Ion mobility spectrometry - Mass spectrometry coupling for synthetic polymers

Using ion mobility spectrometry to understand signal dilution during tandem mass spectrometry sequencing of digital polymers: Experimental evidence of intramolecular cyclization

RATIONALE

Optimizing the structure of digital polymers is an efficient strategy to ensure their tandem mass spectrometry (MS/MS) readability. In block-truncated poly(phosphodiester)s, homolysis of C-ON bonds in long chains permits the release of smaller blocks amenable to sequencing. Yet the dissociation behavior of diradical blocks was observed to strongly depend on their charge state.

METHODS

Polymers were ionized in negative mode electrospray and activated in-source so that blocks released as primary fragments can be investigated using ion mobility spectrometry (IMS) or sequenced in the post-IMS collision cell. Collision cross sections (CCS) were derived from arrival times using a calibration procedure developed for polyanions using the IMSCal software. A multistep protocol based on quantum methods and classical molecular dynamics was implemented for molecular modeling and calculation of theoretical CCS.

RESULTS

Unlike their triply charged homologues, dissociation of diradical blocks at the 2- charge state produces additional fragments, with +1 m/z shift for those holding the nitroxide α-termination and -1 m/z for those containing the carbon-centered radical ω-end. These results suggest cyclization of these diradical species, followed by H• transfer on activated reopening of this cycle. This assumption was validated using IMS resolution of the cyclic/linear isomers and supported by molecular modeling.

CONCLUSIONS

Combining IMS with molecular modeling provided new insights into how the charge state of digital blocks influences their dissociation. These results permit to define new guidelines to improve either ionization conditions or the structural design of these digital polymers for best MS/MS readability.

Elucidation of Dithiol-yne Comb Polymer Architectures by Tandem Mass Spectrometry and Ion Mobility Techniques

Polymers have a wide range of applications depending on their composition, size, and architecture. Varying any of these three characteristics can greatly impact the resulting chemical, physical, and mechanical properties. While many techniques are available to determine polymer composition and size, determining the exact polymer architecture is more challenging. Herein, tandem mass spectrometry (MS/MS) and ion mobility mass spectrometry (IM-MS) methods are utilized to derive crucial architectural information about dithiol-yne comb polymers. Based on their unique fragmentation products and IM drift times, dithiol-yne oligomers with distinct architectures were successfully differentiated and characterized. Additionally, experimental collision cross-sections (Ω) derived via IM-MS were compared to theoretically extracted Ω values from molecular dynamics simulated structures to deduce the architectural motif of these comb oligomers. Overall, this work demonstrates the benefits of combining various mass spectrometry techniques in order to gain a complete understanding of a complex polymer mixture.

Investigations into the characterization, degradation, and applications of biodegradable polymers by mass spectrometry

Biodegradable polymers have been getting more and more attention because of their contribution to the plastic pollution environmental issues and to move towards a circular economy. Nevertheless, biodegradable materials still exhibit various disadvantages restraining a widespread use in the market. Therefore, additional research efforts are required to improve their performance. Mass spectrometry (MS) affords a relevant contribution to optimize biodegradable polymer synthesis, to confirm macromolecular structures, to examine along the time the progress of degradation processes and highlight advantages and drawbacks in the extensive applications. This review aims to provide an overview of the MS investigations carried out to support the synthesis of biodegradable polymers, with helpful information on undesirable products or polymerization mechanism, to understand deterioration pathways by the structure of degradation products and to follow drug release and pharmacokinetic. Additionally, it summarizes MS studies addressed on environmental and health issues related to the extensive use of plastic materials, that is, potential migration of additives or microplastics identification and quantification. The paper is focused on the most significant studies relating to synthetic and microbial biodegradable polymers published in the last 15 years, not including agro-polymers such as proteins and polysaccharides.

End group analysis of poly(methylmethacrylate)s using the most abundant peak in electrospray ionization-ion mobility spectrometry-tandem mass spectrometry and Fourier transform-based noise filtering

RATIONALE

We recently developed the characterization method for synthetic polymers weighing more than a few tens of kilodalton using electrospray ionization-ion mobility spectrometry-tandem mass spectrometry, in which the m/z value of the most abundant peak was used for characterization. However, the identification of the most abundant peak from the isotopic peaks was often difficult due to the background noise.

METHODS

Here, we employed a noise reduction method using Fourier transform (FT) filtering. In the power spectrum obtained using FT of the mass spectrum of the multiple charged analytes, the significant signals in the low-frequency region and at frequency z are observed for the analytes of z charges. When the signals in both regions were used for inversed FT (i.e., the signals in other regions were zero padded), a noise-filtered mass spectrum was obtained.

RESULTS

In the analysis of poly(methylmethacrylate)s weighing 13-17 kDa, mass spectra without noise filtering with relatively high-intensity noise (than signal) were complicated to identify the most abundant peak. On the contrary, the most abundant peak was clearly identified from the mass spectra after FT-based noise filtering, and end group composition was estimated successfully.

CONCLUSIONS

The proposed FT-based noise filtering for the mass spectrum is effective to characterize multiply charged synthetic polymers weighing more than a few tens of kilodalton using electrospray ionization-ion mobility spectrometry-tandem mass spectrometry.

