Eluent Cation Research Articles

One-pot Synthesis of Polyethylene glycol-based Polymeric Monolithic Stationary Phases for Capillary Ion Chromatography

Polyethylene glycol-based polymeric monolithic stationary phases (capillary columns of 0.32 mm I.D.) were successfully prepared via one-pot-single-step reaction and were use in ion chromatography for the separation of several common inorganic anions. It should be noted that the prepared polyethylene glycol-monoliths were able to retain and separate the anions even though there were no ion-exchange sites. The retention mechanism was found to be based on the eluent cations trapped among the polyethylene glycol chains and worked as the anion-exchange sites. Several parameters such as the reaction conditions, monomer:porogen ratios, type of eluents, etc., were investigated. The relative standard deviations obtained for the retention times and signal intensities were less than 5 and 10% respectively. Theoretical plate numbers obtained for the separation of these anions were calculated to be in the range of 700-4300 plates for capillary columns of 10 cm in length.

Separation of Inorganic Anions Using an 18-Crown-6-ether-modified Organic Polymer Monolithic Stationary Phase in Capillary Ion Chromatography.

In this study, a monolithic organic polymer stationary phase was modified using 18-crown-6-ether for use in capillary ion chromatography. Its use in the separation of inorganic anions was investigated. The monolithic stationary phase was obtained by chemically bonding 2-aminomethyl-18-crown-6-ether to a polymer skeleton comprising glycidyl methacrylate and ethylene glycol dimethacrylate. The optimum level of the loading of 2-aminomethyl-18-crown-6-ether onto the stationary phase was investigated. The resulting stationary phase was used to investigate the influence of the eluent cation, the concentration of the eluent, and the pH of the eluent on the separation of inorganic anions.

Capillary Ion Chromatography Using 18-Crown-6 Ether as Eluent Additive for the Separation of Anions

The retention behavior of inorganic anions was investigated on a hydrophobic stationary phase dynamically modified with 18-crown-6 ether (18C6) in capillary ion chromatography. Styrene–divinylbenzene copolymeric (polystyrene) packing material was used as the hydrophobic stationary phase to retain 18C6. The eluent contained 18C6, salt and acetonitrile; 18C6 was retained on the stationary phase and the analytes anions were separated by the cations, which were trapped on the 18C6 and worked as the anion-exchange sites. The effect of eluent conditions, such as eluent cation species, acetonitrile (ACN), 18C6, and salt concentrations (in this case, potassium chloride was used), was investigated. The retention times of inorganic anions decreased with increasing concentration of ACN. In case ACN concentration was less than 4%, the retention times decreased with the passage of time. The retention time of inorganic anions increased with increasing concentrations of 18C6 and potassium chloride concentrations. By contrast, high concentration of 18C6 and chloride in eluent promotes elution of inorganic anions, and the retention time decreased. Furthermore, a contactless conductivity detector was used to replace the conventional UV detector to obtain higher sensitivity detection. Separation of inorganic anions was achieved with the eluent containing potassium hydroxide and enhanced peak intensity was observed using a suppressed contactless conductivity detection.

Enigmatic ion-exchange behavior of myo-inositol phosphates.

The separation of myo-inositol mono-, di-, tri-, tetra-, pentakis-, and hexakisphosphate (InsP1, InsP2, InsP3, InsP4, InsP5, InsP6) was carried out using hydroxide eluent ion chromatography. Acid hydrolysis of InsP6 (phytate) was used to prepare a distribution of InsPs, ranging from InsP1 to InsP5's and including unhydrolyzed InsP6. Counting all possible positional isomers (many of which have stereoisomers that will not be separable by conventional ion exchange), 40 chromatographically separable peaks are possible; up to 22 were separated and identified by mass spectrometry. InsPs show unusual ion-exchange behavior in two respects: (a) the retention order is not monotonically related with the charge on the ion and (b) at the same hydroxide eluent concentration, retention is greatly dependent on the eluent metal cation. The retention of InsP3-InsP6 was determined to be controlled by steric factors while elution was influenced by eluent cation complexation. These highly phosphorylated InsPs have a much greater affinity for alkali metals (Li(+) > Na(+) > K(+)) than quaternary ammonium ions. This difference in cation affinity was exploited to improve separation through the use of a tetramethylammonium hydroxide-sodium hydroxide gradient.

