Microdroplet Fusion Chemistry for Charge State Reduction of Synthetic Polymers via Bipolar Dual Spray with Anionic Reagents

Online Bipolar Dual Spray for the Charge State Reduction and Characterization of Complex Synthetic Polymers

Charge reduction mass spectrometry (CR/MS) hyphenated to liquid chromatography (LC) couples liquid-phase compound separation and mass spectral decompression to resolve and characterize multicomponent systems. LC/CR/MS has proven to be effective for complex mixture analysis, particularly synthetic polymers. A newer charge manipulation approach called bipolar dual spray has previously been demonstrated to reduce the observed charge state distribution of ammoniated polyethene glycol. In this approach, two electrospray emitters, in close proximity and of opposite polarity, fuse droplets from their electrospray plumes, which allows the subsequent chemistry. In this work, we investigate the ability of bipolar dual spray to reduce the charge of synthetic polyols, thereby simplifying complex mixture analysis and generating new compositional information only available through the coupling of charge reduction with LC/MS analysis. This work also represents the first demonstration of online charge reduction via dual spray. Polyethylene glycol (PEG) 7.2K subjected to LC/MS with dual spray reduced the average charge state from 8.2+ to 4.4+. LC/MS with dual spray was also applied to the characterization of an end-group-modified PEG 10K (i.e., aminated) containing several reaction impurities. This approach allowed for the identification of low-level starting material, tosylated PEG, and PEG mono(amine), where both LC/MS and direct infusion dual spray did not detect the impurities. Overall, the results demonstrated that bipolar dual spray can be incorporated into an LC/MS analysis and affords the ability to reduce the charge state distribution of PEG cations, decompress the m/z axis, lower spectra complexity, and enable/simplify data interpretation.

Surface Activity of Amines Provides Evidence for the Combined ESI Mechanism of Charge Reduction for Protein Complexes

Charge reduction reactions are important for native mass spectrometry (nMS) because lower charge states help retain native-like conformations and preserve noncovalent interactions of protein complexes. While mechanisms of charge reduction reactions are not well understood, they are generally achieved through the addition of small molecules, such as polyamines, to traditional nMS buffers. Here, we present new evidence that surface-active, charge reducing reagents carry away excess charge from the droplet after being emitted due to Coulombic repulsion, thereby reducing the overall charge of the droplet. Furthermore, these processes are directly linked to two mechanisms for electrospray ionization, specifically the charge residue and ion evaporation models (CRM and IEM). Selected protein complexes were analyzed in solutions containing ammonium acetate and selected trialkylamines or diaminoalkanes of increasing alkyl chain lengths. Results show that amines with higher surface activity have increased propensities for promoting charge reduction of the protein ions. The electrospray ionization (ESI) emitter potential was also found to be a major contributing parameter to the prevalence of charge reduction; higher emitter potentials consistently coincided with lower average charge states among all protein complexes analyzed. These results offer experimental evidence for the mechanism of charge reduction in ESI and also provide insight into the final stages of the ESI and their impact on biological ions.

Mass spectrometry as a tool to advance polymer science

In contrast to natural polymers, which have existed for billions of years, the first well-understood synthetic polymers date back to just over one century ago. Nevertheless, this relatively short period has seen vast progress in synthetic polymer chemistry, which can now afford diverse macromolecules with varying structural complexities. To keep pace with this synthetic progress, there have been commensurate developments in analytical chemistry, where mass spectrometry has emerged as the pre-eminent technique for polymer analysis. This Perspective describes present challenges associated with the mass-spectrometric analysis of synthetic polymers, in particular the desorption, ionization and structural interrogation of high-molar-mass macromolecules, as well as strategies to lower spectral complexity. We critically evaluate recent advances in technology in the context of these challenges and suggest how to push the field beyond its current limitations. In this context, the increasingly important role of high-resolution mass spectrometry is emphasized because of its unrivalled ability to describe unique species within polymer ensembles, rather than to report the average properties of the ensemble.