Miniaturized mass spectrometric ionization techniques for environmental analysis and bioanalysis

Miniaturized mass spectrometric ionization techniques for environmental analysis and bioanalysis Novel miniaturized mass spectrometric ionization techniques based on atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) were studied and evaluated in the analysis of environmental samples and biosamples. The three analytical systems investigated here were gas chromatography-microchip atmospheric pressure chemical ionization-mass spectrometry (GC-µAPCI-MS) and gas chromatography-microchip atmospheric pressure photoionization-mass spectrometry (GC-µAPPI-MS), where sample pretreatment and chromatographic separation precede ionization, and desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS), where the samples are analyzed either as such or after minimal pretreatment. The gas chromatography-microchip atmospheric pressure ionization-mass spectrometry (GC-µAPI-MS) instrumentations were used in the analysis of polychlorinated biphenyls (PCBs) in negative ion mode and 2-quinolinone-derived selective androgen receptor modulators (SARMs) in positive ion mode. The analytical characteristics (i.e., limits of detection, linear ranges, and repeatabilities) of the methods were evaluated with PCB standards and SARMs in urine. All methods showed good analytical characteristics and potential for quantitative environmental analysis or bioanalysis. Desorption and ionization mechanisms in DAPPI were studied. Desorption was found to be a thermal process, with the efficiency strongly depending on thermal conductivity of the sampling surface. Probably the size and polarity of the analyte also play a role. In positive ion mode, the ionization is dependent on the ionization energy and proton affinity of the analyte and the spray solvent, while in negative ion mode the ionization mechanism is determined by the electron affinity and gas-phase acidity of the analyte and the spray solvent. DAPPI-MS was tested in the fast screening analysis of environmental, food, and forensic samples, and the results demonstrated the feasibility of DAPPI-MS for rapid screening analysis of authentic samples.

Desorptio/fotoionisaatio ilmanpaineessa takavarikoitujen huumausaineiden, steroidien ja räjähdysaineiden tutkimuksessa

Tutkimuksen tarkoituksena oli selvittää desorptio/fotoionisaatio ilmanpaineessa tekniikan (engl. desorption atmospheric pressure photoionization, DAPPI) soveltuvuutta rikosteknisen laboratorion näytteiden analysointiin. DAPPI on nopea massaspektrometrinen ionisaatiotekniikka, jolla voidaan tutkia yhdisteitä suoraan erilaisilta pinnoilta. DAPPI:ssa käytetään lämmitettyä mikrosirua, joka suihkuttaa höyrystynyttä liuotin- ja kaasuvirtausta kohti näytettä. Näytteen pinnan komponentit desorboituvat lämmön vaikutuksesta, jonka jälkeen ionisoituminen tapahtuu VUV-lampun emittoimien fotonien avulla.DAPPI:lla tutkittiin takavarikoituja huumausaineita, anabolisia steroideja ja räjähdysaineita sekä niiden jäämiä erilaisilta pinnoilta. Lisäksi kartoitettiin DAPPI:n mahdollisuuksia ja rajoituksia erilaisille näytematriiseille ilman näytteiden esikäsittelyä. Takavarikoitujen huumausaineiden tutkimuksessa analysoitiin erilaisia tabletteja, jauheita, kasvirouheita, huumekasveja (khat, oopium, kannabis) ja sieniä. Anabolisia steroideja tunnistettiin tableteista sekä ampulleista, jotka sisälsivät öljymäistä nestettä. Jauheet ripoteltiin kaksipuoliselle teipille ja analysoitiin siltä. Muut näytteet analysoitiin sellaisenaan ilman minkäänlaista esikäsittelyä, paitsi nestemäisten näytteiden kohdalla näyte pipetoitiin talouspaperille, joka analysoitiin DAPPI:lla. DAPPI osoittautui nopeaksi ja yksinkertaiseksi menetelmäksi takavarikoitujen huumausaineiden ja steroidien analysoimisessa. Se soveltui hyvin rikoslaboratorion erityyppisten näytteiden rutiiniseulontaan ja helpotti erityisesti huumekasvien ja öljymäisten steroidiliuosten tutkimusta. Massaspektrometrin likaantuminen pystyttiin ehkäisemään säätämällä näytteen etäisyyttä sen suuaukosta. Likaantumista ei havaittu huolimatta näytteiden korkeista konsentraatioista ja useita kuukausia jatkuneista mittauksista. Räjähdysaineiden tutkimuksessa keskityttiin seitsemän eri räjähdysaineen DAPPI-MS-menetelmän kehitykseen; trinitrotolueeni (TNT), nitroglykoli (NK), nitroglyseriini (NG), pentriitti (PETN), heksogeeni (RDX), oktogeeni (HMX) ja pikriinihappoä Nämä orgaaniset räjähteet ovat nitraattiyhdisteitä, jotka voidaan jakaa rakenteen puolesta nitroamiineihin (RDX ja HMX), nitroaromaatteihin (TNT ja pikriinihappo) sekä nitraattiestereihin (PETN, NG ja NK). Menetelmäkehityksessä räjähdysainelaimennokset pipetoitiin polymetyylimetakrylaatin (PMMA) päälle ja analysoitiin siitä. DAPPI:lla tutkittiin myäs autenttisia räjähdysainejäämiä erilaisista matriiseista. DAPPI:lla optimoitiin jokaiselle räjähdysaineelle sopiva menetelmä ja yhdisteet saatiin näkymään puhdasaineina. Räjähdysainejäämien analysoiminen erilaisista rikospaikkamateriaaleista osoittautui haastavammaksi tehtäväksi, koska matriisit aiheuttivat itsessään korkean taustan spektriin, josta räjähdysaineiden piikit eivät useimmiten erottuneet tarpeeksi. Muut desorptioionisaatiotekniikat saattavat soveltua paremmin haastavien räjähdysainejäämien havaitsemiseksi.

