Capacity Factors Of Solutes Research Articles

Transport Characteristics of Aromatic Acids in the Capillary Electrochromatography with Mixed Magnetic Nanoparticles Coating as Stationary Phase

In this paper, novel open-tubular electrochromatographic system with core/shell magnetic nanoparticles modified by amino and C18 functional groups as mixed stationary phase by using a series of permanent magnets to immobilize nanoparticles into the capillary were investigated. The new system showed high separation ability and efficiency due to high specific surface area of nanomaterial and mixing separation mechanism. The transport equations of aromatic acids using the open-tubular electrochromatographic column with magnetic nanoparticle coating as stationary phases were derived under isocratic and double stepwise gradient elutions, and the retention characteristics of organic acids separated by the columns were further investigated. The results showed that the electroosmotic flow gradually decreased with the increase of Fe3O4@SiO2-NH2 in the mixed stationary phases, and the direction of electroosmotic flow reversed eventually. The capacity factor was relevant to the ratio of ion-exchange and reversed-phase stationary phases. Furthermore, experiments confirmed that the capacity factor of solute had a good linear relationship with the ratio of mixed stationary phases, and the correlation coefficient was greater than 0.9166 after fitting. Under the double stepwise gradient elution condition, the migration time of solute can be predicted by its elution time under isocratic elution. After the solutes were eluted by phosphate buffer stepwise gradient elution, the relative errors between predicted retention time and actual retention time of solute were less than 8.0%. After they were eluted by organic modifier concentration stepwise gradient elution, the relative errors between two values of solute were less than 10%. After they were eluted by pH value stepwise gradient elution, the relative errors between two val- ues of solute were less than 5.8%. Rhizoma gastrodiae was separated by the constructed system under the optimized chroma- tographic condition, the main components of gastrodin, vanilla alcohol and citric acid were identified based on retention time of standards, and the relative errors between predicted value and actual value of citric acid was 11%. The open-tubular elec- trochromatographic system shows great potential in the method development of analysis of complicated samples. Keywords gradient elution; transport characteristic; mixed stationary phase 1 引言

Preparation and Chromatographic Characterization of Zirconia Modified with Dodecylamine‐N,N‐dimethylenephosphonic Acid

A new zirconia‐based stationary phase was prepared for the first time by modified zirconia with dodecylamine‐N,N‐dimethylenephosphonic acid (DDPA). The adsorption mode that DDPA adsorbed on zirconia with only one phosphonic group was studied by the coloration reaction. The resultant stationary phase (DPZ) has not only long alkyl groups, but also amino group and free phosphonic groups. The chromatographic characterization of the stationary phase was strongly influenced by these polar groups. Despite the lower surface coverage of the DPZ, the capacity factors of polar solutes on DPZ are greater than that on ODS due to the various retention mechanism provided by the polar groups. The free phosphonic group imparts the negative charges to the stationary phase at high pH and blocks most of the Lewis acid sites on the surface of zirconia, so excellent separations were obtained not only for PAHs and amines, but also for phenols. Separations of benzoic acids and positional isomers of dihydroxylbenzene and nitrophenol on DPZ were improved greatly.

Alternating voltage capillary electrochromatography.

A phenomenon was found whereby the application of an alternating voltage to a capillary column can vary the capacity factor of a sample solute. The alternating voltage-induced variation in the capacity factor was studied using an anion exchange mini-bed (a short capillary column 12 mm long). The capacity factor varied according to both the amplitude and frequency of an applied alternating voltage. The variation greatly depended on the kinds of sample solutes and packing materials. A new separation mode for capillary electrochromatography using an alternating voltage, that is, alternating voltage capillary electrochromatography (AV-CEC), was proposed as an application of this phenomenon to control the retention of a sample solute. The chromatographic behavior of three organic acids (benzoic acid, phthalic acid, and salicylic acid) was studied in AV-CEC using an anion exchange column.

Investigation of host-guest stability constants of calix[n]arenes complexes with aromatic molecules by RP-HPLC method.

Under reversed-phase high-performance liquid chromatographic conditions [Spherisorb ODS 1 stationary phase, UV detection at 254 nm, and acetonitrile-dichloromethane-acetic acid-methyl-tert-butylether (84.6/4.5/0.9/10, v/v/v/v) as the mobile phase], adding p-tert-butylcalix[8] (10(-5)-3.10(-5))-[12]arenes (10(-5)-4.10(-5) mol/L) to the mobile phase leads to decreased sorption of aromatic solutes on the surface of the sorbent because of the formation of host-guest inclusion complexes between the calixarenes and the aromatic molecules. Stability constants of the complexes (781-9338M(-1)) are determined from the relationship between the solute capacity factors and the calixarene concentration in the mobile phase.

