Retention Equation Research Articles

Mechanistic interpretation of ionic azo dye flux decline through compacted Na-montmorillonite membrane

The decrease in filtrate flux of ionic azo dyes through compacted Na-montmorillonite membrane can be adequately described by (i) analytical and empirically derived transport equations, (ii) solute rejection and intrinsic retention equations and (iii) X-ray diffraction. Experiments were performed using two ionic azo dyes, Orange G (7-hydroxy-8-phenlyazo-1,3-naphthalenedisulfonic acid disodium salt), a molecular biology reagent and brilliant yellow (C.I. 24890 direct yellow 4). Filtrate samples were collected using a static head setup consisting of a longitudinal flow acrylic cylinder cell through compacted Na-montmorillonite membrane. The results indicated that there was initial rapid flux reduction for the first 3 days due to surficial fouling owing to affinity of hydrophilic organic compounds rapidly getting attached to exchange sites at the dye–membrane interface before slowing down to a quasi-steady-state (5–7 days) and gradual gelation period (8–16 days) during steady-state. The ratio of flux reduction was approximately 3:5 for brilliant yellow to Orange G. The flux values were higher than mass transfer coefficients. Flux decline for the dyes followed an exponential decay described by J ∝ 1/ t n , with n = 0.9556 for yellow dye and n = 0.8923 for Orange G. X-ray diffraction indicated that both the lower molecular weight Orange G (452.4 g mol −1) and higher molecular weight brilliant yellow (624.6 g mol −1) dyes are less preferentially retained within the basal spacing of Na-montmorillonite due to only a slight shift in the range 5.8–6° 2 θ region. The results offer a preliminary study in the fate, transport and recyclability of these ionic azo dyes through smectitic membranes.

Predicting gas chromatographic retention times of 209 polybrominated diphenyls (PBBs) for different temperature programs

A method has been developed to predict the retention times of 209 individual polybrominated diphenyl congeners for different temperature programs. The retention equations lnk'=A+B/T of five PBBs in gas chromatography (GC) were used to evaluate the properties of the regression coefficients A and B, which are widely accepted as being highly reliable chromatographic retentions. The quantitative relationships between the A and B values of PCBs and those of PBBs were found. The regression equations derived have coefficients of determination greater than 0.999. The A, B values of any PBB can be predicted by using the A, B values of the PCB according to these relationships. Using these predicted A and B values, the retention times of all PBBs can be predicted. This is an important advance in the identification of PBBs because at present there are only a few PBB standards available.

Use of lipophilic ion adsorption isotherms to determine the surface area and the monolayer capacity of a chromatographic packing, as well as the thermodynamic equilibrium constant for its adsorption

A method that champions the approaches of two independent research groups, to quantitate the chromatographic stationary phase surface available for lipophilic ion adsorption, is presented. For the first time the non-approximated expression of the electrostatically modified Langmuir adsorption isotherm was used. The non approximated Gouy–Chapman (G–C) theory equation was used to give the rigorous surface potential. The method helps model makers, interested in ionic interactions, determine whether the potential modified Langmuir isotherm can be linearized, and, accordingly, whether simplified retention equations can be properly used. The theory cultivated here allows the estimates not only of the chromatographically accessible surface area, but also of the thermodynamic equilibrium constant for the adsorption of the amphiphile, the standard free energy of its adsorption, and the monolayer capacity of the packing. In addition, it establishes the limit between a theoretical and an empirical use of the Freundlich isotherm to determine the surface area. Estimates of the parameters characterising the chromatographic system are reliable from the physical point of view, and this greatly validates the present comprehensive approach.

