Mass and Charge Distribution Analysis in Negative Electrosprays of Large Polyethylene Glycol Chains by Ion Mobility Mass Spectrometry

The mass spectrometric (MS) complexity associated with the quasi-continuous distribution of mass and charge (m, z) of electrosprayed industrial polymers may be moderated by use of ion mobility spectrometry (IMS) and MS in series. However, when the high charge levels typical of polar polymers stretch the gas phase ions into linear configurations, the mobility Z tends to be closely correlated with m/z, and IMS-MS does not yield spectra more readily interpretable than pure MS spectra. Here we note that the usual high charge states observed in the ESI of polyethylene glycol (PEG) arise because the stretched gas phase chain is able to strongly bind solution cations. We weaken this binding and therefore moderate the charge level by electrospraying in negative mode (NESI). This produces exclusively globular gas phase ions. IMS-MS then readily separates into distinct bands the different z-states, enabling an unambiguous assignment of all ions and simplifying the determination of mass distributions f_z(m) for each charge state. The measured probability p_z(m) that a polymer ion of given mass m will carry z charges spans a surprisingly narrow z range, each mass being present at most in two charge states. PEG ions of a given charge state z become unstable at a critical mass, below which they shed just one elementary charge, evidently by ion evaporation. We argue that NESI-IMS-MS offers significant analytical advantages over alternative methods previously demonstrated, particularly at increasing masses, when individual ion peaks can no longer be discerned.