Coupling Electron Capture Dissociation and the Modified Kendrick Mass Defect for Sequencing of a Poly(2-ethyl-2-oxazoline) Polymer

Mass spectrometry of polymers: A tutorial review

Ever since the inception of synthetic polymeric materials in the late 19th century, the number of studies on polymers as well as the complexity of their structures have only increased. The development and commercialization of new polymers with properties fine-tuned for specific technological, environmental, consumer, or biomedical applications requires powerful analytical techniques that permit the in-depth characterization of these materials. One such method with the ability to provide chemical composition and structure information with high sensitivity, selectivity, specificity, and speed is mass spectrometry (MS). This tutorial review presents and exemplifies the various MS techniques available for the elucidation of specific structural features in a synthetic polymer, including compositional complexity, primary structure, architecture, topology, and surface properties. Key to every MS analysis is sample conversion to gas-phase ions. This review describes the fundamentals of the most suitable ionization methods for synthetic materials and provides relevant sample preparation protocols. Most importantly, structural characterizations via one-step as well as hyphenated or multidimensional approaches are introduced and demonstrated with specific applications, including surface sensitive and imaging techniques. The aim of this tutorial review is to illustrate the capabilities of MS for the characterization of large, complex polymers and emphasize its potential as a powerful compositional and structural elucidation tool in polymer chemistry.

Stochasticity of poly(2-oxazoline) oligomer hydrolysis determined by tandem mass spectrometry

Understanding modification of synthetic polymer structures is necessary for their accurate synthesis and potential applications. In this contribution, a series of partially hydrolyzed poly(2-oxazoline) species were produced forming poly[(2-polyoxazoline)-co-(ethylenimine)] (P(EtOx-co-EI)) copolymers; EI being the hydrolyzed product of Ox. Bulk mass spectrometry (MS) measurements accurately measured the EI content. Tandem mass spectrometry analysis of the EI content in the copolymer samples determined the distribution of each monomer within the copolymer and corresponded to a theoretically modelled random distribution. The EI distribution across the polymers was shown to be effected by the choice of terminus, with a permanent hydrolysis event observed at an OH terminus. A neighbouring group effect wasn't observed at the polymer length analysed (approximately 25-mer species), suggesting that previously observed neighbouring group effects require a larger polymer chain. Although clearly useful for random polymer distribution this approach may be applied to many systems containing non-specific modifications to determine if they are directed or random locations across peptides, proteins, polymers, and nucleic acids.

Tandem mass spectrometry can be used to better understand modification sites of synthetic polymer structures providing more complete chemical knowledge which is necessary for their accurate synthesis and potential applications.

Characterization Across a Dispersity: Polymer Mass Spectrometry in the Second Dimension

Due to the natural dispersity that is present in synthetic polymers, an added complexity is always present in the analysis of polymeric species. Tandem mass spectrometry analysis requires the isolation of individual precursors before a fragmentation event to allow the unambiguous characterization of these species and is not viable at certain levels of complexity due to achievable isolation widths. Two-dimensional mass spectrometry (2DMS) fragments ions and correlates fragments with their corresponding precursors without the need for isolation. In this study, 2DMS electron capture dissociation (ECD) fragmentation of a polyoxazoline and polyacrylamide species was carried out, resulting in the analysis of byproducts and individual polymer species without the use of chromatographic techniques. This study shows that 2DMS ECD is a powerful tool for the analysis of polyacrylamide and polyoxazoline species and offers a new dimension in the characterization of polymers.

An arsenal of tools based on Kendrick mass defects to process congested electrospray ionization high-resolution mass spectra of polymers with multiple charging

RATIONALE

Electrospray ionization (ESI) favors the multiple charging of high molecular weight polymer samples and allows their high-resolution mass analysis in the low-mass range. It also induces the detection of numerous ion series at different charge states with different adducts complicating the interpretation of the mass spectrum which should be facilitated by an appropriate data processing.

METHODS

An arsenal of tools based on the Kendrick mass defect (KMD) is proposed to process congested ESI high-resolution mass spectra of poly(propylene oxide) (PPO) samples. The combination of regular, charge-dependent, and resolution-enhanced KMD plots in addition to a "remainders" plot and a new three-dimensional plot offers unrivaled capabilities of filtering for any minor series among thousands of points. The sequential data processing is conducted using Kendo, a spreadsheet developed in-house for an advanced KMD analysis.

RESULTS

The charge-state distribution is easily evaluated by counting the parallel lines in a regular KMD plot. A charge-dependent resolution-enhanced KMD plot instantly reveals the variation of adducted ions at a given charge state, helping the user to choose the best analytical conditions. Ion series at different charge states from PPO oligomers carrying different end-groups are also efficiently extracted using several combinations of KMD and remainders plots and assigned using a new simulator tool.

CONCLUSIONS

The innovative combination of existing and new KMD-related plots, selection tools, and simulator all combined in a single spreadsheet dramatically facilitates the processing and interpretation of complex ESI mass spectral data. The presented tools may be extended to any other class of homo-, co- and terpolymers.

The Kendrick analysis for polymer mass spectrometry

The mass spectrum of a polymer often displays repetitive patterns with peak series spaced by the repeating unit(s) of the polymeric backbones, sometimes complexified with different adducts, chain terminations, or charge states. Exploring the complex mass spectral data or filtering the unwanted signal is tedious whether performed manually or automatically. In contrast, the now 60-year-old Kendrick (mass defect) analysis, when adapted to polymer ions, produces visual two-dimensional maps with intuitive alignments of the repetitive patterns and favourable deconvolution of features overlaid in the one-dimensional mass spectrum. This special feature article reports on an up-to-date and theoretically sound use of Kendrick plots as a data processing tool. The approach requires no prior knowledge of the sample but offers promising dynamic capabilities for visualizing, filtering, and sometimes assigning congested mass spectra. Examples of applications of the approach to polymers are discussed throughout the text, but the same tools can be readily extended to other applications, including the analysis of polymers present as pollutants/contaminants, and to other analytes incorporating a repetitive moiety, for example, oils or lipids. In each of these instances, data processing can benefit from the application of an updated and interactive Kendrick analysis.