Localized and analyte-specific normalization for imaging-MALDI-MS - towards quantitative imaging of lipids in tissue

• Applicant: Privatdozent Dr. Jens Soltwisch
• Subject Area: Analytical Chemistry
• Term: from 2018 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 400912714

MALDI-MSI has in recent years become a heavily used technique in imaging mass spectrometry with dedicated platforms now readily available. For the analysis of tissue samples, (phospho-)lipids are often the analyte of choice. Next to a high abundance and preferable ionization efficiencies, their vital role in the metabolism makes them a prime target of biomedical research. To unveil the role of a specific lipid species in the context of a specific tissue, it is important to correctly depict its distribution within the sample. In order to do so, a molecular identity has to be assigned to the determined m/z-value and the respective signal intensities have to be translated to quantities of material for each pixel. While sophisticated hard- and software solutions allow for the identification of lipid signals based on accurate mass, quantification in MALDI-MSI remains a difficult task. This is due to a number of factors inherent to sample preparation and the MALDI-process itself, that greatly influence ion yields in MALDI and that may vary for different regions. This greatly hampers a straightforward conversion of signal intensity to the underlying amount of material across the tissue. One important phenomenon in this regards is the ion suppression effect that arises from the competition for charge between different analytes during the MALDI-process. Especially apparent in the analysis of complex mixtures the presence of an easily ionizable analyte can significantly decrease the signal intensity of a second compound that is less susceptible for ionization. On top of that, physical and chemical properties of the tissue, but also the choice of matrix and preparation method influence the extraction of analyte into the applied matrix layer and therefore also the resulting signal intensity. The proposed project will thoroughly investigate and characterize both the ion suppression effect as well as the extraction of analyte from tissue under MALDI-MSI conditions. For that lipid extracts of specific tissue regions will be produced and quantitatively characterized with ESI-MS. This way, signal intensities in MALDI-MSI can be directly compared to the underlying amounts of substance. In a second approach, artificial tissue with well-defined lipid content can be constructed and examined. These highly simplified samples allow for the systematic and quantitative analysis of the ion suppression effect between different lipid classes. Building on the broad basis of collected data we will develop individualized normalization strategies. These will include the use of external non-endogenous lipid standards suitable to normalize for ion suppression effects as well as location and analyte specific correction factors accounting for the extraction from the tissue. As a result we hope to deliver a robust and practical method that enables the direct translation of MALDI-MSI data to the visualization of relative amounts of material within the analyzed tissue.

• DFG Programme: Research Grants