Development Of Droplet Spray Ionization Mass Spectrometry And Its Application In Mechanism Study Of Advanced Oxidation Processes

Advanced oxygen processes?AOPs?are one of the most important technique in pollution control.However,the reactions information acquisition of AOPs and pollutant degradation is incomplete,which restricts the improvement of removal efficiency and the understanding of the migration and transformation of pollutants in environment.The reasons for this respect are that the analytical methods including spectrophotometry,infrared,nuclear magnetic resonance and chromatography-mass spectrometryat are operated offline,and thus,they have incapacity of capturing reaction intermediates with short lifetime and low concentration,which leads to inaccurate mechanism of AOPs.The hypothesis underlying this study is to develop droplet spray ionization mass spectrometry to seek the reaction mechanism of AOPs and pollutants.Different workflows were constructed,the performance parameters of DSI-MS were investigated,and ionization mechanism was discussed.The application of DSI-MS for real-time study of mechanism of free radical formation was demonstrated.Based on this,droplet spray ionization photochemical reactor syetem was established.The application in real-time capturing of reactive intermediates of pollutant degradation in Photo-Fenton system was investigated to elucidate the degradation mechanism.Droplet spray photocatalytic reactor system was further established.The application in capturing and identifying intermediates of pollutant degradation in photocatalytic reaction was demonstrated,and the degradation mechanism was proposed.Droplet spray ionization mass spectrometry in which a corner of a glass cover slip was used as spray substrate was developed,and two workflow of real-time and continuously on-line modes for chemical reaction study were established.DSI-MS had the advantages of ease to use,easy set-up,low-cost and no interference and pollution to samples.DSI-MS used the corner of the cover slip as the microreactor.The reaction was carried out at the corner,the intermediates and products formed in the reaction were directly sprayed and ionized from the corner.The reaction information was captured without delay,which realized real-time study of reaction processes.This benefits for capturing and identifying intermediates with a half-life on the order of microseconds.The influence factors of DSI-MS for real-time study of chemical reactions were investigated.The polarity between methanol and fluorobenzene could be used as a DSI-MS spray solvent.When the spray angle was4°,the sample solution could be continuously drove to the corner to form a stable spray.The corner of the cover slip could hold a capacity of 70?L of sample solution and achieve a spray time of 18 min with RSD of approximately 6%,which indicated that DSI-MS had the ability of long-time scale and stable spray.Once the spray corner was placed in the range of 10×6×5 mm,a spray could be formed,and optimization and adjustment for obtaining a satisfactory signal was easy.A three-dimensional moving stage was installed to ensure the easibility and stability of the platform.DSI was one variant of electrospray ionization.The glass substrate merely functioned as substrate for sample loading and did not participate in the formation of the spray.The sample solution on the corner formed a droplet with a sharp tip,and then the spray was lanuched and converted to small charged droplets under a strong electric field,which undergo desolvation and coulomb explosions and finally ionized for detection by MS.DSI-MS was firstly used for real-time study of mechanism of peroxymonosulfate?PMS?activated by quinones,and p-benzoquinone,2,6-dimethyl-p-benzoquinone,2,6-dichloro-p-benzoquinone and 2,3,5,6-tetrafluoro-p-benzoquinone were selected as model compounds.Firstly,the reactive species formed during activation of PMS were investigated.The results showed that PMS activated by different quinones all generated singlet oxygen?¹O₂?.The intermediate formed during the activation was captured.By combing tandem mass spectrometry?MS/MS?,isotope labeling and high accurate mass,the structure of intermediate was characterized.The mechanism of activation of PMS by different quinones was consistent,and a possible mechanism was proposed accordingly.The C=O of quinones could be regarded as ketone structure.Nucleophilic addition reaction of PMS with carboxy group of quinones formed adduct intermediates,which were converted into conjugate base intermediates under alkaline conditions.The conjugate base intermediates then undergo intramolecular nucleophilic substitution reaction and formed a dioxirane-like intermediate.Finally,under the nucleophilic attack by SO₅^2-,this intermediate reformed the quinones and simultaneously released ¹O₂ for pollutant degradation.Then,by coupling DSI-MS with illumination,droplet spray photochemical reactor system was developed.The parameters were optimized.The resulst showed that illumination did not cause photoionization and had no effect on the target ionization.The degradation Cu?II?-EDTA in H₂O₂/UV system was selected as model reaction,the intermediates formed during the degradation under different pH were captured in real-time.At neutral conditions,cleavage of amino group from Cu?II?-EDTA was confirmed based on the obtained reaction intermediates.At acidic conditions,loss of carboxyl group from Cu?II?-EDTA was found according to the generated reaction intermediates.It was confirmed for the first time that the degradation of Cu?II?-EDTA at acidic conditions was initiated at carboxyl site,not an acetate group.Finally,the degradation mechanism of Cu?II?-EDTA via carboxyl group,amino group and decomplexation was proposed.Furthermore,on the basis of photochemical reactor system,droplet spray photocatalytic reactor system was developed by simply replacing the cover slip with one slip coated with catalyst.The operation parameters were investigated and optimized.The degradation of cyclophosphamide?CP?in TiO₂/UV system was used as a model reaction.Firstly,the types of free radicals for CP degradation were investigated,and free·OH was found as the main reactive species that promoted CP degradation.During the degradation,the reaction intermediates generated were captured by DSI-MS,which were further characterized by tandem mass spectrometry.Finally,the degradation pathway of CP in TiO₂/UV system was proposed,in which CP was initiated mainly by dehydrogenation and hydroxylation reaction,and subsequently achieved dechlorination by the cleavage of N-P bond.In conclusion,droplet spray ionization mass spectrometry was developed to investigate the formation mechanism of free radicals and to study the mechanism of pollutant degradation in Photo-Fenton and photocatalytic systems.This offers a new method and approach for exploring the pathway of AOPs and involved pollutants in real time,and has significant meaning for the field of pollutant control.