Improvement of atmospheric pressure ion sources for mass spectrometry

In the first project, a new nebulization system for a liquid chromatography – mass spectrometry application (LC-MS) utilizing an inverse low temperature plasma (iLTP) ion source was developed. Instead of a standard atmospheric pressure chemical ionization nebulizer, two commercial nebulizers were disassembled and re-configured to spray the liquid chromatograph eluent onto the ionizing plasma in front of the mass spectrometer inlet. Since both devices, a medical inhaler by Omron and a bottle humidifier by Daiso, demonstrated irregular vibration and nebulization properties as well as a lack of mechanical robustness, another ultrasonic spraying device was developed in cooperation with Hitachi High-Tech corporations. For this application, focusing cones were developed to transport the solvent into gas phase and to focus the spray onto the plasma region to ensure better ionization. A design of experiments was performed to find the best operating parameters, and calibration curves were established to compare both the standard nebulizer and the Hitachi ultrasonic nebulizer. In general, the standard nebulizer proved advantageous which leaves further room for improvement of the ultrasonic nebulization for mass spectrometry applications. Furthermore, the iLTP ion source was compared to a tube plasma ionization configuration. Even though the tube plasma ionization ion source was less robust, its sensitivity was higher than that of the iLTP ion source.

In a second project, a repeller electrode was inserted into an electrospray ionization (ESI) housing to improve the ion transfer from the electrospray capillary to the mass spectrometer inlet. Different repeller designs were manufactured and their position and applied voltage optimized. It was found that the m/z of the investigated molecules and the flow rate of the supplied liquid influenced the peak intensity and the required repeller voltage to reach a signal intensity maximum. Smaller repellers were discovered to give better results and a concave repeller with a diameter of 12 mm was found optimal. A design of experiments provided optimal spatial conditions and repeller voltage. However, the effect that the repeller had on the overall results at the applied liquid chromatography flow rate (450 µL min‑1) was rather small in contrast to lower flow rates. A subsequent analysis of human plasma spiked with testosterone and reserpine also showed little improvement with the repeller electrode. It was furthermore discovered that the repeller electrode did not only aid the transfer of ions from the electrospray capillary into the mass spectrometer inlet, but that it was capable of ionizing molecules without any other ionization source present.

In the third project, the iLTP and a nanoESI ion source were coupled in a dual ion source design in front of a single quadrupole mass spectrometer. Due to the unshielded set-up, the experiments remained prone to environmental influences, but the parameters of both ion sources influencing the measurements were successfully investigated and subsequently optimized. The integration of a plasma ion source between ESI capillary and MS inlet showed a significant increase in the total ion current and the signal response of the test compound reserpine.



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