MALDI-MS for polymer characterization – Recent developments and future prospects

Applications of MALDI mass spectrometry in forensic science

Forensic chemistry literature has grown exponentially, with many analytical techniques being used to provide valuable information to help solve criminal cases. Among them, matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), particularly MALDI MS imaging (MALDI MSI), has shown much potential in forensic applications. Due to its high specificity, MALDI MSI can analyze a wide variety of compounds in complex samples without extensive sample preparation, providing chemical profiles and spatial distributions of given analyte(s). This review introduces MALDI MS(I) to forensic scientists with a focus on its basic principles and the applications of MALDI MS(I) to the analysis of fingerprints, drugs of abuse, and their metabolites in hair, medicine samples, animal tissues, and inks in documents.

Identification and quantification of polymeric impurity in block copolymer by one-dimensional and two-dimensional liquid chromatography coupled to high-resolution mass spectrometry and evaporative light scattering detector

Identifying and quantifying polymeric impurities in a polymeric material is critical for understanding material quality and performance, but it remains a challenge requiring developing new characterization methods. In this work, a comprehensive two-dimensional liquid chromatography method with simultaneous evaporative light scattering and high-resolution mass spectrometry detection was developed to separate and identify a polymeric impurity in alkyl alcohol-initiated polyethylene oxide/polybutylene oxide diblock copolymer. Size exclusion chromatography was implemented in the first dimension, and gradient reversed-phase liquid chromatography using a large-pore C4 column was applied in the second dimension using an active solvent modulation valve as the interface to minimize the polymer breakthrough. The two-dimensional separation significantly reduced the complexity of the mass spectra data compared to the one-dimensional separation, and the combination of retention time and mass spectral interpretation led to the successful identification of the water-initiated triblock copolymer impurity. This identification was confirmed by comparison with the synthesized triblock copolymer reference material. A one-dimensional LC method with evaporative light scattering detection was employed to quantify the triblock impurity. The impurity level in three samples made with the different processes was determined to be in the range of 9-18 wt% using the triblock reference material as the standard.

A Series of Green Oxovanadium(IV) Precatalysts with O, N and S Donor Ligands in a Sustainable Olefins Oligomerization Process

Designing catalyst systems based on transition metal ions and activators using the principles of green chemistry is a fundamental research goal of scientists due to the reduction of poisonous solvents, metal salts and organic ligands released into the environment. Urgent measures to reduce climate change are in line with the goals of sustainable development and the new restrictive laws ordained by the European Union. In this report, we attempted to use known oxovanadium(IV) green complex compounds with O, N and S donor ligands, i.e., [VO(TDA)phen] • 1.5 H₂O (TDA = thiodiacetate), (phen = 1,10-phenanthroline), oxovanadium(IV) microclusters with 2-phenylpyridine (oxovanadium(IV) cage), [VOO(dipic)(2-phepyH)] • H₂O (dipic = pyridine-2,6-dicarboxylate anion), (2-phepyH = 2-phenylpyridine), [VO(dipic)(dmbipy)] • 2H₂O (dmbipy = 4,4'-dimethoxy-2,2'-dipyridyl) and [VO(ODA)(bipy)] • 2 H₂O (ODA = oxydiacetate), (bipy = 2,2'-bipyridine), as precatalysts in oligomerization reactions of 3-buten-2-ol, 2-propen-1-ol, 2-chloro-2-propen-1-ol and 2,3-dibromo-2-propen-1-ol. The precatalysts, in most cases, turned out to be highly active because the catalytic activity exceeded 1000 g mmol^-1·h^-1. In addition, the oligomers were characterized by Fourier-transform infrared spectroscopy (FTIR), matrix-assisted laser desorption/ionization (MALDI-TOF-MS), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques.

