Matrix assisted laser desorption/ionization mass spectrometry (MALDI MS) and the corresponding visualization technique MALDI MS Imaging (MSI) have emerged as important analytical tools in biochemical sciences, e.g., for drug development or to trace the metabolomic changes in cancerous tissues. Initially developed for the detection of high molecular weight compounds (HMWC; M > 1000 Da), in recent years the reliable and reproducible detection of low molecular weight compounds (LMWC; M < 1000 Da) has gained high attention, e.g., in the research fields of metabolomics and lipidomics. By using a protective matrix, the MALDI technique is capable of soft ionization of analytes to prevent their fragmentation or degradation. This matrix is responsible for the spatial separation of the analyte molecules, their protection from the strong laser shots, and their ionization. Commonly used matrices are small organic matrices (SOMs; M < 500 Da), which are utilized in HMWC analytics and recently also in LMWC analytics since they show sufficient absorption of the laser radiation, high crystallinity, and good ionization efficiency. However, their utilization can cause several drawbacks: (i) High background interferences below m/z = 1000 (not MALDI silent), which is disadvantageous specifically for LMWC analytics; (ii) low vacuum stability, which is especially problematic for standard instruments operated under high vacuum (HV); (iii) challenging homogeneous thin-layer coating, potentially causing inconsistent measurement conditions; and (iv) usually no suitability for dual polarity mode experiments, i.e., carrying out positive and negative mode measurements with the same matrix.
Polymeric materials are promising candidates for MALDI silent matrices, as the large variety of possible molecular layouts potentially allows to meet all prerequisites of a MALDI matrix: (a) Sufficient ultra-violet (UV) laser radiation absorption, implemented by introducing conjugated π-electron systems in the polymer backbone or side chains; (b) high ionization efficiency, enhanced by adding acidic and/or basic functional groups to the polymer’s molecular structure, potentially also allowing dual polarity mode measurements; (c) MALDI silence, enabled by the high molar mass of the polymer chains; (d) high vacuum stability, also granted by the polymer’s molar mass; and (e) homogeneous thin-films, achieved by multiple available coating methods. Yet, despite their high potential only a handful of polymeric matrices were reported in literature and so far, investigations to develop conscious design strategies are missing.
The target of this thesis is to contribute to the field of MALDI silent matrices by developing and investigating different polymers as macromolecular MALDI MS and MSI matrices for LMWC analytics. Therefore, two different strategies were explored: (i) Investigating conjugated polymers, and (ii) polymerizing SOMs. For the first strategy, five conjugated polymers were tested as MALDI matrices for the detection of various LMWCs. Among these, four were found to be excellent matrices, with sufficient ionization efficiencies and rare dual polarity mode suitability and allowed LMWC detection with low background interferences (MALDI silent). A high crystallinity of the matrix (SOM) is reported to be crucial to ensure successful measurements, yet conjugated polymer matrices (CPMs) are semi-crystalline, i.e., they contain crystalline and amorphous domains. Hence, the analytes are expected to be incorporated in the crystalline domains of the CPMs, depending on their degree of crystallization. Therefore, two amorphous CPMs were synthesized and tested, showing similar matrix performances (e.g., ionization efficiencies, dual polarity mode, MALDI silence) as a structurally related semi-crystalline CPM. This indicates that the analytes are incorporated in the amorphous parts of the CPM. The second strategy towards polymeric matrices (PMs) is the polymerization of standard SOMs. As the matrix performance of the corresponding SOMs is known, the performance of the respective polymerized SOMs (P(SOMs)) can be validated against this benchmark. At the same time, polymerization can induce the properties needed to enable efficient LMWC analytics. Therefore, two standard SOMs were modified and polymerized, resulting in P(SOMs), which were vacuum stable and MALDI silent, and showed similar optical properties, analyte scopes and ionization efficiencies in benchmark tests with their respective SOMs.
For the fast and facile comparison of the matrix performances of PMs and standard matrices, the graphing software OriginPro was used to visualize, process, and evaluate the acquired mass spectra. To automatize these tasks, a script was programmed using the OriginPro-native programming languages LabTalk and OriginC: X Functions.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:76850 |
| Date | 01 December 2021 |
| Creators | Horatz, Kilian |
| Contributors | Lissel, Franziska, Voit, Brigitte, Michels, Dominik L., Technische Universität Dresden, Leibniz Institut für Polymerforschung Dresden e.V. |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | 10.1021/jacs.8b06637, 10.1002/cplu.201900203, 10.1021/acsapm.1c00665 |
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