CHARACTERIZATION OF PERFLUOROPOLYETHERS BY DESORPTION CHEMICAL IONIZATION AND TANDEM MASS-SPECTROMETRY

Desorption chemical ionization mass spectrometry (DCI-MS) was investigated as a technique for obtaining the structural characterization of perfluorinated polyethers (PFPE). Negative-ion DCI mass spectra were dominated by sequences of oligomeric alkoxy fragment ions, each retaining one of the terminal groups of the polymeric chain. Ion abundances showed Gaussian distributions, which were ascribed to low-energy deposition during desorption. The most Important process in the gas-phase ion chemistry of PFPEs resulted in the cleavage of C-O bonds, associated with charge retention on the oxygen; only low-intensity peaks were due to formation of carbanions. The high selectivity of this process produced extremely simple mass spectra, even for complex copolymers. The type and molar fraction of monomeric units and terminal groups contained in the analyzed PFPEs were easily inferred from their DCI mass spectra. Tandem mass spectrometry of oligomeric alkoxy ions and carbanions produced by DCI showed that the most favored dissociation was the loss of monomeric units. The mechanism of other fragmentation processes was also investigated. An interesting difference was found between metastable and collisionally activated loss of CnF2nO: the former occurs via fluorine rearrangement, whereas the latter involves a direct a cleavage at the charge site.