Retention coefficient

Guo, D, Mant, C. T., Taneja, A. K, Parker, J. M. R., and Hodges, R. S., Prediction of peptide retention times in reversed-phase high-performance liquid chromatography. I. Determination of retention coefficients of amino acid residues of model synthetic peptides, /. Chromatogr., 359, 499, 1986. 

Figure 4.1 Correlation of predicted and observed retention times in reversed-phase chromatography. The predicted retention times for 58 peptides of 2 to 16 residues in length were obtained by summation of retention coefficients for each residue in the peptide. Retention coefficients were determined from the retention of model synthetic peptides with the structure Ac-Gly-XX-(Leu)3-(Lys)2-amide, where X was substituted by the 20 protein amino acids. (Reproduced from D. Guo, C.T. Mant, A.K. Taneja, and R.S. Hodges, J. Chromatogr., 359 519 [1986]. With permission from Elsevier Science.)...

Separation of fully protected, neutral peptides by CEC occurs predominantly by chromatographic interaction with the RPC sorbent, with the observed retention coefficient,... 

The retention times of peptides with fewer than 20 residues in reversed-phase chromatography can be predicted with a high degree of accuracy based on their amino acid composition and the characteristics of their N-terminal and C-terminal amino acids. A number of researchers (66 -75) have studied the role of amino acids in peptide retention and have established retention coefficients for the different amino acids. The retention coefficient value of each amino acid is normally calculated by regression analysis of the retention times for peptides of known composition. 

Table 3 (73) compares the retention coefficients for synthetic peptides from various sources. To ensure comparability, the data has been standardized with respect to lysine and assigned a value of 100. The table shows that there are discrepancies between the results obtained using different chromatographic systems. Predictions of retention times should therefore be made using chromatographic systems similar to those used to calculate the retention coefficients for the amino acids. Casal et al. (75a) have made a comparative study of the prediction of the retention behavior of small peptides in several columns by using partial least squares and multiple linear regression analysis. 

The retention time predicted for a given peptide is equal to the sum of the retention coefficients for each residue and the terminal group, plus t0 (the elution time for an unretained compound) and ts (the difference between the observed ts and the calculated tr for a standard peptide). The retention times predicted for peptides with more than 20 residues are poorly correlated with experimental data, because the retention coefficients do not take steric factors into account. Moreover, the peptides may be denatured through the action of the solvents. 

Table 3 Comparison of Predicted Retention Coefficients of Amino Acid Residues...

Let mp s and mp be the polymer mass of the considered P-mer in the sol and gel, respectively, cp s and cp the corresponding concentrations, u(P) = (dz/dt)p the constant elution rate of the P-mer transported along the z-axis of the vertical PDC-column, and v the mean overall linear rate of the column liquid then, a trivial integration of the chromatographic transport Equation (i.e. the thermodynamically and hydrodynamically defined retention coefficient)... 

These results also suggest that the sulfate model of Baker et al. (KL52), which uses a single, lake-wide retention coefficient should be refined to reflect two mechanisms of sulfate retention with different controls. 

The value of the concentration modulus depends on the convective velocity and the mass-transfer coefficient of the concentration boundary layer (D/ i) that means that on the membrane structure and the hydrodynamic conditions. If the retention coefficient is equal to 1, then c /ch = exp(Pe). The larger convective velocity (or smaller diffusion coefficient) causes higher concentration polarization on the membrane interface. 

Differences in the molecular characteristics and interactive behavior of biosolutes can thus be revealed from these HPLC separations, through quantitative evaluation of the thermodynamic and extrathermodynamic differences manifested in the interaction of the biosolutes with the sorbent, under a defined set of mobile phase conditions at specified temperature and pressure. Exploitation of these principles forms the basis for the evaluation of the retention coefficients of polypeptides1,53,62,143 146,214 (through the dependency of linear free energy relationships given by group retention indices such... 

Retention coefficient of the first eluded enantiomer Retention coefficient of the second eluded enantiomer Separation factor... 