Surface analysis of lipids by mass spectrometry : More than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated. © 2013 Elsevier Ltd. All rights reserved.

Development of atmospheric pressure ionization ion mobility spectrometry and ion mobility spectrometry mass spectrometry

This study is focused on the development and evaluation of ion mobility instrumentation with various atmospheric pressure ionization techniques and includes the following work. First, a high-resolution drift tube ion mobility spectrometer (IMS), coupled with a commercial triple quadrupole mass spectrometer (MS), was developed. This drift tube IMS is compatible with the front-end of commercial Sciex mass spectrometers (e.g., Sciex API-300, 365, and 3000) and also allows easy (only minor modifications are needed) installation between the original atmospheric pressure ion source and the triple quadrupole mass spectrometer. Performance haracteristics (e.g.,resolving power, detection limit, transmission efficiency of ions) of this IMS-MS instrument were evaluated. Development of the IMS-MS instrument also led to a study where a proposal was made that tetraalkylammonium ions can be used as chemical standards for ESI-IMS. Second, the same drift tube design was also used to build a standalone ion mobility spectrometer equipped with a Faraday plate detector. For this highresolution (resolving power about 100 shown) IMS device, a multi-ion source platform was built, which allows the use of a range of atmospheric pressure ionization methods, such as: corona discharge chemical ionization (CD-APCI), atmospheric pressure photoionization (APPI), and radioactive atmospheric pressure chemical ionization (R-APCI). The multi-ion source platform provides easy switching between ionization methods and both positive and negative ionization modes can be used. Third, a simple desorpion/ionization on silicon (DIOS) ion source set-up for use with the developed IMS and IMS-MS instruments was built and its operation demonstrated. Fourth, a prototype of a commercial aspiration-type ion mobility spectrometer was mounted in front of a commercial triple quadrupole mass spectrometer. The set-up, which is simple, easy to install, and requires no major modifications to the MS, provides the possibility of gathering fundamental information about aspiration mobility spectrometry.