Development of Chemically Bonded Crown Ether Stationary Phases in Capillary Ion Chromatography

Three types of crown ethers (i.e. 1-aza-15-crown-5-ether, 1-aza-18-crown-6-ether and 2-aminomethyl-18-crown-6-ether) were chemically bonded to silica gels via a simple 2-step reaction and the resulted stationary phases were evaluated for the separation of both anions and cations in capillary ion chromatography. It was found that the chemically bonded stationary phases were most stable and relatively sharper peaks were obtained when a mixture of toluene:acetonitrile = 7:3 was used as the reaction solvent in the second step reaction. The size of crown ether cavity as well as the presence of nitrogen atom within the cavity affected the retention behaviors of the ions. As for the separation of anions, it was found that the eluent cations were trapped on the stationary phases and acted as the anion-exchange site, viz. the anions were separated in the ion-exchange mode. Without the obstruction of nitrogen atom, 2-aminomethyl-18-crown-6-ether could easily form complexes with metal cations, and the retention as well as elution order were dependant on the complex formation constant between crown ether and the metal cations. Eluent with higher pH favored the retention and separation of the metal cations.

Poly(ethylene oxide)-bonded stationary phase for separation of inorganic anions in capillary ion chromatography

A tosylated-poly(ethylene oxide) (PEO) reagent was reacted with primary amino groups of an aminopropylsilica packing material (TSKgel NH2-60) in acetonitrile to form PEO-bonded stationary phase. The reaction was a single and simple step reaction. The prepared stationary phase was able to separate inorganic anions. The retention behavior of six common inorganic anions on the prepared stationary phase was examined under various eluent conditions in order to clarify its separation/retention mechanism. The elution order of the tested anions was iodate, bromate, bromide, nitrate, iodide, and thiocyanate, which was similar as observed in common ion chromatography. The retention of inorganic anions could be manipulated by ion exchange interaction which is expected that the eluent cation is coordinated among the PEO chains and it works as the anion-exchange site. Cations and anions of the eluent therefore affected the retention of sample anions. We demonstrated that the retention of the analyte anions decreased with increasing eluent concentration. The repeatability of retention time for the six anions was satisfactory on this column with relative standard deviation values from 1.1 to 4.3% when 10mM sodium chloride was used as the eluent. Compared with the unmodified TSKgel NH2-60, the prepared stationary phase retained inorganic anions more strongly and the selectivity was also improved. The present stationary phase was applied for the determination of inorganic anions contained in various water samples.

Separation of Inorganic Anions on a Chemically Bonded 18-Crown-6 Ether Stationary Phase in Capillary Ion Chromatography

Inorganic anions were separated on a chemically bonded 18-crown-6 ether (18C6E) stationary phase in capillary ion chromatography. In this study 18C6E groups were chemically bonded on silica gel via reaction with 3-glycidyloxypropyltrimethoxysilane, followed by reaction with 2-aminomethyl-18C6E. Analyte anions were separated in the ion-exchange mode based on the fact that the eluent cation was trapped on the 18C6E and it worked as the anion-exchange site. The eluent cation and eluent anion as well as the eluent concentration affected the retention of the analyte anions. Different selectivity was achieved by using an acetonitrile-rich eluent. The present stationary phase was applied for the determination of UV-absorbing anions contained in saliva samples.

Polyoxyethylene as the Stationary Phase in Ion Chromatography

The present article reviews the use of polyethylene glycol (PEG) or polyoxyethylene (POE) as the stationary phase for the separation of inorganic anions in ion chromatography and discusses about the retention mechanisms involved in the separation of anions on the novel stationary phases. PEG permanently coated on a hydrophobic stationary phase retained anions in the partition mode and allowed us to use high-concentration eluents because the retention of anions increased with increasing eluent concentration for most of the eluents. This situation was convenient to determine trace anions contained in seawater samples without any disturbance due to matrices. Chemically bonded POE stationary phases retained not only anions but also cations. Anions were retained in the ion-exchange mode, although POE chains possess no ion exchange sites. The retention behavior suggested that eluent cations could be trapped among multiple POE chains via ion-dipole interaction, and that the trapped cations worked as the anion-exchange sites. Anions could be separated using crown ether, i.e., cyclic POE, as the eluent additive with a hydrophobic stationary phase, where analyte anions were retained via electrostatic interaction with the eluent cation trapped on the crown ether.

