Interpretation of ion-exchange chromatographic retention based on an electrical double-layer model

Application of surface complexation modeling on adsorption of uranium at water-solid interface: A review

Precise prediction of uranium adsorption at water-mineral interface is of great significance for the safe disposal of radionuclides in geologic environments. Surface complexation modeling (SCM) as a very useful tool has been extensively investigated for simulating adsorption behavior of metals/metalloids at water-mineral interface. Numerous studies concerning the fitting of uranium adsorption on various adsorbents using SCM are well documented, but the systematic and comprehensive review of uranium adsorption using various SCM is not available. In this review, we briefly summarized the rationale of SCM, including constant-capacitance-model (CCM), diffuse-layer-model (DLM), triple-layer-model (TLM); The recent progress in the application of SCM on the fitting of uranium adsorption towards metal (hydr)oxides, clay minerals and soil/sediments was reviewed in details. This review hopefully provides the beneficial guidelines for predicting the transport and fate of uranium in geologic environments beyond laboratory timescales.

Avoiding pitfalls when modeling removal of per- and polyfluoroalkyl substances by anion exchange

Per- and polyfluoroalkyl substances (PFAS) are receiving a great deal of attention from regulators, water utilities, and the general public. Anion-exchange resins have shown high capacities for removal of these substances from water, but there is currently a paucity of ion-exchange treatment models available to evaluate performance. In this work, important theoretical and practical considerations are discussed for modeling PFAS removal from drinking water using gel-type, strong-base anion-exchange resin in batch and column processes. Several important limitations found in the literature preclude movement toward model development, including the use of inappropriate isotherms, inappropriate kinetic assumptions, and experimental conditions that are not relevant to drinking water conditions. Theoretical considerations based on ion-exchange fundamentals are presented that will be of assistance to future researchers in developing models, designing batch and column experiments, and interpreting results of batch and column experiments.

Synergistic effect of temperature and background counterions on ion-exchange equilibria

The effects of temperature and background counterions on ion-exchange selectivity for alkali metal ions and tetraalkylammonium ions on strongly acidic cation-exchange resins have been investigated using superheated water ion-exchange chromatography (SW-IEC). We have found out that alkali metal ions show reversal in the order of the distribution coefficient (K D), from Li+ < Na+ < K+ < Rb+ in water at ordinary temperature to Rb+ < K+ < Na+ < Li+ in superheated water, when a relatively large cation such as cesium ion is used as the background counterion. The effect of counterion on the ion-exchange selectivity is enhanced with the ion-exchange resins of higher ion-exchange capacity and cross-linking degree. Tetraalkylammonium ions chosen as model ions for poorly hydrated ions also exhibit reversal in the order of K D at around 430 K in superheated water. However, the effect of the nature of alkali metal counterions on the change in K D values of tetraalkylammonium ions is rather small compared with the effect on the K D of alkali metal ions. These results are attributed to the change in local hydration structures of the ions in the ion-exchange resin due to dehydration of alkali metal ions enhanced by interionic contacts of the analyte ion with the coexisting counterion and lower hydration energy of the ions at elevated temperatures. Although it has been considered that temperature is not effective at changing the ion-exchange separation selectivity, significant selectivity changes can be achieved by SW-IEC.

Micro- and Nano-Liquid Phases Coexistent with Ice as Separation and Reaction Media

Ice has a variety of scientifically interesting features, some of which have not been reasonably interpreted despite substantial efforts by researchers. Most chemical studies of ice have focused on the elucidation of its physicochemical nature and its roles in the natural environment. Ice often contains impurities, such as salts, and in such cases, a liquid phase coexists with solid ice over a wide temperature range. This impure ice also acts as a cryoreactor, governing the circulation of chemical species of environmental importance. Reactions and phenomena occurring in this liquid phase show features different from those seen in normal bulk aqueous solutions. In the present account, we discuss the chemical characteristics of the liquid phase that develops in a frozen aqueous phase and show how novel analytical systems can be designed based on he features of the liquid phase which are predictable in some cases but unpredictable in others.

