Study of dendrimers by MALDI and ESI-MS

Although MALDI-MS plays an outstanding role in dendrimer analysis, additional use is also made of modern ESI mass spectrometers for monitoring syntheses, for determination of relative molecular masses, and for studying the purity and polydispersity of dendrimers, including those of higher generations [34]. 

Recent comparative studies on the various ionisation techniques in the case of POPAM dendrimers and POPAM dendrimers bearing peripheral sulphona-mide groups nevertheless show that both ionisation methods can erroneously indicate a sample composition which does not correspond to reality [39]. 

Thus the interpretation of ESI-MS spectra of POPAM dendrimers recorded on an FT-ICR (Fourier-Transform Ion Cyclotron Resonance) mass spectrometer leads to drastic overestimation of defects in the sample molecules (Fig. 7.4). The 

MALDI-TOF spectra of the sulphonamide dendrimers erroneously suggest formation of defect structures during synthesis in fact, these are only generated during the ionisation process, primarily due to reaction of the peripheral sulphonamide groups with the acid matrix. An apparently unsatisfactory result should therefore always be checked by use of a second ionisation method or different MALDI matrices should be used to exclude misinterpretation of the spectra and to gain certainty about the true composition of the sample. 

Another problem is that dendrimers often crystallise as solvates or clathrates, that is with inclusion of solvent molecules. Such crystals are often rather unstable and decompose with release of the solvent As a consequence of these properties of dendrimers, suitable single crystals have so far only been grown for just a few dendrimers up to the second generation at most 

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