Development Of Laser-based Ion Source Time-of-flight Mass Spectrometry And Its Applications In Solid Analysis And Co-registered Chemical And Topographical Imaging Of Single Cells

Laser-based ionization sources time-of-flight mass spectrometry has been widely applied in many fields,such as elemental and molecular imaging of biological tissues.Nevertheless,many bottleneck problems have hindered them from high lateral resolution mass spectrometry imaging techniques due to relatively low detection sensitivity(e.g.,sampling,ionization,and transmission efficiency)and optical diffraction limit(low lateral resolution).To this end,a novel laser ionization source with co-registered chemical and topographical imaging at sub-micrometer scale was developed to improve lateral resolution.Moreover,laser postionization source was introduced to improve ionization efficiency,facilitating high-resolution co-registered chemical and topographical imaging in single cells.In addition,other laser-based ion sources time-of-flight mass spectrometric techniques were also developed and applied in the analysis of elements and molecules,e.g.,direct elemental analysis and depth profiling of solid samples,simultaneous acquisition of elemental,fragmental,and molecular information on organometallic compounds,and gas-phase interactions of peptides with"naked" metal ions.1.Laser desorption/ionization time-of-flight mass spectrometer(LDI-TOFMS)was developed with the resolving power of about 7000.To acquire higher ionization efficiency,a 266 nm laser was introduced to avoid matrix interference.Thus,a laser desorption laser postionization time-of-flight mass spectrometer(LD-LPI-TOFMS)was developed and used for direct microtrace determination of rare earth elements(REEs)in residues,depth profiling of nanometer thin-layer,as well as elemental analysis in metal alloys.LD-LPI source can achieve low average ablation rate(AAR)down to 0.026 nm in depth per laser pulse and thickness measurement.And outstanding limit of detection(LOD)and absolute limit of detection(ALOD)ofng/mL and?femtogram can be obtained,respectively,for fifteen REEs with more uniform relative sensitivity coefficients(RSCs)ranging from 0.5 to 2.5.Moreover,low LODs of 100-7-10-8 g/g is attainable for trace elemental detection in metal alloys by LD-LPI-TOFMS.2.A newly developed nanoscale aperture tip desorption ionization time-of-flight mass spectrometer(Nano-ATDI-TOFMS)for simultaneously co-registered chemical and topographical imaging within an individual cell is demonstrated.With the integrated advantages of ionization source in high vacuum and 157 nm laser postionization,transmission and ionization efficiency have been greatly improved,achieving ALOD down to 0.44 amol.On the premise of the detectable MS signals,a series of ablated craters with the diameter of 350 nm,full width at half maximum(FWHM)of 270 nm,and a depth of 70 nm on the proflavine molecular layers can be obtained by this technique.Based on the 16%-84%rule,this technique can achieve the imaging resolution of 250 nm in 2000 mesh TEM copper grids for physical vapor deposition(PVD)of proflavine molecules onto silicon substrate.Moreover,the mean diameter of ablation spots on the cellular surfaces by Nano-ATDI source is 400 nm.A chemical image of Hela cells cultivated with proflavine drug concentration of 50 ?M can be acquired at a pixel size of 250 nm.And three-dimensional reconstructed chemical imaging of the real single-cell surface can be also obtained using nano-ATDI-TOFMS platform.It opens up new possibility of mass spectrometry imaging techniques at the single-cell level.3.A solvent-free and matrix-free strategy in the analysis of metallophthalocyanine,metalloporphyrin,and peptides has been developed via a home-in-built high irradiance laser ionization time-of-flight mass spectrometer(HILI-TOFMS).It allows the simultaneous acquisition of metal/nonmetal,fragmental,and molecular information by simply changing He gas pressure in the ion source.And thermal diffusion desorption mechanism is proposed to explain the suival of intact molecular ions under high laser irradiance.Proton attachment mechanism is involved in the thermal diffusion desorption process,which expedites the matrix-free identification of peptides.In near future,it is expected to be a comprehensive method with the integrated features of inorganic,organic,and biological mass spectrometry.4.A novel strategy for the generation of metal-peptide complexes in the gas phase was proposed via atmospheric pressure laser ionization electrospray ionization source(LI-ESI),which is of great values for probing the interactions of "naked" metal ions with peptide molecules."Naked" metal ions are generated from the metal target by laser ionization in open air,whereas gas-phase peptide ions are electrosprayed separately,facilitating the formation of gas-phase metal-peptide complexes.Compared with the conventional electrospray ionization(ESI)method,higher reaction yields and stabilities of metal-peptide complexes can be obtained by a direct gas-phase reaction of peptides with "naked" metal ions devoid of counter-ions and surrounding solvent due to stronger noncovalent interactions,such as coulomb interactions and charge-dipole interactions.A diffusion model is introduced to expound on the high reaction yield of metal-peptide complexes and distribution evolution of metal ions,verified by both calculated and experimental results.In addition,the influence of the number and position of basic residues in peptides on the binding sites of metal ions and CID fragmentation patterns of complexes is explored and discussed.Plausible mechanisms responsible for fragments remote from the initial binding sites of metal ions are proposed.