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Mechanistic studies on the retention of carboxylic acids in ion-exclusion chromatography

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Ng, Kai Ling 2002 , 'Mechanistic studies on the retention of carboxylic acids in ion-exclusion chromatography', PhD thesis, University of Tasmania.

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Abstract

This work presents a comprehensive study on the retention mechanism of aliphatic and
aromatic carboxylic acids in ion-exclusion chromatography (ICE) by considering
simultaneous electrostatic repulsion effects and hydrophobic adsorption effects, as well
as other factors which affect retention. These factors can be divided into three
categories: analyte effects, mobile phase effects and stationary phase effects.
The analyte properties including pKa values, charge, hydrophobicity and size were found
to have the most significant effect in its retention. Furthermore, the retention mechanism
could be controlled by varying the mobile phase conditions from 1 o-6 to 10-3 M sulfuric
acid and 0 to 20% methanol, which in turn affected the degree of ionisation of the acid,
as well as reducing the adsorption oflong-chain and aromatic carboxylic acids.
The stationary phase substrate was also found to have a profound effect on the retention
mechanism. Changing the substrate affected the hydrophobicity of the columns, thereby
increasing or decreasing the contribution of adsorption to the retention mechanism. This
was especially evident for aromatic acids which exhibited very long retention times
when separated on polymeric columns, but were eluted significantly faster on silica
based columns. In addition, the effects of varying the ion-exchange capacity (from 0.5
to 5 mequiv/g) and the degree of cross-linking of the polystyrene-divinylbenzene
substrate (from 4 to 12%) were investigated and found to have a significant effect on the
retention of some acids.
A mathematical retention model which describes the relationship between the retention
factor of the analyte and the mobile phase composition, the type of analyte and physical
characteristics of the stationary phase was derived. Fourteen carboxylic acids
(comprising mono- and divalent, aliphatic and aromatic acids) were chosen and used to
acquire retention data on three different cation-exchange stationary phases (in which the
sulfonate functional groups are bound to polystyrene-divinylbenzene, polymethacrylate
or silica). Thirteen different mobile phase conditions of varying sulfuric acid
concentration and percentage of methanol were employed. These retention data were
used to derive the parameters necessary to solve the retention model using non-linear
regression. A plot of the calculated retention factors against the experimental values
gave a correlation coefficient (r2) calculated by least-squares regression of 0.9755. The
retention model was then applied to optimising the separation of nine carboxylic acids.
When solutions of sulfuric acids were used as the eluent in ICE, the poor buffer capacity
of the eluent introduced a significant error into the mathematical model. Therefore, a
second retention model was derived which accounted for unbuffered eluents and also
considered the degree of hydrophobic adsorption of the undissociated and dissociated
forms of the analyte onto the unfunctionalised polymer surface. The adsorption
coefficients calculated from the model were in accordance with expected trends and
showed that both the undissociated and dissociated forms of the analyte acids were
retained by hydrophobic adsorption effects. The correlation coefficient calculated for
this retention model was 0.9863 and the derived equation was able to accurately predict
the retention of acids which exhibited strong adsorption effects as well as to optimise the
conditions for their separation.

Item Type: Thesis - PhD
Authors/Creators:Ng, Kai Ling
Keywords: Carboxylic acids, Ion exchange chromatography
Copyright Holders: The Author
Copyright Information:

Copyright 2002 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, 2002. Includes bibliographical references

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