Linear dependence of the intensity of the [M + H]+ ion peak in fast atom bombardment mass spectrometry on the basicity of meso-substituted octaethylporphyrins

A fast atom bombardment mass spectrometric study of the meso-substituent effect of Octaethylporphyrins (OEPX, X = NH₂, CH₃, H, CHO, Cl, CN, NO₂) is reported. The intensity of the M + H]⁺ ion peaks was estimated quantitatively using an external standard, which was observed to be correlated linearly with the basicity (pK₃) of the OEPX. The pK₃ value of the OEPXs was obtained by titration with perchloric acid and it was observed to be correlated linearly with Hammett's σ_p value, with a ρ value of 1.38.