Separation of Inorganic Anions by Liquid Chromatography with Crown Ether as Eluent Additive

Inorganic anions were separated on a reversed-phase stationary phase dynamically modified with crown ether as a selector in capillary ion chromatography. The eluent contained crown ether, acetonitrile and a salt. Free and cation-trapped crown ether molecules in the eluent were adsorbed on a hydrophobic stationary phase such as triacontyl-functionalized silica (C30). The eluent cations trapped on crown ether worked as the ion-exchange sites, where the eluent anions and the analyte anions were competing for electrostatic interaction. The sizes of crown ether and the salt cation affected the retention of analyte anions. The concentrations of acetonitrile and crown ether as well as the eluent anion also affected the retention of analyte anions. An aqueous solution containing 18-crown-6-ether, potassium salt and acetonitrile achieved larger retention for analyte anions. Effects of the eluent conditions on the retention of analyte anions were examined in detail.

Multifunctional Separation Mechanism on Poly(oxyethylene) Stationary Phases in Capillary Liquid Chromatography

The retention behavior on poly(oxyethylene) (POE) chemically-bonded stationary phases was investigated in ion-exchange, reversed-phase (RP) and hydrophilic interaction (HILIC) modes in capillary liquid chromatography. The eluent cations fixed on the oxygen atoms of the POE chains by ion-dipole interaction worked as the anion-exchange sites for the separation of inorganic anions. Nonpolar analytes were retained on the POE stationary phases in the RP mode, while polar organic analytes were retained in the HILIC mode. Sodium alkyl sulfate dynamically modified on the stationary phase enhanced the hydrophobic property of the stationary phase. The effect of the oxyethylene unit number on the retention behavior was examined, and it was expected that the eluent cation is trapped among multiple POE chains by ion-dipole interaction.

Development of packed-column suppressor system for capillary ion chromatography and its application to environmental waters

A novel suppressor unit for capillary ion chromatography was designed to reduce the background conductivity and at the same time to increase the analyte signal. Regeneration of the suppressor was carried out on-line by passing an appropriate acidic solution through the column to displace the accumulated eluent cations. By using two 6-port microswitching valves and two packed capillary column suppressors, the background conductivity of sodium carbonate–bicarbonate mobile phase was maintained at low conductivity for continuous chromatographic runs, and the detection limits at low ppb levels were achieved. The relative standard deviations (RSDs) for the retention time, peak area and peak height of six common inorganic anions (0.05 mM each of F −, Cl −, NO 2 −, Br −, NO 3 − and SO 4 2−) were between 0.5–0.9, 1.1–4.6 and 0.7–4.9%, respectively. The present system was successfully applied to the determination of inorganic anions in river water and tap water samples.

Anion-exchange chromatography on short reversed-phase columns modified with amphoteric ( N-dodecyl- N, N-dimethylammonio)alcanoates

Short reversed-phase columns (50 mm × 4.6 mm Gemini C 18) were dynamically coated with carboxybetaines of the general structure, C 12H 25N +(CH 3) 2(CH 2) n COOH, namely ( N-dodecyl- N, N-dimethylammonio)undecanoate, DDMAU ( n = 10) and ( N-dodecyl- N, N-dimethylammonio)butyrate, DDMAB ( n = 3), and investigated for the separation of inorganic anions in ion chromatography. The role of the ionic strength of coating surfactant solutions on their adsorption and resultant column capacity was studied. The retention of inorganic anions was investigated with different eluents at various concentrations and pH. Interestingly, no retention for anions was found with pure water as the eluent, but the addition of small amounts of electrolytes, up to 0.1 mM, caused a sharp increase in the retention of analytes. The effect of increasing anion retention with an increase in eluent cation charge was also observed. Based on this effect a new cation charge gradient concept was proposed and applied to the separation of a standard mixture of anions.

Creation and characteristics of phosphatidylcholine stationary phases for the chromatographic separation of inorganic anions

New stationary phases for chromatographic separation of anions, obtained by loading liposomes made from dimyristolyphosphatidylcholine (DMPC) onto reversed-phase packed columns (C 18 and C 30) are reported. Mono- and divalent anions were used as model analyte ions and retention data for these species were obtained using the DMPC stationary phases and used to elucidate the separation mechanisms involved in this chromatographic system. The DMPC stationary phases can separate anions by either a solvation-dependent mechanism or an electrostatic ion-exchange mechanism, depending upon the relative magnitudes of the negative electrostatic potential ( Ψ (−)) of the phosphate moiety (P −) and the positive electrostatic potential ( Ψ (+)) of the quaternary ammonium groups (N +) on the headgroup of DMPC. If Ψ (+)> Ψ (−), such as in case where Ψ (−) has been reduced either by binding of eluent cations (e.g., H + or divalent cations) onto the P − group of DMPC or by steric screening when a C 30 reversed-phase material was used to support the DMPC, then the overall electrostatic surface potential (and hence also the effective anion-exchange capacity) was generally large and the anions were separated on the basis of an electrostatic mechanism. However, if Ψ (+) was similar to Ψ (−), such as in the case of using a C 18 reversed-phase support and monovalent cations as eluent cations, then the overall electrostatic surface potential and the effective anion-exchange capacity were very small and the analyte anions were separated on the basis of a solvation-dependent mechanism. The DMPC stationary phases were found to be suitable for the direct determination of iodide and thiocyanate in highly saline water samples, such as seawater samples.

