Study On Dual Mass Spectrometer Scheme In Molecular Reaction Control

There are many molecules in nature that have similar structures, especiallythose isomers with similar structures. Though these molecules have similar structures,their chemical or physical properties might differ from each other dramatically, hencea efficient method to distinguish them is needed. Using shaped femtosecond laserpulses and time of flight mass spectrometer, this kind of distinguish faces difficulty.Because the mass spectra from different isomers are almost the same, the spectra fromthe mixture of the two molecules completely overlap, there is no way to let the selflearning method to find a pulse train to get different structure spectrum from differentmolecule (hence achieve the purpose of distinguishing).To tackle the above problem, a scheme using two time of flight massspectrometers to do the genetic algorithm (GA) optimization is proposed andestablished. In the system, two linear time of flight mass spectrometers are used, one shaped femtosecond laser beam is divided into two, then they irradiate on differentmolecules in different spectrometer, hence the difference of the two spectra can beenlarged by GA optimization. During the design and establishment process of thesystem, many aspects are considered to ensure its feasibility and reasonability. Theyinclude the following. The two laser beams must have similar optical path. Byoptimizing second harmonic signal, using multiphoton intrapulse interference phasescan method, the laser pulses at the entrance of the two spectrometers. withoutapplying any phase, are ensured to be transform limited. The laser energy in twobeams are adjusted carefully to ensure laser intensities on molecules are the same.Since one beam is divided into two, to ensure the laser intensity is strong enough onmolecules, a pulse shaping before amplification method is used. GA programs areredesigned according to dual-spectrometer scheme. Data from two spectrometers (twooscilloscopes) are collected and compared. Data acquisitions from the twooscilloscopes are started synchronously and data are collected after the twooscilloscopes are all done with their data acquisition processes, to save time and toensure pulse shapes and pulse energies for the two spectrometers are exactly the same.The average numbers and average times for the two spectrometers are set to be thesame. The dual-spectrometer scheme set up in this thesis can be used not only to distinguish isomers that have similar structures, but also to optimize mass peaks fromtwo different molecule systems (not isomers), so as to enlarge the differences of themass peaks, or to optimize different products’ yields from two molecule systems. Thisscheme can also efficiency of those experiments that need adjusting parametersrepeatedly, preventing the influence from changing of other experimental parametersduring the change of this one.To test the feasibility of above established dual spectrometer system, a numberof molecules are chosen to optimize the ratios of different type of products. Theyinclude: the optimization of ratio of parent ions from isomers, i.e. the optimization ofionization processes; optimization of ratio of fragment ion to parent ion of anotherisomer; optimization of fragment ions from two isomers; optimization of product ionsfrom non-isomer molecules. A number of ratio optimization of ionization and/ordissociation products of different molecules have been successfully performed. After16generations of optimization, the ratio of parent ions from propylene oxide andacetone is enlarged from0.041to0.053. After20generations of optimization, theratio of parent ions from propylene oxide and propylaldehyde is enlarged from0.17to0.26. After18generations of optimization, the ratio of fragment ion m/z=29frompropylene oxide to the parent ion of acetone is enlarged from0.08to0.12. After20 generations of optimization, the ratio of fragment ion m/z=43from propylene oxide tothe parent ion of propyladehyde is enlarged from0.04to0.12. After27generations ofoptimization, the ratio of fragment ion m/z=43from acetone to the parent ion ofpropyladehyde is enlarged from0.003to0.005. After15generations of optimization,the ratio of fragment ion m/z=43from acetone to the fragment ion m/z=27frompropyladehyde is enlarged from0.17to0.34. After18generations of optimization, theratio of fragment ion m/z=43from acetone to the fragment ion m/z=15fromcyclopentanone is enlarged from0.7to1.3. After13generations of optimization, theratio of fragment ion m/z=15from acetone and cyclopentanone is enlarged from0.19to0.40.In this thesis, the main frame of dual-spectrometer GA optimization system isdesigned and established, the feasibility of which is tested thoroughly throughexperiments. The system is expected to play an important role in future studies. Thereare many aspects of the sytem that can be enhanced or modified, e.g. enhancingexperimental efficiency by optimizing GA programs and using apparatus with higheracquisition rate, optimizing optical setup to make sure the two beams have sameoptical path, etc.