Three-Dimensional Mass Spectral Imaging of Polymers Via Laser-Assisted Micro-Pyrolysis Program with Flowing Atmospheric-Pressure Afterglow Ambient Mass Spectrometry

Herein, a novel diode laser-assisted micro-pyrolysis program (LAMP) technique is demonstrated and coupled with flowing atmospheric-pressure afterglow ambient mass spectrometry for instantaneously profiling polymers and polymer additives. Laser power modulation allows thermal separation of additives and different pyrolysis products, as shown through positive- and negative-mode high-resolution mass spectra and Kendrick mass defect plots of homopolymers, copolymers, polymer blends, and complex polymer samples. LAMP allows much faster temperature control through real-time duty cycle changes and gives significantly better spatial confinement compared to typical resistive heating pyrolysis approaches. Finally, MS imaging, with lateral and depth resolution, is demonstrated for a complex polymer pressure-sensitive adhesive tape sample.

High-performance miniature linear time-of-flight mass spectrometry as an advantageous tool in a high mass-to-charge range

A newly developed miniature MALDI-TOF mass spectrometer provides superior performance in a high m/z range.

C60-based chemical labeling strategy for the determination of polyamines in biological samples using matrix-assisted laser desorption/ionization mass spectrometry

Bioactive polyamines play important roles in many biological processes such as gene expression, cell growth, protein synthesis, and signal transduction. Accurate determination of polyamines is helpful for studying their biological functions. Herein, a C₆₀-based chemical labeling strategy was proposed for the determination of polyamines (putrescine, cadaverine, spermidine, and spermine) in biological samples using matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). An N-hydroxysuccinimide ester functionalized C₆₀ (NHS-C₆₀) was used as a labeling reagent and the m/z of the labeled polyamines reached up to more than 900 Da, which avoided matrix interferences in the low m/z region. In addition, as NHS-C₆₀ derivatives, mono- and bis-substituted polyamines were produced simultaneously, which benefited the qualitative analysis of polyamines. The analytical method was validated using NHS-C₆₀ labeled polyamines in cells and mice feces samples. Good linearities were obtained with correlation coefficients ranging from 0.9786 to 0.9982. The limits of quantification were in the range of 0.68-1.48 pmol. Good reproducibility and reliability of our proposed method were confirmed by intra- and inter-day precisions ranged from 2.8 to 16.6%, and the recoveries ranged between 81.8 and 119.9%. Finally, the proposed method was applied to determine polyamines in cells and mice feces. Three polyamines were detected in the cells, and the contents of cadaverine and spermidine in the feces of high-fat diet mice were found to be significantly lower than those in the normal diet mice. The results show that the proposed NHS-C₆₀ labeling coupled with MALDI MS strategy is suitable for the determination of polyamines in biological samples.

Structural elucidation and osteogenic activity of a novel heteropolysaccharide from Alhagi pseudalhagi

Alhagi pseudalhagi, commonly known as camel thorn, is used as an indigenous medicinal plant in China. The present study was designed to elucidate the structure of a novel polysaccharide, APP90-2, isolated from Alhagi pseudalhagi and evaluate its osteogenic activity. A homogeneous polysaccharide (APP90-2) was obtained from A. pseudalhagi via DEAE-52 and Sephacryl S-100 columns, with a molecular weight of 5.9 kDa. Monosaccharide, GC-MS, and NMR analyses showed that APP90-2 consisted of α-l-Rhap-(1→, →3)-α-l-Araf-(1→, →5)-α-l-Araf-(1→, →4)-β-d-Xylp-(1→, α-d-Glcp-(1→, →3,5)-α-l-Araf-(1→, →4)-β-d-GlcAp-(1→, →4)-3-OAc-α-d-Glcp-(1→, →3)-α-d-Galp-(1→, →3)-β-d-GalAp-(1→, →4)-α-d-Galp-(1→, →6)-α-d-Manp-(1→, →4,6)-β-d-Galp-(1→, and →3,6)-β-d-Glcp-(1→ with relative molar ratios of 4.1:1.8:6.1:6.7:1.7:1.0:1.5:2.7:2.4:1.1:2.3:2.6:1.4:2.0. Morphological analyses revealed that APP90-2 interacted with Congo-red and had an obvious honeycomb structure. Additionally, APP90-2 significantly promoted proliferation, differentiation, and mineralization of MC3T3-E1 cells, indicating that APP90-2 exhibited pronounced osteogenic activity. Therefore, our findings suggest that A. pseudalhagi may be used as an alternative medicine or health supplement for the prevention and treatment of osteoporosis.

