Dynamics of peak dispersion

Ga , B., StSdry, M., Rizzi, A., and Kenndler, E., Dynamics of peak dispersion in capillary zone electrophoresis including wall adsorption I. Theoretical model and results of simulation. Electrophoresis, 16, 958,1995. 

In order to achieve a good separation between two compounds with close mobilities, it is important to have each compound migrating in narrow bands (zones) through the capillary (column). These narrow bands in the separation column are reflected as narrow peaks in the final electropherogram. The broadness of bands in the electropherogram of a CE separation is determined by the dispersion of the migrating solute zones in the capillary (in the background buffer electrolyte). CE is a dynamic process therefore, dispersion effects are bound to occur. 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

The properties of the isolated peptides were quite similar in nature, whereby each peptide consisted of 12 amino acids in length and possessed a munber of residues with functional side groups that could stabilize nanoclusters. In many instances, these side chains were the hydroxyl-terminated side chains of serine, threonine, and tyrosine. In two of the peptides (AG3 and AG4), the location of the hydroxylated amino acids was conserved within two of the peptides. Similarly, one proline amino acid was conserved throughout all three of the sequences. Upon incubating each peptide in a solution of silver nitrate with no exogenous reductant, a clearly observable plasmon resonance peak arose at 440 nm for AG3 and AG4, but not with AG5. The peak was quite broad, indicative of a disperse size and shape distribution. The main difference between the active peptides and inactive AG5 was an overall basic isoelectric point for AGS The assays were performed at neutral conditions which would modulate the side-chain dynamics under acidic or basic conditions. 

As already mentioned, the effects of several parameters are often lumped into one (see also Section 6.5.3.1). In this case, all band broadening effects are included in a dispersion coefficient. The so-called apparent dispersion coefficient Dapp is used here to distinguish from the axial dispersion coefficient, Dm, which is assumed to be independent of concentration and only influenced by the quality of the packing. The lumped parameter Dapp includes peak broadening effects caused by the fluid dynamics of the packing (axial dispersion), as well as by all other mass transfer effects that might occur, and was first introduced by van Deemter et al. (1956). 

Peak width at half-maximum height is indicative of sample dispersion and has been often used for determining diffusion coefficients (see 5.2.3.1). Peak width near the baseline, often called peak appearance time [110] is used in flow systems relying on time-based measurements, with the aim of a wider dynamic range or implementation of pseudo flow titrations (see Chapter 8). 

Upon incubating each peptide in a solution of silver nitrate with no exogenous reductant, a clearly observable plasmon resonance peak arose at 440 nm for AGS and AG4, but not with AGS. The peak was quite broad, indicative of a disperse size and shape distribution. The main difference between the active peptides and inactive AGS was an overall basic isoelectric point for AGS. The assays were performed at neutral conditions which would modulate the side-chain dynamics under acidic or basic conditions. 

As an alternative to conventional sorption rate measurements it is also possible to derive diffusional time constants from the dynamic response of a packed column to a change in sorbate concentration. In a chromatographic system the broadening of the response peak results from the combined effects of axial dispersion and mass transfer resistance. By making measurements over a range of gas velocities it is possible to separate the dispersion and mass transfer effects and so to determine the effective overall mass transfer coefficient or the diffusional time constant. Further details are given in Section 8.5. 

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