MALDI TOF mass spectrometry for the characterization of phosphorus-containing dendrimers. Scope and limitations.

Neutral phosphorus-containing dendrimers with aldehyde groups at the periphery have been analyzed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS) up to generation four. Although the expected quasi-molecular ion is generally observed, the mass spectral pattern, presence of fragments and adducts related to the original skeleton, is highly relevant to the sample preparation (nature of the matrix: 2-5-dihydroxybenzoic acid (2.5-DHB), 1,8-dihydroxy-9[10H]-anthracenone (dithranol), 6-azathiothymine, 2,4,6-trihydroxyacetophenon, 7-hydroxycoumarin or 2-anthramine, and addition of alkali metal salts). The dithranol matrix with addition of LiI offers milder conditions; however, abundant fragments are still observed for the higher generation dendrimers. Investigation of these effects in connection with SEC, NMR, and MALDI-TOFMS studies of UV preirradiated dendrimers allows the assumption to be made that fragmentation occurs in MALDI due to the relatively strong absorption of the dendrimers at 337 nm. Fragmentations and formation of adducts involve nitrogen-nitrogen bond cleavage, imine metathesis, and reaction of aldehyde groups with internal imino groups.