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Ion-exchange capillary electrochromatography of inorganic and small organic anions

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Hilder, Emily Frances (2000) Ion-exchange capillary electrochromatography of inorganic and small organic anions. PhD thesis, University of Tasmania.

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Abstract

This work presents a systematic study on the use of packed column ion-exchange
capillary electrochromatography (IE-CEC) for the separation of ionic analytes.
Methods have been developed for preparing packed capillary columns utilising
either silica-based or polymer-based stationary phases. The stationary phase is
retained by futs prepared by sintering either the silica-based packing material or
pure silica. The electroosmotic and pressure-driven properties of frits prepared from
bare or functionalised silica were investigated. Despite the high temperature of
heating of the packing, sufficient residual functional groups remained on the fut
such that distinctively different EOF behaviour was observed for each type of
packing. Frits made from materials providing negative surface charges increased the
magnitude of the cathodic EOF compared to the open capillary, but a substantial
anodic EOF of -1.88 10-8 m2 v-1 s-1 was produced by introduction of a single fut
made from SAX material. An explanation of this behaviour is based on the
hypothesis that the EOF generated by the frit determines the overall flow in the
whole open capillary. The frit is considered to work as a pump and overrides the
EOF generated at the capillary wall. IE-CEC was used for selectivity manipulation in the separation of a number of
inorganic anions using a silica-based anion-exchanger. Changing the type and
concentration of the competing anion in the eluent mediated the ion-exchange (IE)
contribution to the separation mechanism. The use of polymer-based anion
exchangers and a combination of hydrodynamic and electroosmotic flow allowed
both the electrophoretic and/or chromatographic components of the separation
mechanism to be varied and considerable changes in the separation selectivity could
be obtained. With a short packed bed the separation of 8 test analytes in under 2.2
min was possible using pressure-driven flow and a simple step voltage gradient. Using only the application of high voltage allowed many of the same analytes to be
separated in less than 20 s with different separation·selectivity.
A theoretical model was derived from IE and capillary electrophoresis (CE) theory,
which describes the mobility of inorganic anions in IE-CEC. The model was
verified using a mixture of UV absorbing inorganic ions in eluents of differing
eluotropic strength with excellent agreement (r2 > 0.99) obtained. Values of
constants in the model equation determined by non-linear regression were used to
estimate the relative strengths of the interactions of different analytes with the
stationary phase and were found to agree well with elution orders observed in
conventional IE. A mixed-mode C6/SAX stationary phase was used for the simultaneous separation
of acidic, basic and neutral organic compounds and inorganic anions. The analytes
were separated by a mechanism that comprised chromatographic interactions
(hydrophobic interactions, ion-exchange) as well as electrophoretic migration. The
influence of ion-exchange and hydrophobic interactions on the retention/migration
of the analytes could be manipulated by varying the eluent composition.
Conductivity detection was applied to IE-CEC using a capacitively coupled
contactless conductivity detector (C4
D) with detection occurring through the packed
bed. A systematic approach was used to determine suitable eluents for IE-CEC
separations with simultaneous indirect UV and direct conductivity detection.
Homogenous column packing was found to be imperative, and monitoring of the
baseline could be used to assess the homogeneity of the packed bed. Direct
conductivity detection was found to be superior to indirect UV detection with regard
to both baseline stability and detection sensitivity with detection limits of 4-25 µg/L
being obtained.

Item Type: Thesis (PhD)
Keywords: Ion exchange chromatography, Anion separation, Capillary liquid chromatography
Copyright Holders: The Author
Copyright Information:

Copyright 2000 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2001. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:41
Last Modified: 21 Nov 2016 01:24
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