Application of comprehensive gas chromatography (GCxGC) to measurements of volatile organic species in ambient air

The focus of this thesis was the in-situ application of the new analytical technique "GCxGC" in both the marine and continental boundary layer, as well as in the free troposphere. Biogenic and anthropogenic VOCs were analysed and used to characterise local chemistry at the individual measurement sites. The first part of the thesis work was the characterisation of a new set of columns that was to be used later in the field. To simplify the identification, a time-of-flight mass spectrometer (TOF-MS) detector was coupled to the GCxGC. In the field the TOF-MS was substituted by a more robust and tractable flame ionisation detector (FID), which is more suitable for quantitative measurements. During the process, a variety of volatile organic compounds could be assigned to different environmental sources, e.g. plankton sources, eucalyptus forest or urban centers. In-situ measurements of biogenic and anthropogenic VOCs were conducted at the Meteorological Observatory Hohenpeissenberg (MOHP), Germany, applying a thermodesorption-GCxGC-FID system. The measured VOCs were compared to GC-MS measurements routinely conducted at the MOHP as well as to PTR-MS measurements. Furthermore, a compressed ambient air standard was measured from three different gas chromatographic instruments and the results were compared. With few exceptions, the in-situ, as well as the standard measurements, revealed good agreement between the individual instruments. Diurnal cycles were observed, with differing patterns for the biogenic and the anthropogenic compounds. The variability-lifetime relationship of compounds with atmospheric lifetimes from a few hours to a few days in presence of O3 and OH was examined. It revealed a weak but significant influence of chemistry on these short-lived VOCs at the site. The relationship was also used to estimate the average OH radical concentration during the campaign, which was compared to in-situ OH measurements (1.7 x 10^6 molecules/cm^3, 0.071 ppt) for the first time. The OH concentration ranging from 3.5 to 6.5 x 10^5 molecules/cm^3 (0.015 to 0.027 ppt) obtained with this method represents an approximation of the average OH concentration influencing the discussed VOCs from emission to measurement. Based on these findings, the average concentration of the nighttime NO3 radicals was estimated using the same approach and found to range from 2.2 to 5.0 x 10^8 molecules/cm^3 (9.2 to 21.0 ppt). During the MINATROC field campaign, in-situ ambient air measurements with the GCxGC-FID were conducted at Tenerife, Spain. Although the station is mainly situated in the free troposphere, local influences of anthropogenic and biogenic VOCs were observed. Due to a strong dust event originating from Western Africa it was possible to compare the mixing ratios during normal and elevated dust loading in the atmosphere. The mixing ratios during the dust event were found to be lower. However, this could not be attributed to heterogeneous reactions as there was a change in the wind direction from northwesterly to southeasterly during the dust event.

Proton magnetic resonance with parahydrogen induced polarization : Imaging strategies and continuous generation

A major challenge in imaging is the detection of small amounts of molecules of interest. In the case of magnetic resonance imaging (MRI) their signals are typically concealed by the large background signal of e.g. the tissue of the body. This problem can be tackled by hyperpolarization which increases the NMR signals up to several orders of magnitude. However, this strategy is limited for 1H, the most widely used nucleus in NMR andrnMRI, because the enormous number of protons in the body screen the small amount of hyperpolarized ones.Here, I describe a method giving rise to high 1H MRI contrast for hyperpolarized molecules against a large background signal. The contrast is based on the J-coupling induced rephasing of the NMR signal of molecules hyperpolarized via parahydrogen induce polarization (PHIP) and it can easily be implemented in common pulse sequences.rnrnHyperpolarization methods typically require expensive technical equipment (e.g. lasers or microwaves) and most techniques work only in batch mode, thus the limited lifetime of the hyperpolarization is limiting its applications. Therefore, the second part of my thesis deals with the simple and efficient generation of an hyperpolarization.These two achievements open up alternative opportunities to use the standard MRI nucleus 1H for e.g. metabolic imaging in the future.

HCl gas gettering of 3d transition metals for crystalline silicon solar cell concepts

In order to reduce the costs of crystalline silicon solar cells, low-cost silicon materials like upgraded metallurgical grade (UMG) silicon are investigated for the application in the photovoltaic (PV) industry. Conventional high-purity silicon is made by cost-intensive methods, based on the so-called Siemens process, which uses the reaction to form chlorosilanes and subsequent several distillation steps before the deposition of high-purity silicon on slim high-purity silicon rods. UMG silicon in contrast is gained from metallurgical silicon by a rather inexpensive physicochemical purification (e.g., acid leaching and/or segregation). However, this type of silicon usually contains much higher concentrations of impurities, especially 3d transition metals like Ti, Fe, and Cu. These metals are extremely detrimental in the electrically active part of silicon solar cells, as they form recombination centers for charge carriers in the silicon band gap. This is why simple purification techniques like gettering, which can be applied between or during solar cell process steps, will play an important role for such low-cost silicon materials. Gettering in general describes a process, whereby impurities are moved to a place or turned into a state, where they are less detrimental to the solar cell. Hydrogen chloride (HCl) gas gettering in particular is a promising simple and cheap gettering technique, which is based on the reaction of HCl gas with transition metals to form volatile metal chloride species at high temperatures.rnThe aim of this thesis was to find the optimum process parameters for HCl gas gettering of 3d transition metals in low-cost silicon to improve the cell efficiency of solar cells for two different cell concepts, the standard wafer cell concept and the epitaxial wafer equivalent (EpiWE) cell concept. Whereas the former is based on a wafer which is the electrically active part of the solar cell, the latter uses an electrically inactive low-cost silicon substrate with an active layer of epitaxially grown silicon on top. Low-cost silicon materials with different impurity grades were used for HCl gas gettering experiments with the variation of process parameters like the temperature, the gettering time, and the HCl gas concentration. Subsequently, the multicrystalline silicon neighboring wafers with and without gettering were compared by element analysis techniques like neutron activation analysis (NAA). It was demonstrated that HCl gas gettering is an effective purification technique for silicon wafers, which is able to reduce some 3d transition metal concentrations by over 90%. Solar cells were processed for both concepts which could demonstrate a significant increase of the solar cell efficiency by HCl gas gettering. The efficiency of EpiWE cells could be increased by HCl gas gettering by approximately 25% relative to cells without gettering. First process simulations were performed based on a simple model for HCl gas gettering processes, which could be used to make qualitative predictions.