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=NH 2 , CH 3 , H, CHO, Cl, CN, NO 2 ) 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 (pK 3 ) of the OEPX. The pK 3 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