The current and potential applications of Ambient Mass Spectrometry in detecting food fraud

The adulteration of food has received substantial amounts of media attention in the last few years, with events such as the European horsemeat scandal in 2013 sending shockwaves through society. Almost all cases are motivated by the pursuit of profits and are often aided by long and complex supply chains. In the past few years, the rapid growth of ambient mass spectrometry (AMS) has been remarkable, with over thirty different ambient ionisation techniques available. Due to the increasing concerns of the food industry and regulators worldwide, AMS is now being utilised to investigate whether or not it can generate results which are faster yet comparable to those of conventional techniques. This article reviews some aspects of the adulteration of food and its impact on the economy and the public's health, the background to ambient mass spectrometry and the studies that have been undertaken to detect food adulteration using this technology.

Heated Nebulizer Microchips for Mass Spectrometry

Miniaturized analytical devices, such as heated nebulizer (HN) microchips studied in this work, are of increasing interest owing to benefits like faster operation, better performance, and lower cost relative to conventional systems. HN microchips are microfabricated devices that vaporize liquid and mix it with gas. They are used with low liquid flow rates, typically a few µL/min, and have previously been utilized as ion sources for mass spectrometry (MS). Conventional ion sources are seldom feasible at such low flow rates. In this work HN chips were developed further and new applications were introduced. First, a new method for thermal and fluidic characterization of the HN microchips was developed and used to study the chips. Thermal behavior of the chips was also studied by temperature measurements and infrared imaging. An HN chip was applied to the analysis of crude oil – an extremely complex sample – by microchip atmospheric pressure photoionization (APPI) high resolution mass spectrometry. With the chip, the sample flow rate could be reduced significantly without loss of performance and with greatly reduced contamination of the MS instrument. Thanks to its suitability to high temperature, microchip APPI provided efficient vaporization of nonvolatile compounds in crude oil. The first microchip version of sonic spray ionization (SSI) was presented. Ionization was achieved by applying only high (sonic) speed nebulizer gas to an HN microchip. SSI significantly broadens the range of analytes ionizable with the HN chips, from small stable molecules to labile biomolecules. The analytical performance of the microchip SSI source was confirmed to be acceptable. The HN microchips were also used to connect gas chromatography (GC) and capillary liquid chromatography (LC) to MS, using APPI for ionization. Microchip APPI allows efficient ionization of both polar and nonpolar compounds whereas with the most popular electrospray ionization (ESI) only polar and ionic molecules are ionized efficiently. The combination of GC with MS showed that, with HN microchips, GCs can easily be used with MS instruments designed for LC-MS. The presented analytical methods showed good performance. The first integrated LC–HN microchip was developed and presented. In a single microdevice, there were structures for a packed LC column and a heated nebulizer. Nonpolar and polar analytes were efficiently ionized by APPI. Ionization of nonpolar and polar analytes is not possible with previously presented chips for LC–MS since they rely on ESI. Preliminary quantitative performance of the new chip was evaluated and the chip was also demonstrated with optical detection. A new ambient ionization technique for mass spectrometry, desorption atmospheric pressure photoionization (DAPPI), was presented. The DAPPI technique is based on an HN microchip providing desorption of analytes from a surface. Photons from a photoionization lamp ionize the analytes via gas-phase chemical reactions, and the ions are directed into an MS. Rapid analysis of pharmaceuticals from tablets was successfully demonstrated as an application of DAPPI.

Liquid AP-UV-MALDI enables stable ion yields of multiply charged peptide and protein ions for sensitive analysis by mass spectrometry

