Reproducibility of low and high concentration data in reversed-phase liquid chromatography. II. Overloaded band profiles on Chromolith-C18

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for six low molecular weight compounds (phenol, aniline, caffeine, o-toluidine, p-toluidine and propylbenzoate) on one Chromolith-C18 column (#30, Merck, Darmstadt, Germany), using different methanol:water solutions (composition between 60/40 and 15/85 v/v, depending on the solute) as the mobile phase. These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions were also derived and used for the selection of the best isotherm model. Widely different models were obtained for these six compounds, four being convex upward (i.e., Langmuirian) and two having at least one inflection point. Overloaded band profiles corresponding to two different sample sizes (a low and a high loading factor) were recorded on six monolithic columns (#30-35) belonging to the same manufactured lot. These experimental band profiles were compared to the profiles calculated from the isotherm measured by FA on the first column, using the equilibrium-dispersive (ED) model of chromatography. For four of the six columns (#30, #32, #33, and #35), the reproducibility was better than 5 and 2.5% for the low and the high concentration profiles, respectively. On the other two columns (#31 and #34), the bands showed significant and systematic retention time shifts for all six compounds (with nearly identical band shapes), the relative adsorption being between 6 and 15% stronger on column #31 or between 2 and 7% lower on column #34. These differences seem to be correlated with the differences in the total porosities of these columns, which differ by 3% from columns #31 to #34, the higher porosity column giving the stronger adsorption.     

Separation mechanism of nortriptyline and amytriptyline in RPLC

The single and the competitive equilibrium isotherms of nortriptyline and amytriptyline were acquired by frontal analysis (FA) on the C18- bonded discovery column, using a 28/72 (v/v) mixture of acetonitrile and water buffered with phosphate (20 mM, pH 2.70). The adsorption energy distributions (AED) of each compound were calculated from the raw adsorption data. Both the fitting of the adsorption data using multi-linear regression analysis and the AEDs are consistent with a trimodal isotherm model. The single-component isotherm data fit well to the tri-Langmuir isotherm model. The extension to a competitive two-component tri-Langmuir isotherm model based on the best parameters of the single-component isotherms does not account well for the breakthrough curves nor for the overloaded band profiles measured for mixtures of nortriptyline and amytriptyline. However, it was possible to derive adjusted parameters of a competitive tri-Langmuir model based on the fitting of the adsorption data obtained for these mixtures. A very good agreement was then found between the calculated and the experimental overloaded band profiles of all the mixtures injected.     

Heterogeneity of the surface energy on unused C18-chromolith adsorbents in reversed-phase liquid chromatography

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for phenol and caffeine on a new C18-Chromolith column (Merck, Darmstadt, Germany), using a water-rich mobile phase (methanol/water, 15/85, v/v). These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions, based on the expectation-maximization (EM) procedure, were also derived and used for the selection of the best isotherm model. The adsorption energy distributions (AEDs) for phenol and caffeine converged toward a trimodal and a quadrimodal distribution, respectively. Energy distributions with more than two modes had not been reported before for the adsorption of these compounds on packed columns. The third high energy mode observed for both phenol and caffeine seems to be specific of the surface of the monolithic column while the first and second low energy modes have the same physical origin as the two modes detected on packed columns. These results suggest significant differences between the structures of the porous silica in these different materials.     

Repeatability and reproducibility of high-concentration data in reversed-phase liquid chromatography. III. Isotherm reproducibility on Kromasil C18

Single component equilibrium isotherms of six compounds (aniline, caffeine, ethylbenzene, phenol, propranolol, and theophylline) were determined by the inverse method on 10 Kromasil-C18 columns, using water-methanol solutions as the mobile phase. This method offers an economic and fast isotherm determination by means of the overloaded band profiles of the compounds. Five out of the ten columns used in this test come from the same batch whilst the other five columns represent five additional batches. Statistical evaluation was used to assess the reproducibility of the isotherm parameters. We found that the column-to-column reproducibility of the isotherm parameters is of the same magnitude as the batch-to-batch reproducibility (with the exception of one outlier column). In most of the cases, the reproducibilities of the saturation capacities and that of the retention factors are excellent, they are typically between 1.2 and 3%, and very often below 2%. Within the limits of the experimental precision, these results agree with those obtained earlier, using a conventional method of isotherm determination.     

