An overview of Surface-Assisted Laser Desorption/Ionization mass spectrometry and its application to the biological research area

In recent years, there has been a growing interest in the study of small molecules (e.g. amino acids, secondary metabolites, lipids, drugs) as they play crucial roles in many research areas, including biomedicine, biotechnology, environmental science, food safety, clinical diagnosis and drug discovery. Mass spectrometry (MS) has become a predominant technology to analyze these low-molecular weight compounds. In particular, mass spectrometry imaging (MSI) has brought additional highly valuable information as, besides the detection and the identification of various species provided by conventional MS, MSI also allows the visualization of the spatial distribution of the analytes in (generally complex) samples, such as biological tissue sections, microbial colonies and cells. However, the study of small molecules by the established matrix-assisted laser desorption/ionization (MALDI) MS technique can be challenging because of the presence of matrix ions and clusters in the low m/z range (< 500 Da), which might interfere with the signals of low-molecular weight analytes. Therefore, as analytical procedures for the analysis of small molecules are highly demanded, new innovating MS techniques that do not require an organic matrix have been developed. These include surface-assisted laser desorption/ionization (SALDI) MS, which relies on nanostructured substrates (which can be colloidal nanoparticles, sputtered metal nanoclusters, or nanostructured solid substrates including for example porous silicon and nanopillar arrays) to promote the desorption and ionization processes. SALDI nanosubstrates offer many advantages over organic matrices, such as a low chemical background in the low m/z range, a higher signal reproducibility, non-volatility (they are thus stable in vacuum conditions for MSI experiments), and the nanosubstrates are usually effective in both ionization modes, which is an undeniable benefit for the study of molecular species that are preferentially ionized either in the positive or negative ionization mode (e.g. lipids and amino acids). Moreover, SALDI nanosubstrates give access to new sample preparation procedures, including blotting and functionalization of the nanosubstrate surface to enhance the analysis selectivity and sensitivity. Here, we show how we can take advantage of the specificities of the SALDI-MS technique to analyze small molecules in biological samples. Examples include the imaging of the secondary metabolites excreted in agar-based bacterial co-cultures with a new blotting sample preparation involving a DIUTHAME membrane, and the dual-polarity imaging of the lipids present in a mouse brain section with gold nanoparticles. These examples demonstrate the potential of the SALDI-MSI technique to provide unequaled insights to address the modern challenges faced in the biological research area. References Müller, W. H., Verdin, A., De Pauw, E., Malherbe, C., & Eppe, G. (2022). Surface‐assisted laser desorption/ionization mass spectrometry imaging: A review. Mass Spectrometry Reviews, 41(3), 373-420. Müller, W. H., De Pauw, E., Far, J., Malherbe, C., & Eppe, G. (2021). Imaging lipids in biological samples with surface-assisted laser desorption/ionization mass spectrometry: A concise review of the last decade. Progress in Lipid Research, 83, 101114. Müller, W. H., McCann, A., Arias, A. A., Malherbe, C., Quinton, L., De Pauw, E., & Eppe, G. (2022). Imaging Metabolites in Agar‐Based Bacterial Co‐Cultures with Minimal Sample Preparation using a DIUTHAME Membrane in Surface‐Assisted Laser Desorption/Ionization Mass Spectrometry. ChemistrySelect, 7(18), e202200734. Müller, W. H., Verdin, A., Kune, C., Far, J., De Pauw, E., Malherbe, C., & Eppe, G. (2021). Dual-polarity SALDI FT-ICR MS imaging and Kendrick mass defect data filtering for lipid analysis. Analytical and Bioanalytical Chemistry, 413(10), 2821-2830