Enhanced Electrospray Ionization for Mass Spectrometry and Ion Mobility Spectrometry
Electrospray ionization (ESI) has become one of the most commonly used ionization techniques for mass spectrometry (MS) and ion mobility spectrometry (IMS), and efforts continue to improve its performance. ESI-MS is most recognized for its wide application to biomacromolecules where high sensitivity is of paramount importance. However, the major limitation in sensitivity with ESI-MS is due to its low ion transmission efficiency from the ESI source into the sampling orifice and through any stages utilized for transfer of ions from atmosphere to vacuum in the MS. A series of atmospheric pressure ion focusing interfaces were designed and implemented to enhance the performance of ESI-MS. The technical objective of this work was to improve sensitivity and detection limits of ESI-MS using a combination of concentric high velocity converging gas flow (aerodynamic focusing) and regulated external electric field (electrostatic focusing) to assist in focusing and transporting ions from the ESI sprayer tip into the sampling nozzle of the MS. The separation time in IMS, based on differing gas phase ion mobilities, ranges from several hundred microseconds to milliseconds. This allows faster analysis than most other conventional separation techniques, such as gas chromatography (GC), liquid chromatography (LC), and capillary electrophoresis (CE). However, the major limitation in ESI-IMS is its low resolution. It is believed that one of the most important contributions to low resolution in ESI-IMS is unwanted ion penetration through the ion gate. In order to solve this ion penetration problem, two mechanical ion gates were designed and optimized to assist in gating sprayed ions from the ESI source into the drift region of the IMS with improved sensitivity and resolution at atmospheric pressure. Applying a voltage to the ion gate and using a high flow drift gas helped to further improve the performance of ESI-IMS. Reduced pressure IMS should help to eliminate clustering and multiple peaks and, hence, improve experimental resolution when using ESI. Therefore, I report the design, construction and evaluation of new IMS systems for reduced pressures. However, the performance of the reduced pressure IMS was not as good as when using atmospheric pressure IMS.
electrospray ionization, mass spectrometry, ion mobility spectrometry, air amplifier, ion focusing, mechanical ion gate, sensitivity, resolution, Biochemistry, Chemistry
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