Influence of the pH on the behavior of an imprinted polymeric stationary phase--supporting evidence for a binding site model.

The equilibrium isotherms of the two enantiomers of phenylalanine anilide (PA) were measured by conventional frontal analysis at three different pH on a thermally-treated imprinted stationary phase selective for the L enantiomer. The first of these pH (buffer pH=3.0, pH(app)=4.0) is well below the apparent pKa (6.4) of the two solutes, the second (buffer pH=5.8, pH(app)=7.0) slightly below this pKa, and the third (buffer pH=7.0, pH(app)=8.3) well above it. The experimental data were fitted to several isotherm models. The best estimates of the parameters of these models are reported and discussed. The corresponding isotherms are compared with the experimental ones. The contributions of the enantioselective and nonselective interactions could be separated. The results obtained show that the saturation capacity is always smaller for D-PA than for L-PA, the template. The analytical separation is best at pH=3.0 because of a good separation factor (2.82) and short retention times. A good compromise between the resolution and the saturation capacity is obtained at pH=5.8, for which the best preparative separation is found. Both analytical and preparative results are poor at pH=7.0 because the separation factor is low (1.32). At this pH, the isotherm remains nearly linear in the whole concentration range accessible to measurements. The number of nonselective sites increases with increasing mobile phase pH slightly faster than the number of selective sites, suggesting different pKa ranges for the two type of sites. Moreover, the binding energy and the homogeneity of the selective sites decreases with increasing pH. These results agree with a binding site model involving more than one carboxylic acid group, providing charge complementarity and hydrogen bond donors for binding of L-PA.

Adsorption equilibria and overloaded band profiles of basic drugs in a reversed-phase system.

Single-solute adsorption equilibrium isotherms of three basic drugs: buspirone, doxepin and diltiazem were determined by frontal analysis in a reversed-phase system composed of an octadecylsilica packing material and a buffered mobile phase containing acetonitrile. The adsorption data were fitted to the bi-Langmuir model. Within the framework of this model, the adsorption of the drugs is assumed to occur on two distinct kinds of sites with different average adsorption energies. The data are consistent with the assumption that the low energy sites account for the hydrophobic interactions between the solutes and the chemically bonded alkyl chains and the high energy sites account for the ion-exchange interactions between the residual active silanols and the protonated bases. Multisolute, overloaded band profiles were also measured for the three binaries and for mixtures of the three drugs. Theoretical band profiles were calculated using the equilibrium dispersive model and the ideal adsorbed solution theory model which uses the parameters determined from the correlation of the single-solute adsorption data. Good agreement was found between the experimental and calculated overloaded band profiles.

High-concentration band profiles and system peaks for a ternary solute system

High-concentration HPLC band profiles of single solutes and the individual band profiles of the components of binary and ternary mixtures are reported for benzyl alcohol, 2-phenylethanol, and 2-methylbenzyl alcohol. These solutes were eluted from a C18 column by a binary mobile phase (MeOH:H2O = 1:1, v/v). High-concentration system peaks were obtained using mixtures of benzyl alcohol and 2-phenylethanol at different relative concentrations as the feed and 2-methylbenzyl alcohol as the strong mobile phase additive. Band profiles and system peak profiles were calculated using the equilibrium-dispersive model of chromatography. The adsorption equilibrium in the multicomponent system was characterized by the competitive Langmuir model. Excellent quantitative agreement was found between the experimental and the calculated profiles. This work confirms that extremely unusual system peak profiles can be obtained even when the adsorption behavior is quite simple. Under certain circumstances, the use of a properly chosen additive could markedly increase the separation between bands and hence the production rate, the recovery yield, and/or the purity of the fractions.

Modeling of separations by closed-loop steady-state recycling chromatography of a racemic pharmaceutical intermediate.

Closed-loop steady-state recycling (SSR) is a cyclic, one-column process that is similar to simulated moving bed (SMB) chromatography in several respects. Both processes are cyclic. In both SMB and SSR, fractions are collected from the leading and trailing portions of the circulating chromatographic profile, and fresh sample is injected into the interior of the profile. However, SMB is a continuous process whereas SSR is a discontinuous, repetitive process. This paper presents a model for the closed-loop SSR process and its experimental validation in a case of practical importance. For this last purpose, we used the closed-loop SSR separation of the enantiomers of a racemic pharmaceutical intermediate. The experimental determination of the competitive adsorption equilibria was performed by frontal analysis in a system composed of a chiral HPLC column as the stationary phase and pure acetonitrile as the mobile phase. All the adsorption data were well correlated by the Langmuir model. The Langmuir model was used to calculate overloaded band profiles corresponding to the separation of racemic mixtures at both analytical and preparative scales. Theoretical band profiles were calculated using the equilibrium-dispersive model. With proper corrections for the contributions of the sources of extra-column band broadening, the model properly predicts the experimental band profiles obtained in the closed-loop SSR setup and demonstrates that a cyclic steady-state develops after the completion of a finite number of cycles. The results also show that the extra-column effects must be accounted for in order to model the closed-loop SSR process accurately.

[Outbreak of schistosomiasis in a group a travellers returning from Burkina Faso]. / Brote de esquistosomiasis en un grupo de viajeros procedentes de Burkina Fasso.

BACKGROUND: Schistosomiasis in Spain is always an imported disease mainly presented in travellers from endemic areas with a history of having bathed in fresh water and in immigrants from these areas. A group of travellers presenting infection by schistosomes following an 8-day journey to Burkina Faso. PATIENTS AND METHODS: The travellers, residents of Gran Canaria (Spain), who had bathed in fresh water were studied by schistosomiasis serology and parasitologic examination in stools and urine. RESULTS: A total of 29 travellers were studied, 20 (69%) of whom were considered to be infected, on presenting positive serology and/or coproparasitologic examination. Fourteen of the infected patients presented clinical symptomatology compatible with the Katayama syndrome, while 6 were asymptomatic. The mean time to appearance of the symptoms was 27 days and these were most frequently fever, headache, neck pain, diarrhea and arthromyalgia together with eosinophilia achieving a mean value of 3,513 eosinophils/microliter. All the infected travellers presented positive serology and in 5 eggs of a Schistosoma mansoni were observed in stools. All the infected patients were treated with praziquantel. CONCLUSIONS: Limited exposure by bathing in fresh water in this group of travellers led to a high rate of infection. The diagnosis of schistosomiasis should be considered in any traveller from an endemic area with a history of having bathed in fresh water. In the case of an outbreak, investigation of all the travellers who had bathed is recommendable given that the infection may be asymptomatic.

Extension of a Jovanovic-Freundlich isotherm model to multicomponent adsorption on heterogeneous surfaces.

A recently proposed Jovanovic-Freundlich isotherm model for single component adsorption without lateral interactions on heterogeneous surfaces is extended to account for lateral interactions and for competitive adsorption. The model is tested using previously reported single component and competitive adsorption data of 2-phenylethanol and 3-phenylpropanol on ODS-silica with methanol-water as the mobile phase. A comparison is made regarding the ability of the Jovanovic-Freundlich and Langmuir-Freundlich models to predict competitive equilibria using the single component identified parameters. Fair predictions of the competitive data were obtained when using heterogeneous-surface models which do not take into account the possible interactions of phenylalcohols in the adsorbed phase via hydrogen bonding. Markedly improved predictions were obtained with models which account simultaneously for the two main sources of adsorbed phase nonideal behavior, i.e. adsorbate-adsorbate interactions and heterogeneity of the adsorbent surface.