ≫ Cu²⁺ /H₂ O₂  > Mⁿ⁺ /H₂ O₂ (Mⁿ⁺  = Fe³⁺ , Co²⁺ , Mn²⁺ ) ~ Mⁿ⁺ (Mⁿ⁺  = Fe²⁺ , Fe³⁺ , Co²⁺ , Cu²⁺ , Mn² ). The superior capability of Fe²⁺ /H₂ O₂ in causing the degradation of sildenafil indicates that Fe²⁺ efficiently decomposes H₂ O₂ to yield hydroxyl radicals, quite reactive species that cause the substrate oxidation. The results also indicate that H₂ O₂ 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, Fe³⁺ , Co²⁺ , Cu²⁺ , and Mn²⁺ . A unique oxidation by-product was detected in the reaction between Fe²⁺ /H₂ O₂ 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.