Effect of the mobile phase composition on the isotherm parameters and the high concentration band profiles in reversed-phase liquid chromatography

Single-component adsorption isotherm data were acquired by frontal analysis for phenol on a C18-Kromasil packed column, under reversed-phase liquid chromatography conditions, using various methanol-water solutions (30-60%, v/v, methanol). The isotherm model accounting best for these data was the biLangmuir model. With increasing methanol content, the two saturation capacities decrease, particularly that of the high-energy sites, the adsorption constant of the low-energy sites decreases significantly and that of the high-energy sites decreases strongly. These results allow a quantitative investigation of the properties of the high-energy sites (which are not necessarily the so-called active sites), a feature rarely discussed yet. The band profiles calculated with the numerical values of the isotherm model parameters derived by fitting the frontal analysis data to the model and using the equilibrium-dispersive model agree very well with the experimental band profiles in the whole concentration range.     

Retention of ionizable compounds in high-performance liquid chromatography. 14. Acid-base pK values in acetonitrile-water mobile phases

Using competitive frontal analysis, the binary adsorption isotherms of the enantiomers of 1-phenyl-l-propanol were measured on a microbore column packed with a chiral stationary phase based on cellulose tribenzoate. These measurements were carried out using only the racemic mixture. The experimental data were fitted to four different isotherm models: Langmuir, BiLangmuir, Langmuir-Freundlich and Tóth. The BiLangmuir and the Langmuir-Freundlich models accounted best for the competitive adsorption data. An excellent agreement between the experimental and the calculated overloaded band profiles for various samples of racemic mixture was obtained when the equilibrium dispersive model of chromatography was used together with the BiLangmuir competitive isotherm. The isotherm parameters measured under competitive conditions were used to calculate the overloaded band profiles of large samples of the pure S- and R-enantiomers, too. A satisfactory agreement between the experimental and calculated band profiles was observed when using in the computation the corresponding single component BiLangmuir isotherm derived from the binary isotherm previously determined. Thus oniy data derived from the racemic mixture are required for computer optimization of the preparative chromatography separation of the enantiomers.     

Comparison of the adsorption equilibrium of a few low-molecular mass compounds on a monolithic and a packed column in reversed-phase liquid chromatography

Adsorption isotherm data were acquired by frontal analysis for several low-molecular mass compounds (3-phenyl 1-propanol, 4-tert.-butylphenol, butylbenzene, and butyl benzoate) on a classical packed column and a monolithic column using methanol-water RP-HPLC conditions. These columns have similar characteristics (C18-bonded silica, close specific surface areas and bonding densities). In each case, the isotherm model best accounting for the data was the same on both columns. The solute polarity determines the class of this model. For the two -OH compounds it was a Langmuirian adsorption isotherm. The hydrocarbon data were best modeled by an anti-Langmuir convex-downward isotherm model. The adsorption data for the aromatic ester exhibited a nearly linear behavior, depending on the methanol concentration of the mobile phase. A slightly convex downward isotherm was obtained at high methanol concentrations while the best fitting was obtained with a liquid-solid extended multilayer B.E.T. isotherm model at low concentrations. The validation of these models is discussed in detail. In all cases, similar values of the adsorption-desorption constants were found, underlining the closeness of the adsorption energies on both columns. By contrast, the adsorption capacity of the monolithic column was found to be approximately 1.4 greater than that of the packed column in spite of the close values of the surface areas of the silica in both columns.     

Modeling of the separation of two enantiomers using a microbore column

A microbore column packed with Chiralcel OB (cellulose tribenzoate coated silica) was used for the measurement of the single and competitive equilibrium-isotherm data of the 1-indanol enantiomers by frontal analysis. The amount of sample needed for the isotherm data acquisition was about 20 times less than that required with a conventional column. The data obtained were fitted to different single and competitive isotherm models. Both the single and the competitive data sets fitted best to the same Bilangmuir (BL) isotherm model with small differences in the numerical values of the parameters. The best fitted Bilangmuir single and competitive isotherm models were used to predict the overloaded experimental profiles of both pure enantiomers, of the racemic mixture, and of different enantiomeric mixtures. All the calculated profiles were in excellent agreement with the experimental ones. This agreement confirms that in many chiral separations, the competitive isotherms can be derived from data acquired from the mere racemic mixture with a sufficient accuracy for a correct prediction of the band profiles of all kinds of enantiomer mixtures, making possible the computer-assisted optimization of the experimental conditions.     