Non-suppressed Ion Chromatography of Copper (II) Cation Using Thiacalix[4]arene as Cation-exchange Group

Thiacalix [4] arene derivatives have unique complexahion nabme that they can coordinate weakly to transition metal rations and hardly to alkali and alkaline earth metal rations. To apply this nature to ion chromatography, p-tert-octylthiacalix [4] arene was coated onto reversed Phase high performance liquid chromatography column and cation-chromatographic property of thiacalpc [4] arene-type cation-exchange group was researched. As a resulk it was found that the coated column can be used for selective separation and conductivity detection of copper (II) ration with using proton as eluent cation.

Electrostatic ion chromatography using a carboxybetaine-type zwitterionic surfactant as the stationary phase.

A carboxybetaine-type zwitterionic stationary phase obtained by immobilizing Mitsubishi Reagent EF-700 (C(8)F(17)SO(2)NHC(3)H(6)N(+) (CH(3))(2)-C(2)H(4)-COO(-)) onto a reversed-phase column was used for chromatographic separation of ions. When aqueous electrolyte solutions having higher pH values (>8) were used as eluents, the model analyte ions (NO(2)(-), H(2)PO(4)(-), Cl(-), Br(-), NO(3)(-), ClO(3)(-), I(-) and SCN(-)) were co-eluted and appeared at the void volume of this HPLC system. However, when aqueous electrolyte solutions having lower pH values (<5.5) were used as eluents, these anions were well retained and separated. Furthermore, when acetate buffers (NaAc/HAc) were used as eluents, plots of log k' (k', retention factor) versus pH of eluents (at constant [NaAc+HAc]), and log k' versus log [NaAc+HAc] (at constant pH), were linear with negative slopes. Breakthrough curves for acid solutions obtained using conductivity detection showed that H(+) ions and their conjugate anions were both retained on the stationary phase and the degree of binding was found to be independent of the acid species used. The degree to which the eluent cation was bound onto the carboxylate functionality of the zwitterion was found to exert a major effect on the retention of analyte anions. A strongly bound cation, such as H(+), reduced electrostatic repulsion effects exerted by the carboxylate functionality on analyte anions, so that they could freely access the quaternary ammonium sites on the zwitterion. It is concluded based on these experimental results that both the charges on the zwitterionic stationary phase make meaningful contributions to the separation of the analyte ions.

Capacity effects of the eluent cation on anion separation in ion chromatography using octadecylsilica coated with hexadecylphosphocholine as the stationary phase.

In the field of ion chromatography (IC) development, attention in recent years has been directed to the on-column modulation of the capacity of the separation column towards the targeted species. These increased efforts have been motivated by concern over the fact that the given capacity of a certain separation column for ions of low affinity is generally insufficient, but for ions of high affinity it is overabundant. The so-called capacity-gradient technique designed by Lamb and coworkers 1-4 was discussed in earlier reports concerning the on-column capacity modulating strategies. They established a macrocycle-based separation column, and observed that the column anion capacity was a function of the interaction occurring between the cation in eluent and the macrocycle group on the stationary phase. 1-4 Complex formation between borate compounds and vicinal diols is another strategy introduced by Yamamoto et al. 5 for the on-column modulation of the capacity. This technique was established based on the fact that most of the chemically bonded anion-exchange resin having hydroxyl groups on the resin surface; masking the hydroxyl groups by increasing concentration of the borate compound in the eluent could result in minimizing the apparent ion-exchange capacity of the column for anions. Another strategy presented by Sekiguchi and Suzuki at a local conference 6 also deserves mention. They used an anion-exchange column with crown-ether groups on the resin surfaces for anion separation, and observed that the column capacity was changeable by changing the eluent cation. The capacity effect of the eluent cation on the anion capacity of this kind of column was based on the fact that cations being held dynamically by the crown ethers made meaningful contributions to the anion separation. By using any of the above-mentioned techniques, anions of lower affinity could be base-line separated, yet the anions of high affinity could be forced to be eluted rapidly. Here, we report on a zwitterion-type anion separation column which was established by loading hexadecylphosphocholine micelles onto a reversed-phase octadecylsilica (ODS) stationary phase, by which the capacity for targeted ions could also be modulated on-column by simply switching the eluent cation.