Probing Polyester Branching by Hybrid Trapped Ion-Mobility Spectrometry-Tandem Mass Spectrometry

Trapped ion-mobility spectrometry combined with quadrupole time-of-flight mass spectrometry (TIMS-QTOFMS) was evaluated as a tool for resolving linear and branched isomeric polyester oligomers. Solutions of polyester samples were infused directly into the ion source employing electrospray ionization (ESI). TIMS-MS provides both mobility and m/z data on the formed ions, allowing construction of extracted-ion mobilograms (EIMs). EIMs of polyester molecules showed multimodal patterns, indicating conformational differences among isomers. Subsequent TIMS-MS/MS experiments indicated mobility differences to be caused by (degree of) branching. These assignments were supported by liquid chromatography-TIMS-MS/MS analysis, confirming that direct TIMS-MS provided fast (500 ms/scan) distinction between linear and branched small oligomers. Observing larger oligomers (up to 3000 Da) using TIMS required additional molecular charging to ensure ion entrapment within the mobility window. Molecular supercharging was achieved using m-nitrobenzyl alcohol (NBA). The additional charges on the oligomer structures enhanced mobility separation of isomeric species but also added to the complexity of the obtained fragmentation mass spectra. This complexity could be partly reduced by post-TIMS analyte-decharging applying collision-induced dissociation (CID) prior to Q1 with subsequent isolation of the singly charged ions for further fragmentation. The as-obtained EIM profiles were still quite complex as larger molecules possess more possible structural isomers. Nevertheless, distinguishing between linear and symmetrically branched oligomers was possible based on measured differences in collisional cross sections (CCSs). The established TIMS-QTOFMS approach reliably allows branching information on isomeric polyester molecules up to 3000 Da to be obtained in less than 1 min analysis time.

Molecular Correlative Material Characterization: Advantages for Polymer Analysis Using Liquid Chromatography

This article discusses innovation around the molecular structure of polymeric materials—an indispensable part of modern society—with a specific focus on sustainability. This field of research, so-called molecular correlative material characterization (MCMC), will enhance the transition into new sustainable functional copolymers.

Using Ion Mobility-Mass Spectrometry to Extract Physicochemical Enthalpic and Entropic Contributions from Synthetic Polymers

Ion mobility-mass spectrometry (IM-MS) experiments are mostly used hand in hand with computational chemistry to correlate mobility measurements to the shape of the ions. Recently, we developed an automatable method to fit IM data obtained with synthetic homopolymers (i.e., collision cross sections; CCS) without resorting to computational chemistry. Here, we further develop the experimental IM data interpretation to explore physicochemical properties of a series of nine polymers and their monomer units by monitoring the relationship between the CCS and the degree of polymerization (DP). Several remarkable points of the CCS evolutions as a function of the DP were found: the first observed DP of each charge state (ΔDP_{first DP}), the DPs constituting the structural rearrangements (ΔDP_rearr), and the DPs at the half-rearrangement (DP_half-rearr). Given that these remarkable points do not rely on absolute CCS values, but on their relative evolution, they can be extracted from CCS or raw IM data without accurate IM calibration. Properties such as coordination numbers of the cations, steric hindrance, or side chain flexibility can be compared. This leads to fit parameter predictions based on the nature of the monomer unit. The interpretation of the fit parameters, extracted using solely experimental data, allows a rapid screening of the properties of the polymers.

Assessing the Structural Heterogeneity of Isomeric Homo and Copolymers: an Approach Combining Ion Mobility Mass Spectrometry and Molecular Dynamics Simulations

Synthetic polymers occupy a unique place in the field of ion mobility mass spectrometry (IMS-MS). Indeed, due to their intrinsic dispersity, they have the asset to offer a broad range of homologous ions with different lengths that can be detected in several charge states. In addition, the gas-phase structure of polymer ions mostly depends on their ability to screen the adducted charges. Several works dealing with linear, cyclic, and star-shaped polymers have already shown that the gas-phase structure of polymer ions heavily relies on the polymer architecture, i.e., the primary structure. In the present work, we move a step further by evaluating whether a relationship exists between the primary and secondary structures of synthetic homo and copolymers. The IMS-MS experiments will be further complemented by MD simulations. To highlight the effectiveness of IMS separation, we selected isomeric homo and copolymers made of lactide (LA) and propiolactone (PL) units. In this way, the mass analysis becomes useless since isomeric comonomer sequences can coexist for any given chain length. An UPLC method was implemented in the workflow to successfully separate all PL-LA comonomer sequences before infusion in the IMS-MS instrument. The analysis of doubly charged copolymers showed that the comonomer sequence has an impact on the IMS response. However, this only holds for copolymer ions with precise sizes and charge states, and this is therefore not a rule of thumb.

Detection challenges in quantitative polymer analysis by liquid chromatography

Accurate quantification of polymer distributions is one of the main challenges in polymer analysis by liquid chromatography. The response of contemporary detectors is typically influenced by compositional features such as molecular weight, chain composition, end groups, and branching. This renders the accurate quantification of complex polymers of which there are no standards available, extremely challenging. Moreover, any (programmed) change in mobile-phase composition may further limit the applicability of detection techniques. Current methods often rely on refractive index detection, which is not accurate when dealing with complex samples as the refractive-index increment is often unknown. We review current and emerging detection methods in liquid chromatography with the aim of identifying detectors, which can be applied to the quantitative analysis of complex polymers.

Can IM-MS Collision Cross Sections of Biomolecules Be Rationalized Using Collision Cross-Section Trends of Polydisperse Synthetic Homopolymers?

In the past, we developed a method inferring physicochemical properties from ion mobility mass spectrometry (IM-MS) data from polydisperse synthetic homopolymers. We extend here the method to biomolecules that are generally monodisperse. Similarities in the IM-MS behavior were illustrated on proteins and peptides. This allows one to identify ionic species for which intramolecular interactions lead to specific structures.