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.     

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

Summary 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 G° = –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.     

Effects of temperature on retention of chiral compounds on a ristocetin A chiral stationary phase

The isocratic retention of enantiomers of chiral analytes, i.e. tryptophan, 1,2,3,4-tetrahydroisoquinoline and gamma-butyrolac tone analogs, was studied on a ristocetin A chiral stationary phase at different temperatures and with different mobile phase compositions, using the reversed-phase, polar-organic and normal-phase modes. By variation of the both mobile phase composition and the temperature, baseline separations could be achieved for these enantiomers. The retention factors and selectivity factors for the enantiomers of all investigated compounds decreased with increasing temperature. The natural logarithms of the retention factors (ln k) of the investigated compounds depended linearly on the inverse of temperature (1/T). van't Hoff plots afforded thermodynamic parameters, such as the apparent change in enthalpy (deltaH(o)), the apparent change in entropy (deltaS(o)) and the apparent change in Gibbs free energy (deltaG(o) ) for the transfer of analyte from the mobile to the stationary phase. The thermodynamic parameters (deltaH(o), deltaS(o) and deltaG(o)) were calculated in order to promote an understanding of the thermodynamic driving forces for retention in this chromatographic system.     

Speciation of arsenical species by anion-exchange and ion-pair reversed-phase liquid chromatography

Summary Speciation and quantitative analysis of arsenical compounds are performed by using high-performance liquid chromatography (HPLC) with direct UV detection. Ion chromatography has been used to separate mixtures of arsenical compounds (arsenite, MMA, DMA, arsenate) on an anion-exchange column using phosphate buffer (1 mmol/l, pH=5.3) as eluent. Ion -pair reversed-phase chromatography has been investigated to resolve mixtures of arsenite, arsenate, MMA, DMA, arsenobetaine and arsenocholine on an octadecyl-bonded silica column using water as mobile phase (pH=7.3) and tetrabutylammonium cation as ion-pairing reagent. The influence of several parameters (pH, the ion-pairing reagent concentration or the amount of methanol in the mobile phase) has been studied to determine the best chromatographic conditions.     

The dipole approach in ion-interaction chromatography of zwitterions

Summary The chromatographic behavior of zwitterions in Ion-interaction chromatography (IIC) is, investigated theoretically for the first time. The modification of the stationary phase in the presence of Ion-interaction reagent (IIR), and adsorption competition between test analytes and IIR for inner layer sites are shown theoretically to change the partition coefficient for zwitterions. Experimental results from the literature concerning retention behavior of zwitterions in IIC, were used to test the new thermodynamic theory. Very reasonable estimates of (i) ΔG ^o values for the IIR adsorption onto the stationary phase (II) total ligand concentration, and (iii) dipolar moments validate the present thermodynamic model for the IIC of zwitterionic analytes. Retention equations are compared to those which can be obtained, if the net charge of the analyte is zero, from the most important retention models in IIC. None of them is able to explain, even in a qualitative way, the retention behavior of zwitterions in IIC whereas, the present model is quantitatively able to do this.     

Preconcentration of the herbicide glyphosate and its metabolite AMPA by immobilised metal ion affinity chromatography (IMAC)

A method based on Immobilised Metal Ion Affinity Chromatography (IMAC) using a chelating resin (Spheron Oxine 1000) loaded with Pd(II) is evaluated for the extraction and preconcentration of glyphosate and AMPA from natural water samples. The efficiency of the metal-loaded resin in retaining glyphosate and AMPA and the ability of different reagents to recover them is investigated. The most effective elution solution for the recovery of both analytes from the resin is found to be a mixture of 0.1 M HCI and 1 M NaCl. The effect of flow rate, analyte concentration, and sample volume is evaluated. The optimised experimental conditions are then used in the extraction of the analytes from spiked natural waters. The use of the Pd loaded resin led to recoveries ranging from 80-90% for glyphosate and 60-80% for AMPA.     

Electrostatic interaction mechanism on the separation of phenols by non-aqueous capillary electrophoresis

The electrostatic interaction between additive and analyte is of great importance to non-aqueous cap- illary electrophoresis(NACE)separation.Three tetraalkylammonium bromides and acetonitrile were applied as additives and running solvent respectively.The effect of alkyl chain length and concentra- tion of additive on electrostatic interactions was investigated by the separation of phenols.The sepa- ration ability was found to increase with decreasing alkyl chain length of the additive,and the resolu- tion values were increased with increasing additive concentration.The separation was seriously dete- riorated after a little amount of water was added in the running solution.Furthermore,the electrostatic interaction is strong under the conditions of low electron cloud density,weak steric hindrance and multi-interaction sites.Thus,the separation result can be predicted by theoretical analysis,which is helpful for the separation of other substances in NACE based on electrostatic interaction.     