Adsorption-desorption isotherm hysteresis of phenol on a C18-bonded surface

Single component adsorption and desorption isotherms of phenol were measured on a high-efficiency Kromasil-C18 column (N = 15000 theoretical plates) with pure water as the mobile phase. Adsorption isotherm data were acquired by frontal analysis (FA) for seven plateau concentrations distributed over the whole accessible range of phenol concentration in pure water (5, 10, 15, 20, 25, 40, and 60 g/l). Desorption isotherm data were derived from the corresponding rear boundaries, using frontal analysis by characteristic points (FACP). A strong adsorption hysteresis was observed. The adsorption of phenol is apparently modeled by a S-shaped isotherm of the first kind while the desorption isotherm is described by a convex upward isotherm. The adsorption breakthrough curves could not be modeled correctly using the adsorption isotherm because of a strong dependence of the accessible free column volume on the phenol concentration in the mobile phase. It seems that retention in water depends on the extent to which the surface is wetted by the mobile phase, extent which is a function of the phenol concentration, and of the local pressure rate, which varies along the column, and on the initial state of the column. By contrast, the desorption profiles agree well with those calculated with the desorption isotherms using the ideal model, due to the high column efficiency. The isotherm model accounting best for the desorption isotherm data and the desorption profiles is the bi-Langmuir model. Its coefficients were calculated using appropriate weights in the fitting procedure. The evolution of the bi-Langmuir isotherm parameters with the initial equilibrium plateau concentration of phenol is discussed. The FACP results reported here are fully consistent with the adsorption data of phenol previously reported and measured by FA with various aqueous solutions of methanol as the mobile phase. They provide a general, empirical adsorption model of phenol that is valid between 0 and 65% of methanol in water.     

New thermodynamically consistent competitive adsorption isotherm in RPLC

A new equation of competitive isotherms was derived in the framework of the ideal adsorbed solution (IAS) that predicts multisolute adsorption isotherms from single-solute isotherms. The IAS theory makes this new isotherm thermodynamically consistent, whatever the saturation capacities of these single-component isotherms. On a Kromasil-C(18) column, with methanol-water (80/20 v/v) as the mobile phase, the best single-solute adsorption isotherm of both toluene and ethylbenzene is the liquid-solid extended multilayer BET isotherm. Despite a significant difference between the monolayer capacities of toluene (370 g/l) and ethylbenzene (170 g/l), the experimental adsorption data fit very well to single-component isotherms exhibiting the same capacities (200 g/l). The new competitive model was used for the modeling of the elution band profiles of mixtures of the two compounds. Excellent agreement between experimental and calculated profiles was observed, suggesting that the behavior of the toluene-ethylbenzene adsorbed phase on the stationary phase is close to ideal. For example, the concentrations measured for the intermediate plateau obtained in frontal analysis differ by less than 2% from those predicted by the IAS model.     

Effect of temperature on the adsorption behavior of tryptophan in reversed-phase liquid chromatography

Single-component adsorption isotherm data of l-tryptophan on a C(18)-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 5% of acetonitrile at five different temperatures between 23 and 62 degrees C. The non-linear fitting of these data provided the bi-Moreau model for all temperatures as the best isotherm model. The inverse method (IM) was used to derive the parameters at these temperatures from the parameters of the 25 degrees C isotherm. The adsorption constants and the saturation capacities of the low and high-energy sites decreases by increasing the temperature, while the adsorbate-adsorbate parameters of both sites increase. An excellent agreement was found between the experimental and calculated overloaded band profiles at all the temperatures used. The breakthrough curves obtained and the overloaded band profiles obtained were found to have different shapes according to the range of concentration studied and the temperatures. At low concentration 0.05-0.5 g/L the breakthrough curves and the overloaded band profiles have a front shock and diffuse rear, which indicates langmuirian behavior, but at intermediate 1-2 g/L and high concentration 8 g/L they start to have diffuse fronts and shocks at the rear or more than one shock at the rear which indicates non-langmuirian behavior. At 23 degrees C the isotherm has another langmuirian part, which appears at high concentration. The behavior of the breakthrough curves is explained by the shape of the isotherm in which all of the isotherms have a langmuirian part (the isotherm is concave upward) and an antilangmuirian part (the isotherm is concave downward). The temperature affected the breakthrough curves by decreasing the time of the appearance of the fronts for all concentration ranges studied, and by decreasing the time difference between the highest concentration and lowest concentration of the fronts, especially the low concentration range at 0.5 g/L. The fronts of the breakthrough curves at high concentration seems to be the most affected by temperature.     