Modeling electrostatic interactions of baculovirus vectors for ion-exchange process development

Product-related impurities constitute a major burden in the production of recombinant viral vectors for gene therapy and vaccination; it impairs not only the biological efficacy of the preparation but the process yield/productivity. Recombinant baculovirus was used as an enveloped virus model to address this issue. Given that ion-exchange chromatography is a process of choice for purification of viral vectors, the analysis of the electrostatic behavior can be instrumental for the improvement of impurity removal. The main species, product (infective virus particle) and product-derived impurities (dsDNA-, glycoprotein-, and envelope-deprived baculovirus particles), were isolated and correspondent zeta potentials were analyzed through dynamic light scattering. A model of the virus based on the viral components critical for biological function is proposed. The contribution of these viral components to the overall particle electrostatic interaction energy profile (calculated between the particle and a putative ion-exchange surface) was assessed as a function of ionic strength and pH. This resulted in a deterministic tool capable of distinguishing the electrostatic properties of the infective virus particle from the major virus-related impurities. Within an ion-exchange bind-elute process, this knowledge helps narrow the optimization space in early stage process development for viral vectors by predicting the best selectivity conditions.

Retention controlling and peak shape simulation in anion chromatography using multiple equilibrium model and stochastic theory

The stochastic theory of chromatography and an equilibrium based approach were used for the prediction of peak shape and retention data of anions. This attempt incorporating the potential advantages of two different chromatographic phenomena for analytical purposes. It is an integrated method to estimate kinetic and thermodynamic properties for the same chromatographic run of ions. The stochastic parameters of eluted anions, such as the residence time of the molecule on the surface of the stationary phase, and the average number of adsorption steps were determined on the basis of a retention database of organic and inorganic anions (formate, chloride, bromide, nitrate, sulphate, oxalate, phosphate) obtained by using carbonate/bicarbonate eluent system at different pHs (9-11) and concentrations (7-13 mM). In the investigated IC system the residence times are much higher and the average number of sorption steps is somewhat smaller than in RP-HPLC. The simultaneous application of the stochastic and the multispecies eluent/analyte model was utilized to peak shape simulation and the retention controlling of various anions under elution conditions of practical importance. The similarities between the measured and the calculated chromatograms indicates the predictive and simulation power of the combined application of the stochastic theory and the multiple species eluent/analyte retention model.

Onboard determination of submicromolar nitrate in seawater by anion-exchange chromatography with lithium chloride eluent

Ion-exchange chromatography using a high-capacity anion exchanger with UV detection was applied to the determination of nitrate in seawater. Major ions in seawater samples did not affect the peak shape and the retention time of the nitrate when an alkaline metal cation-chloride solution was used as an eluent at high concentrations (0.5-2 mol/l). At a wavelength of 220 nm, the peak of bromide was very small because of low absorption, while its separation from the nitrate peak was good at high concentrations. Among the eluents tested, lithium chloride gave the best separation of nitrate from bromide. It was estimated that the lithium ion had the least potential for ion-pair formation with nitrate, and its retention time was prolonged compared with the retention times when using other cations; with bromide and nitrite, such an effect was not observed. The results of shipboard seawater nitrate determination by our method in the South Pacific Ocean and Antarctic Sea showed good agreement with those by the conventional photometric method using continuous flow.

Imprinted polymer-modified hanging mercury drop electrode for differential pulse cathodic stripping voltammetric analysis of creatine

The molecularly imprinted polymer [poly(p-aminobenzoicacid-co-1,2-dichloroethane)] film casting was made on the surface of a hanging mercury drop electrode by drop-coating method for the selective and sensitive evaluation of creatine in water, blood serum and pharmaceutical samples. The molecular recognition of creatine by the imprinted polymer was found to be specific via non-covalent (electrostatic) imprinting. The creatine binding could easily be detected by differential pulse, cathodic stripping voltammetric signal at optimised operational conditions: accumulation potential -0.01 V (versus Ag/AgCl), polymer deposition time 15s, template accumulation time 60s, pH 7.1 (supporting electrolyte< or =5 x 10(-4)M NaOH), scan rate 10 mV s(-1), pulse amplitude 25 mV. The modified sensor in the present study was found to be highly reproducible and selective with detection limit 0.11 ng mL(-1) of creatine. Cross-reactivity studies revealed no response to the addition of urea, creatinine and phenylalanine; however, some insignificant magnitude of current was observed for tryptophan and histidine in the test samples.

