Page 187 - Chiral Separation Techniques

6.5 Adsorption–Desorption Kinetics and Chromatographic Band Broadening 165
6.5 Adsorption-Desorption Kinetics and Chromatographic
Band Broadening

For most applications of specific molecular recognition elements, a rapid association
dissociation kinetics in the ligand receptor binding is important. In chemical sensors
the response time depends on the association rate between the sensor-bound recep-
tor and the target analyte, whereas the dissociation rate determines if, and how
quickly, the sensor can be regenerated [51]. The kinetics thus influences the sample
throughput of the analysis, i.e. how many samples that can be analyzed in a certain
time interval. Furthermore, in catalysis the binding kinetics will determine the max-
imum rate of the chemical transformation, and in chromatographic separations it will
influence the spreading of the chromatographic peaks.
When a solute band passes a chromatographic column it is broadened continu-
ously due to various dispersion processes [52]. These include processes that show
little or no flow rate dependence, such as eddy-diffusion or extracolumn effects and
flow rate-dependent processes such as axial diffusion, mass transfer processes
including mobile phase, intraparticle and stationary phase diffusion and slow kinetic
processes upon interaction with the stationary phase. Other factors such as nonlinear
binding isotherms and slow desorption kinetics instead affect the shape of the peak
[53]. Altogether, these processes counteract the separation of two compounds and
lead to lower resolutions. An understanding of their origin is important in order to
improve the separations as well as to gain insight into the kinetics and mechanism
of solute retention.
The dependence of the chromatographic parameters on flow rate and sample load
was studied in enantiomer separations of d- and l-phenylalanine anilide (D,L-PA) on
L-PA-imprinted chiral stationary phases (CSPs) [54].Using a thermally annealed sta-
tionary phase, a strong dependence of the asymmetry factor (A ) of the l-form on
s
sample load and a weak dependence on flow rate suggested that column overloading
contributed strongly to the peak asymmetry (Fig. 6-7). This is to be expected in view
of the site heterogeneity discussed in the previous section. However, slow kinetic
processes is another contributing factor to the pronounced band broadening in the
chromatography using MIP-based columns. In view of the high binding constants
observed for MIPs, the desorption rate at the high-energy binding sites should be
much slower than that at the low-energy sites. The mass transfer rate coefficients,
estimated using a MIP prepared in dichloromethane as diluent, were small and
strongly dependent on the temperature and concentration, in particular the rate coef-
ficients corresponding to the imprinted L-enantiomer [42]. Recent related studies of
the retention mechanism of both enantiomers of dansyl-phenylalanine on a dansyl-
L-phenylalanine MICSP led to similar conclusions [55], although these processes are
strongly dependent on the system studied, i.e. template-monomer system, crosslink-
ing monomer, porogen and method of polymerization.