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.     

Adsorption isotherms of the fullerenes C60 and C70 on a tetraphenylporphyrin-bonded silica

The single-component adsorption isotherms of the C60 (from 0 to 15 g/L) and C70 (from 0 to 8 g/L) buckminsterfullerenes on a tetraphenylporphyrin-bonded silica were acquired by frontal analysis, using a solution of toluene-1-methylnaphthalene (40:60, v/v) as the mobile phase. The best isotherm model derived from the fitting of these adsorption data was the bi-Langmuir model, a choice supported by the bimodal affinity energy distribution (AED) obtained for C60. The isotherm parameters derived from the inverse method (IM) of isotherm determination (by fitting calculated profiles to experimental overloaded band profiles of C60 and C70) are in very good agreement with those derived from the FA data. According to the isotherm parameters found by these three methods (FA, AED, IM), the tetraphenylporphyrin-bonded silica can adsorb 54 and 42 mmol/L of C60 and C70 fullerenes, respectively, a result that is consistent with the relative molecular size of these two compounds. The 20% lower surface accessibility for C70 is compensated by a three times higher equilibrium constant on the low-energy sites, giving a selectivity alpha(C70/C60) = 3.6. Large volumes (0.2, 0.8 and 1.7 mL) of mixtures of C60 (3.2 g/L) and C70 (1.3 g/L) were injected and their elution profiles compared to those calculated from the competitive bi-Langmuir model derived from the single-component isotherm data. A good agreement is obtained between calculated and experimental profiles, which supports the two-site adsorption mechanism derived from the single-component adsorption data. The measurements of the influence of the pressure on the retention of C60 and C70 demonstrate that the partial molar volumes of the two buckminsterfullerenes are 12 mL/mol larger in the stationary than in the mobile phase.     

Effect of the ionic strength of the solution and the nature of its ions on the adsorption mechanism of ionic species in RPLC III. Equilibrium isotherms and overloaded band profiles on Kromasil-C18

In two companion papers, we have described the influence of the concentration and the nature of completely dissociated salts dissolved in the mobile phase (methanol:water, 40:60, v/v) on the adsorption behavior of propranolol (R'-NH2+-R, Cl-) on XTerra-C18 and on Symmetry-C18. The same experiments were repeated on a Kromasil-C18 column to compare the adsorption behavior of this ionic compound on these three different RPLC systems. The adsorption data of propranolol hydrochloride were first measured by frontal analysis (FA) using a mobile phase without salt. These data fit best to the Bi-Moreau model. Large concentration band profiles of propranolol were recorded with mobile phases containing increasing KCl concentrations (0, 0.002, 0.005, 0.01, 0.05, 0.1 and 0.2 M) and the best values of the isotherm coefficients were determined using the numerical solution of the inverse problem of chromatography. The general effect of a dissociated salt in the mobile phase was the same as the one observed earlier with XTerra-C18 and Symmetry-C18. However, obvious differences were observed for the shape of the band profiles recorded at low column loading (1.5 g/L, 250 microL injected). A long shoulder is visible at all salt concentrations and the band broadening is maximum at low salt concentrations. A slow mass transfer kinetics on the high-energy sites of the bi-Moreau model might explain this original shape. Five other salts (NaCl, CsCl, KNO3, CaCl2 and Na2SO4) were also used at the same ionic strength (J = 0.2 M). As many different band profiles were observed, suggesting that specific solute-salt interactions take place in the adsorbed phase.     

Band splitting in overloaded isocratic elution chromatography II. New competitive adsorption isotherms

The equations of two new binary competitive isotherms models are derived. The first of these models assumes that the isotherms of the two pure, single compounds have distinct monolayer capacities. Its derivation is based on kinetic arguments. The ideal adsorbed solution (IAS) framework was applied to derive the second model that is a thermodynamically consistent competitive isotherm. This second model predicts the competitive adsorption isotherm behavior of a mixture of two compounds that have single-component adsorption behavior following a BET and/or a Langmuir isotherms. Both models apply well to the binary adsorption of ethylbenzoate and 4-tert.-butylphenol on a Kromasil-C18 column (with methanol-water, 62:38, v/v, as the mobile phase). The best single-solute adsorption isotherms of these two compounds are the liquid-solid extended multilayer BET and the Langmuir isotherms, respectively. The kinetic and thermodynamic new competitive models were compared, regarding the accuracy of their prediction of the elution band profiles of mixtures of these two compounds. A better agreement between experimental and calculated profiles was observed with the kinetic model. The IAS model failed because the behavior of the ethylbenzoate/4-tert.-butylphenol adsorbed phase mixture is probably non-ideal. The most striking result is the qualitative prediction by these models of the peak splitting of 4-tert.-butylphenol during its elution in presence of ethylbenzoate.     