In biological mass spectrometry (MS), two ionization techniques are predominantly employed for the analysis of larger biomolecules, such as polypeptides. These are nano-electrospray ionization [1, 2] (nanoESI) and matrix-assisted laser desorption/ionization [3, 4] (MALDI). Both techniques are considered to be “soft”, allowing the desorption and ionization of intact molecular analyte species and thus their successful mass-spectrometric analysis. One of the main differences between these two ionization techniques lies in their ability to produce multiply charged ions. MALDI typically generates singly charged peptide ions whereas nanoESI easily provides multiply charged ions, even for peptides as low as 1000 Da in mass. The production of highly charged ions is desirable as this allows the use of mass analyzers, such as ion traps (including orbitraps) and hybrid quadrupole instruments, which typically offer only a limited m/z range (< 2000–4000). It also enables more informative fragmentation spectra using techniques such as collisioninduced dissociation (CID) and electron capture/transfer dissociation (ECD/ETD) in combination with tandem MS (MS/MS). [5, 6] Thus, there is a clear advantage of using ESI in research areas where peptide sequencing, or in general, the structural elucidation of biomolecules by MS/MS is required. Nonetheless, MALDI with its higher tolerance to contaminants and additives, ease-of-operation, potential for highspeed and automated sample preparation and analysis as well as its MS imaging capabilities makes it an ionization technique that can cover bioanalytical areas for which ESI is less suitable. [7, 8] If these strengths could be combined with the analytical power of multiply charged ions, new instrumental configurations and large-scale proteomic analyses based on MALDI MS(/MS) would become feasible.

New analytical LC-mass spectrometry methodologies for the quali-quantitative determination of natural substances and drugs in complex matrices

This thesis reports an integrated analytical and physicochemical approach for the study of natural substances and new drugs based on mass spectrometry techniques combined with liquid chromatography. In particular, Chapter 1 concerns the study of Berberine a natural substance with pharmacological activity for the treatment of hepatobiliary and intestinal diseases. The first part focused on the relationships between physicochemical properties, pharmacokinetics and metabolism of Berberine and its metabolites. For this purpose a sensitive HPLC-ES-MS/MS method have been developed, validated and used to determine these compounds during their physicochemical properties studies and plasma levels of berberine and its metabolites including berberrubine(M1), demethylenberberine(M3), and jatrorrhizine(M4) in humans. Data show that M1, could have an efficient intestinal absorption by passive diffusion due to a keto-enol tautomerism confirmed by NMR studies and its higher plasma concentration. In the second part of Chapter 1, a comparison between M1 and BBR in vivo biodistribution in rat has been studied. In Chapter 2 a new HPLC-ES-MS/MS method for the simultaneous determination and quantification of glucosinolates, as glucoraphanin, glucoerucin and sinigrin, and isothiocyanates, as sulforaphane and erucin, has developed and validated. This method has been used for the analysis of functional foods enriched with vegetable extracts. Chapter 3 focused on a physicochemical study of the interaction between the bile acid sequestrants used in the treatment of hypercholesterolemia including colesevelam and cholestyramine with obeticolic acid (OCA), potent agonist of nuclear receptor farnesoid X (FXR). In particular, a new experimental model for the determination of equilibrium binding isotherm was developed. Chapter 4 focused on methodological aspects of new hard ionization coupled with liquid chromatography (Direct-EI-UHPLC-MS) not yet commercially available and potentially useful for qualitative analysis and for “transparent” molecules to soft ionization techniques. This method was applied to the analysis of several steroid derivatives.

Elucidation of Nucleic Acid–Drug Interactions by Tandem Mass Spectrometry

In continuation of the long tradition of mass spectrometric research at the University of Bern, our group focuses on the characterization of nucleic acids as therapeutic agents and as drug targets. This article provides a short overview of our recent work on platinated single-stranded and higher-order nucleic acids. Nearly three decades ago the development of soft ionization techniques opened a whole new chapter in the mass spectrometric analysis of not only nucleic acids themselves, but also their interactions with potential drug candidates. In contrast to modern next generation sequencing approaches, though, the goal of the tandem mass spectrometric investigation of nucleic acids is by no means the complete sequencing of genetic DNA, but rather the characterization of short therapeutic and regulatory oligonucleotides and the elucidation of nucleic acid–drug interactions. The influence of cisplatin binding on the gas-phase dissociation of nucleic acids was studied by the means of electrospray ionization tandem mass spectrometry. Experiments on native and modified DNA and RNA oligomers confirmed guanine base pairs as the preferred platination site and laid the basis for the formulation of a gas-phase fragmentation mechanism of platinated oligonucleotides. The study was extended to double stranded DNA and DNA quadruplexes. While duplexes are believed to be the main target of cisplatin in vivo, the recently discovered DNA quadruplexes constitute another promising target for anti-tumor drugs owing to their regulatory functions in the cell cycle.

