The Development of Laser-Heated Multiple Electrospray Emitters for Nano-Electrospray Ionization Mass Spectrometry (Nano-ESI-MS)

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Groper, Emily
Mass Spectrometry , Multiple Electrospray , Light-Assisted Desolvation , Nano-Electrospray Ionization (ESI) , Microstructured Fibers (MSFs) , Wet-Chemical Etching
Nano-electrospray ionization mass spectrometry (nano-ESI-MS) is a valuable technique for detecting larger biomolecules such as oligosaccharides, glycosides and glycoproteins. The reliability of nano-ESI-MS, however, depends largely on the spray emitter properties. Commercial single-aperture, tapered nano-ESI emitters are limited by poor robustness, high clogging tendencies and a limited range of experimental flow rates. In order to overcome these limitations and yield mass spectrometry (MS) methods with higher sensitivity, this thesis seeks to develop novel emitters capable of generating multiple electrosprays (MESs) from custom-designed microstructured fibers (MSFs). A wet chemical etching method with hydrofluoric acid (HF), previously established by Bachus in 2017, was employed to fabricate a series of MES emitters with different nozzle profiles. These emitters were then characterized via scanning electron microscopy (SEM) and image analysis with respect to their nozzle length, post angle and axicon angle values. By using an offline ESI setup, parameters including the hydrophobic coating, voltage, emitter- to-orifice distance, and solvent flow rate were all examined in order to identify the optimal conditions for nano-ESI-MS analysis and data acquisition. The performance of the MES emitters was then tested using these optimal parameter settings. Preliminary results show an enhanced ionization efficiency and robustness compared to single-channel, commercial alternatives. Moreover, this thesis confirms the MES emitters’ ability to guide and focus light, and thereby, operate similarly to optical axicon lenses. We then investigate whether near-infrared (IR) light, that co-propagates with the liquid flow and is focused in front of each axicon microlens, can further amplify the ion spray current. Most notably, the expected enhancement in ionization efficiency was realized by coupling 1064 nm radiation into the MES emitter using a class IIIB laser. This work demonstrates that higher sensitivity and lower detection limits in ESI-MS are achievable by using the MES emitter employed in this report in tandem with light-assisted desolvation.
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