Use of OPLC to study the biological activity of organic compounds

Solute capacity factors in pure water, logkw, determined by the new method based on Ościk's equation, are proposed as descriptors of the biological activity of some benzamides. Values of logkw determined by the new method and by parabolic extrapolation were compared with the minimum inhibitory concentration (MIC) of the compounds against six different dermatophytes, and correlations between logkw and log (1/MIC) values of the same substances were analysed. For all the dermatophytes and all the compounds very good linear correlations were obtained for retention factors determined by the method proposed in this study.

Effect of pressure on solute capacity factor in HPLC using a non-porous stationary phase

The pressure applied in liquid chromatography (LC) can influence the capacity factor of analytes. Using commercial equipment, column (33×4.6 mm) and 1.5 μm non-porous, tetradecyl (C14) coated silica particles, a moderate but significant change in the capacity factor was observed with increasing pressure. Depending on the experimental conditions the relationship was found to be either linear or non-linear. In the latter case the retention increased at first, but later tended to decrease at still higher pressure. The results direct attention to the role of the pressure (and hence of flow rate and heat of friction) in determining capacity factors.

Properties and Applications of Mixed Packing Capillary Electrochromatography

Mixed packing capillary electrochromatography (MP CEC) with the stationary phase comprising a physical mixture of strong cation exchange (SCX) phase and octadecysilyl (ODS) phase was developed. With the existence of a sulfonic acid group on the surface of SCX, not only could the electroosmotic flow (EOF) remain high at low pH, but also the hydrophilicity of the stationary phase was increased greatly, leading to broad adaptable ranges of both pH and organic modifier concentration in the mobile phase. At the same time, with the coexistence of C18 on the surface of ODS, both the retention and the resolution of samples were improved. Accordingly, MP CEC combined the advantages of both SCX and ODS columns. Effects of operation parameters on EOF and the capacity factors of solutes as well as the retention mechanism of such a column were studied systematically. In addition, MP CEC columns were used in the analysis of strong polar solutes as well as for the high speed separation of acidic, basic, and neutral compounds in a single run.

Properties and Applications of Mixed Packing Capillary Electrochromatography

Mixed packing capillary electrochromatography (mp cec) with the stationary phase comprising a physical mixture of strong cation exchange (scx) phase and octadecpilyl (ods) phase was developed. with the existence of a sulfonic acid group on the surface of scx, not only could the electroosmotic flow (eof) remain high at low ph, but also the hydrophilicity of the stationary phase was increased greatly, leading to broad adaptable ranges of both ph and organic modifier concentration in the mobile phase. at the same time, with the coexistence of c-18 on the surface of ods, both the retention and the resolution of samples vr ere improved. accordingly, mp cec combined the advantages of both scs and ods columns. effects of operation parameters on eof and the capacity factors of solutes as well as the retention mechanism of such a column were studied systematically. in addition, mp cec columns were used in the analysis of strong polar solutes as well as for the high speed separation of acidic, basic, and neutral compounds in a single fun.

Pressure-controlled high-performance liquid chromatographic study on the influence of rim chemistry on partial molar volume differences between free and complexed cyclodextrins

Pragmatic comparison of pressure dependent retention for differing cyclodextrin rim chemistries is assessed using controlled-pressure HPLC. For pressure differences of <300 bar, systematic shifts in solute capacity factor are observed for both native and methylated β-cyclodextrin stationary phases. In addition to the importance of this observation for the practice of liquid chromatography, this technique can also be implemented in the fundamental determination of the influence of rim chemistry on the cyclodextrin partial molar volume both with and without solute inclusion. That is, pressure-controlled measurements provide a direct comparison between the partial molar volumes for native cyclodextrin (CD) and methylated cyclodextrin (MCD) in the presence and absence of the complexing solute (comp). Surprisingly, direct comparison of the measured partial molar volumes for the two rim chemistries indicates that the presence of neutral solutes does not contribute significantly to the volumetric component of complexation, V comp,CD− V comp,MCD≈ V CD− V MCD. In contrast, their ionized counterparts are shown to exhibit marked rim chemistry differences in the partial molar volume of cyclodextrins with and without anion inclusion, V comp(−),CD− V comp(−),MCD < V CD− V MCD. Not previously demonstrated by direct chromatographic measurement, these results have interesting implications for advancing the fundamental understanding of host–guest solvation properties.