In order to be able to predict the retention behavior of peptides of different composition, of peptides of the same composition but different sequence (positional isomers), and of diastereoisomeric peptides, a knowledge of the incremental contribution of each amino acid to the overall contact area term is required not only for each well-defined stationary phase but also for each mobile-phase condition. Group retention coefficient summation approaches based on the assumption that selectivity differences can be ascribed predominantly to amino acid sequence differences, have been developed by Meek (46a, 52b) and Su et al. (45a). These treatments have subsequently been applied to a number of different elution systems (52c-52e). A comparative analysis of the different amino acid group contribution coefficients derived for phosphate, perchlorate, pyridine/acetate, trifluoroacetate, and bicarbonate buffer systems has been reported (52f). 

The generic permselectivity of a membrane can be described by the retention coefficient for liquid phase or the separation factor for gas phase. Separation factor will be defined and discussed in Chapter 7. In the case of liquid-phase membrane separation, the retention coefficient of the membrane can be characterized by some commonly used model molecules such as polyethylene glycol (PEG) polymers which have linear chains and arc more flexible or dextians which arc slightly branched. The choice of these model molecules is due to their relatively low cost. They are quite deviated from the generally... 

Membrane constituent pH of soaking solution Salt retention coefficient (%) Water permeation rate(cm -cm -hr ... 

Cf and Cq are the concentration of species, i, in the filtrate and the feed solutions, respectively T is usually expressed as an equivalent parameter R is the retention coefficient, given by... 

The experiments were conducted in the temperature range 35°C-80°C at feed inlet, and 5°C-30°C at distillate inlet, and with feed and distillate flow rates up to 1500 dm /h. Under these conditions permeate stream was 10-50 dm /h (60-300 dm /m day). During the experiment run the activity of the distillate was stable on the level of natural background radioactivity and the concentrating of radioactive compounds took place in retentate. Retention of radioactive ions in retentate was almost complete (decontamination factors oo. Table 30.13). Most of radionuclides were not detected in distillate only trace amounts of Co-60 and Cs-137 were present. Also retention coefficients of non-active ions were high (Table 30.14). 

Eq. 2 demonstrates that the plate resolution, as in other forms of chromatography, depends on the number of theoretical plates N, the selectivity, and the retention coefficient of the solute for the particular layer concerned. 

One of the basic retention parameters (i.e., the solute s retention coefficient k) can be expressed as a function of the solubility parameters, S ... 

High MW species are too large to enter the pores of the gel, and so elute with the void volume. Very low MW species enter all pores, and are the last to elute. Species of intermediate MW are separated based on their average abilities to enter and exit the pores of the gel, with larger molecules tending to be excluded more often. The retention coefficient is given by... 

To test for either adsorptive or electrostatic interactions, the SEC separation is performed at a variety of temperatures. If the separation occurs by size alone, the retention coefficient R(= V0/Ve) is independent of temperature very small variations may be observed as a result of gel swelling or microstructural changes to the gel. The presence of a significant dependence of R on T indicates the presence of a mechanism other than size exclusion. While R should not vary with 7 diffusion coefficients increase with T and so zone broadening occurs, leading to decreased resolution with increasing separation temperatures. 

Analytical chromatography aims to achieve an adequate, not necessarily a maximum, resolution of solute bands to identify and to quantify the analytes based on their retention coefficient, peak height and peak area. Information on the analytes is the target. 

The number of theoretical plates N is a measure of the peak broadening of a solute during the separation process (for definitions see Chapter 2). The efficiency of a column can be given for any solute of a test mixture but is strongly dependent on the retention coefficient of the solute. 

Material dp (jim) ratio) Optimum linear velocity u (mm s" ) Number of theoretical plates N Retention coefficient k of solute Reduced plate height h... 

All these parameters depend on the mass loadability of the column and change significantly when a critical loadability is reached. The critical mass loadability of analytical columns is usually reached at a 10% reduction of the retention coefficient or at 50 % decrease of column plate number. At higher values the column is, in chromatographic terms, overloaded. 

Rejection or retention coefficient This describes the ability of the membrane to retain the desired species from the feed on the membrane surface. Since the rejection is often dependent on membrane characteristics and operating parameters, these must be clearly stated so that a fair comparison can be made between different types of membranes for a given application. It is defined as / = 1- Cp/Cn, where, Cp is the concentration of the species in permeate and Q is its concentration in the retentate. If a significant passage of the species occurs, then an average concentration is used. 

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