MALDI-MS Imaging Method Based On Nanocarbon-Polydopamine And Nanoplatinum Pre-coated Matrix

Matrix-assisted laser desorption ionization mass spectrometry(MALDI-MS)imaging technology visualizes molecules with a specified mass number in the sample area to obtain the relative content and spatial distribution characteristics of the surface of the sample to be measured.In this thesis,we use Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR-MS)to study nanocarbon-polydopamine and platinum nanomaterials as pre-coated matrixes for MALDI-MS analysis and imaging of small molecular compounds.(1)Nanocarbon-polydopamine nanocomposite pre-coated matrix were prepared using carbon nanomaterials modified by dopamine self-polymerisation under weak alkaline conditions,which formed uniform films on the surface of stainless steel plates using the immersion method for small molecule MALDI-MS analysis in animal tissue sections.To investigate the effect of experimental conditions on analytical results when carbon nanomaterials and polydopamine are used as pre-coated substrates.In positive ion mode,isopropanol was chosen as the solvent to disperse 3 mg/m L of conductive nanocarbon powder,and the best analyte signal intensity was obtained at 50%laser intensity and 25 scans.The feasibility of using polydopamine(PDA)as a MALDI-MS matrix was explored and investigated.A thin film was formed by self-polymerization on the surface of stainless steel plates by solution oxidation.The effects of different reaction conditions,such as dopamine concentration and reaction time,on the signal intensity were examined.The best signal intensity of the PDA film was obtained with good reproducibility by polymerization for 12 h at a DA concentration of 1 mg/m L.The tissue sections of Panax ginseng were analysed using PDA films,in which 17 compounds,including oligosaccharides and saponins,were identified.A composite pre-coated matrix prepared by combining the optimised carbon nanomaterials and polydopamine improved the signal intensity of the analytes and reduced background noise compared to using both alone.Sucrose,cholesterol and palmitic acid were analysed under selected conditions using composite pre-coated matrix films with inter-point repeatability<5.6%and intra-point repeatability<5.0%.The linear correlation coefficients R~2>0.992 between peak intensity and concentration in the range of 0.05-1.0 mg/m L for both palmitic acid and cholesterol,and their limits of detection were 2.0 and 1.9μg/m L,respectively,showing good potential for quantitative analysis.A total of 103 compounds,including oligosaccharides,fatty acids and lipids,were identified using this composite pre-coated matrix for the analysis of pig liver tissue sections.(2)The platinum nano-materials prepared by ion sputtering instrument were studied as the pre-coated matrix for MALDI-MS.The preparation method is simple and the substrate is attached stably,which can effectively eliminate the"dessert"effect.The influence of sputtering platinum nano-materials on different substrates(cover glass,silicon plate and ceramic plate)on the signal intensity of analytes was compared.The experimental results showed that sputtering platinum signal on cover glass was the best.Sputtered platinum was compared with prepared platinum nanoflowers,which are complicated to prepare and easy to dislodge,and the signal intensity was not as good as that of sputtered platinum nanomaterials.Optimized experimental conditions for sputtering platinum.A sputtering time of 1 min,a laser intensity of 50%and 25 scans were chosen to analyse Palmitic acid,Raffinose,Abscisic acid and Triglyceride with inter-point repeatability was less than 5.3%,and the intra-point repeatability was less than 4.5%.The linear correlation coefficient R~2of peak intensity versus concentration for both Palmitic acid and Raffinose were determined to be greater than 0.993in the range of 0.05-1.0 mg/m L,and their limits of detection were 1.17 and 0.48μg/m L respectively,showing a good quantitative analysis potential.Using platinum pre-coated matrix,the extracts of Radix Bupleuri,Polygonum cuspidatum and soybean were analyzed.In the extract of Radix Bupleuri,30 compounds such as saikosaponin,sterol,fatty acid and sugar were detected and identified.In the extract of Polygonum cuspidatum,12 compounds including polydatin,resveratrol and sugar were detected and identified.The soybean extract mainly detected and identified 24 compounds such as oligosaccharides and triglycerides.Oligosaccharides and triglycerides in germinated wheat and fresh sweet corn kernels were analyzed by MALDI-MS imaging,which showed that triglycerides were mainly distributed in the embryo of germinated wheat and corn kernels,and oligosaccharides were mainly distributed in the endosperm.(3)The desorption mechanism of three substrates(carbon nanomaterials,PDA films and platinum nanomaterials)were briefly investigated using a chemical thermometer benzylpyridinium ion(BP~+)and hole sensitivity probe tetrabutylammonium iodide(TBAI).According to the total ion intensity,survival yield(SY)and mass spectral peak intensity of TBA~+,it is hypothesised that nano carbon powder has two mechanisms:thermal-driven desorption and non-thermal-driven desorption.PDA film is mainly thermal-driven desorption,while Platinum is mainly non-thermal-driven desorption.