Influence of the addition of modifiers on solute-micelle interaction in hybrid micellar liquid chromatography

In reversed-phase micellar liquid chromatography (MLC) organic modifiers are usually added to the mobile phase to modify the eluent strength and to increase the efficiency of the chromatographic peaks. The effect of the modifiers methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, acetonitrile and tetrahydrofuran on the critical micelle concentration (cmc) of the anionic surfactant sodium dodecylsulphate (SDS) has been studied to enable understanding of the interactions between solute, micelles and bulk liquid in such hybrid eluents. Methanol, acetonitrile and tetrahydrofuran increased thecmc, whereas the other alcohols reduced its value. For butanol and pentanol, which partition into the micelle, thecmc barely changed for alcohol concentrations above 4% and 1.5%, respectively. This behaviour indicated that the micelle was mainly modified at lower alcohol concentrations by introduction of the molecule into the micelle palisade. When the ratio (number of molecules of alcohol in the micelle)/(number of molecules of surfactant), was >3, new molecules of the alcohol probably dissolved in the micelle core. The dependence of the capacity factors of several solutes on the concentration of modifier was similar to that found for thecmc, implying that the effects that change thecmc values upon addition of alcohol to a surfactant solution are, at least partially, those that induce reduced retention in MLC—modification of the nature of bulk water and micelle. The change in retention observed with SDS mobile phases, when butanol and pentanol are added, are mainly a result of modification of the structure of the micelle.

Effect of Octakis(diethoxyphosphoryloxy)-tert-butyl-calix[8]arene in Mobile Phase on the Reversed-Phase Retention Behavior of Aromatic Compounds: Host-Guest Complex Formation and Stability Constants Determination

Under reversed-phase high-performance liquid chromatographic conditions (Separon SGX C 18 stationary phase, ultraviolet detection at 254 nm, and acetonitrile—water [86:14, v/v] as the mobile phase), addition of octakis(diethoxyphosphoryloxy)-tert-butylcalix[8]arene in concentrations of 1×10 -4 to 4 ×10 4M to the mobile phase leads to decreasing sorption of aromatic solutes on the sorbent's surface due to the formation of host-guest inclusion complexes between the calixarenes and the aromatic molecules. Stability constants of the complexes (608-2795M-1) are determined from the relationship between the solute capacity factors and the calixarene concentration in the mobile phase.

Enhancements in micellar electrokinetic chromatographic separations with decanonyl-N-methylalkanamide micelles through the addition of alkyl sulfates

Sodium dodecyl sulfate (SDS) and sodium tetradecyl sulfate (STS) are effective additives for improving the efficiency of the micellar electrokinetic chromatography (MEKC) with micelle forming solutions of decanonyl-N-methylglucamide (MEGA 10) with boric acid, pH 10. MEGA 10, a nonionic surfactant, is one of a series of micelle forming compounds for which the surface charge density of the micelle and electrophoretic migration behavior can be varied through the extent of complexation with a charged species, i.e., borate or boronate ions. Using aniline and a series of 11 substituted anilines as model solutes, the addition of 0–10 m M concentrations of SDS or 0–8 m M STS to buffers containing 20 m M or 40 m M MEGA 10 and boric acid (1:4 concentration ratio) results in an increase in the solute capacity factors and an improvement in separation efficiencies with only small changes in the rate of electroosmotic flow and size of the elution range. Further addition of the sodium alkyl sulfate surfactants results in a large expansion of the elution range. The effects of SDS and STS are attributed to changes in the structure of the MEGA 10/borate micelles resulting in a mixed surfactant micelle structure which is more permeable for the solutes.

