Capacity Factors Of Solutes Research Articles

Correlation between retention and the C18 silica surface coverage in reversed-phase liquid chromatography

Members of three homologous series and five non-homologous solutes with various functional groups were chromatographed on a series of well-characterized C18 reversed stationary phases with a range of methanol-water mobile phases. Measured capacity factors of solutes were correlated with the concentration of C18 ligands on the stationary phase. A linear relationship with the slope depending on a solute molecular structure and the volume fraction of methanol in the mobile phase was obtained. A method for the evaluation of phase ratio is also proposed.

Column “dead volume” in liquid chromatography

The physical and chromatographic volumes associated with a liquid chromatography column have been considered theoretically and their inter-relationship is discussed. The two chromatographically important dead volumes, the kinetic dead volume and the thermodynamic dead volume, have been identified and it is shown that, whereas the kinetic dead volume (the volume of the moving phase) was common to all solutes, the thermodynamic dead volume was unique to each solute. The solute dependence of the thermodynamic dead volume resulted from any exclusion characteristics that the stationary phase possessed. Stationary phases based on silica exhibited strong exclusion properties and hence the thermodynamic dead volume for silica-based phases was strongly dependent on solute size. It followed that the pertinent thermodynamic dead volume had to be determined strictly for each solute. It also followed that correlation of retention with solvent composition could not be carried out employing k′(solute capacity factor) values, as these were dependent on accurate dead volume measurements. It is recommended that for such a correlation, corrected retention volumes are employed and then only where the solute is eluted at sufficiently high k′ values. Under such circumstances any errors that would result from incorrect dead volume measurements are minimised. Experimental methods are given for the evaluation of all chromatographically pertinent volumes and, as an example, the procedures were applied to a Zorbax C8 reversed-phase column. The data obtained are discussed in the light of the theory presented.

Preparation and retention characteristics of different phenyl phases for reversed-phase liquid chromatography

Three different phenyl phases were prepared. The amount of organic moiety bound on silica support was determined from thermogravimetric curves of the modified silica gel. The specific surface areas of gels were obtained from nitrogen sorption measurement at −196°C. The effect on separation and selectivity of the different bonded-phenyl functional groups was studied. The selectivity of the phenyl-bonded silica gels was examined and compared with octadecyl (ODS) silica in liquid chromatography. Corresponding to the high surface concentration of functional groups, the capacity factors of solutes, normalized to unit surface area of the adsorbent, k′/SBET were found to decrease in the sequence phenylmethyl>diphenyl>triphenyl. Polar solutes are retained in greater extent on the phenyl phases than on the ODS phase.

On the Fundamental Retention Equation in Gradient Elution Liquid Chromatography

Abstract Basic integral equations used to predict the solute retention times or volumes in gradient elution liquid chromatography are carefully examined. In order to simplify the integration procedure, one strongly suggests to select a convention independent of the solvent composition for defining the solute capacity factor. Most of these equations make use of the volume V′ of mobile phase which has passed through the peak center (or band maximum) as the integration variable. It is shown that, instead of V′, one can use the volume V flowing from the column in these equations as the integration variable in combination with retention conditions prevailing at the column inlet rather than actually within the column, provided that the gradient is displaced without modification through the column, i.e. without retention of the mobile phase components. The general retention equation is derived for a binary gradient where the strong component of the mobile phase is retained in such conditions that its distributio...

Single-column anion chromatography with indirect uv detection using pyromellitate buffers as eluents

Ion chromatography can be conveniently carried out with conventional high-performance liquid chromatographic equipment and classical ion exchangers using a single column and a UV-absorbing eluent ion. Solute peaks are detected as vacancy peaks (corresponding to a decrease in eluent absorbance). Benzenepolycarboxylate buffers such as those obtained from pyromellitic acid are used as eluents for anion analysis; owing to the four carboxylic acid groups, the eluent ion charge varies with pH, and the eluting power can be adjusted within a wide range. A linear relationship holds between the logarithm of the solute capacity factor and the logarithm of the concentration of the eluting species; the slope observed is - x/y, here x and y are the charges on the solute and eluent anion, respectively. At a constant analysis time, the best sensitivity is achieved when the initial absorbance of the eluent (determined by the detection wavelength) is high but its concentration is low, and therefore its charge is high enough to give the required eluting power. Within the linearity range, quantitative analysis requires only one calibration graph when peak-area measurements are performed. The detection limits are about 2–5 ng injected and compare favourably with other detection modes, considering the simplicity of the method. Various examples of separations of inorganic and organic anions are given.

