Brett J. Stanley

Determination of single component isotherms and affinity energy distribution by chromatography.

Adsorption isotherm data were acquired by frontal analysis (FA) and large sample-size band profiles were recorded for phenol and caffeine. For both compounds, the isotherm data fit well to the Langmuir, Toth, and Bi-Langmuir models of adsorption. The Langmuir model must be dismissed because it does not predict accurately the overloaded band profiles. However, profiles calculated using the unimodal Toth and the bimodal Bi-Langmuir models are indistinguishable. The expectation-maximization procedure was used to calculate directly the affinity energy distribution (AED) from the raw FA data points. For both compounds, the AED converges to a bimodal distribution at high numbers of iterations. This result, which shows the high sensitivity of the EM method, suggest that the Bi-Langmuir model makes better physical sense than the Toth model. This model also permits a detailed investigation of the properties of active sites, a feature often evoked in chromatography but so far rarely the topic of a quantitative investigation.

On the reproducibility of column performance in liquid chromatography and the role of the packing density

The packing behavior of a typical 10 μm C18 stationary phase was studied in terms of the resultant column efficiency and capacity factor. The column-to-column reproducibility of these parameters under identical packing procedures is assessed. Correlation of these parameters and the column void volume to the column packing density is reported. Two regimes were studied: that of poor-and well-packed columns. For poorly packed columns, the column-to-column variability is high, but a concomitantly poor same-column reproducibility of measurement suggests that little statistically significant difference exists between different columns packed with the same procedure. There is also no statistically significant correlation between the column parameters and the packing densities, however, the poorest columns showed a degradation of performance after the drying procedures used to obtain the column masses. Well-packed columns showed much less degradation upon drying. For the well-packed columns, statistically significant column-to-column differences were observable, mainly due to a high same-column precision of measurement. Analysis of the results suggests that even well-packed columns are not optimally packed and that regions of high and low density coexist along the column. The results are compared to those achieved with semi-preparative columns packed with the same slurry procedure and preparative columns packed under dynamic axial compression. Poor day-to-day, same-column reproducibility (degradation) under ambient conditions was observed in conjunction with a high column-to-column variability for the semi-preparative columns.

Consolidation of the packing material in chromatographic columns under dynamic axial compression IV. Mechanical properties of some packing materials

Dynamic axial compression columns were packed with three C18 silica materials used as statinalry phases in preparative chromatography, one made of irregular and two of spherical particles. Measurements of column lenght vs. time and axial compression pressure, obtained for several columns, have been previously reported. These results are discussed here in terms of packing density, internal porosity, and external porosity, as allowed by the use of independent measurements of the density of the silica skeleton and the pore volume. The compressibilities and Young moduli of the columns are calculated and compared to results obtained in soils mechanics for various soil types. The results indicate that the preparative columns packed with silica particles exhibit a behavior intermediate between that of uniform fine and medium-sized sands. The angular silica material exhibits higher compressibility than both spherical silica materials on first compression. Upon recompression, one of the spherical silicas studied exhibits high compressibility and thus elasticity, suggesting a looser packing structure than the other spherical material; this is supported by calculation of the external porosities of the two materials. Results with respect to the kinetic theory of consolidation in soils mechanics are also presented.

Estimation of the adsorption energy distributions for the Jovanovic–Freundlich isotherm model with Jovanovic local behavior

Abstract The successful design of many modern separation processes as the overloaded chromatographic separations require an accurate knowledge of the associated adsorption equilibria. It is well known that the characterization of the adsorbent heterogeneity plays an important role in the understanding of the adsorption equilibria. In a previous paper, a new Jovanovic–Freundlich isotherm model for single-component adsorption on heterogeneous surfaces was proposed. Also, the adsorption energy distributions (AEDs) corresponding to the new model for Langmuir local behavior were derived and evaluated for the specific case of the adsorption of a series of chlorinated hydrocarbons on silica gel. In the present study, the adsorption energy distributions corresponding to the Jovanovic–Freundlich model with Jovanovic local behavior are evaluated for the same set of experimental data. In order to provide a basis for comparison, adsorption energy distributions were also estimated for the given set of data via an established expectation–maximization procedure which does not assume any particular model for the overall isotherm. The results presented in these paper complement the previous findings and provide new insights into the features of the Jovanovic–Freundlich isotherm model, which could be useful both in the correlation of single-component adsorption data and in the prediction of multicomponent equilibria.

Reliable Analysis of the Interaction between Specific Ligands and Immobilized Beta-2-Adrenoceptor by Adsorption Energy Distribution

Although a comparatively robust method, immobilized protein-based techniques have displayed limited precision and inconsistent results due to a lack of strategy for the accurate selection of drug adsorption models on the protein surface. We generated the adsorption data of three drugs on immobilized beta-2-adrenoceptor (β2-AR) by frontal affinity chromatography-mass spectrometry (FAC-MS) and site-specific competitive FAC-MS. Using adsorption energy distribution (AED) calculations, we achieved the best adsorption models for the binding of salbutamol, terbutaline, and pseudoephedrine to immobilized β2-AR. The Langmuir model proved to be desirable for describing the adsorptions of salbutamol and terbutaline on immobilized β2-AR, while the bi-Langmuir model was favorable to characterize the adsorption of pseudoephedrine on the receptor. Relying on the accurate determination of association constants, we presented an efficient approach for β2-AR ligand screening based on the loss of breakthrough time of an indicator drug caused by the inclusion of competitive drugs in the mobile phase. We concluded that the current strategy enables the reliable and accurate analysis of G protein-coupled receptor (GPCR)-drug interaction. The percentage change in the breakthrough time for drugs can provide useful information for estimating their binding affinity to the receptor. This approach builds a powerful platform for high-throughput ligand screening.