Recent developments in pre-treatment and analytical techniques for synthetic polymers by MALDI-TOF mass spectrometry

A great deal of effort has been expended to develop accurate means of determining the properties of synthetic polymers using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). Many studies have focused on the importance of sample pre-treatment to obtain accurate analysis results. This review discusses the history of synthetic polymer characterization and highlights several applications of MALDI-TOF MS that recognize the importance of pre-treatment technologies. The subject area is of significance in the field of analytical chemistry, especially for users of the MALDI technique. Since the 2000s, many such technologies have been developed that feature improved methods and conditions, including solvent-free systems. In addition, the recent diversification of matrix types and the development of carbon-based matrix materials are described herein together with the current status and future directions of MALDI-TOF MS hardware and software development. We provide a summary of processes used for obtaining the best analytical results with synthetic polymeric materials using MALDI-TOF MS.

Characterization of Polyethylene Branching by Thermal Analysis-Photoionization Mass Spectrometry

The rising demand for more and more specialized polyethylene represents a challenge for synthesis and analysis. The desired properties are dependent on the structure, but its elucidation is still intricate. For this purpose, we applied thermal analysis hyphenated to single photon ionization mass spectrometry (STA-SPI-MS). The melting and pyrolysis behavior of different types of polyethylene were tracked by DSC and mass loss. Crystallinity and melting point give hints about the branching but are also influenced by the molecular weight distribution. The evolving gas analysis patterns obtained by SPI-MS however, contain specific molecular information about the samples. Shifts in the summed spectra, which can be clearly observed with our technique, result from differently favored degradation reactions due to the respective structure. Pyrolysis gas chromatography mass spectrometry (Py-GC-EI-MS) was used to support the assignment of pyrolysis products. Principal component analysis was successfully applied to reduce the complexity of data and find suitable markers. The obtained grouping is based on the molecular fingerprint of the samples and is strongly influenced by short-chain branching. Short and medium alkenes and dienes have the strongest impact on the first four principal components. Thus, two marker ratios could be defined, which also give a comprehensible and robust grouping. Butene and pentene were the most abundant signals in our set of samples. With STA-PI-MS, a broad range of pyrolysis products can be measured at the same time, possibly extending the range for quantifiable short-chain branches to more than six carbon atoms for PE. Unfortunately, no clear trend between long-chain branching and any grouping was observed. The quite universal and soft single photon ionization enables access to many different compound classes and hence other polymers can be studied.

New chromium(III)-based catalysts for ethylene oligomerization

The report focuses on the new precatalysts for ethylene oligomerization. The five chromium(III) complex compounds containing the following ligands: dipicolinate anion, oxalate anion, 5-aminopyridine-2-carboxylate anion, 2,2'-bipyridine and 4,4'-dimethoxy-2,2'-bipyridine have been examined towards catalytic activity for ethylene oligomerization. The chromium(III) complexes have been activated by modified methylaluminoxane. The obtained oligomers have been investigated by MALDI-TOF-MS, thermal analysis and infrared spectroscopy. The results revealed that the examined chromium(III) complexes are highly active catalysts for ethylene oligomerization. The values of catalytic activities of the examined complexes are in the range 1860 - 3798 g∙mmol^-1∙h^-1∙bar^-1.

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.

Mechanistic Studies on Cationization in MALDI-MS Employing a Split Sample Plate Set-up

In the analysis of polymers by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), a commonly observed ionization pathway is cation-adduct formation, as polymers often lack easily ionizable (basic/acidic) functional groups. The mechanism of this process has been hypothesized to involve gas-phase cation attachment. In previous experiments, a split sample plate set-up has been introduced, enabling separate deposition of the components on individual MALDI plates. The plates are divided by a small gap of a few micrometers, allowing simultaneous laser irradiation from both plates, while precluding the possibility of any other interactions prior to ablation. Here, we extend on these studies by using different polymer-salt combinations to test the generalizability of a gas-phase ionization process. Clear evidence for in-plume ionization is presented for the model polymers poly (methyl methacrylate) and polystyrene. Furthermore, the contribution of in-plume processes to the overall ion formation by cationization is gauged, providing a first estimate for the importance of this pathway.