Tailoring Elution of Tetraalkylammonium Ions. Ideal Electrostatic Selectivity Elution Order on a Polymeric Ion Exchanger

Although ion exchange is often depicted as a process driven by electrostatic forces, ionic solvation or hydrophobic forces contribute greatly to ion exchange selectivity and is often the dominant factor. On a variety of commercial anion exchange columns, monovalent ClO4- elutes after doubly charged SO42- and even triply charged PO43-. For identically charged alkali metal ions, electrostatic charge densities based on crystal radii would suggest Li+ to be the most strongly retained on a cation exchanger. In practice, it is typically the least strongly held cation on most cation exchangers, because of its very high hydration energy and with most eluents its capacity factor approaches zero. Even when the ion is very poorly solvated, as with tetraalkylammonium (NR4+) cations, there has never been a report on a polymeric ion exchanger of an ideal electrostatic selectivity order where NR4+ cations elute in their increasing charge density order: R = n-butyl first, followed by n-propyl, ethyl, and last, methyl. We show that this selectivity order is easily achieved on recently described methracrylate-based monolithic capillary cation exchange columns (Ueki, Y.; Umemura, T.; Li, J. X.; Odake, T; Tsunoda, K. Anal. Chem. 2004, 76, 7007-7012) with minor amounts of hydroorganic modifiers. Indeed, under such conditions, Li+ (and other alkali cations) elutes after NMe4+.

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.

Alternative high-performance liquid chromatographic peptide separation and purification concept using a new mixed-mode reversed-phase/weak anion-exchange type stationary phase

This article describes a new complementary peptide separation and purification concept that makes use of a novel mixed-mode reversed-phase/weak anion-exchange (RP/WAX) type stationary phase. The RP/WAX is based on N-(10-undecenoyl)-3-aminoquinuclidine selector, which is covalently immobilized on thiol-modified silica particles (5 microm, 100 A pore diameter) by radical addition reaction. Remaining thiol groups are capped by radical addition with 1-hexene. This newly developed separation material contains two distinct binding domains in a single chromatographic interactive ligand: a lipophilic alkyl chain for hydrophobic interactions with lipophilic moieties of the solute, such as in the reversed-phase chromatography, and a cationic site for anion-exchange chromatography with oppositely charged solutes, which also enables repulsive ionic interactions with positively charged functional groups, leading to ion-exclusion phenomena. The beneficial effect that may result from the combination of the two chromatographic modes is exemplified by the application of this new separation material for the chromatographic separation of the N- and C-terminally protected tetrapeptide N-acetyl-Ile-Glu-Gly-Arg-p-nitroanilide from its side products. Mobile phase variables have been thoroughly investigated to optimize the separation and to get a deeper insight into the retention and separation mechanism, which turned out to be more complex than any of the individual chromatography modes alone. A significant anion-exchange retention contribution at optimal pH of 4.5 was found only for acetate but not for formate as counter-ion. In loadability studies using acetate, peptide masses up to 200 mg could be injected onto an analytical 250 mm x 4 mm i.d. RP/WAX column (5 microm) still without touching bands of major impurity and target peptide peaks. The corresponding loadability tests with formate allowed the injection of only 25% of this amount. The analysis of the purified peptide by capillary high-performance liquid chromatography (HPLC)-UV and HPLC-ESI-MS employing RP-18 columns revealed that the known major impurities have all been removed by a single chromatographic step employing the RP/WAX stationary phase. The better selectivity and enhanced sample loading capacity in comparison to RP-HPLC resulted in an improved productivity of the new purification protocol. For example, the yield of pure peptide per chromatographic run on RP/WAX phase was by a factor of about 15 higher compared to the standard gradient elution RP-purification protocol.

