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.