Biomedical subjects

Tarab Ahmad

Publications and source records attributed to Tarab Ahmad.

4 recordsLinked to original sources

PubMed2006

Numerical determination of the adsorption isotherms of tryptophan at different temperatures and mobile phase compositions.

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for tryptophan on a C(18)-Kromasil packed column, using acetonitrile-water solutions of various compositions (2.5, 5, and 7.5% ACN+1% acetic acid) and at five different temperatures between 25 and 65 degrees C. The adsorption isotherm model accounting best for these data is the bi-Moreau model, showing that two types of adsorption sites coexist on the surface and that strong adsorbate-adsorbate interactions take place. Large concentration band profiles of tryptophan were obtained for the three mobile phase compositions, at five different temperatures and the best values of the adsorption isotherm coefficients were determined by the inverse method (IM) of chromatography. The advantages and drawbacks of using the FA and the IM for determining the coefficients of the adsorption isotherm of tryptophan under the experimental conditions selected are discussed. The results of the FA and IM measurements are in good agreement. Both indicate that the retention time of tryptophan decreases rapidly with increasing acetonitrile concentration in the mobile phase as well as the saturation capacities of the two types of adsorption sites, with the highest values of the two saturation capacities being found for the lowest ACN content and the lowest temperature. The adsorption constant on the low-energy sites decreases with increasing acetonitrile content and temperature. In contrast, the adsorption constant on the high-energy sites increases with increasing ACN content of the mobile phase but decreases with increasing temperature. The solute-solute interaction parameters for the low and the high-energy adsorption sites increase rapidly with increasing ACN concentration in the mobile phase and with increasing temperature.

Adsorption↗

PubMed2006

Effect of temperature on the adsorption behavior of tryptophan in reversed-phase liquid chromatography.

Single-component adsorption isotherm data of l-tryptophan on a C(18)-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 5% of acetonitrile at five different temperatures between 23 and 62 degrees C. The non-linear fitting of these data provided the bi-Moreau model for all temperatures as the best isotherm model. The inverse method (IM) was used to derive the parameters at these temperatures from the parameters of the 25 degrees C isotherm. The adsorption constants and the saturation capacities of the low and high-energy sites decreases by increasing the temperature, while the adsorbate-adsorbate parameters of both sites increase. An excellent agreement was found between the experimental and calculated overloaded band profiles at all the temperatures used. The breakthrough curves obtained and the overloaded band profiles obtained were found to have different shapes according to the range of concentration studied and the temperatures. At low concentration 0.05-0.5 g/L the breakthrough curves and the overloaded band profiles have a front shock and diffuse rear, which indicates langmuirian behavior, but at intermediate 1-2 g/L and high concentration 8 g/L they start to have diffuse fronts and shocks at the rear or more than one shock at the rear which indicates non-langmuirian behavior. At 23 degrees C the isotherm has another langmuirian part, which appears at high concentration. The behavior of the breakthrough curves is explained by the shape of the isotherm in which all of the isotherms have a langmuirian part (the isotherm is concave upward) and an antilangmuirian part (the isotherm is concave downward). The temperature affected the breakthrough curves by decreasing the time of the appearance of the fronts for all concentration ranges studied, and by decreasing the time difference between the highest concentration and lowest concentration of the fronts, especially the low concentration range at 0.5 g/L. The fronts of the breakthrough curves at high concentration seems to be the most affected by temperature.

Acetates↗

PubMed2006

Effect of the mobile phase composition on the adsorption behavior of tryptophan in reversed-phase liquid chromatography.

Single-component adsorption isotherm data of l-tryptophan on a C18-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 2.5, 5, and 7.5% of acetonitrile (ACN) or 7, 10, 15, and 20% of methanol (MeOH). Most of these isotherms have two inflection points and three different parts. The low and the high concentration parts exhibit langmuirian behavior. The intermediate part exhibits anti-langmuirian behavior. The inflection points shift toward higher concentrations with increasing mobile phase concentration in ACN or MeOH, which causes the differences in the isotherm profiles. The nature of the organic modifier and its concentration affect only the isotherm profile and the numerical values of its parameters, not the nature of the best model, which is the bi-Moreau model in all cases. The isotherm profiles depend on the experimental conditions because they affect the intensity of the adsorbate-adsorbate interactions. Overloaded band profiles of tryptophan were recorded with the seven mobile phase compositions. They were used to determine the best values of the isotherm coefficients by the inverse method (IM) of chromatography. There is an excellent agreement between the values of these parameters obtained by FA and by IM. Increasing the concentration of either ACN or MeOH in the mobile phase causes a slight decrease in the saturation capacities of the low and the high energy sites, and in the adsorption constant of the low energy sites. The adsorption constant of the high energy sites increases with increasing concentration of either solvent or is little affected. The adsorbate-adsorbate interaction constants of both low and high energy sites increase for both solvents. Saturation capacities of the high energy sites are higher for ACN than for MeOH.

Acetonitriles↗

PubMed2004

Single-component shock layer analysis in elution chromatography.

One of the fundamental results of the theory of nonlinear chromatography is that a propagation velocity is associated with each concentration. This velocity is related to the slope of the isotherm at the corresponding concentration. It follows that if a continuous concentration gradient is injected into a column, the gradient profile will not propagate in a mere translation but will progressively change shape. In the most common case of a convex upward isotherm (e.g., Langmuir), a linear gradient will become curved upward, the high concentrations migrating faster than the low ones. However, high concentrations cannot pass low ones, so the concentrations pile up, a concentration shock forms, and its height increases. In practice, axial dispersion and the mass-transfer resistances combine and prevent the formation of a true shock. A shock layer, a region where the concentration gradient is very steep, is formed. This shock layer migrates at the same velocity as the ideal shock would. Many characteristics of concentration shocks and shock layers have been determined previously, but not the time that it takes for a continuous gradient to turn into a shock layer, the circumstances of the birth of the shock layer, and of its growth. Yet, this is important to know to understand certain aspects of gradient elution. We have derived simple equations relating the circumstances of the birth of shocks to the phase equilibrium isotherm and to the column characteristics. The results of experimental measurements made with a high-efficiency analytical column are in excellent agreement with these theoretical predictions.

Journal Article↗