Extended thermodynamic approach to ion interaction chromatography. A mono‐ and bivariate strategy to model the influence of ionic strength

Recent breakthroughs in the theory of ion interaction chromatography (IIC) permit new analyses of the dependence of retention on different interdependent factors. The influence of the ionic strength / on the surface potential, the Donnan effect, and salting effects are taken into account to model the chromatographic behaviour of charged analytes in IIC. The most reliable experimental results found in the literature were used to test the retention equations that were developed following both a monovariate (/ changes as the concentration of H, ion interaction reagent, changes) and a bivariate (/ changes because of the simultaneous variation of H and of the background electrolyte concentrations) approach. The present extended thermodynamic model builds on the sound intuition of the electrostatic approach and proves to provide the most successful and exhaustive quantitative explanation of experimental evidence. It is also able to rationalise the less extensive agreement between the pure electrostatic approach predictions and experimental results. The adequacy of the model is supported by physically reliable estimates of the adjustable constant (ion-pair constants, deltaG degrees). Moreover statistical practice demonstrates that all the adjustable parameters (three at most) are statistically significant. A linear, zero crossing function with unit slope is obtained when k(pred) is plotted against k(exp). The mean percent error between k(pred) and k(exp) is 4.5% at most. In the absence of H the present retention equation reduces, as expected, to the relationship that describes the influence of / on the retention behaviour in reversed-phase liquid chromatography.

Stability of lycopene during spray drying of tomato pulp

Lycopene is the principal pigment found in tomatoes and is important not only because of the color it imparts but also because of the recognized health benefits associated with its presence. Degradation of lycopene not only affects the attractive color of the final products but also their nutritive value. The objectives of this study were to determine the retention of lycopene during spray drying of tomato pulp and study the effect of drying conditions on the lycopene content of tomato powder. Sixty-four different experiments were conducted keeping constant the feed rate, the feed temperature and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying air and the air inlet temperature. Tomato powders were analysed for lycopene content. Lycopene content varied from 1016.05 to 1181.30μg/g total solids, whereas lycopene loss ranged between 8.07 and 20.93%. Analysis of experimental data yielded correlations between lycopene retention and the variable operating conditions. Lycopene loss increased with increases in air inlet temperature and in both drying and compressed air flow rate. Multiple regression analysis was used to develop a predictive equation for final lycopene retention during spray drying. The decrease in lycopene content reported here was due to an actual degradation of lycopene, rather than to a progressive conversion from the all-trans-lycopene to a less strongly colored, less intensely absorbing cis form.

Ion-interaction chromatography of zwitterions. The fractional charge approach to model the influence of the mobile phase concentration of the ion-interaction reagent

The chromatographic behavior of zwitterions in ion-interaction chromatography (IIC) was investigated theoretically, on the basis of the following picture of the retention mechanism. The zwitterionic analyte is considered to interact with the charged stationary phase via an effective fractional charge, opposite to the surface one. Adsorption competitions between analytes and the ion interaction reagent (H) concur to explain the retention factor of zwitterions. The present model is quantitatively able to explain the retention behavior of zwitterions in IIC and also to account for the influence of the zwitterion ionization degree, according to the mobile phase pH, on the course of retention as a function of the H concentration. The approach we used is simple and epistemologically interesting because the retention equation for zwitterions may also be obtained from the general retention equation of our extended thermodynamic approach to IIC. Estimated magnitudes of the effective charges are very reasonable and show the same trend as that of the molecular dipole moments, as expected; the total ligand concentration compares well with the bonded phase coverage of the two columns used. For the homologous series from 4-aminobutyric-, to 8-aminocaprylic acid, the estimated effective charge always increases with increasing chain length and this results in parallel growth of the analyte retention increase upon H addition, so that the retention increase for the highest member of the series compares with that of a positively charged analyte. The estimated dipole for this analyte compares excellently to the estimate obtained via a quantum mechanics calculation. The influence of the mobile phase pH on retention was taken into account for the very first time. The good predictive abilities of the retention equations, and the reliability of the estimated constants that make sense physically confirm that retention modeling is able, at the thermodynamic level, to disclose the complexity of the IIC system.