Roles of Organic Modifiers in Micellar Electrokinetic Capillary Chromatography

In the present work, four organic modifiers, Pit. urea, dioxane, methanol, and tetrahydrofuran, were comparatively and systematically studied in terms of their effects on electrokinetic migration behavior and the retention mechanism of homologous solutes in MECC. The results showed that the electroosmotic mobility, mu(eo), and the electrophoretic mobility of a micelle, mu(cp,mc), decrease linearly with increasing organic modifier concentration. The ability of organic modifiers to lower mu(eo) is greater than their ability to lower mu(ep,mc). The negative values of the slopes of these linear relationships, D-eo and D-cp,D-mc, increase along the series the order urea < methanol < dioxane < tetrahydrofuran. The logarithm of the capacity factor (In k') of uncharged homologous solute, which is mainly determined by the hydrophobic interaction, decreases linearly with increasing organic modifier concentration, due not only to the decrease in the partition coefficient but also to the decrease in the phase ratio. A linear relationship was observed between the slope of the plot of ln k' vs. organic modifier concentration and the carbon number of homologous compound. The slope of such a relationship can characterize the hydrophobicity of the organic modifier. The hydrophobicity of such organic modifiers is also found to increase along the series urea < methanol < dioxane < tetrahydrofuran.

Effect of mobile-phase composition on pressure-induced shifts in solute retention for LC separations using β-cyclodextrin stationary phases

Modest pressure (<350 bars) has a significant impact on reversed-phase liquid chromatographic retention using β-cyclodextrin stationary phase. Demonstrated here for separations of the positional isomers of nitrophenol and naphthol as well as the enantiomers of hexobarbital and mephobarbital, the effect of small pressure perturbations on solute capacity factor ranges from +11 to −14%. These changes in retention arise from pressure-induced shifts in solute equilibria and are used to estimate the change in molar volume upon solute binding with β-cyclodextrin (ΔVcomp). In this article, the effect of the mobile phase on pressure-induced shifts in solute retention is examined. Mobile-phase composition is observed to have a significant influence on the change in solute capacity factor with pressure and therefore on ΔVcomp. As the composition of water in the mobile phase increases, the change in partial molar volume of complexation is observed to become more positive for both positional isomer and chiral separations. Although these studies imply that changes in solvent composition have a general effect on the observed ΔVcomp and therefore on the change in pressure perturbation with solvent composition, it is clear that more studies are needed to establish the exact nature and breadth of this effect. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 647–652, 1998

Effect of mobile‐phase composition on pressure‐induced shifts in solute retention for LC separations using β‐cyclodextrin stationary phases

Modest pressure (<350 bars) has a significant impact on reversed-phase liquid chromatographic retention using β-cyclodextrin stationary phase. Demonstrated here for separations of the positional isomers of nitrophenol and naphthol as well as the enantiomers of hexobarbital and mephobarbital, the effect of small pressure perturbations on solute capacity factor ranges from +11 to −14%. These changes in retention arise from pressure-induced shifts in solute equilibria and are used to estimate the change in molar volume upon solute binding with β-cyclodextrin (ΔVcomp). In this article, the effect of the mobile phase on pressure-induced shifts in solute retention is examined. Mobile-phase composition is observed to have a significant influence on the change in solute capacity factor with pressure and therefore on ΔVcomp. As the composition of water in the mobile phase increases, the change in partial molar volume of complexation is observed to become more positive for both positional isomer and chiral separations. Although these studies imply that changes in solvent composition have a general effect on the observed ΔVcomp and therefore on the change in pressure perturbation with solvent composition, it is clear that more studies are needed to establish the exact nature and breadth of this effect. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 647–652, 1998

Theoretical and Experimental Studies of the Effect of Pressure on Solute Retention in Liquid Chromatography

Pressure is often assumed to have a negligible influence on solute retention in liquid chromatography because of the small compressibility of the mobile and stationary phases. The range of pressures commonly encountered in reversed-phase separations is considerable, however, and may give rise to significant changes in solute capacity factor. In this study, the retention of model solutes is measured directly along the chromatographic column as a function of the local pressure. The model solutes, a homologous series of derivatized fatty acids, exhibit a significant increase in capacity factor ranging from +9.3% for n-C(10) to +24.4% for n-C(20) for inlet pressures from 1500 to 5000 psi. These experimental results are compared with a thermodynamic model derived from regular solution theory. This model suggests that state effects alone are not sufficient to describe the measured change in solute retention and that variations in interaction energy with density must also be considered. By using the simple relationship of van der Waals for the interaction energy (E ∝ 1/V), the change in capacity factor with density is slightly underestimated. However, by using an extended relationship that better describes polar fluids (E ∝ 1/V(2)), good agreement is observed. Finally, the correlation of experimental results with this thermodynamic model reveals that all components in the chromatographic system, including the solute, mobile phase, and stationary phase, must be considered compressible. The results of this study have clear implications for the determination of fundamental physicochemical parameters, as well as for the everyday practice of liquid chromatography.