Chromatographic probing of electrostatic potential

Electrostatic potential in the vicinity of the surface of a cation-exchange resin has been evaluated by modeling chromatographic retention. Binary mixtures of K+ and its crown ether complex in methanol are used as mobile phases, and two types of solutes, that is, cationic and crown ether probes, have been examined. The cationic probes show the sigmoidal retention changes with increasing concentration of a crown ether incorporated into the mobile phase, whereas crown ether probes give retention maximums. The model derived from the Poisson-Boltzmann theory well explains these specific changes in probe retention and gives the electrostatic potential at the closest approach of each probe molecule. The closest approaches for probe molecules correlate well with their molecular sizes. In addition, changes in retention of cationic probes also correlate well with the electrostatic potential changes at the closest approaches of probe molecules, indicating that simple sensing of the electrostatic potential is feasible using probe retention. The reduction of crown ether complexation occurs in the vicinity of the cation-exchange resin surface and causes the specific retention behaviors of crown ether probes in the mobile-phase systems composed of K+ and its complex with a modifier crown ether.

Determination of cefaclor by selective sample enrichment/clean-up on silica gel bonded polyelectrolyte in ion-exchange column chromatography

A silica gel-bound cationic polyelectrolyte, poly[N-chloranil N,N,N',N'- tetramethylethylene diammonium dichloride], modified as ion-exchanger capable of molecular recognition of beta-lactam antibiotic, was used in solid phase extraction through column chromatography for a sample clean-up and enrichment of analyte from a dilute solution. The optimum and selective sorption conditions for a model antibiotic, cefaclor, were established. The high selectivity of polymer at pH 9.5 and flow rate as high as 5 ml/min were observed for the quantitative sorption of cefaclor. The desorption by 0.1 N HCl at flow rate of 0.1 ml/min and subsequent heating at 80 degrees C for 2 h allowed the antibiotic to be detected as corresponding oxazolone form in UV-spectrophotometric and differential pulse adsorptive stripping voltammetric measurements. The potential of the suggested approach was illustrated by estimating cefaclor in urine and blood plasma samples.

Retention of ions on nonporous charged stationary phases

The interactions between ion-exchange resins and counterions consist of several mechanisms, such as ion-pair formation between active sites and counterions, specific adsorption, solvation changes, and double-layer accumulation. The double-layer accumulation of ions, which is a typical nonstoichiometric mechanism, is an important factor governing overall ion-exchange chromatographic retention when a major part of the stationary-phase surface is in contact with eluent flows. Nonporous stationary phases, where solutes are accessible to the surfaces by convection as well as by diffusion, possibly highlight this nonstoichiometric contribution through the coupling of a flow profile with an electrostatic potential function. The retention of ions on nonporous stationary phases has been interpreted by a model derived on the basis of the Poisson-Boltzmann equation including solvation change terms. Unusual retention behaviors have been confirmed only for anions, and can be explained by the model including the assumption that anions undergo solvation changes in a thin layer (approximately 5 nm thickness) at the vicinity of the stationary phase; the thickness should be a function of eluent flow rates. This strongly suggests that there is a difference in solvation nature between cations and anions. It can be inferred that water molecules interacting with polymer domains of the stationary phase behave like single molecules and cannot form a stable hydration shell around an anion as usually seen in bulk solution.

Interpretation of chromatographic behavior of ions based on the electric double-layer theory

A retention model based on electrostatic theories is applied to the analysis of the ion-exchange chromatographic separation of ions. The adsorption of counterions and the ion-pair formation between ion-exchange sites and counterions are included in the model; these represent separation selectivity. A nonstoichiometric contribution, the accumulation of ions in an electrical double layer, is also involved in the model. The retention of ions is calculated by assuming these ionic properties for both eluent and solute ions. The comparison of calculated retention factors with experimental values gives insight into the ion-exchange nature of ions; e.g. a strongly adsorbed ion should have higher ion-pair formation ability, and vice versa.

Acoustic effects in chromatography

An acoustic field effectively affects chromatographic retention in some separation modes and, thus can be a novel factor controlling retention. After being transmitted into the column, ultrasound energy is mostly converted into heat as a result of absorption by stationary and mobile phases. Thus, ultrasound brings about temperature increases. However, actual temperature increases measured by thermosensors are smaller than those calculated from chromatographic retention changes. In addition, larger ultrasound effects are observed in chromatographic modes involving ionic interactions. These results possibly imply that ultrasound directly influences ionic interactions involved in retention mechanisms.