Extended Thermodynamic Approach to Ion Interaction Chromatography: Influence of Organic Modifier Concentration

The influence of organic modifier and ion interaction reagent (IIR) concentrations on analyte retention in ion interaction chromatography (IIC) was investigated via a bivariate approach. The system examined is as follows: ODS-Hypersils was used as stationary phase; the eluent was a variable mixture of a phosphate buffer pH 2.1 and methanol ranging 0%–40% (v/v). The organic modifier decreases both the analyte and the IIR free energy of adsorption hence their interaction is lower when the organic modifier concentration is higher. Retention equations describe how these two factors interact with each another. The estimated equilibrium constant for the IIR adsorption corresponds to ΔG° = −17.3 KJ mol−1. This value is reliable because it makes sense physically. The chi-squared ∑(k calc − k exp)2 is 1.11E + 00 and 2.85E-02, for octopamine and 2-naphthalenesulfonate, respectively. Retention equations were compared with those obtained from the outstanding retention models in IIC. For the modelled system, the extended thermodynamic approach is able to explain experimental evidence that cannot be rationalised by existing retention models: it proves superior for fitting experimental data and is also able to predict when the purely electrostatic approach may work properly. In the absence of IIR the present retention equation reduces, as expected, to the relationship that describes the influence of organic modifier on retention behaviour in reversed-phase liquid chromatography.

Onset of sample concentration effects on retention in field‐flow fractionation

AbstractIn field‐flow fractionation (FFF), when the sample amount is increased, a shift in retention occurs. Positive as well as negative deviations of the retention have been observed, depending on the type of sample and on the experimental conditions. Two concentration effects have been invoked to explain this retention shift: the viscosity effect (the velocity profile of the analyte suspension or solution is modified by the concentration dependence of the viscosity) and the mean distance effect (the mean distance of the analyte molecules or particles to the accumulation wall and the steady state cross‐sectional concentration profile of the analyte are affected by concentration‐dependent interactions between analyte molecules or particles). The general FFF retention equation has been solved in the case where the reciprocal of the viscosity and the FFF analyte‐field interaction parameter, λ, are linearly related to concentration. The two proportionality coefficients involved in these relationships are identified as the intrinsic viscosity and twice the second virial coefficient of the osmotic pressure, respectively. A general expression of the derivative of the analyte average relative velocity, R, with respect to the cross‐sectional average analyte concentration, 〈c〉, has been obtained as a function of these two coefficients, and of λo, the limit of λ for infinite dilution. A survey of retention effects reported in FFF literature for various sample types and FFF methods is presented.

Retention equations of nonionic organic chemicals in soil column chromatography with methanol-water eluents.

Research efforts dealing with chemical transportation in soils are needed to prevent damage to ground water. Methanol-containing solvents can increase the translocation of nonionic organic chemicals (NOCs). In this study, a general log-linear retention equation, log k' = log k'w - Sphi (Eq. [1]), was developed to describe the mobilities of NOCs in soil column chromatography (SCC). The term phi denotes the volume fraction of methanol in eluent, k' is the capacity factor of a solute at a certain phi value, and log k'w and -S are the intercept and slope of the log k' vs. phi plot. Two reference soils (GSE 17204 and GSE 17205) were used as packing materials, and were eluted by isocratic methanol-water mixtures. A model of linear solvation energy relationships (LSER) was applied to analyze the k' from molecular interactions. The most important factor determining the transportation was found to be the solute hydrophobic partition in soils, and the second-most important factor was the solute hydrogen-bond basicity (hydrogen-bond accepting ability), while the less important factor was the solute dipolarity-polarizability. The solute hydrogen-bond acidity (hydrogen-bond donating ability) was statistically unimportant and deletable. From the LSER model, one could also obtain Eq. [1]. The experimental k' data of 121 NOCs can be accurately explained by Eq. [1]. The equation is promising to estimate the solute mobility in pure water by extrapolating from lower-capacity factors obtained in methanol-water mixed eluents.

