Multimodal mass spectrometry - Raman imaging reveals solvent dependent structural and chemical tissue alterations

Mass spectrometry imaging (MSI) and Raman spectroscopy imaging are two chemical imaging techniques that can reveal and visualize the chemical distributions in thin tissue sections. The two techniques are complementary with Raman spectroscopy detecting the Raman shift of molecular bonds while MSI detects mass-to-charge of ionized molecules. Furthermore, MSI requires molecules to be desorbed, ablated or sputtered from the surface, making Raman spectroscopy less destructive in comparison. With a multimodal analysis workflow, the advantages of both techniques can be combined to yield enhanced chemical information of surfaces, such as thin tissue sections. Here, we combine the two imaging modalities and map the chemical signature in the exact same location on thin tissue section to uncover MSI derived chemical alterations in tissue.

In this study, we present a multimodal workflow integrating pneumatically assisted nanospray desorption electrospray ionization (PA nano-DESI) MSI and Raman spectroscopy on the same tissue section. This requires some adaptation of MSI to the sample restrictions of Raman. We demonstrate successful integration and the power of combined analysis in characterization of tissue chemistry. Raman spectroscopy proved highly effective for identifying solvent-induced chemical alterations in tissue caused by PA nano-DESI MSI with methanol or acetonitrile-based solvents. First, we describe chemical alterations connected to different sampling modes, such as different probe speeds and oversampling. Following, we characterize chemical changes in the resulting tissue exposed to up to five repeated imaging experiments. The results from the two modalities were in excellent agreement and showed that methanol desorbs more material from the tissue compared to acetonitrile, and that both solvents cause protein denaturation in the tissue.

This study presents the establishment of a multimodal workflow combining imaging with PA nano-DESI MSI and Raman spectroscopy from the exact same tissue sections and locations. The combination enabled chemical characterization of the tissue after MSI and revealed previously unknown alterations in tissue chemistry upon MSI. Overall, our findings offer new insights into how exposure to solvents impact tissue chemistry and highlight the potential of combining MSI with Raman spectroscopy for in depth chemical characterization of tissue sections.