Development, characterization, and application of flowing atmospheric-pressure afterglow ionization for mass spectrometric analysis of ambient organic aerosols

Addressing current limitations of state-of-the-art instrumentation in aerosol research, the aim of this work was to explore and assess the applicability of a novel soft ionization technique, namely flowing atmospheric-pressure afterglow (FAPA), for the mass spectrometric analysis of airborne particulate organic matter. Among other soft ionization methods, the FAPA ionization technique was developed in the last decade during the advent of ambient desorption/ionization mass spectrometry (ADI–MS). Based on a helium glow discharge plasma at atmospheric-pressure, excited helium species and primary reagent ions are generated which exit the discharge region through a capillary electrode, forming the so-called afterglow region where desorption and ionization of the analytes occurs. Commonly, fragmentation of the analytes during ionization is reported to occur only to a minimum extent, predominantly resulting in the formation of quasimolecular ions, i.e. [M+H]+ and [M–H]– in the positive and the negative ion mode, respectively. Thus, identification and detection of signals and their corresponding compounds is facilitated in the acquired mass spectra. The focus of the first part of this study lies on the application, characterization and assessment of FAPA–MS in the offline mode, i.e. desorption and ionization of the analytes from surfaces. Experiments in both positive and negative ion mode revealed ionization patterns for a variety of compound classes comprising alkanes, alcohols, aldehydes, ketones, carboxylic acids, organic peroxides, and alkaloids. Besides the always emphasized detection of quasimolecular ions, a broad range of signals for adducts and losses was found. Additionally, the capabilities and limitations of the technique were studied in three proof-of-principle applications. In general, the method showed to be best suited for polar analytes with high volatilities and low molecular weights, ideally containing nitrogen- and/or oxygen functionalities. However, for compounds with low vapor pressures, containing long carbon chains and/or high molecular weights, desorption and ionization is in direct competition with oxidation of the analytes, leading to the formation of adducts and oxidation products which impede a clear signal assignment in the acquired mass spectra. Nonetheless, FAPA–MS showed to be capable of detecting and identifying common limonene oxidation products in secondary OA (SOA) particles on a filter sample and, thus, is considered a suitable method for offline analysis of OA particles. In the second as well as the subsequent parts, FAPA–MS was applied online, i.e. for real time analysis of OA particles suspended in air. Therefore, the acronym AeroFAPA–MS (i.e. Aerosol FAPA–MS) was chosen to refer to this method. After optimization and characterization, the method was used to measure a range of model compounds and to evaluate typical ionization patterns in the positive and the negative ion mode. In addition, results from laboratory studies as well as from a field campaign in Central Europe (F–BEACh 2014) are presented and discussed. During the F–BEACh campaign AeroFAPA–MS was used in combination with complementary MS techniques, giving a comprehensive characterization of the sampled OA particles. For example, several common SOA marker compounds were identified in real time by MSn experiments, indicating that photochemically aged SOA particles were present during the campaign period. Moreover, AeroFAPA–MS was capable of detecting highly oxidized sulfur-containing compounds in the particle phase, presenting the first real-time measurements of this compound class. Further comparisons with data from other aerosol and gas-phase measurements suggest that both particulate sulfate as well as highly oxidized peroxyradicals in the gas phase might play a role during formation of these species. Besides applying AeroFAPA–MS for the analysis of aerosol particles, desorption processes of particles in the afterglow region were investigated in order to gain a more detailed understanding of the method. While during the previous measurements aerosol particles were pre-evaporated prior to AeroFAPA–MS analysis, in this part no external heat source was applied. Particle size distribution measurements before and after the AeroFAPA source revealed that only an interfacial layer of OA particles is desorbed and, thus, chemically characterized. For particles with initial diameters of 112 nm, desorption radii of 2.5–36.6 nm were found at discharge currents of 15–55 mA from these measurements. In addition, the method was applied for the analysis of laboratory-generated core-shell particles in a proof-of-principle study. As expected, predominantly compounds residing in the shell of the particles were desorbed and ionized with increasing probing depths, suggesting that AeroFAPA–MS might represent a promising technique for depth profiling of OA particles in future studies.