Evaluation of vacuum filtration testing for geotextile tubes

High water content (or low solids content) materials consist of marine sediments, industrial, wastewater, and water treatment sludges, and agricultural waste. Geotextile tubes are emerging as a new technology that can dewater high water content material. The goal of this study was to utilize vacuum filtration testing to evaluate the filtration and retention capacity of woven fabrics used for geotextile tubes and to provide design guidance in selecting appropriate geotextiles to increase solids retention while reducing excessive fines migration. A procedure for analysis of vacuum filtration test results is provided. The results of this study revealed that woven geotextiles provided adequate retention of solids. In addition, empirical equations for retention, clogging, and permeability were assessed for applicability to geotextile tube design.

THE DIPOLE APPROACH IN ION INTERACTION CHROMATOGRAPHY OF ZWITTERIONS. USE OF THE LINEARIZED POTENTIAL EXPRESSION FOR LOW SURFACE POTENTIAL

The chromatographic behaviour of zwitterionic analytes in Ion Interaction Chromatography (IIC) was theoretically investigated for low surface potential. For the first time, simplified retention equations were obtained via the linearized potential expression. They can be used to model zwitterions retention as a function of both the mobile and stationary phase concentration of the Ion-Interaction Reagent (IIR), if the surface potential is below 25 mV.

The dipole approach in the lon-interaction chromatography of zwitterions— Use of a potential approximation to obtain a simplified retention equation

In ion-interaction chromatography (IIC) the zwitterionic analyte is believed to interact with the stationary phase potential as a result of its electrical dipole, and to compete with the ion-interaction reagent (IIR) for available inner layer sites. The most reliable set of literature data relating to the retention behavior of zwitterions in IIC has been used for successful testing of a high surface-potential approximation which enables the fitting of retention data without detailed information about the physical and chemical properties of the mobile phase. This approach can explain quantitatively, very simply, the retention behavior of zwitterions in IIC.

Extended thermodynamic approach to ion-interaction chromatography for high surface potential: Use of potential approximation for simplified retention equations

The chromatographic behaviour of charged analytes in ion interaction chromatography (IIC) has been investigated theoretically. A potential approximation for high surface potential was used to obtain simplified retention equations that are able to model analyte retention as a function of both the mobile and stationary phase concentrations of the ion-interaction reagent (IIR). The main advantage of using this potential approximation is that it allows calculation of the surface potential, without needing detailed information on physical and chemical properties of the mobile phase.

EXTENDED THERMODYNAMIC APPROACH TO ION INTERACTION CHROMATOGRAPHY FOR LOW SURFACE POTENTIAL. USE OF THE LINEARIZED POTENTIAL EXPRESSION

The chromatographic behaviour of charged analytes in Ion Interaction Chromatography (IIC) was theoretically investigated. Simplified retention equations were obtained via the linearized potential expression. They can be used to model analyte retention as a function of both the mobile and stationary phase concentration of the Ion-Interaction reagent (IIR), if the surface potential is below 25 mV. Simplified retention equations were compared to those, which can be obtained from two of the most important retention models in IIC. They reduce to stoichiometric or electrostatic retention model equations if the surface potential or pairing equilibria are respectively neglected.

Histidine as a dipolar eluent component in cation chromatography: II. Prediction of retention data for alkaline and alkaline-earth ions

The utility of cation chromatography has been developed by the application of l-histidine as a multiprotic and dipolar (zwitterionic) eluent component. The method simplifies the cation analysis. The chromatographic characteristics of this system were studied in detail with a view to determining the selectivity and the mechanism by which the cations (Na +, K +, Mg 2+, Ca 2+) are retained. Complete separations were observed in the isocratic run over the eluent concentration range 3.0–6.0 m M at pH below 2.0. Sensitive detection was achieved using suppressed conductivity at the pH of isoelectric point of the histidine. Retention equations are derived for mono- and divalent cations eluted from ion-exchange separation column with multiple ionic eluents. The theory is based on the extension of ion-exchange equilibrium by protonation equilibria. The selectivity data for analyte and eluent species are determined using the model from the experimental retention data by computer-assisted iterative calculations. The model was utilized to predict retention data. The results in three-dimensional retention surfaces together with species distribution graphs are presented.

