Characterization of Homopolymer Distributions via Direct Infusion ESI-MS/MS using Wide Mass-to-Charge Windows and Gas-Phase Ion/Ion Reactions

A hallmark of electrospray ionization (ESI) of large polymeric molecules is its tendency to generate charge state distributions. When a distribution of polymers is subjected to ESI, the charge state distribution of each component can lead to a mass spectrum composed of a highly congested mixture of ions with overlapping mass-to-charge (m/z) ratios. When the polymers are composed of a common monomeric unit (i.e., a homopolymer), the overlap of the charge state distributions of the polymer components can give rise to striking spectral patterns with a dense central cluster of peaks having similar m/z values and wing-like patterns on either side. We refer to the central cluster of peaks as an "Emerald City," with a nod to the Wizard of Oz, combining the wings as an "Emerald City pattern". The Emerald City pattern can appear in the mass spectrum of any homopolymer with distributions of charge states and sizes. Various parameters were studied individually for their contributions to the appearance of Emerald City patterns. Dextran samples were used to demonstrate the spectral pattern experimentally, and a web-based tool was developed to validate the findings. We also proposed to use direct infusion ESI-MS coupled with segmented m/z windows that encompass Emerald Cities followed by gas-phase proton transfer reactions for characterizing poly disperse synthetic polymer samples. Poly(ethylenimine) samples were used as model systems to demonstrate the approach. The proposed strategy improves sample characterization relative to conventional zero-charge deconvolution or proton transfer reactions without prior mass-selected m/z windows.