Modeling the combined effect of temperature and organic modifier content on reversed-phase chromatographic retention: Effectiveness of derived models in isocratic … (original) (raw)
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Journal of Chromatography A, 2002
A semi-thermodynamic treatment is adopted to account for adsorption or partition of solute molecules from aqueous mobile phases on / in reversed-phase liquid chromatography stationary phases. The theoretical expressions of ln k9 versus organic modifier content are tested against 10 data sets covering a variety of solute molecules. It is shown that the mean field approximation, adopted widely in previous studies, is marginally valid in aqueous mobile phases, especially in the presence of solute molecules, and the lattice model approximation, which is also used in relevant studies, is a poor approximation. Clear conclusions about the validity of either the adsorption or the partition model for the retention mechanism could not be drawn. The equations of the adsorption model describe all data sets absolutely satisfactorily and yield a physically reasonable picture about the behavior of modifier and solvent at the adsorbed layer. However, the high applicability of the adsorption model may not safely entail the validity of the adsorption mechanism at a molecular level, especially in the case of solutes with small and non-polar molecules, where our analysis gives strong indications about the validity of the partition mechanism. The next steps needed for the final elucidation of the retention mechanism in reversed-phase chromatographic columns are indicated.
Journal of Chromatography A, 2009
The polarity parameter model previously developed: log k = (log k) 0 + p(P N m − P N s ) has been successfully applied to study several chromatographic systems involving new generation RPLC columns (Luna C18, Resolve C18, XTerra MSC18, and XTerra RP18). In this model the retention of the solutes (log k) is related to a solute parameter (p), a mobile phase parameter (P N m ) and two chromatographic system parameters [P N s and (log k) 0 ]. The studied systems have been characterized with different acetonitrile-water and methanol-water mobile phases, using a set of 12 neutral solutes of different chemical nature. The polarity parameter model allows prediction of retention of any solute in any mobile phase composition just using the retention data obtained in one percentage of organic modifier and the polarity parameters established in the characterization of the chromatographic systems. This model also allows the solute polarity data transference between RPLC characterized systems, so it is possible to predict the retention in various RPLC systems working experimentally with just one of them. Moreover, the global solvation parameter model has also been applied to the same chromatographic systems using a wide set of solutes in order to compare its predictive ability with the one of the polarity parameter model. The results clearly show that both models predict retention with very similar accuracy but the polarity parameter model requires much less preliminary experimental measurements to achieve equivalent results than the global solvation approach.
Journal of Separation Science, 2012
Recent developments in HPLC methods have focused on various strategies in order to increase the speed of analysis. One area of impressive growing is column technology. Today, analytical methods that propose the use of short columns packed with sub-2 mm particles installed in ultra high-pressure LC instruments are not uncommon. Another strategy consisted of heating thermally resistant columns to temperatures well above of 1001C in order to reduce eluent viscosities and, therefore, column backpressure. We discuss experimental conditions for achieving high-throughput analysis using standard instruments with a few simple modifications. The chromatographic performance of two particulated and a silica-based monolithic column operated at moderate temperatures and flow rates are compared. The monolithic column proved to be stable over several thousands column volumes at 601C. More important, its resistance to mass transfer at this temperature was significantly reduced. Very fast separations of two different mixtures of pharmaceutical compounds, anti-inflammatory drugs and b-blockers, were achieved with the three columns at 601C by using ACN/buffer at 5 mL/min. Excellent peak shapes of basic solutes and quite reasonable resolutions were achieved in very short analysis times with columns operated at temperatures moderately higher than the usual room temperature.
Journal of Chromatography A, 2002
Six equations that express the combined effect of mobile phase pH and organic modifier content on sample retention in reversed-phase liquid chromatography (RPLC) are developed based on either the adsorption or the partition model for retention. The equations are tested against five retention data sets taken from literature. In the tests two pH scales are used, w s pH and pH. It is shown that a new seven-parameter equation works more satisfactorily, because it exhibits good numerical w s behavior, gives low values of the sum of squares of residuals and represents the experimental retention surfaces successfully. In addition, the danger of overfitting, which leads to the prediction of physically meaningless retention surfaces, is minimized by using the proposed new seven-parameter equation. Finally, the possibility of obtaining reliable pK values of weak acids or bases chromatographically by means of the derived equations is also considered and discussed.