Measurement of polycyclic aromatic hydrocarbons in ambient air by fluorescence spectroscopic techniques

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Prompt electron emission and collisional ionization of ambient gas during pulsed laser ablation of silver

A silver target kept under partial vacuum conditions was irradiated with focused nanosecond pulses at 1:06 mm from a Nd:YAG laser. The electron emission monitored with a Langmuir probe shows a clear twin-peak distribution. The first peak which is very sharp has only a small delay and it indicates prompt electron emission with energy as much as 60 5 eV. Also the prompt electron emission shows a temporal profile with a width that is same as that for the laser pulse whereas the second peak is broader, covers several microseconds, and represents the low-energy electrons (2 0:5 eV) associated with the laser-induced silver plasma as revealed by time-of-flight measurements. It has been found that prompt electrons ejected from the target collisionally excite and ionize ambient gas molecules. Clearly resolved rotational structure is observed in the emission spectra of ambient nitrogen molecules. Combined with time-resolved spectroscopy, the prompt electrons can be used as excitation sources for various collisional excitation–relaxation experiments. The electron density corresponding to the first peak is estimated to be of the order of 1017 cm?--3 and it is found that the density increases as a function of distance away from the target. Dependence of probe current on laser intensity shows plasma shielding at high laser intensities.

Twin peak distribution of electron emission profile and impact ionization of ambient molecules during laser ablation of silver target

Laser-induced plasma generated from a silver target under partial vacuum conditions using the fundamental output of nanosecond duration from a pulsed Nd:yttrium aluminum garnet laser is studied using a Langmuir probe. The time of flight measurements show a clear twin peak distribution in the temporal profile of electron emission. The first peak has almost the same duration as the laser pulse while the second lasts for several microseconds. The prompt electrons are energetic enough ('60 eV) to ionize the ambient gas molecules or atoms. The use of prompt electron pulses as sources for electron impact excitation is demonstrated by taking nitrogen, carbon dioxide, and argon as ambient gases.

Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Characterization of Primary Amine End-Functionalized Polystyrene and Poly(methyl methacrylate) Synthesized by Living Anionic Polymerization Techniques

The reaction of living anionic polymers with 2,2,5,5-tetramethyl-1-(3-bromopropyl)-1-aza-2,5- disilacyclopentane (1) was investigated using coupled thin layer chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Structures of byproducts as well as the major product were determined. The anionic initiator having a protected primary amine functional group, 2,2,5,5-tetramethyl- 1-(3-lithiopropyl)-1-aza-2,5-disilacyclopentane (2), was synthesized using all-glass high-vacuum techniques, which allows the long-term stability of this initiator to be maintained. The use of 2 in the preparation of well-defined aliphatic primary amine R-end-functionalized polystyrene and poly(methyl methacrylate) was investigated. Primary amino R-end-functionalized poly(methyl methacrylate) can be obtained near-quantitatively by reacting 2 with 1,1-diphenylethylene in tetrahydrofuran at room temperature prior to polymerizing methyl methacrylate at -78 °C. When 2 is used to initiate styrene at room temperature in benzene, an additive such as N,N,N',N'- tetramethylethylenediamine is necessary to activate the polymerization. However, although the resulting polymers have narrow molecular weight distributions and well-controlled molecular weights, our mass spectra data suggest that the yield of primary amine α-end-functionalized polystyrene from these syntheses is very low. The majority of the products are methyl α-end-functionalized polystyrene.

