Residue mass plot and abundance plot: detection of isobaric interferences in DE-MALDI-TOF-mass spectra of complex polymer mixtures.

Mass spectra of complex polymer mixtures often disturbed by overlapping homologue peak series have been interpreted by means of the novel techniques of the residue mass plot and the abundance plot. The model substance used for the investigations is so far poorly characterized non-ionic emulsifier Cremophor EL (polyoxyl 35 castor oil) (CrEL), a heterogeneous polyethoxylate mixture. Because of its high amount of hydrophobic and hydrophilic components, CrEL was separated into two fractions, aqueous and methanolic, by cation exchange (CCaEx) chromatography. CrEL was then subjected to delayed extraction matrix-assisted laser desorption/ionization time of flight mass spectrometry (DE-MALDI-TOF-MS). Evaluation of the mass spectra was performed by comparing the residue masses of the homologue peak series with the calculated residue masses of potential components of the excipient cationized with Na+ and K+. A number of these series are overlapping because they differ in their theoretical residue masses by about 0.05 Da. The detection of these isobaric interferences was the basic requirement for our analysis method. This goal was achieved by high mass accuracy of the measurements (obtained by internal calibration) in combination with two newly developed evaluation methods, the residue mass plot and the abundance plot. Using this combined technique, generally applicable for complex polymer mixtures, it was shown that the aqueous CCaEx fraction contains hydrophobic components such as di- and triesters of ricinoleic acid and polyethylene glycol as well as glycerol polyoxyethylene di- and triricinoleates, whereas the methanolic fraction contains hydrophilic components, mainly polyethylene glycol (PEG) and glycerol polyoxyethylene ether. Moreover, free PEG was shown to consist of PEG 800 in contrast to the value of 600 Da described so far in literature.