Ion generation, ion collection and ionic reactions outside the mass spectrometer

Abraham K Badu-Tawiah, Purdue University


Traditionally, the formation, deposition and reactions of molecular ions are conducted in high vacuum environment, or at least, at reduced pressure. In this thesis, molecular ions are considered as ordinary organic molecules and they are generated, transmitted and reacted with vapors or adsorbates on surfaces, all in the ordinary open environment. For example, ion soft landing was successfully implemented under ambient conditions. In ambient ion soft landing, ions generated by electrospray ionization are passed pneumatically through a heated metal drying tube, their polarity is selected using ion deflectors, and the dry selected ions are deposited onto a selected surface at a specified location. Unlike the corresponding vacuum soft-landing experiment, where ions are mass-selected and soft-landed within a mass spectrometer, here the ions to be soft-landed are selected through the choice of a compound that gives predominantly one ionic species upon ambient ionization. No mass analysis is performed during the soft landing experiment. The purified ions were typically used for surface preparation and modification. However, when utilized as reagents for organic reactions (e.g., pyrylium cation conversion into pyridinium cation) in a solvent-free environment, the ionic species provided a regioselective route to the expected reaction product when employing bifunctional reagents (e.g., D-lysine and ethanolamine) compared with the corresponding bulk solution-phase reaction conditions. Charged microdroplets associated with incompletely dried ions could also be selected for surface reactions by choice of the temperature of the drying tube inserted between the ion source and the electrical ion deflectors. This reaction condition allowed at least one order of magnitude increase in reaction rate compared with the bulk solution phase reaction conducted using the same quantities of starting reagents. The droplet reaction condition was further developed in three different ways: (i) nanoelectrospray emitter array, (ii) contained-electrospray apparatus and (iii) electrospray synthetic apparatus with high efficient product collection system. These are envisioned to impact the field of synthetic chemistry at larger scales due to the dramatic reaction rate enhancements. Several reaction systems have been studied under the charged microdroplet condition including [3,3]-sigmatropic rearrangement (e.g., Borsche-Drecsel cyclization), C-N (e.g., Girard condensation) and C-C (e.g., Claisen-Schmidt condensation) bonds formation. The droplet reaction was extended to thin film reactions which represent an alternative version of limited-volume reactors. The aza-Michael addition and the Mannich condensation were observed to occur in thin films deposited on ambient surfaces. Fundamentals of ion generation in the ambient environment were also studied, particularly the mechanism of desorption electrospray ionization (DESI). Critical evidence based on studies of surfactants and non-aqueous solvents has been provided for the proposed DESI (droplet pick-up) mechanism.




Cooks, Purdue University.

Subject Area

Analytical chemistry

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