Complementary mass spectrometric techniques for the characterization of the organic fraction in atmospheric aerosols

Aerosol particles are strongly related to climate, air quality, visibility and human health issues. They contribute the largest uncertainty in the assessment of the Earth´s radiative budget, directly by scattering or absorbing solar radiation or indirectly by nucleating cloud droplets. The influence of aerosol particles on cloud related climatic effects essentially depends upon their number concentration, size and chemical composition. A major part of submicron aerosol consists of secondary organic aerosol (SOA) that is formed in the atmosphere by the oxidation of volatile organic compounds. SOA can comprise a highly diverse spectrum of compounds that undergo continuous chemical transformations in the atmosphere.rnThe aim of this work was to obtain insights into the complexity of ambient SOA by the application of advanced mass spectrometric techniques. Therefore, an atmospheric pressure chemical ionization ion trap mass spectrometer (APCI-IT-MS) was applied in the field, facilitating the measurement of ions of the intact molecular organic species. Furthermore, the high measurement frequency provided insights into SOA composition and chemical transformation processes on a high temporal resolution. Within different comprehensive field campaigns, online measurements of particular biogenic organic acids were achieved by combining an online aerosol concentrator with the APCI-IT-MS. A holistic picture of the ambient organic aerosol was obtained through the co-located application of other complementary MS techniques, such as aerosol mass spectrometry (AMS) or filter sampling for the analysis by liquid chromatography / ultrahigh resolution mass spectrometry (LC/UHRMS).rnIn particular, during a summertime field study at the pristine boreal forest station in Hyytiälä, Finland, the partitioning of organic acids between gas and particle phase was quantified, based on the online APCI-IT-MS and AMS measurements. It was found that low volatile compounds reside to a large extent in the gas phase. This observation can be interpreted as a consequence of large aerosol equilibration timescales, which build up due to the continuous production of low volatile compounds in the gas phase and/or a semi-solid phase state of the ambient aerosol. Furthermore, in-situ structural informations of particular compounds were achieved by using the MS/MS mode of the ion trap. The comparison to MS/MS spectra from laboratory generated SOA of specific monoterpene precursors indicated that laboratory SOA barely depicts the complexity of ambient SOA. Moreover, it was shown that the mass spectra of the laboratory SOA more closely resemble the ambient gas phase composition, indicating that the oxidation state of the ambient organic compounds in the particle phase is underestimated by the comparison to laboratory ozonolysis. These observations suggest that the micro-scale processes, such as the chemistry of aerosol aging or the gas-to-particle partitioning, need to be better understood in order to predict SOA concentrations more reliably.rnDuring a field study at the Mt. Kleiner Feldberg, Germany, a slightly different aerosol concentrator / APCI-IT-MS setup made the online analysis of new particle formation possible. During a particular nucleation event, the online mass spectra indicated that organic compounds of approximately 300 Da are main constituents of the bulk aerosol during ambient new particle formation. Co-located filter analysis by LC/UHRMS analysis supported these findings and furthermore allowed to determine the molecular formulas of the involved organic compounds. The unambiguous identification of several oxidized C 15 compounds indicated that oxidation products of sesquiterpenes can be important compounds for the initial formation and subsequent growth of atmospheric nanoparticles.rnThe LC/UHRMS analysis furthermore revealed that considerable amounts of organosulfates and nitrooxy organosulfates were detected on the filter samples. Indeed, it was found that several nitrooxy organosulfate related APCI-IT-MS mass traces were simultaneously enhanced. Concurrent particle phase ion chromatography and AMS measurements indicated a strong bias between inorganic sulfate and total sulfate concentrations, supporting the assumption that substantial amounts of sulfate was bonded to organic molecules.rnFinally, the comprehensive chemical analysis of the aerosol composition was compared to the hygroscopicity parameter kappa, which was derived from cloud condensation nuclei (CCN) measurements. Simultaneously, organic aerosol aging was observed by the evolution of a ratio between a second and a first generation biogenic oxidation product. It was found that this aging proxy positively correlates with increasing hygroscopicity. Moreover, it was observed that the bonding of sulfate to organic molecules leads to a significant reduction of kappa, compared to an internal mixture of the same mass fractions of purely inorganic sulfate and organic molecules. Concluding, it has been shown within this thesis that the application of modern mass spectrometric techniques allows for detailed insights into chemical and physico-chemical processes of atmospheric aerosols.rn