Comparison of the binary equilibrium isotherms of the 1-indanol enantiomers on three high-performance liquid chromatography columns of different sizes

The competitive isotherm data for the enantiomers of 1-indanol were measured on three columns, a microbore column (15 cm x 0.1 cm), a conventional analytical column (15 cm x 0.46 cm), and a semi-preparative column (20 cm x 1.0 cm), packed with Chiralcel OB. The sets of isotherm data measured on each one of these three columns could be fitted well by a bi-Langmuir isotherm model. The experimental elution band profiles of mixtures of the 1-indanol isomers were recorded on the three columns. The isotherm model, combined with the equilibrium dispersive model of chromatography, gave calculated profiles that are in excellent agreement with the experimental profiles in all cases investigated. It was found that the value of the inner diameter of the column is an important parameter in the calculation of the isotherm parameters from the measured isotherm data. In order to use isotherm data obtained on one column to account for the phase equilibrium on another one, the inner diameters of these columns must be measured accurately. The diameters of the three columns were all slightly off their nominal value. Without correction, an important systematic error was made on the isotherm data obtained with the microbore column while only negligible errors were made on the data obtained with the other two columns. After due correction for this effect, the relative difference between the isotherm data for the microbore and the semi-preparative column is still, on the average, about 10%, a difference that might be explained by the limited precision of the measurement of the microbore column diameter. The relative difference between the isotherm data for the analytical and the semi-preparative columns was about 1%, a reasonable value, since the two columns came from different batches of the same packing material.     

Effect of the temperature on the isotherm parameters of phenol in reversed-phase liquid chromatography

Adsorption isotherm data of phenol from an aqueous solution of methanol onto a C18-bonded silica (Symmetry-C18) were acquired by frontal analysis (FA) at six different temperatures, in a wide concentration range. The non-linear fitting of these data provided the bi-Langmuir model as best isotherm model, a conclusion further supported by the results of the calculation of the affinity energy distribution (AED). The isotherm parameters were obtained using several methods, the fitting of FA isotherm data, the calculation of the AED, and the inverse method, that uses overloaded elution band profiles. The different values obtained are in close agreement. They allow a quantitative investigation of the separate properties of the low- and the high-energy sites on the adsorbent surface. Increasing the temperature decreases the saturation capacity of the low-energy adsorption sites and the adsorption constant of the high-energy sites. In contrast, increasing the temperature does not cause any significant changes in either the saturation capacity of the high-energy sites or the adsorption constant of the low-energy sites.     

Physical origin of peak tailing on C18-bonded silica in reversed-phase liquid chromatography

