Application Of Compound Identification And Imaging Of Complicated Samples Based On Laser Desorption Laser Postionization Time-of-Flight Mass Spectrometry

Laser desorption laser postionization time-of-flight mass spectrometry(LDPI-TOFMS),as an emerging technology for molecular detection and imaging,stands out in terms of detection sensitivity,spatial resolution and quantitative analysis,and is becoming more and more attractive.In recent years,many attempts have been made to make breakthroughs in universal soft ionization and improved spatial resolution,but spatial resolution remains to be a few microns.To this end,an innovative nanoscale aperture tip desorption laser postionization time-of-flight mass spectrometer(Nano-ATDPI-TOFMS)was developed.Briefly,a tuning fork based atomic force microscope(AFM)system was introduced into the LDPI source for narrowing tip-to-sample distances to a near-field region(typically a few nanometers),achieving submicron-scale imaging resolution.As a newly developed high resolution mass spectrometry imaging technology,some benchmarks and applications of Nano-ATDPI-TOFMS are urgently needed to fully highlight its superior performance.Besides,the focus of LDPI-MS has been put on acquiring molecular information,while imaging of complicated samples is still in its infancy.Meanwhile,many'unwanted' characteristic fragments obtained by LDPI source play crucial roles in compound characterization and structure identification.However,few systematic studies have been conducted.Hence,in this dissertation,some novel functions and applications of LDPI-TOFMS were presented,enabling LDPI-MS with the performance of 'tandem mass spectrometry' and high resolution imaging at sub-micrometer scale.Relative woks were briefly demonstrated as follows:1.LDPI-TOFMS with the capability of 'tandem mass spectrometry' was newly developed for structure identification and isomer discrimination of organic compounds.In dissertation,'unwanted' fragments obtained using LDPI-TOFMS were innovatively renewed.The influences of the wavelength and laser power of postionization laser on molecular species and structural fragments were systematically investigated.Furthermore,molecular,fragmental and elemental information was obtained simultaneously with the minimum sample amount.Moreover,unambiguous identification and isomeric discrimination of organic compounds have been largely simplified and expedited.The enormous potentials of LDPI-MS for molecular identification and structural analysis were fully explored.Additionally,signal intensities of some metal elements were 1-2 orders of magnitude higher than those of molecular ions for poorly soluble and involatile organometallic compounds were analyzed.The detection sensitivity of targeted molecules could be greatly improved due to higher molecular utilization of LDPI,when their unique elements regarded as marker elements.2.LDPI-TOFMS was applied for unambiguous discrimination of inks,as well as molecular imaging of equivocal documents.Thanks to minimized matrix effects,higher molecular coverage and non-destructive nature of the LDPI source,many ink ingredients,such as organic dyes or pigments,organic solvents and additives were successfully acquired with a single laser shot,facilitating the direct determination and comprehensive understanding of chemical ingredients in inks.Additional discriminatory information on ink components offered a high level of confidence to discriminate various ballpoint and gel inks.Importantly,spatially resolved images of the investigated documents via LDPI made it more confirmatory to visually discriminate the forged portions and decipher the concealed contents.Minimized sample consumption and destructive nature can be achieved by virtue of the postionization laser,thereby,keeping the evidence intact.Direct laser desorption from complex sample surfaces without solvent or matrix involved will effectually reduce the overall detection time and impro ve analytical throughput.3.Single-cell targeting drugs distribution has been investigated using a novel Nano-ATDPI-TOFMS.Sample preparation has proven to be a key step for submicron-scale mass spectrometry imaging of single cell,and as yet there was no consensus on what,thus different requirement must be adapted in accordance with the specificity of the analysis.Therefore,the influences of two sample preparation methods on the distribution of molecules in cells and cell morphology were firstly examined.The results demonstrate that chemical fixation is beneficial to in-situ preservation of molecules accumulated in cells and cellular morphology.Fluorescence imaging results confirm that proflavine drugs are mainly accumulated in lysosomes,which remain almost unchanged after treatment,laying the foundation for the subsequent visualization for organelle-targeted drugs via high resolution MS imaging.Finally,the chemical and morphological imaging of proflavine and neutral red in HeLa cells with a pixel size of 500 nm was provided via Nano-ATDPI-TOFMS.The distribution of molecules accumulated in cells was clearly visible in the MS images,in accordance with that observed in the optical maps and morphological images using home-in-built AFM system.