High-performance liquid chromatography of amino acid peptides and proteins

The retention behaviour of seven globular proteins ranging in molecular weight from 12,000 to 69,000 was investigated using Mono-Q anion-exchange resin as the stationary phase and sodium chloride as the displacer salt. In particular the influence of changes in ionic strength and mobile phase pH on the isocratic retention properties was assessed. Several proteins were found to have significant retention when the pH of the mobile phase was below the reported pl values of the proteins. This behaviour results from the non-uniform charge distribution on the protein surface, which allows interaction with the charged stationary phase even though the protein net charge is equal to or greater than zero. The influence of pH and ionic strength on experimentally observed bandwidths was also investigated. The dependence of the effective reduced plate height on solute capacity factor was found to vary significantly with the mobile phase pH, a behaviour consistent with the interplay of complex multisite binding kinetics. These results provide a basis for further detailed investigations into the mechanism of interaction of proteins not only with charged surfaces associated with adsorptive chromatographic media but also with other macromolecules.

Experimental Characterization of Strongly Nonlinear Elution Peaks in Liquid Chromatography

Abstract The shape of nonlinear elution peaks was studied experimentally in various chromatographic modes and a wide sample size range encompassing the case where the distribution isotherm curvature is the prevailing factor of band broadening. It is shown that as sample size is varied, the tailing part of all peaks adjoins a common curve that can be fitted with an exponential function whose parameters are independent of mobile phase linear velocity and stationary phase particle size. The effect of solute capacity factor on these parameters was also investigated. These findings provide valuable help for the optimization of linear velocity, particle size, as well as sample size in preparative liquid chromatography under mass overload conditions.

Unified Theory of Absorption Chromatography: Gas, Liquid, and Supercritical Fluid Mobile Phases

Abstract Based on a simple theoretical model which exploits the isomorphism pervading critical phenomena, a general equation is derived for the solute capacity factor, k′, in a chromatographic system where the stationary phase is an absorbent and the mobile-phase fluid may be an ideal gas, moderately nonideal gas, supercritical fluid or liquid. This comprehensive equation is in the form 2nk′ = 2nk′o + F(TR, ρR), where TR and ρR are the reduced temperature and reduced density of the mobile phase and where 2nk'o, the stationary-phase contribution, corresponds to ideal gas-liquid chromatography (ρR = 0) and F(TR, ρR), the mobile-phase contribution (eqn. 14), is a quadratic function of [sgrave]R and a linear function of TR -1 and solute carbon number (homologous solute series). The limitations and advantages, molecular interpretation, predictions, practical implications and applications, and general utility of this unified theory are discussed, with special emphasis on supercritical fluid chromatography (and ...

Model for partition chromatography of ionic solutes in the presence of eluent electrolytes

A general model that does not depend upon a specific mechanism is proposed for describing the retention of ionic solutes in partition chromatography in the presence of electrolytes. The effect of various types of 1∶1 and 1∶2 eluent electrolytes on the retention of inorganic ions on a cross-linked polyacrylamide is successfully explained in terms of the model. Capacity factors of ionic solutes can be estimated semiempirically on the basis of the theoretical equation presented.

Conductivity and indirect UV absorption detection of inorganic cations in non-suppressed ion chromatography using aromatic bases as eluents : I. Principles of operation

An equation is developed to describe the retention behaviour of inorganic cations in non-suppressed ion chromatography and it is shown that a linear relationship exists between the logarithm of the concentration of the eluting species in the eluent and the logarithm of the solute capacity factor. Moreover, the negative slope of this relationship is equal to the ratio of the charges on the solute and eluent ions, provided only one eluent species is responsible for elution. The conductivity change accompanying elution of a solute cation can be positive or negative, depending on whether the eluent cation has a high or low limiting equivalent ionic conductance. When an aqueous solution of an aromatic base is used as eluent, the direction of the conductivity signal when a solute cation elutes is dependent on the relative eluting roles of hydrogen ion and the protonated base cation. When both of these species contribute to the elution of a solute, detection sensitivity is decreased in comparison to the situation where only one species is responsible for elution. An equation for the conductivity change occurring on elution of a solute cation is presented. Indirect UV absorption detection is also possible when aromatic bases are used as eluents, provided the role of hydrogen ion as an eluting species is minimised. This can be achieved using an eluent comprising an aqueous solution of an aromatic base with p K a value in excess of 7.5, and by maintaining the eluent pH in the neutral to alkaline region. Benzylamine at pH 7.15 is shown to be a suitable eluent for indirect UV absorption and conductivity detection and using this eluent, detection limits in the low parts per billion range (for an injection volume of 10 μl) were obtained for lithium, sodium, ammonium and potassium, with both detection methods.

