ORCID Identifier(s)

0000-0001-9299-2825

Graduation Semester and Year

2021

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry

Department

Chemistry and Biochemistry

First Advisor

Purnendu K Dasgupta

Abstract

This dissertation aims to improve the functionality of open tubular capillary ion chromatography (OTIC) in terms of the lower limit of detection (LOD) and higher sensitivity by fabricating a microchannel suppressor and integrating it into an OTIC system. To reach the goal, an ion-exchange membrane (IEM) was formulated and developed using aqueous solutions environmentally friendly chemicals. The PVA-SS polymer is made from poly vinyl alcohol (PVA) as the polymer matrix and styrene sulfonate sodium salt (SS) as the ionic functional group (the salt-form polymer is eventually converted to acidic form), referred to here mostly as PVA-SS polymers. The new IEM has high ion-exchange capacity (IEC), at the high end, 2 times more than best known commercial cation exchange membrane, Nafion. The PVA-SS prepolymer solution can be uniquely cast around fine wires as mandrels, polymerized, and thus generate a smooth circular micro-size channel after removing the wire. This allowed us to succeed in fabricating suppressors with diameters size close to those of actual OT columns in use sizes. The second chapter of the dissertation reviews the broad area of analytical applications of ion-exchange membranes (IEM) including their applications in IC in fabricating suppressors, the main focus of this dissertation. It provides a complete history of IEM-based suppressors in IC and how they have been evolved. It reviews different geometries of chemically and electrodialytically regenerated suppressors from macro to micro size. In addition, it covers other analytical applications of IEMs, such as electrodialytic eluent generation, water purification, charge detection, matrix isolation, and reagent introduction, Donnan dialysis separation, and preconcentration. Chapter 3 discuses formulating, developing, and characterizing PVA-SS as a novel moldable strong cation-exchange polymer. It centers on the synthesis of PVA-SS polymers and their characterization for water sorption capacity, specific conductance values, and ion-exchange capacities across the different polymer compositions. IEMs exhibit anisotropy regarding their electrical conductance. The two different specific conductance values of IEMs (normal and tangential) were thoroughly discussed including experimental values of PVA-SS and compared with those for several commercial IEMs. PVA-SS polymers were found to absorb water (as moles of water/mole H+) 5- 10 times greater than does Nafion. The IEC of PVA-SS polymers at the high SS-content end is > 2x of Nafion. The PVA-SS polymers survive hour-long boiling in water and alcohols and maintains their high IECs over repeated regeneration cycles. This chapter also describes the fabrication of microchannel ~ 30 μm in diameter from casting the prepolymer solution around the 25 μm wire as a mandrel. The microchannel permitted facile flow of water and was shown to withstand at least 300 psi pressure. Chapter 4 covers how a fabricated 45 μm bore microchannel in a PVA-SS polymer was utilized as a microsuppressor in OTIC. The microsuppressor was characterized using tandem mass spectrometry (MS) and conductance measurements to determine any potential leaching of the polymer or its monomeric components. A commercial electrodialytically regenerated suppressor was also characterized in a similar manner by MS. The PVA-SS microchannel suppressor showed that with rare exceptions (possibly in the m/z range of 675-750), no significant leachates over blank water were observed for the whole range of m/z 30-1500. The blank was water that went directly through the system bypassing the suppressor. However, for the commercial suppressor, the story was completely different. The PVA-SS polymer simply has far less detectable MS background to permit extended use in mass spectrometry. The microsuppressor was next integrated with the column and detector to realize its practical application as a chemically regenerated suppressor in an OTIC system. The microsuppressor was placed between an admittance detector and a conductivity following the separation column. The two separate detectors provide orthogonal information about the analytes and also permit evaluation of the dispersion caused by the microsuppressor. The results indicated that the induced dispersion is perceptible even with the suppressor of this bore and the additional dispersion of a 700 μm long suppressor can discerned over that caused by a 400 μm long suppressor. However, the considerable increase in peak height and decrease in baseline noise in suppressed IC results indicate that adding the microsuppressor leads us to the lower LOD and higher sensitivity. A 700 μm long microsuppressor was tested at a flow rate of 168 nL/min (typical of flow rates currently used in OTIC) for its ability to suppress (quantitively exchange K+ for H+) a KOH solution. The microsuppressor could suppress 30 mM KOH; comparison, most practical eluent concentrations in OTIC is less than 10 mM. The capabilities of a PVA-SS microsuppressor is therefore adequate. The last chapter addresses potential future work in this area. Ways to improve the performance by modifying PVA-SS microsuppressors are suggested and discussed.

Keywords

Analytical chemistry, Liquid chromatography, Ion chromatography polymers, Ion exchange membranes, Ion exchange polymers, Suppressors, Open tubular ion chromatography, Capillary ion chromatography, Innovation, Novel Synthesis, Polymer synthesis, Ion exchange, membrane synthesis

Disciplines

Chemistry | Physical Sciences and Mathematics

Comments

Degree granted by The University of Texas at Arlington

Included in

Chemistry Commons

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