Retention profiles and mechanism of anion separation on latex-based pellicular ion exchanger in ion chromatography

A retention model based on stoichiometric approach has been developed in order to describe analyte retention of anions on latex-based pellicular ion exchanger. The chromatographic process entails two stepwise and complex equilibria, first is ion-pair forming of analyte or eluent ion with ion-exchange sites under the effect of electrostatic forces due to the sulfonic layer behind the aminated functional groups of stationary phase. Second component is the ion-exchange between the analyte and eluent ions. As a new parameter of the fractional electrostatic coefficient of the ion exchange capacity was introduced to develop retention profiles of anions. Analysis of the dependence of the capacity factors on the eluent concentrations at different values of fractional coefficient shed light on the possible complex mechanism. Extensive experimental retention data were obtained for 14 anions (formate, acetate, propionate, pyruvate, lactate, chloride, nitrate, oxalate, malonate, succinate, tartarate, fumarate, maleate, sulphate) using hydroxide eluents of varying concentration. The ion-pair formation and ion-exchange selectivity constants for analyte and eluent species are determined using derived retention equation from experimental data by nonlinear iterative calculation. The model was utilized to predict retention data under elution conditions of practical importance. The predicted and obtained retention factors are in good agreement, which confirms the predictive power of the model.     

Thermodynamic study of the second-stage dissociation of N,N-bis-(2-hydroxyethyl)glycine (bicine) in water at different ionic strength and different solvent mixtures

The second stage dissociation constant pK2 of N,N-bis-(2-hydroxyethyl)glycine (bicine) has been determined in aqueous solution at different ionic strengths and different temperatures, using pH-metric technique. The thermodynamic quantities (deltaG(o), deltaH(o), and deltaS(o)) have been studied and discussed. Evaluation of the effect of organic solvent of the medium on the dissociation processes have also been reported and discussed. The organic solvents used were methanol, dimethylsulfoxide (DMSO), and dioxane. The pK2 for the ionization in water +10, +20, +30, +40, and +50 wt % dioxane has been determined at five different temperatures from 15 to 35 degrees C at intervals of 5 degrees C. The thermodynamic quantities were calculated. The implications of the results with regard to specific solute-solvent interactions (particularly stabilization of zwitterionic species) are also discussed.     

Unusual Retention Behavior of Some Cationic-Type Aldoximes Used as AChE Reactivators Under Ion-Pairing Liquid Chromatographic Mechanism

Retention of some cationic-type aldoximes used as AChE reactivators was studied under ion-pairing (IP) liquid chromatographic conditions, using alkyl sulphonates as IP agents. An unusual chromatographic retention behavior was observed: the functional dependencies between the logarithm of the capacity factor and the methanol content in the mobile phase followed a binomial pattern (U-shaped), with a minimum positioned within the interval 50–80% methanol. The tautomerism of the investigated structures and the stationary phase morphological modification caused by the adsorption of the ion-pairing agent may explain the experimental findings. The study focused on the influences brought by the pH, the alkyl chain length of the ion-pair agent, and the organic modifiers on the retention behavior of these compounds.     

Solubilization study for aggregates of sodium dodecyl sulfate and cationic polymer of high charge density

Interaction of sodium dodecyl sulfate (SDS) with a cationic polymer (polydiallyldimethylammonium chloride, PDADMAC) was investigated. The surface tension of SDS/PDADMAC solution ([PDADMAC]=100 ppm) decreased from 72 to ca. 40 mN m(-1) with increasing SDS concentration at 298.2 K, where the SDS concentration, 0.6 mmol dm(-3), at 40 mN m(-1) was less than cmc/10 of SDS. From the relatively high value of I1/I3, ca. 1.5, in the pyrene fluorescence spectrum, which is larger than the value in SDS micelles, the aggregation number is suggested to be lower than that of SDS micelles. The maximum additive concentration for n-alkylbenzenes as solubilizate increased with the increase in SDS concentration and with decreasing alkyl chain length of the solubilizates. The Gibbs energy changes for their solubilization from the phase separation model were almost the same as those from the mass action model for longer chain solubilizates, due to their smaller solubilized amounts in the micelles. The Gibbs energy change for the solubilization decreased with increasing alkyl chain length of the solubilizates. The Gibbs energy decrease per CH2 group (deltaG(CH2)0) was larger in magnitude than for micelles of single-surfactant systems, which was substantiated by the absorption spectrum change of the solubilizates.     