Computer-based numerical integration for the calculation of retention times in gradient high-performance liquid chromatography

An approach to the calculation of solute retention under gradient elution conditions using numerical approximations on a DEC VAX 11/780 computer is presented. This approach eliminates the need for exact solutions of the inherently complex gradient integrals. The programs developed can be used for any solvent composition vs. solute capacity factor or solvent composition vs. time relationships and are, therefore, universal in nature. Gradient microbore HPLC in the reversed-phase mode was used to demonstrate the accuracy of the approach. The assumption of a quadratic dependency between k' and solvent composition allowed for the least-squares fitting of solute isocratic retention with a minimal number of experiments. By use of this approach and the corresponding FORTRAN program, prediction of solute retention under complex multiple-linear gradient shapes was achieved with an accuracy of better than 5%. 20 references, 4 figures, 6 tables.

Solute Retention in Column Liquid Chromatography. VI. Enthalphy Entropy Compensation: The Column Temperature and Mobile-Phase Composition

Abstract The concept of enthalpy-entropy compensation in “non-polar” reverse-phase (ODS) column liquid chromatography is examined in terms of the column temperature (10[ddot]-35[ddot]C) as well as mobile-phase composition (acidified water/tetrahydrofuran) for the solutes: phenol, p-nitrophenol, 2,4-dinitrophenol, o-chlorophenol, o-nitrophenol, 2,4-dimethylphenol, 2,4- dinitro-o-cresol, 4-chloro-m-cresol, and 2,4-dichlorophenol. Plots of log (solute capacity factor, k') against reciprocal column temperature, T−1 exhibit approximate linearity only over the volume-fraction based mobile phase compositional range ⊘THF = 0 to 0.2. Substantial deviations occur beyond ⊘THF = 0.3. Plots of In k' against -ΔH[ddot] (compensation plots) yield a random distribution of data, indicating that what enthalpy-entropy compensation may exist for the systems studied is masked both in terms of temperature and mobile-phase composition. It is concluded that interaction(s) of the solutes with the stationary sorbent are more comple...

Variation of gas chromatographic retentions with carrier pressure and composition

The second-interaction cross virial coefficients B/sub 1M/ are calculated for the solutes n-hexane, trans-3-hexene, benzene, 3,3-dimethylpentane, and cyclohexane with hydrogen, helium, nitrogen, carbon monoxide, carbon dioxide, argon, water, methane, ethane, carbon tetrachloride, and fluorotrichloromethane. The results are then used to evaluate the regression of In (solute specific retention volume) (V/sub g/..pi..) with carrier pressure and composition, and it is shown theoretically as well as demonstrated experimentally that retentions as well as retention order can accordingly be altered. Plots of solute V/sub g//sup 0/ or of (V/sub g/..pi..)/sup -1/ against mole fraction of the fluoroalkane mobile phase in admixture with hydrogen are found to be curved, although the curvature is slight at column pressure drops of 1.1 atm and less. The linear variation of solute capacity factors K' against partial pressure of steam + nitrogen as reported by Pretorius is thereby explained.

The synthesis and characterization of aliphatic sulfonic acid cation exchangers for HPLC

Two oxidation procedures for the synthesis of silicabased aliphatic sulfonic acid cation-exchangers for HPLC are described. Several reaction conditions for oxidizing a terminal-bonded thiol with organic peroxides are provided along with their yields, capacities, reproducibility, and chromatographic behavior. Also, three methods for determining exchange capacity are discussed, and their relationship to the capacity factors of charged solutes is compared. The maximum capacity determined by the titration of the bonded silica was 0.34 meq/gram. The general problems of synthesizing cation-exchangers, measuring their efficiencies and exchange capacities, and evaluating their overall performances are outlined.