Determination of trace alkaline earth metals in brines using chelation ion chromatography with an iminodiacetic acid bonded silica column

The chromatographic behaviour of alkaline earth metals on iminodiacetic acid bonded silica was studied. It was found that the ionic strength of the eluent greatly affected both retention time and selectivity by controlling the extent to which either simple ion exchange or surface complexation was responsible for retention. With a 0.1 M KNO 3 eluent, the retention order was Mg(II), Sr(II), Ca(II) and Ba(II), indicating a strong contribution to retention from ion exchange. However, when using a 1.5 M KNO 3 eluent, Ba(II) was found to elute first, indicating complexation to be more dominant under these conditions (pH 4.2). The effect of the ionic strength of the sample was also studied and it was found that by matching the eluent cation with that of the sample matrix, efficient separations of alkaline earth metals in 1.0 M NaCl and KCl brines could be obtained without matrix system peaks. Using post-column reaction with o-cresolphthalein complexone, trace levels of Ca(II) and Mg(II) were determined in medicinal NaCl saline solution and laboratory-grade KCl.

Sigmoidal retention changes caused by crown ether complexation with eluent cations in ion-exchange chromatography

Addition of a crown ether to the mobile phase in cation-exchange chromatography causes specific sigmoidal changes in probe retention. An electrostatic model, including surface ion-pair formation as a retention mechanism, is developed to explain this unusual retention change. In most cases, electrostatic terms play minor roles in the determination of overall retention changes, and thus can be neglected for the discussions outlined here. This simplification allows us to estimate the order of ion-pair formation constants, their ratios, and the stoichiometry of crown ether complexation. In aprotic solvents, this approximation gives a sufficient description of the interactions taking place at the surface of an ion-exchange resin. In contrast, it is inapplicable to some systems, e.g. Na+ eluents in methanol, implying significant contributions from electrostatic effects.

Direct Determination of Bromide, Nitrate, and Iodide in Saline Matrixes Using Electrostatic Ion Chromatography with an Electrolyte as Eluent

A new ion chromatographic method that is applicable to the direct determination of UV-absorbing inorganic anions in saline matrixes is described. An octadecylsilica column modified with a zwitterionic surfactant (3-(N,N-dimethylmyristylammonio)propanesulfonate) is used as the stationary phase, and an electrolytic solution is used as the eluent. Under these conditions, the matrix species (such as chloride and sulfate) are only retained weakly and show little or no interference. It is proposed that a binary electrical double layer (EDL) is established by retention of the eluent cations on the negatively charged (sulfonate) functional groups of the zwitterionic surfactant (forming a cation-EDL) and by retention of eluent anions on the positively charged (quaternary ammonium) functional groups of the zwitterionic surfactant (forming an anion-EDL). Sample anions are able to distribute into the cation-EDL and to form ion pairs with the EDL cations, while at the same time experiencing repulsion from the anion-EDL. Anions are therefore eluted in order of increased propensity to form ion pairs. The method has been applied to the determination of bromide, nitrate, and iodide in artificial seawater, giving detection limits of 0.75 ppb for bromide, 0.52 ppb for nitrate, and 0.8 ppb for iodide using UV absorbance detection at 210 nm and relative standard deviations of <1.2%. Real seawater samples have also been analyzed successfully.

Complexation of Polyethers in Cation Exchange Resin and Application to Chromatographic Determination of Complexation Constants in Solution

The complex formation of crown ethers with counterions in cation-exchange resin is chromatographically evaluated with a model based on the Poisson−Boltzmann−Gouy−Chapman−Stern theory. In the model, two contributions are taken into account to explain distribution ratios (or chromatographic capacity factors) of crown ethers; one is the ion-pair formation of a cation−crown ether complex with a sulfonate group chemically bonded on silica gel, and the other is the accumulation of cationic complexes in the diffuse layer. To describe this system, at least three equilibrium constants are necessary; a complex formation constant and two ion-pair formation constants between a cation-exchange site and a countercation and between a cation-exchange site and a cation complexed by a crown ether. The combination of two chromatographic methods, i.e. the crown ether retention and the retention of a probe cation under effects of the crown ether complexation with eluent cations, allows quantitative evaluation of equilibrium constants necessary for numerical calculation. Thus, crown ethers can probe the equilibria not evaluated by other approaches. The results of the calculation are used for the evaluation of the validity of a simple equation previously reported for the chromatographic determination of the complex formation constants of polyethers in solution.