Design,Construction And Application Of Laser Induced Acoustic Desorption Time-of-Flight Mass Spectrometry

Mass spectrometry has developed into a highly versatile analytical technology that has been applied to many fields such as environment,medical treatment,energy and so on.From the initial isotope detection to the current biological macromolecule analysis,the application scope of mass spectrometry has been continuously expanded.The more complex types of analytes continue to challenge the existing mass spectrometry technology.Laser-induced acoustic desorption(LIAD)technology uses a pulsed laser to irradiate the metal foil to generate sound waves to promote the analyte deposited on the other side of the metal foil into the gas phase.Since this process only transmits the energy near the desorption threshold,it can realize the nondestructive desorption of analyte molecules.Meanwhile,as a pure desorption method,LIAD can flexibly combine various types of ionization methods and mass analyzers to deal with various application scenarios,and has the potential to become a general-purpose mass spectrometry technology.In this paper,substrate enhanced laser-induced acoustic desorption(SE-LIAD)technology with higher desorption efficiency was developed on the basis of traditional lid.At the same time,compact substrate enhanced laser-induced acoustic desorption time of flight mass spectrometry(SE-LIAD-TOFMS,500 x 500 x 400 mm,resolution～1500)was constructed.The desorption efficiency of SE-LIAD is 5.6 times higher than that of traditional LIAD in electron ionization(EI)mode and 13.8 times higher in single photon ionization(SPI)mode.The improvement of desorption efficiency of SE-LIAD mainly comes from 1.High intensity of constrained stress wave;2.Adding quartz substrate can form the superposition of multiple sound waves before sample desorption.The detection of limit(LOD)of SE-LIAD-TOFMS was investigated by melamine.The LOD of melamine in El mode was 2-6 pg,and the detection limit of melamine in SPI mode was 0.5 ng.In order to investigate the mass spectrum analysis characteristics of complex analytes and trace analytes by SE-LIAD-TOFMS,it was applied to(1)the composition and structure analysis of Xinjiang Naomaohu coal and(2)the analysis of trace process products in the formation of nitrogen doped carbon materials.1)Chemical Structure Analysis of Naomaohu(NMH)Coal.The organic small molecular components in NMH coal are obtained based on microwave-assisted ultrasonic extraction technology.On this basis,the extract is further separated into four fractions:saturated hydrocarbon,aromatic hydrocarbon,lipid and asphaltene by column chromatography.Combined with the mass spectrum information of each fraction provided by SE-LIAD/VUVSPI-TOFMS,the aromatic ring distribution characteristics of each fraction provided by synchronous fluorescence spectrum and the cross contamination law between fractions,it is finally determined that NMH coal extract contains at least 248 compounds,including 46 saturated hydrocarbons embedded in coal skeleton structure,132 aromatic hydrocarbons embedded in coal skeleton structure and 59 possible coal skeleton unit compounds,and 11 compounds that may be aromatics or coal skeleton unit structure。2)Study on the transformation mechanism of nitrogen functional groups of nitrogen doped carbon materials.The pyrolysis of PANI,a common precursor of nitrogendoped carbon materials,was used as the model reaction for the preparation of nitrogen-doped carbon materials.The in-situ pyrolysis process of PANI was investigated by in-situ pyrolysis time-of-flight mass spectrometry(IN-situ PyTOFMS),and the initial pyrolysis products were characterized.Combined with SELIAD/VUVSPII-TOFMS,the structural characteristics of trace products(pyrolysis residues)during the pyrolysis process were investigated so as to realize the complete reaction process characterization of nitrogen functional groups in nitrogen-doped carbon materials by mass spectrometry.The main pyrolysis reaction of PANI is mainly composed of three basic reactions:1.The C-N bond breaks to produce aniline polymer and oligomeric NH radical;2.Aromatic ring cleavage(delocalization π bond fracture),the formation of small molecular hydrocarbons and the main formation pathways of nitrogen-containing heterocycles;3.Amino based aromatization reaction to form carbazole structure.Nitrogen-containing heterocyclic compounds mainly appear in the form of pyrrole nitrogen and pyridine nitrogen in the initial products,while graphite nitrogen does not appear in the initial stage of pyrolysis reaction,and its formation mainly depends on the addition reaction of the initial pyrolysis products.The formation and transformation of nitrogen functional groups at the edge of carbon materials is the main reaction in the process of nitrogen doping.Meanwhile,it is also the main reason for the low nitrogen doping rate of nitrogen doped carbon materials and the complex form of nitrogen functional groups.On the basis of SE-LIAD-TOFMS,the primary integration of SE-LIAD-TOFMS is achieved by replacing the traditional desktop laser with a self-developed compact all solid state passively Q-switched laser(Wavelength 1064 nm,maximum energy 550 μJ,Laser pulse width 1.5 ns).