A LASER DESORPTION FOURIER-TRANSFORM MASS-SPECTROMETRIC STUDY OF DIMETHYL 8-ACETYL-3,7,12,17-TETRAMETHYLPORPHYRIN-2,18-DIPROPANOATE

Laser desorption Fourier transform ion cyclotron resonance positive- and negative-ion mass spectra are presented for dimethyl 8-acetyl-3,7,12,17-tetramethylporphyrin-2,18-dipropanoate. The 248-nm laser ionization thresholds for both positive and negative ions are observed to be about 2.5 MW cm-2. The M+. molecular ion is assigned to the base peak in the low-power spectra whereas it is the M-. ion for the corresponding anion spectra. Increased intensities of [M + H]+ and [M - H]- are observed with increased laser fluences of up to 38 MW cm-2. At high laser powers the negative-ion results reveal that a series of carbon-nitrogen cumulene and polyacetylene cluster ions are formed. Laser evaporation/multiphoton ionization/ and thermal evaporation/electron impact ionization/collision-induced dissociation experiments carried out on the porphyrin M+. and [M + H]+ ions over a range of translational kinetic energies and delay times after acceleration are compared and used to obtain mechanistic and structural information. In contrast to the electron impact experiments, which show only side-chain cleavage, the laser-based collision-induced dissociation experiments reveal that, in addition to side-chain cleavage, it is possible to cleave the porphyrin ring to various extents depending on the ion translational energy selected.