Single component isotherm data of caffeine and phenol were acquired on two different stationary phases for RPLC, using a methanol/water solution (25%, v/v, methanol) as the mobile phase. The columns were the non-endcapped Waters Resolve-C18, and the Waters XTerra MS C18. Both columns exhibit similar C18 -chain densities (2.45 and 2.50 micromol/m2) and differ essentially by the nature of the underivatized solid support (a conventional, highly polar silica made from water glass, hence containing metal impurities, versus a silica-methylsilane hybrid surface with a lower density of less acidic free silanols). Thirty-two adsorption data points were acquired by FA, for caffeine, between 10(-3) and 24 g/l, a dynamic range of 24,000. Twenty-eigth adsorption data points were acquired for phenol, from 0.025 to 75 g/l, a dynamic range of 3000. The expectation-maximization procedure was used to derive the affinity energy distribution (AED) from the raw FA data points, assuming a local Langmuir isotherm. For caffeine, the AEDs converge to a bimodal and a quadrimodal distribution on XTerra MS-C18 and Resolve-C18, respectively. The values of the saturation capacity (q(s,1) approximately equal to 0.80 mol/l and q(s,2) approximately equal to 0.10 mol/l) and the adsorption constant (b1 approximately equal to 3.11/mol and b2 approximately equal to 29.1 l/mol) measured on the two columns for the lowest two energy modes 1 and 2, are comparable. These data are consistent with those previously measured on an endcapped Kromasil-C18 in a 30/70 (v/v), methanol/water solution (q(s,1) = 0.9 mol/l and q(s,2) = 0.10 mol/l, b1 = 2.4 l/mol and b2 = 16.1 l/mol). The presence of two higher energy modes on the Waters Resolve-C18 column (q(s,3) approximately equal to 0.013 mol/l and q(s,4) approximately equal to 2.6 10(-4) mol/l, b3 approximately equal to 252 l/mol and b4 = 13,200 l/mol) and the strong peak tailing of caffeine are explained by the existence of adsorption sites buried inside the C18-bonded layer. It is demonstrated that strong interactions between caffeine and the water protected bare silica surface cannot explain these high-energy sites because the retention of caffeine on an underivatized Resolve silica column is almost zero. Possible hydrogen-bond interactions between caffeine and the non-protected isolated silanol groups remaining after synthesis amidst the C18-chain network cannot explain these high energy interactions because, then, the smaller phenol molecule should exhibit similarly strong interactions with these isolated silanols on the same Resolve-C18 column and, yet, the consequences of such interactions are not observed. These sites are more consistent with the heterogeneity of the local structure of the C18-bonded layer. Regarding the adsorption of phenol, no matter whether the column is endcapped or not, its molecular interactions with the bare silica were negligible. For both columns, the best adsorption isotherm was the Bilangmuir model (with q(s,1) approximately equal to 2 and q(s,2) approximately equal to 0.67 mol/l, b1 0.61 and b2 approximately equal to 10.3 l/mol). These parameters are consistent with those measured previously on an endcapped Kromasil-C18 column under the same conditions (q(s,1) = 1.5 and q(s,2) = 0.71 mol/l, b1 = 1.4 l/mol and b2 = 11.3 l/mol). As for caffeine, the high-energy sites are definitely located within the C18-bonded layer, not on the bare surface of the adsorbent.     

Study of the competitive isotherm model and the mass transfer kinetics for a BET binary system

The competitive adsorption behavior of the binary mixture of phenetole (ethoxy-benzene) and propyl benzoate in a reversed-phase system was investigated. The adsorption equilibrium data of the single-component systems were acquired by frontal analysis. The same data for binary mixtures were acquired by the perturbation method. For both compounds, the single-component isotherm data fit best to the multilayer BET model. The experimental overloaded band profiles are in excellent agreement with the profiles calculated with either the general rate model or the modified transport-dispersive models. The competitive adsorption data were modeled using the ideal adsorbed solution (IAS) theory. The numerical values of the coefficients were derived by fitting the retention times of the perturbation pulses to those calculated using the IAS theory compiled with the coherence conditions. Finally, the elution profiles of binary mixtures were recorded. They compared very well with those calculated. As a characteristic feature of this case, an unusual retainment effect of the chromatographic band of the more retained component by the less retained one was observed. The combination of the General Rate Model and the adsorption isotherm model allowed an accurate prediction of the band profiles.     

Study of the selectivity of reversed-phase columns for the separation of polycarboxylic acids and polyphenol compounds

A systematic investigation was undertaken into the relative separation performance of five reversed-phase chromatography columns including some commercially new hybrid packed columns for a series of polycarboxylic acids and polyphenol compounds. Information theory (IT) and factor analysis (FA), together with a basic evaluation of retention information (band shape, retention factor and elution order) were used to compare four columns to a conventional C18 column. The results revealed very little difference in retention behaviour between the Phenomenex Aqua C18 column, the Waters XTerra RP C18 column, and the conventional Phenomenex Luna C18 column. However, there were notable differences in the retention processes between the Phenomenex Synergi polar-RP column, which is an ether-linked phenyl base with polar endcapping, and the Luna C18 column. The most significant differences were observed between the Luna C18 column and a Phenomenex Luna Cyano column. However, the limited degree of retention of the polycarboxylic acids and polyphenol compounds on the Luna Cyano column permits only limited use for the separation of these types of compounds. Overall, the Phenomenex Synergi polar-RP column exhibited the best performance for the separation of the test solutes compared to that of the conventional C18 column, with IT yielding an Informational Similarity of 0.99 and FA a moderate correlation coefficient of 0.70. The Phenomenex Synergi polar-RP column gave the best peak shape and offered substantial selectivity differences thereby providing a good alternative over the conventional C18 column for separating polycarboxylic acids and polyphenols.