Ambient desorption/ionization mass spectrometry (ADI-MS) has emerged as an important field for direct sample analysis since the mid-2000s, when desorption electrospray ionization (DESI) and direct analysis in real time (DART) were introduced. The main goal of ADI-MS is to minimize sample preparation procedures (e.g., solvent extraction) and to overcome the long analysis times, often encountered with chromatographic separations, by directly desorbing the target analytes from the sample surface.
In the first part of this thesis, a brief summary and review will highlight the main achievements for the optimization of plasma-based ADI sources, which can be found in peer-reviewed journal articles. These ADI source optimization parameters are important to improve the methods sensitivity for fast analyte screening and direct surface analysis. Subsequently to this literature overview, own studies for the optimization of instrumental parameters for the flowing atmospheric-pressure afterglow (FAPA) source will be presented in a following chapter.
In the second section of the thesis, DART-MS, FAPA-MS, and low-temperature plasma mass spectrometry (LTP-MS), are compared for the direct analysis of liquid-crystalline materials by MS. With this approach sample preparation is minimized for quick and cheap liquid crystal display (LCD) failure analysis. For DART-MS limits of detection (LODs, absolute amount) in the ng range and for LTP- and FAPA-MS LODs in the pg range are obtained. Additionally, mass-spectral imaging of LCDs by FAPA-MS and laser ablation halo-FAPA-MS (LA-h-FAPA-MS) will be presented. This instrumental setup enables the analysis of very low sample volumes inside an LCD and first experiments suggest an estimated spatial resolution of 1-2 mm and 0.6 mm for FAPA-MS and LA-h-FAPA-MS, respectively. Ultimately, these methods shall help to discriminate between different LCD components and to identify defects within an LCD by lateral profiling.
Despite the versatile application possibilities for ADI-MS methods, these techniques are very sensitive to matrix effects. Thus, thin-layer chromatography (TLC) is used to quickly separate matrix components and reaction byproducts from the target analytes before spatial resolved and quantitative detection by FAPA-MS. Furthermore, different TLC stationary phases (e.g., normal phase, reversed phase, and cyano phase) as well as different particle sizes of the stationary phase are compared for best performance. Moreover, a validated TLC-FAPA-MS method is developed for quantification of 0.32 mg/mL caffeine in energy drinks via stable isotope dilution analysis. A second application involves the separation and individual detection of active agents in antimalarial preparations.
Finally, FAPA-MS is applied for the analysis of a P4O10 in DMSO mixture, to elucidate the reaction mechanism between both compounds. With these measurements the presence of the important C2H5S+ intermediate product could be confirmed.