Behaviour of different groups of pesticides during liquid-solid chromatography on silica gel with ternary mobile phases

Carbamate insecticides and carbamate, thiocarbamate, substituted phenylurea, amide, and miscellaneous herbicides, and some fungicides were chromatographed in a liquid-solid system by high-performance liquid chromatography with a silica gel column and ternary mobile phases of acetonitrile, dichloromethane, and heptane. The log k′ values, where k′ is the solute capacity factor, were found to be linear functions of the logarithm of the mole fraction of acetonitrile (Nb) in the respective mobile phases. The slope and y-intercept values of log k′ vs log Nb plots are interpreted in terms of the solute molecular structure.

High-performance liquid chromatographic column efficiency as a function of particle composition and geometry and capacity factor

Reduced plate height ( h) vs. reduced velocity ( v) plots have been measured over a wide range of v for 36 high-performance liquid chromatographic systems. Column type was varied over wide limits and solute capacity factor ( k′) values were changed over the range 0.6–22. Resulting data can be accurately described by the Knox equation h = Av 1/3 + B/v + Cv, where A is roughly constant ( A = 0.5–0.8) for all columns studied, but values of B and C are strongly dependent on column type and solute k′ values. These observations can be rationalized by a quantitative model that recognizes two effects: (a) surface diffusion of solute molecules in the stationary phase (along the pore wall) and (b) restricted diffusion of small solute molecules within particles having narrow pores and long alkyl chains bonded to their surface.

Performance evaluation of slurry-packed capillary columns for liquid chromatography

Fused-silica capillary columns with 200 μm internal diameter were efficiently packed with both 3 μm and 5 μm spherical C18 bonded-phase particles. Optimum packing conditions were determined and column performance was then critically evaluated in terms of the reduced plate height as a function of both reduced velocity and solute capacity factor. Characteristic values for various column performance indices, such as the Knox separation impedance and the interparticle porosity, were also determined and found to be highly reproducible and similar to those of conventional columns. At optimum velocity, the column contribution to reduced plate height values appeared very close to the theoretical. The columns were found to tolerate samples of up to 5 μg with little loss of efficiency and were used in the high-resolution separation of coal-derived fluids.

Effects of Surfactants on Capacity Factors of Nonionic Solutes in Reversed-Phase Liquid Chromatography

Effects of Surfactants on Capacity Factors of Nonionic Solutes in Reversed-Phase Liquid Chromatography

Isohydric solvents in liquid-solid column chromatography : Importance for the reproducibility of chromatographic separations and application to the experimental determination of mobile phase polarity

Isohydric solvents are defined as solvents with water contents corresponding to the same activity of the adsorbent. The influence of the water content of the mobile phase on the activity of the adsorbent and on the reproducibility of separations was studied. A practical means of determining suitable water contents of solvents and solvent mixtures is described. The use of isohydric solvents greatly improves the reproducibility of chromatographic separations. The water contents of isohydric solvents may constitute a scale of relative polarities. In mixtures of isohydric solvents, good linear relationships were obtained between solute capacity factors ( k′) and the inverse of the molar fraction of water (1/ N H 2O isoh).

Recent developments in the evaluation of chemically bonded silica packings for liquid chromatography

The basic variables and conditions in the synthesis of chemically bonded silica packings (CBSPs), obtainable by means of a surface reaction between porous silica and an organochlorosilane or organoethoxysilane as a modifier, yielding a dense monolayer of bonded functional groups, are discussed. The CBSPs can also be prepared by a novel procedure termed bulk modification, which involves a co-hydrolysis and co-condensation of tetraethoxysilane or poly(ethoxysiloxane) and a corresponding organotriethoxysilane. In this way, the organic functional groups become a constituent of the bulk phase as well as of the surface. The retention behaviour of solutes on columns packed with both types of packings was examined in normal and reversed-phase liquid-solid chromatography. Corresponding to the high surface concentration of functional groups, the capacity factors of solutes normalized to unit surface area of the adsorbent, k′/ S BET, were found to be relatively larger on bulk- than on surface-modified silica packings for a given bonded group. For n-alkyl bonded phases in reversed-phase chromatography, the plot of log k′/ S BET versus the number of carbon atoms in the n-alkyl chain was linear for each solute. Further, it is shown that the selectivity of solutes on polar modified packings is largely controlled by the type of polar functional groups substituting the silanol groups.