Infoscience

Thesis

High sensitivity field asymmetric ion mobility spectrometer

Field Asymmetric IonMobility Spectrometry (FAIMS) is a very promising technique for chemical detection. FAIMS systems are portable, highly sensitive, relatively inexpensive and can work at atmospheric conditions. These attributes make it a very desirable technique to replace current bulky and expensive detection systems, such as Mass Spectrometry devices, especially for on-site experiments. Therefore, this work focuses on improving the FAIMS limits of detection (LoD) in order to expand the application fields for FAIMS technology. With this aim, the different factors affecting the FAIMS sensitivity and resolution were analyzed by theoretical and experimental studies. An ion mobility simulation software was developed based on the physical models for FAIMS proposed in literature. This software is able to simulate the ion trajectories inside the ion filter and the expected ion spectra, making it an important tool for theoretically determining the effect of different parameters in FAIMS measurements. It also helps the user to predict and understand the experimental results. The possibility of improving the fabrication process was evaluated by implementing two different techniques: PCB manufacturing and screen printing. The PCB plates were fabricated on FR4 substrates with copper electrodes while the screen printed plates used alumina substrates and silver-palladium electrodes. The prototypes developed with PCB techniques showed a very similar behavior to the standard alumina systems. Therefore, the PCB fabrication approach is a good option to reduce the fabrication complexity and costs of the FAIMS devices. Two atmospheric pressure ionization sources, corona discharge and UV, were tested for possible use in the FAIMS setup. Both sources showed similar ionization efficiency levels. Between the two sources, the UV is a simpler system that requires very low optimization and has very stable ionization rates. Consequently, it was selected as the ionization method for this work. All the required electronics for the ions filtering and detection were also developed. The designed separation voltage circuit, based on the power nMOS inverter configuration, is able to produce rectangular waveforms of up to 1000 Vpp in the MHz range. The compensation voltage (CV) circuit can scan the ion spectra by generating a voltage ramp from -20 to 20 V. The fabricated low current amplifier has a gain of 10^11 with a noise of about 4 fA/Hz^0.5 limited by the thermal noise of the feedback resistor. The experimental parameters can be controlled and monitored by a custom user interface software developed in LabVIEW®. This software also digitalizes, displays and save the measured data. Finally, several measurements were performed to determine the influence of the different experimental parameters in the FAIMS sensitivity and resolution. In order to have a better understanding of the phenomena occurring within the FAIMS, the parameters¿ effects on each of the FAIMS section were studied separately. Also, for the first time, the effects of a synchronous modulation technique (lock-in) in the FAIMS ion current detection were analyzed. Based on the results of these studies, the LoD of the FAIMS system was significantly improved. The extrapolated acetone LoD levels, from the experimental signal amplitude and base line noise, of the developed device are 900ppt and 40ppt at carrier gas flows of 3.05 and 9.5 l/min respectively.

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