Extended thermodynamic approach to ion interaction chromatography.

The chromatographic behavior of charged analytes in ion interaction chromatography (IIC) is theoretically investigated. The chemical modifications of the stationary and mobile phases in the presence of ion interaction reagent (IIR) are theoretically shown to change the partition coefficient for charged molecules. The most reliable literature experimental results concerning retention behavior of charged molecules in IIC were used to test the new theory. Retention equations are compared with those that can be obtained from the most important retention models in IIC. The present exhaustive retention model, which is well-founded in physical chemistry, goes further than the previous ones whose retention equations can be viewed as limiting cases of the present theory. The present extended thermodynamic approach reduces to stoichiometric or electrostatic retention models if the surface potential or pairing equilibria are respectively neglected. Moreover, it is able to quantitatively explain experimental evidences that cannot be rationalized by the existing retention models.

PREDICTION OF PARAMETERS c AND a IN REVERSED-PHASE HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY USING RETENTION PARAMETERS IN GAS LIQUID CHROMATOGRAPHY

The relationship of retention mechanism between gas liquid chromatography (GLC) and reversed-phase high-performance liquid chromatography (RP-HPLC) is discussed. A model to predict the retention parameters c and a in retention equation in RP-HPLC using retention parameters in GLC is established, and it is verified by experimental data. By comparing the interaction forces in GLC and RP-HPLC, it was found that the hydrogen-bonding interaction plays an important role in determining the retention mechanism of a solute in RP-HPLC, and the form of hydrogen-bonding is the eluent acting as hydrogen-bonding donor acidity and the solute acting as hydrogen-bonding acceptor basicity. The refractive force is important in both GLC and RP-HPLC, but the dipole moment interaction can be ignored.

Interpretation of the excess adsorption isotherms of organic eluent components on the surface of reversed-phase adsorbents: Effect on the analyte retention

The excess adsorption isotherms of acetonitrile, methanol and tetrahydrofuran from water on reversed-phase packings were studied, using 10 different columns packed with C 1–C 6, C 8, C 10, C 12 and C 18 monomeric phases, bonded on the same type of silica. The interpretation of isotherms on the basis of the theory of excess adsorption shows significant accumulation of the organic eluent component on the adsorbent surface on the top of “collapsed” bonded layer. The accumulated amount was shown to be practically independent of the length of alkyl chains bonded to the silica surface. A model that describes analyte retention on a reversed-phase column from a binary mobile phase is developed. The retention mechanism involves a combination of analyte distribution between the eluent and organic adsorbed layer, followed by analyte adsorption on the surface of the bonded phase. A general retention equation for the model is derived and methods for independent measurements of the involved parameters are suggested. The theory was tested by direct measurement of analyte retention from the eluents of varied composition and comparison of the values obtained with those theoretically calculated values. Experimental and theoretically calculated values are in good agreement.

Calculation of retention factors for charged solutes in capillary electrochromatography

An equation describing the capillary electrochromatographic (CEC) retention factor is derived using the fundamental chromatographic concept of retention factor. This retention equation couples both the chromatographic retention factor and the electrophoretic velocity factor, which can explain well the charged solute retention in CEC. The known CEC retention expressions are compared. A simple experimental method for obtaining the CEC retention factors for charged solutes is demonstrated.

ION INTERACTION CHROMATOGRAPHY OF NEUTRAL MOLECULES. USE OF A POTENTIAL APPROXIMATION TO OBTAIN A SIMPLIFIED RETENTION EQUATION

The chromatographic behavior of neutral molecules in Ion Interaction Chromatography (IIC) was theoretically investigated. The use of a potential approximation to obtain a simplified retention equation was successfully tested. The most reliable experimental literature results, concerning the retention behavior of neutral molecules in IIC, were successfully fitted by the retention equation. The wide variability among them was elucidated on the basis of the developed retention equation. A new separation strategy, to improve the selectivity of the chromatographic method, for difficult to separate mixtures of neutral eluates was proposed.