The Development And Application Of Tip-enhanced Ablation/Ionization Time-of-Flight Mass Spectrometry

Currently,mass spectrometry imaging(MSI)technologies can be applied to sample component analysis,and obtian the component distribution of sample.MSI has the merits of high sensitivity,low sample consumption,rapid muti-elemental analysis and lable-free operation,and so on.Develpoing MSI technologies have been a hot issue in scientific research field.Spectroscopic methods with nanoscale lateral resolution are becoming essential in the fields of physics,chemistry,geology,biology,and materials science,ect.However,the lateral resolution of laser-based mass spectrometry imaging(MSI)techniques has so far been limited to the microscale.There is a scientific trend calling for developing MSI technologies that can offer nanoscale lateral resolution.Due to the localized surface plasmon resonance(LSPR)effect of apertureless tips,the intensity of incident light with a proper wavelenght can be effectively enhanced underneath the tip apex.This phenomenon is called near-field enhancement effect of apertureless tips.By utilizing the near-field enhancement of apertureless tips,and introducing the scanning tunneling microscope into the MS system for precisely controlling the tip-sample distance,the tip-enhanced ablation/ionization time-of-flight mass spectrometry(TEAI-TOFMS system)was developed.In the TEAI-TOFMS system,the near field enhancement of apertureless tips was used initially to ablate and ionize sample,resulting in nanoscale lateral resolution.This is an innovative study.TEAI-TOFMS system can offer nanoscale lateral resolution in the analysis of solid metals,residues of inorganic salts as well as in MSI experiment.Brief introductions of these investigations were given as follows:1.Developing the TEAI-TOFMS system.This work describes the vaccum system,ion source,and important components of mass spectrometer.In the design of TEAI-TOFMS system,the whole system is placed in vacuum,and the ion transmission parts were removed in order to improve the utilization rate of ions.In our design of ion source,a scanning tunneling rmicroscope(STM)system was introduced into the mass spectrometer to control the tip-smaple distance.Mass spectra of analytes can be obtained by TEAI-TOFMS system,providing a guarantee for the follow-up study.2.Tip-enhanced ablation/ionization experiments were performed by using nano-second laser(ns-laser)and femto-second laser(fs-laser).Both ns-laser and fs-laser sources were used to induce the near-field enhancement of apertureless tips,the related experimental results were reported here.Different metal coatings on single cystal gold surface were analyzed by TEAI-TOFMS system,and nanoscale near-field craters can be generated.The formation of near-field craters was studied,indicating that the repeatability of TEAI-TOFMS system was good enough for MSI experiments.In the experiment of rapid analysis of inorganic salt residues,near-field mass spectra were obtained.Comparison of analysis results obtained by secondary ion mass pectrometry(SIMS)and TEAI-TOFMS showed that mass spectra of residues obtained by TEAI-TOFMS were more clear and distinct,with no matrix interference.Near-field MSI with a 50 nm lateral resolution was achieved in this study.3.Developing a 3-D model using finite element method.In order to explain the results of tip-enhanced ablation/ionization experiment through a theoretical perspective,a 3-D model was developed using finite element method(FEM)to simulate the electromagnetic field of regeion between the silver tip and the sample.The relationship between the tip-sample distance and near-field enhanced factor as well as the relationship between different metal coatings and near-field enhanced factor were studied.The simulated results were consistent with the experimental results,indicating that the 3-D model was correct and reliable.