Journal of Chromatography A, 2005
An extension of the treatment adopted in a recent paper [P. Nikitas, A. Pappa-Louisi, P. Agrafiotou, J. Chromatogr. A 946 33] was used to derive expressions describing the variation of solute retention k with composition in ternary reversed phase liquid chromatography, RP-LC, solvent systems. The equation of the partition model obtained in this way for a ternary mobile phase was identical to that previously derived using the solubility parameter concept. This equation as well as two new expressions of ln k versus organic modifiers content were tested in a variety of ternary solvent systems in order to examine the possibility of predicting retention behavior of solutes under ternary solvent mixture elution conditions from known retention characteristics in binary mobile phases. It was demonstrated the superiority of both new equations derived in this paper to that previously proposed and applied to date in ternary solvent mixtures.
Recent applications of retention modelling in liquid chromatography
2020
Recent applications of retention modelling in liquid chromatography (2015–2020) are comprehensively reviewed. The fundamentals of the field, which date back much longer, are summarized. Retention modeling is used in retention‐mechanism studies, for determining physical parameters, such as lipophilicity, and for various more‐practical purposes, including method development and optimization, method transfer, and stationary‐phase characterization and comparison. The review focusses on the effects of mobile‐phase composition on retention, but other variables and novel models to describe their effects are also considered. The five most‐common models are addressed in detail, i.e. the log‐linear (linear‐solvent‐strength) model, the quadratic model, the log–log (adsorption) model, the mixed‐mode model, and the Neue–Kuss model. Isocratic and gradient‐elution methods are considered for determining model parameters and the evaluation and validation of fitted models is discussed. Strategies in ...
… of Chromatography A, 2004
Two-parameter equations that describe the dependence of ln k upon ϕ, where k is the retention factor and ϕ the volume fraction of the organic modifier in the mobile phase, are examined in what concerns the underlying approximations and their performance to fit experimental data obtained from reversed-phase liquid chromatography. Using 293 experimental systems, it was found that the performance of these equations to describe ln k versus ϕ data is rather low, since the percentage of the systems that can be described satisfactorily ranges from 40 to 60% depending on the fitting equation. This percentage may be raised to 75%, if the discreteness effect is properly taken into account. A further improvement to 90% of the systems studied can be achieved only by the use of three-parameter equations, which may arise by refinements of the rough approximations of the two-parameter equations. Although the refinements do not lead always to better equations, we developed a new three-parameter expression of ln k that works more satisfactorily, since it combines simplicity, linearity of its adjustable parameters and the highest applicability.
Four retention models for the effect of aliphatic alcohol additives on the retention of analytes in reversedphase liquid chromatography have been developed following either a semi-thermodynamic treatment or an empirical approach. Their performance was tested using the experimental retention times of six non-polar analytes (alkylbenzenes) and ten o-phthalaldehyde derivatives of amino acids under different isocratic chromatographic runs when a small amount of ethanol, 1-propanol, 1-butanol, 1-pentanol, 1hexanol or 1-heptanol was added to methanol/water mixtures containing a constant amount of methanol. It was shown that for the structurally simple alkylbenzenes all the models can be adopted for retention prediction with good results. In contrast, just one out of four models, that with the fewest approximations, predicts satisfactorily the retention properties of amino acids derivatives. However, the most interesting feature is that this model can predict the effect of an alcohol-additive on the retention properties of solutes, even if this additive was not used in chromatographic runs done for the fitting procedure, provided that it belongs to the same homologous series of alkanols. This feature is also observed in all models described the retention of alkylbenzenes.
Analytica Chimica Acta, 1994
Retention data (log k') for 14 benzene and 18 phenol derivatives have been obtained in a C18 column for the full range of methanol-water and acetonitrile-water mobile phase compositions. From the equations developed in an earlier work a new solvent parameter has been calculated from the retention data in the two mobile phases studied. General linear equations have been established which describe chromatographic retention in the full range of mobile phase compositions by a single solute parameter and the new solvent parameter. The applicability of the new solvent parameter to different solutes and C18 columns has been tested with literature data.