Studies into the thermodynamic origin of negative cooperativity in ion-pairing molecular recognition

The association of synthetic receptors to target guests often proceeds through the cooperative action of multiple binding forces. An investigation into the thermodynamic origin of cooperativity in ion-pairing host-guest binding in water is described. The binding affinities of 1,2,3,4-butanetetracarboxylate, tricarballate, glutarate, and acetate to a C(3)(v) symmetric metallo-host (1) are characterized in terms of the binding constants (K(a)) and the thermodynamic parameters deltaG degrees, deltaH degrees, and deltaS degrees, as determined by isothermal titration calorimetry (ITC). These values are used to determine the individual contributions of the binding interaction to the overall binding. Several ways to view the combination of the individual binding events that make up the whole are analyzed, all of which lead to the conclusion of negative cooperativity. Combined, the data were used to evaluate the thermodynamic origin of negative cooperativity for this series of guests, revealing that entropy is the largest contributing factor. An interpretation of this result focuses upon differences in the number of water molecules displaced upon binding.     

Influence of electrolyte nature on the separation selectivity of amphetamines in nonaqueous capillary electrophoresis: protonation degree versus ion pairing effects

The simultaneous analysis of Ecstasy and its derivatives in an acetonitrile-methanol (80:20 v/v) mixture was previously shown to be strongly dependent on the nature of the electrolyte (acetate versus formate). To elucidate the phenomena involved, systematic experiments were conducted in this solvent medium. Conductivity measurements allowed to evaluate the ion-pairing rate in the background electrolyte (BGE) and thereby distinguish between electrolyte concentration and ionic strength. The influence of electrolyte concentration on analyte effective mobilities micro(eff)) was studied by capillary electrophoresis (CE). As micro(eff) extrapolated to infinite dilution proved to be independent of the nature of the electrolyte, selectivity changes could not be attributed to a modification in the protonation degree of amphetamines. Experimental mobility data were then confronted to existing theoretical mobility models to discriminate between ion pairing or simple ionic strength effect. Ion-pair formation in a BGE containing acetate was highlighted with an ion-pairing model and ion-pair formation constants between each amphetamine and acetate ion were calculated.     

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

Summary 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.     

Single column ion chromatographic determination of inorganic acids in workroom places

Single Column Ion Chromatography (SCIC) combines low capacity-columns, low conductivity eluents and a detector with considerable electronic supression capability and enables quantitation of ions at the ppm level without the need of a suppressor column associated with other forms of Ion Chromatography. Vapors and aerosols of inorganic acids are collected on filter and silica gel tubes and analyzed directly by SCIC. A conventional liquid chromatograph with a low capacity column and a conductimetric detector is used to analyze aerosols of Cl-, Br-, NO-3 and SO=4 with good results.     

High-capacity cation-exchange column for enhanced resolution of adjacent peaks of cations in ion chromatography

One of the advantages of ion chromatography [Anal Chem. 47 (1975) 1801] as compared to other analytical techniques is that several ions may be analyzed simultaneously. One of the most important contributions of cation-exchange chromatography is its sensitivity to ammonium ion, which is difficult to analyze by other techniques [J. Weiss, in: E.L. Johnson (Ed.), Handbook of Ion Chromatography, Dionex, Sunnyvale, CA, USA]. The determination of low concentrations of ammonium ion in the presence of high concentrations of sodium poses a challenge in cation-exchange chromatography [J. Weiss, Ion Chromatography, VCH, 2nd Edition, Weinheim, 1995], as both cations have similar selectivities for the common stationary phases containing either sulfonate or carboxylate functional groups. The task was to develop a new cation-exchange stationary phase (for diverse concentration ratios of adjacent peaks) to overcome limitations experienced in previous trails. Various cation-exchange capacities and column body formats were investigated to optimize this application and others. The advantages and disadvantages of two carboxylic acid columns of different cation-exchange capacities and different column formats will be discussed.     

