Resolving oligomers from fully grown polymers with IMS-MS

Ion mobility and mass spectrometry techniques, combined with electrospray ionization, have been used to examine distributions of poly(ethylene glycols) (PEG) with average molecular masses of 6550 and 17900 Da. The analysis provides information about the polymer size distributions as well as smaller oligomers existing over a wide range of charge states and sizes (i.e., [HO(CH2CH2O)xH + nCs](n+), where x ranges from 21 to 151 and n = 2 to 11 for the 6550 Da sample; and, x ranges from 21 to 362 and n = 2 to 23 for the 17 900 Da sample). The present data show that oligomer distributions also fall into families, corresponding to much narrower size distributions for individual charge states; this dramatically simplifies data analysis. For example, we show evidence for baseline resolution of the +10 charge state of polymers. Unlike the charge-state trends reported previously for peptide ion families, which show generally increasing mobilities with increasing charge state (for a given m/z value), the mobilities of [HO(CH2CH2O)xH + nCs]n+ families generally decrease with increasing charge state. This requires that the addition of charges leads to substantial changes in the average structures of the ions. Comparisons of cross section calculations from molecular modeling results for multiply cesiated PEG ions with experimental cross sections indicate that these ions adopt highly extended (in many cases nearly linear) conformations, except for the high degree of coordination of the charged sites.