MALDI matrix sublimation for sensitivity enhancement in molecular SIMS

Mass spectrometry imaging (MSI) becomes a powerful tool to characterize the spatial distribution of biomolecules in thin tissues. The brain is frequently investigated because there is an interest in the identification of lipid-mediated signaling processes in health and disease. For biological applications such as disease diagnostics, drug screening or metabolomics, it is essential to be able to increase the extraction of intact molecular ions from a sample. Indeed, sensitivity to molecular ions remain a limiting factor for high resolution imaging of organic and biological materials. In this work, sublimation to deposit matrix as a surface modification method has been investigated to enhance sensitivity of secondary ion mass spectrometry (SIMS) for large molecules. A sublimation device has been built to sublimate a thin layer of two conventional matrices used in matrix-assisted laser ionization mass spectrometry (MALDI MS): $\alpha$-Cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB). Then, two biomolecular models have been studied: (i) spin-coated cardiolipin, a heavy lipid; (ii) spin-coated amino acids, smaller molecules with well-known properties. For cardiolipin, a ionization enhancement of about 10 times has been obtained for both matrices. For amino acids, smaller molecules, ionization enhancements have also been shown. In addition, the results with this model indicate that the ionization process can not be only explained by gas phase basicity as in MALDI. Finally, depth profiling has been used to study the distribution and intensities of these analytes within the matrix layer. Moreover, differences between the two matrices are also highlighted. After that, sections of brain tissue have been investigated with and without matrix. Without matrix, several intact lipids have been detected in positive and negative modes, but the intensity is often low excepted for cholesterol and fatty acids producing higher intensities due to their lower mass. With matrix, the signal intensity is definitely increased in positive mode for all lipids. Depth profiling and imaging have been performed. These results are discussed in regards to the potential ability of the matrix to improve the resolution. Finally, a variant of matrix-enhanced SIMS (ME-SIMS) in which the transfer of matrix molecules to the sample is conducted \textit{in situ} (Transferred matrix SIMS or TM-SIMS) has been studied. This new approach can be compatible in the future with 3D imaging and frozen samples. The first results of this method are presented in this work and are promising.