Gas chromatographic determination of the interconversion energy barrier for dialkyl 2,3-pentadienedioate enantiomers

The enantiomers of dialkyl 2,3-pentadienedioate undergo interconversion during gas chromatographic separation on chiral stationary phases. In this paper the on-column apparent interconversion kinetic and thermodynamic activation data were determined for dimethyl, diethyl, propylbutyl and dibutyl 2,3-pentadienedioate enantiomers by gas chromatographic separation of the racemic mixtures on a capillary column containing a polydimethylsiloxane stationary phase coupled to 2,3-di-O-methyl-6-O-tertbutyldimethylsilyl-beta-cyclodextrin. A deconvolution method was used to determine the individual enantiomer peak areas and retention times that are needed to calculate the interconversion rate constants and the energy barriers. The apparent rate constants and interconversion energy barriers decrease slightly with an increase in the alkyl chain length of the dialkyl 2,3-pentadienedioate esters. The optimum conformation of the dialkyl 2,3-pentadienedioate molecules, their separation selectivity factors and apparent interconversion enthalpy and entropy data changes with the alkyl chain length. The dependence of the apparent interconversion energy barrier (deltaG(app)(a-->b), deltaG(app)(b-->a)) on temperature was used to determine the apparent activation enthalpy (deltaH(app)(a-->b), deltaH(app)(b-->a)) and apparent entropy (deltaS(app)(a-->b), deltaS(app)(a-->b)) (where a denotes the first and b second eluted enantiomer). The comparison of the activation enthalpy and entropy (deltaS(app)(a-->b), deltaS(app)(a-->b)) indicated that the interconversion of dialkyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series is an entropy driven process at 160 degrees C. Data obtained for dimethyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series at 120 degrees C (deltaG(app)(a-->b) = 123.3 and deltaG(app)(b-->a) = 124.4 kJ mol(-1)) corresponds (at the 95% confidence interval) with the value of deltaG(#) = 128+/-1 kJ mol(-1) found at this temperature by gas chromatography using a two-dimensional stop flow technique on an empty capillary column [V. Schurig, F. Keller, S. Reich, M. Fluck, Tetrahedron: Asymmetry 8 (1997) 3475].     

Electrostatic correlations and fluctuations for ion binding to a finite length polyelectrolyte

A statistical mechanical model is presented which explicitly accounts for the fluctuations, the electrostatic, and the excluded volume correlations for ions bound to a polyelectrolyte such as DNA. The method can be employed to treat a wide range of ionic conditions including multivalent ions. The microscopic framework of the theory permits the use of realistic finite length and grooved structural model for the polyelectrolyte and modeling of the finite size of the bound ions. Test against Monte Carlo simulations suggests that the theory can give accurate predictions for the ion distribution and the thermodynamic properties. For multivalent ions, the theory makes improved predictions as compared with the mean-field approach. Moreover, for long polyelectrolyte and dilute salt concentration, the theory predicts ion binding properties that agree with the counterion condensation theory.     

On-line coupled ion chromatography-ICP-(AES, MS) and electro thermal vaporisation-ICP-MS in use for ultra trace analysis of high purity rhenium

This work presents the benefits of coupling techniques such as Electro Thermal Vaporisation (ETV)-ICP-MS and Ion Chromatography (IC)-ICP-(AES, MS) for ultra trace analysis in a high purity rhenium powder sample. Direct analysis using ICP-AES suffers from poor detection limits and allows trace analysis only above 1 g/g for most analytes. ICP-MS analysis of trace elements is more sensitive, but signal depression caused by the heavy Re-ions limits trace analysis to concentrations of 50–100 ng/g analyte in the solid sample. Coupling Ion Chromatography with ICP-spectrometers, combined with time resolved measurement (IC-ICP-TRM) of the elution signals, was used to enhance the sensitivity of both ICP-AES and ICP-MS. Resulting detection limits are in the very low ng/g to pg/g range. Coupling of ETV and ICP-MS offers the possibility of eliminating the volatile Re₂O₇ matrix by thermal pretreatment and allows ICP-MS measurements without matrix interferences caused by Re. Results from these methods are compared with Glow Discharge Mass Spectrometry (GDMS) analysis, a semiquantitative solid state technique. The results are also compared with the manufacturers' specifications to show the power of modern routine analysis using ICP-AES or FAAS.