Published:Journal of Chromatographic Science,
ISSN 0021-9665Volume
39, Number 3, March 2001, pp. 87-92
Oligoacetic
Acid Characterization by Isocratic and Linear Salt Gradient Anion-Exchange Chromatography
Hong
Li[1],[2] and Costas S. Patrickios[1],[3]
[1]Department of Chemical Engineering, University of Manchester Institute of
Science and Technology (UMIST), P.O. Box 88, Manchester M60 1QD, U.K.;
[2}Department of Chemistry, University of Manchester, Oxford Road, Manchester
M13 9PL, U.K.;
[3]Department of Chemistry, Faculty of Pure and Applied Sciences, University
of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus
A homologous
series of four oligomers of acetic acid (namely acetic acid, succinic acid,
tricarballylic acid, and tetracarboxylic acid) is characterized using isocratic
and linear salt gradient anion-exchange chromatography. The double logarithmic
plot of the isocratic retention factors versus the salt concentration gives
straight lines for all samples. These straight lines (with the exception of
the line for the strongly retained succinate peak) have a common intersection
point—something which is proved to be a direct consequence of the stoichiometric
mass-action ion-exchange model. The characteristic charge and the equilibrium
ion-exchange constant (and the corresponding Gibbs free energy of ion exchange,
or DGexchange) are determined from the isocratic experiments. The characteristic
charge agrees satisfactorily with the number of carboxylic acid groups in the
samples, and the DGexchange value decreases linearly with the characteristic
charge. Succinic acid always gives two chromatographic peaks despite the proven
chemical purity of the sample. The characteristic charge that is calculated
for both of the succinic acid peaks is approximately two. The DGexchange value
calculated for the weakly retained succinic acid peak falls in the free energy
versus characteristic charge straight line defined by the other homologues.
The DGexchange value of the strongly retained peak is lower than that of the
weakly retained peak by 1.85 kJ/mol. The two succinic acid peaks are explained
in terms of an equilibrium between two conformers in solution—one binding
the solution counterions tightly and the other loosely. An analysis of all samples
under a linear salt gradient provides retention times that increase linearly
with the number of functional groups. Using an appropriate model (along with
the isocratically determined characteristic charge and ion-exchange constant),
we predict theoretically the linear gradient retention times, which agree reasonably
well with the experimental ones.
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