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A family of models is proposed for the description of skewed chromatographic peaks, based on the modification of the standard deviation of a pure Gaussian peak, by the use of a polynomial function, h(t) = He-(1/2)([ t - t R ]/[ s 0 + s 1 ( t - t R )+ s 2 ( t - t R )2+...])2 , where H and t R are the height and time at the peak maximum, respectively. The model has demonstrated a high flexibility with peaks of a wide range of asymmetry and can be used to accurately predict the profile of asymmetrical peaks, using the values of efficiency and asymmetry factor measured on experimental chromatograms. This possibility permits the simulation of chromatograms and the optimization of the separation of mixtures of compounds producing skewed peaks, where both the position and peak shape are considered. A first-degree polynomial was adequate for peaks of moderate asymmetry, but higher degree polynomials were preferable for peaks showing a high asymmetry, including those with negative skewness. The proposed models can be employed in the resolution of overlapped peaks in binary and ternary mixtures of compounds, or to improve the accuracy in the evaluation of peak shape parameters. The results obtained with the proposed models were comparable or even superior to those achieved with the exponentially modified Gaussian model.

A general systematic approach is proposed for the numerical calculation of multi-equilibrium problems. The approach involves several steps: (i) the establishment of balances involving the chemical species in solution (e.g., mass balances, charge balance, and stoichiometric balance for the reaction products), (ii) the selection of the unknowns (the concentration of selected chemical species at equilibrium), (iii) the estimation of the concentration of the other species based on the selected species and the equilibrium expressions, and (iv) the minimization of the sum of the squared balances (search of the optimal combination of the unknowns). The application of the systematic approach to cases of diverse complexity, involving acid−base reactions, redox reactions, and the formation of complexes and solids in aqueous solution, using the Solver option of the Excel spreadsheet is shown. The approach is useful to solve problems of high complexity. It might be useful to undergraduate students in the field of solution equilibria, and they would also gain experience with a method for solving simultaneous equations.

Countercurrent chromatography was used to determine the octanol-water partition coefficients (P o/w ) of 23 diuretic drugs. The measured P o/w values ranged over 4 orders of magnitude from 0.05 to 550 (-1.3 < log P o/w < +2.7). All the compounds, except spironolactone, were ionizable. The P o/w values were strongly depending on the aqueous-phase pH. A theoretical model linking these values with the pH was derived for four cases: (i) molecular acid-anionic base, (ii) cationic acid-molecular base, (iii) biprotic systems with two acidic or basic groups showing the same charge sign, and (iv) biprotic systems with ionizable groups showing different charges with special attention to the amino acid case. In each case, hypothetical compounds are graphically studied through log P o/w vs pH curves. The apparent P o/w coefficients of the diuretics were determined at pH 2.58, 5.86, and 7.39 using 0.01 M ammonium phosphate buffers. The results were analyzed using the theoretical model. The P o/w coefficients of the molecular forms agreed with the literature values. The P o/w coefficients of the ionic forms were obtained. The model also allows establishment of the acidity constants of the studied molecules.

The reversed-phase liquid chromatographic (RPLC) behavior (retention, elution strength, selectivity, efficiency, and peak asymmetry) for a group of basic drugs (beta-blockers), with mobile phases containing the anionic surfactant sodium dodecyl sulfate (SDS) and acetonitrile, revealed different separation environments, depending on the concentrations of both modifiers: hydro-organic, submicellar at low surfactant concentration and high concentration of organic solvent, micellar, and submicellar at high concentration of both surfactant and organic solvent. In the surfactant-mediated modes, the anionic surfactant layer adsorbed on the stationary phase interacts strongly with the positively charged basic drugs increasing the retention and masks the silanol groups that are the origin of the poor efficiencies and tailing peaks in hydro-organic RPLC with conventional columns. Also, the strong attraction between the cationic solutes and anionic SDS micelles or monomers in the mobile phase enhances the solubility and allows a direct transfer mechanism of the cationic solutes from micelles to the modified stationary phase, which has been extensively described for highly hydrophobic solutes.

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Micellar liquid chromatography makes use of aqueous solutions or aqueous-organic solutions containing a surfactant, at a concentration above its critical micelle concentration. In the mobile phase, the surfactant monomers aggregate to form micelles, whereas on the surface of the nonpolar alkyl-bonded stationary phases they are significantly adsorbed. If the mobile phase contains a high concentration of organic solvent, micelles break down, and the amount of surfactant adsorbed on the stationary phase is reduced, giving rise to another chromatographic mode named high submicellar liquid chromatography. The presence of a thinner coating of surfactant enhances the selectivity and peak shape, especially for basic compounds. However, the risk of full desorption of surfactant is the main limitation in the high submicellar mode. This study examines the adsorption of the anionic surfactant sodium dodecyl sulfate under micellar and high submicellar conditions on a C18 column, applying two methods. One of them uses a refractive index detector to obtain direct measurements of the adsorbed amount of sodium dodecyl sulfate, whereas the second method is based on the retention and peak shape for a set of cationic basic compounds that indirectly reveal the presence of adsorbed monomers of surfactant on the stationary phase.