Chemical Ionization laser Time-of-flight mass spectrometry

Volatile organic compounds (VOC) are emitted both from biogenic and anthropogenic sources followed by complex oxidation processes. These processes involve formation of tropospheric ozone and the formation of secondary organic aerosol particles, both affecting human health and climate. Our understanding of this chemistry has advanced substantially over the last decades and has always been accompanied by instrumentation allowing direct observation of the complex chemistry. The myriad of different species being present in the atmosphere at extremely low concentrations only one molecule in a thousand billion ones may still have huge impact atmospheric chemistry makes atmospheric measurements in particularly challenging. Modern approaches rely on charging initially neutral molecules (Chemical Ionization), followed by mass spectrometric detection and quantification. However, a mass spectrometer measures the amount of species having a particular mass and thus cannot distinguish between species having the same molecular sum formula and therefore the same molecular mass but different chemical structures. The structure of a molecule does define its chemical and physical properties, making a measurement technique being able to specifically distinguish molecules having different structures desirable. The current project Chemical Ionization laser time-of-flight mass spectrometry aims to close this observational gap by combining latest developments in chemical ionization mass spectrometry with spectroscopic techniques. Infrared laser spectroscopy is based on the absorption of light of specific wavelength by different chemical building blocks of molecules therefore being able to distinguish between different structures. However, as a stand-alone technique, infrared spectroscopy is not suited for complex mixtures such as ambient air containing thousands of different species, since interpretation of the obtained spectrums is impossible due to overlapping absorption bands. A combination with mass spectrometry on the other hand introduces a second dimension: having both the molecular mass and the structural information on every individual molecular mass present in the mass spectrum, makes interpretation possible. Only recently, with the development of powerful benchtop infrared laser systems, laser spectroscopy of charged molecules combined with mass spectrometry became feasible. The development of an atmospheric measurement platform being able to measure concentrations of organic species in the atmosphere present at extremely low concentrations and at the same time being structure-specific will advance atmospheric science by providing hitherto inaccessible observational data of the composition of the atmosphere.

Besides carbon dioxide (CO2) and nitrous gases, volatile organic compounds play a key role in our atmosphere. These volatile organic compounds are emitted both by biogenic and anthropogenic sources, while biogenic emissions by far exceed the source strength of anthropogenic sources. Once emitted, chemical reactions lead to a myriad of oxidized organic compounds, leading to the complex composition of our atmosphere. Many of these oxidation products are significantly less volatile, which enables condensation of these molecules onto pre-existing aerosols. This process leads to so-called secondary organic aerosol, influencing climate and human health via changing both chemical properties and the size distribution of ambient aerosol. Due to the development of novel instrumentation to detect the species involved, our knowledge about these processes has dramatically increase in recent years. These measurements are challenging for a number of reasons: ambient air is a complex mixture of hundreds or thousands of chemical species; many of them are present in minute concentration levels, often less than 1 molecule in 100 billion other molecules. Within this project, methods and instruments were developed with the goal to close some observational gaps. Modern instruments are based on a method called Chemical Ionization, in which neutral organic molecules are ionized (electrically charged) and subsequently seperated by their mass and detected by means of a mass spectrometer. On the basis of prior research conducted at the University of Innsbruck and a commercially available high performance mass spectrometer, a novel ion source was designed to further improve overall instrument performance. The new design improved previous instrument sensitivities by a factor of 5, enabling more precise measurements of substances present in minute concentrations. Several challenges need to be adressed in order to perform both qualitative and quantitative measurements: fragmentation (break-up of molecules within the instruments in smaller pieces) should be avoided or minimzed in order to being able to interpret the resulting mass spectrum in complex gas mixtures such as ambient air. Furthermore, in order to being able to determine the amound (concentration) of detected species, the instrument's response factor (its sensitivity) towards the myriad of individual organic compounds needs to be well known. We develped a method which both reduces fragmentation and allow to estimate the sensitivties towards all measured substances within a mass spectrum at the same time. Furthermore, a method was developed, which allows the detection of short-lived radical species. These radical species are formed during the oxidation process and are difficult to detect due to their high reactivity thus short life time. By adding a so-called spin-trap to the incoming air stream to be analyzed, these highly reactive species form a stable complex with a high enough life time to be detected with existing state-of-the-art instrumentation.