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Zwitterionic ion chromatography


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Cook, Helmy Alicia 2003 , 'Zwitterionic ion chromatography', PhD thesis, University of Tasmania.

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This work presents a comprehensive study on Zwitterionic Ion Chromatography (ZIC)
for the separation of inorganic ions.
The elution characteristics of anions on a sulfobetaine-type stationary phase are
examined and it is shown that the Hofineister series could be used to predict the elution
order, with retention increasing with increasing polarisability, as well as the effect of
the mobile-phase anion on retention of anions. A mobile-phase anion with a greater
polarisability than the analyte anion reduced retention, and vice versa. An increase in
anion retention was also observed in changing the mobile-phase cation from Na+ to
Mg2+ to Ce3+ .
Electro-osmotic flow measurements in an analogous capillary electrophoresis system
revealed a zeta potential on the stationary phase that could be modulated from positive
(+40.2 mV for CeCI3) to negative (-53.4 mV for NaCI04) depending on the mobilephase
anion and cation. Thus the zwitterionic stationary phase is only neutral for
mobile phases where the mobile-phase anion and cation equally shield the charges on
the stationary phase.
A new retention mechanism for ZIC was developed, based the ability of an analyte to
penetrate the repulsion effects of a Donnan membrane (established by the zeta
potential) and to interact directly with the inner charge of the zwitterion. This
mechanism was established on the basis of experimental data obtained for the anion
system, and an analogous mechanism was proposed for a cation system that utilised a
phosphocholine-type stationary phase, where a systematic study of the elution effects
of this system was found to mirror that of the anion system.
Manipulation of the separation selectivity of inorganic anions in ZIC was achieved by
controlling the ratio of cationic and zwitterionic surfactants in the stationary phase
coating solution. Even at a ratio of 2:8 cationic:zwitterionic surfactant, a large
contribution of an ion-exchange mechanism from the cationic surfactant occurred, in
particular for small, highly-charged analytes. This was evident in the slopes of log k'
versus log [mobile phase] plots. For example, the slope for the analyte SO42- changed
from 0.23 to —1.22 when the ratio was changed from 1:10 to 2:8.
Hard and soft mathematical models were investigated to quantitatively describe the
mechanism of ZIC. The non-stoichiometric hard model was successful in describing
general experimental trends, but further work is required to achieve accurate
predictions of retention factors. The experimental data obtained for the anion system
were used successfully to train an artificial neural network capable of predicting
retention factors for a wide range of mobile phases. Plots of calculated versus
experimental retention factors for a test set of analytes gave an r2 of 0.985.

Item Type: Thesis - PhD
Authors/Creators:Cook, Helmy Alicia
Keywords: Inorganic ion exchange materials, Zeta potential, Anion separation, Ion exchange chromatography
Copyright Holders: The Author
Copyright Information:

Copyright 2003 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, 2003. Includes bibliographical references

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