Apparatus And Applications Of Vacuum Ultraviolet Laser Desorption/Ionization Mass Spectrometry Imaging

Mass spectrometry imaging（MSI）has important applications in material research,biology and medicine.Mass spectrometry images allow the acquisition of the molecular composition and distribution of the sample to further investigate the physical,biological and chemical phenomena.The commonly used MSI methods are secondary ion mass spectrometry（SIMS）and the matrix-assisted laser desorption ionization（MALDI）method.The SIMS-MSI method can detect molecular fragments（m/z<1000）with submicron lateral resolution.Although the MALDI technique produces less molecular fragmentation than the SIMS method,its lateral resolution is usually worse than 10μm due to the matrix effect.As a result,new MSI techniques that can offer the advantages of both SIMS（submicron spatial resolution）and MALDI（less molecular fragmentation）are required.After working on this project for more than four years,we have built a new instrument of vacuum ultraviolet laser desorption/ionization mass spectrometry imaging（VUVDI-MSI）with submicron spatial resolution.We have generated high intensity VUV laser via four-wave mixing method.Based on VUV laser desorption/ionization,we have obtained many VUVDI-MS and VUVDI-MSI results of biological samples,e.g.,standards,tissues and single cells:（a）The VUV laser is prepared based on the four-wave mixing method using three collimated laser beams and a heated Hg cell.The spatial resolution of VUVDI-MSI is2×5μm².The new instrument was tested by the mass spectra of Nile red and FGB（Fibrinogen beta chain）samples and mass spectrometric images of a fly brain section.For the tested samples,the VUVDI method offers lower levels of molecular fragmentations and higher sensitivities than those of the UVLDI method and SIMS imaging method using a Bi₃⁺beam.（b）VUV laser desorption/ionization（VUVDI）method was used to measure the mass spectrum of the cholesterol standard,the mass spectrometry images（MSI）of parent cholesterols in a mouse esophagus tissue section and mouse zygotes.The VUVDI mass spectrum of the cholesterol standard shows dominant peaks from the parent molecules.In the case of the mouse esophagus tissue section,the ion images reveal that the cholesterols populate mainly in mucosae.The experimental results indicate that the VUVDI method may provide a new MSI tool to measure the intact cholesterols in tissues and cells.（c）We proposed a new MSI methodology of vacuum ultraviolet laser desorption/ionization（VUVDI）with 500 nm spatial resolution,demonstrating significant improvement on ion yield,fragmentation and image contrast compared with ToF-SIMS.Additionally,we demonstrated its powerful capabilities,in which fluorescence image and mass spectrum were obtained simultaneously,species in mixed system analyzed sequentially,MS/MS spectrum could be attained by tuning laser fluence and molecular signal was enhanced by using matrix.These profits suggest that VUVDI could be a potential submicron spatial resolution MSI method for analyzing the underlying molecular mechanisms of biological process in single cells.