Novel Ambient Soft Lonization Techniques And The Application In Mass Spectrometric Analysis Of Organic Aerosols

Novel ambient soft ionization techniques are usually derived from traditional electrospray ionization (ESI). With simpler source configuration and easier execution than ESI, ambient soft ionization techniques have been applied to direct mass spectrometry analysis with no pretreatment in many fields. In the field of atmospheric chemistry, off-line mass spectrometry has quickly become the most essential approach to indepth understanding of the physicochemical properties of organic aerosols, especially the secondary organic aerosols. Since aerosol samples, collected and deposited on substrates, usually have low mass loading but complicated chemical compositions, ambient soft ionization techniques are very suitable for off-line mass spectrometric analysis of organic aerosols. One popular ionization technique is nanospray desorption electrospray ionization (nano-DESI). In this dissertation, our study is based on the nano-DESI source. On the one hand, nano-DESI is coupled to high-resolution mass spectrometry (HR-MS) for molecular-level analysis of organic aerosols in Shanghai and Los Angeles. On the other hand, through furtherly simplification of the nano-DESI source configuration, we developed a novel ambient rapid ionization source, which is referred to as electrostatic field induced spray ionization (EFISI). Thus the content of this dissertation can be divided into two parts.Part I(nano-DESI HR-MS of organic aerosols in Shanghai and Los Angeles): Collected in the urban areas of Shanghai and Los Angeles when the air mass was stagnated, PM1 aerosol samples were analyzed by nano-DESI HR-MS in both positive and negative modes, with the mass range of 150-500. Methods such as second-order Kendrick mass defect analysis were used for rapid formulae assignment of the detected organic compounds. Mixtures of acetonitrile/water and acetonitrile/toluene were both used as desorption solvents in nano-DESI. The results show that acetonitrile/toluene enables to extract and ionize the higher molecular-weight, less-polar organic aerosol compounds. In Los Angeles sample, organic aerosols include amines/imines, organonitrates, organosulfates, and carbonyl/hydroxyl and carboxyl compounds. But in Shanghai sample, the majority of observed compounds are organosulfates, including nitrooxy organosulfates, indicating the highly aged aerosols in Shanghai. Few amines/imines, carbonyl/hydroxyl and carboxyl compounds are observed, and they distinctly differ from those in Los Angeles in their higher molecular weight and fewer oxygen-containing functional groups. Moreover, the dominant organosulfates in the two samples are of different types. Compared with Los Angeles sample in which as- known monoterpene-derived organosulfates dominate, Shanghai sample had distinctive abundance of organosulfates with high molecular weight (C atom number>8), few oxygen-containing functional groups (O atom number<3, not including the O atoms in nitrate and sulfate groups) and low degree of unsaturation (double bonds number<3, not including the double bonds in nitrate and sulfate groups). According to their assigned formulae, e.g. (CH2)nO5S, they are long alkyl-chain organosulfates with few oxygen-containing functional groups. The use of acetonitrile/toluene solvent facilitates identification of these special organosulfates, suggesting they could be missed in previous work relying sample extraction using common polar solvents. Their possible formation route is the sulfate esterification of long alkyl-chain carbonyl/hydroxyl compounds in the liquid phase of the aerosols when water is being evaporated and acidity is very high. But the source of the long alkyl-chain carbonyl/hydroxyl compounds is still under investigation. Given their special molecular structure, these special organosulfates have greater surfactant properties than as-known humic-like substances, and have potential to affect the light-absorption and hygroscopicity of the atmospheric particulate matter.Part II(development of EFISI):A simplified electrospray ionization source based on electrostatic field induction is developed on the home-made nano-DESI source. The EFISI is facilely performed using a needle electrode and a capillary spray emitter, independent of any other assistant equipment. High voltage is only applied to the needle electrode which is contactless to the sample or the capillary. Only 1μL sample droplet is needed for analysis with no or little pretreatment. The influences of needle electrode, capillary and droplet solvent on the ionization process are investigated. The change of charge distribution in the droplet solution, by the induction of external electrostatic field from the needle tip, is proposed to be the main cause of ion formation. We demonstrated the feasibility of EFISI MS in the rapid characterization of a wide range of standard chemicals and real samples. When coupled with the surface hydrophobic treatment, EFISI can also used for desorption ionization of surface sample. In addition, EFISI MS enables the detection of multichannel samples and observation of the chemical reaction in the droplet, indicating their promising potential in microfluidic analysis and organic reaction study.