Pressure-Dependent Retention and Selectivity in Reversed-Phase Liquid Chromatographic Separations Using β-Cyclodextrin Stationary Phases

The influence of pressure on solute retention in liquid chromatography is commonly ignored due to the small compressibility of polar mobile phases. However, the equilibrium processes driving solute retention may be significantly affected by pressure, even under the modest conditions commonly encountered in HPLC (<350 bar). In this paper, we examine the role of pressure in separations where the primary mechanism for solute retention is inclusion complexation. Using the positional isomers of nitrophenol as model solutes, pressure-induced decreases in solute capacity factor ranging from −2.1% to −35.1% are observed experimentally for pressures from 40 to 340 bar. Individual contributions of pressure-induced solute ionization and complexation to this pressure-dependent solute retention are isolated by controlling mobile-phase pH. Pressure-induced dissociation of the cyclodextrin−solute complex appears to play the primary role in determining the pressure dependence of solute retention. Experimentally observed selective perturbation in solute retention with pressure is shown to have a direct impact on chromatographic resolution. In addition, the magnitude of the pressure-induced decrease in solute retention is demonstrated to be a function of the mobile-phase solvent strength. This previously underappreciated pressure effect has clear implications for the practical application of cyclodextrin stationary phases, as well as for the fundamental interpretation of those thermodynamic parameters central to the separation process.

Practical Examination of a Nonporous Silica Stationary Phase for Reversed-Phase Fast LC Applications

The performance of a nonporous silica reversed-phase high-performance liquid chromatographic stationary phase (1.5 μm) is examined and contrasted with that of a conventional stationary phase. The conventional column exhibits classic van Deemter behavior with respect to the impact of flow rate on efficiency, but the “fast” liquid chromatographic (LC) column’s efficiency is not as strongly affected by flow rate because of its lessened contributions of longitudinal diffusion and stagnant mobile phase mass transfer effects and greater sensitivity to system void volume effects. Both columns exhibit roughly equivalent efficiencies on a per meter basis; however, the efficiency of the fast LC column was more strongly influenced by solute capacity factor. Retention by the fast LC column is more strongly influenced by mobile phase composition. In general, the fast LC column produces multispecies separations more rapidly with a weaker mobile phase than does the conventional column, which results in considerable reduction in solvent usage without a significant loss of efficiency. The performance of the fast LC column is strongly influenced by the system configuration, especially with respect to total system void volume.

Immobilized-artificial-membrane chromatography: measurements of membrane partition coefficient and predicting drug membrane permeability

Immobilized artificial membranes (IAMs) are chromatographic surfaces prepared by covalently immobilizing cell membrane phospholipids to solid surfaces at monolayer densities. IAM surfaces mimic fluid cell membranes. For 23 structurally unrelated compounds, solute capacity factors [log ( k′ IAM)] measured on IAM columns correlate very well with the solute equilibrium partition coefficients [log ( K m)] measured in fluid liposome systems ( r = 0.907). This indicates that solute partitioning between the IAM bonded phase and the aqueous mobile phase is similar to the solute partitioning between liposomes and the aqueous phase. IAMs also predicted oral drug absorption in mice and drug permeability through Caco-2 cells. IAM chromatography is experimentally simple and large volume screening of experimental compounds for drug absorption is possible. Solute retention on IAMs was found to be dominated by a partitioning mechanism. The structural requirements for HPLC bonded to predict solute-membrane partitioning are briefly discussed.

High-performance liquid chromatography with non-aqueous solvents Relative elution strength on a polystyrene—divinylbenzene column

A polymeric resin-based stationary phase was used to study the retention of alkylphenols, alkylbenzenes, substituted benzenes and fused-ring compounds with different eluents. Various 100% organic solvents were used as eluents in order to observe the relative ability to solvents to elute compounds of given chemical structure. Plots of log k′ vs. number of carbons in a chain attached to the aromatic ring showed linear correlations with slopes that are dependent on solvent type and solute functionality. Of the solvents studied, acetonitrile exhibited the highest solvating ability for the benzene ring and for several functional groups. Ethanol was shown to have the strongest eluting power for the methylene group. The relative ability of several solvents to elute polynuclear aromatic hydrocarbons in solid-phase extraction was determined. The functional group contribution (τ) was calculated for selected compounds and acceptable agreement was found in the correlation between experimental and predicted values for solute capacity factor.