Eluent Cation Research Articles

Electrochemically regenerated ion suppressors, new suppressors for ion chromatography

A new electrochemically regenerated suppressor, which is based on solid-phase chemistry is described. An electrochemical process is used to regenerate the solid-phase suppressor for continuous unattended operation. Electrolysis of the detector effluent generates hydronium or hydroxide ions which automatically replace the eluent cations or anions on the suppressor. This suppressor improves and simplifies the instrumentation for suppressor-based ion chromatography.

Elution behaviour of anions and cations on a mixed-bed stationary phase of anion and cation exchange resins

The retention mechanism of anions and cations on a mixed-bed column packed with a mixture of strong anion and cation exchange resins has been studied. Organic acids and inorganic anions have showed the same elution behaviour on the mixed-bed column as on a single anion exchange column. In contrast, different elution behaviours for cations have been observed between the mixed-bed column and a single cation exchange column. On the mixed-bed column, cations would be eluted by a mixed retention mechanism. The primary retention mechanism is ion exchange with eluent cation (H+) for all cations. The ion exchange mechanism on the sites occupied by anion exchange groups makes a greater contribution for monovalent cations. A partition mechanism between the stationary phase and mobile phase contributes to the separation of Mg2+ and Ca2+ by forming neutral compounds.

Use of step gradients on different polymeric substrates in the separation of anions by macrocycle-based ion chromatography

Macrocycle-based ion-exchange columns have been used in our laboratory for the separation of anions. Column anion capacity is determined by the degree to which column macrocycles are bound with mobile phase cations. Capacity gradient separations have previously been performed by gradually changing the eluent cation from one with a high affinity for the macrocycle to one with a lower affinity over the course of the separation. In this work, we demonstrate that gradient separations can also be performed in step rather than linear fashion by switching eluent cations at the start of the separation and allowing the more strongly bound cation to slowly bleed from the column, reducing the column capacity during the separation. The column capacity is reduced at a rate determined by the rate of loss of the first cation from the column. Two different substrates were used as the basis for the macrocycle-based columns, Dionex MPIC and unsulfonated AS10 resins. The MPIC-based column showed retention characteristics similar to those that we have previously described with ramp gradients, while the AS10-based systems showed improved column efficiencies. Separations achieved with step gradients on these two substrates are comparable to linear gradients achieved with the same chemical systems, eliminating the need for pumps with gradient capabilities. Fourteen anions of widely varying character were separated on the AS10-based D222 column using a step gradient from NaOH to LiOH in just over 10 min.

Anion chromatography with a crown ether-based stationary phase and an organic modifier in the eluent

The unique ability of macrocyclic ligands, such as the crown ethers and cryptands, to selectively complex alkali metal cations can be used as the basis for chromatographic separations of anions. Specifically, macrocycles which are adsorbed onto a reversed-phase column, form positively charged anion-exchange sites when they combine with eluent cations. Previously we have demonstrated gradient anion separations based on changing the column capacity during the course of the separation by altering the eluent cation, temperature, or organic modifier content using cryptand-based columns. Herein we report that excellent separations can also be achieved using 18-crown-6 based columns. In this column, anion retention increases with increasing eluent strength and organic modifier content. This observation is in keeping with the relatively moderate affinity of crown ethers for alkali metals when compared to cryptands. The separation of anions achieved by optimizing mobile phase variables shows that isocratic separations of anions on the crown-based column are almost as good as separations achieved only under gradient conditions on cryptand-based columns. Cation gradients provide additional improvements on the separations using the crown-based column.

Ion chromatography of polyhydroxy aliphatic amines of pharmaceutical interest

An ion chromatographic method has been developed for the purity testing of 1-(butylamino)-1-deoxy-D-glucitol (BDG), a unique polyhydroxy aliphatic amine of pharmaceutical interest, in the presence of related amines and n-butylamine. Utilizing cation exchange chromatography, a comparative study of dilute molar solutions of hydrochloric acid (HCl) with millimolar amounts of DL-2,3-diaminopropionic acid (DAP) as eluent was performed to optimize the separation. An eluent of 10 mM HCl containing 0.01 mM DAP was found to be superior for the separation of the given analytes. The method was validated for the determination of BDG. The validation addressed linearity, accuracy, precision, and ruggedness of the method. The selectivity of the method was compared to an existing ion-pairing high-performance liquid chromatography method. In addition, the retention of these analytes was determined as a function of the reciprocal of the concentration of the exchangeable eluent cation. A plot of capacity factors of the organic cations vs the reciprocal of the eluent ion concentration showed good linearity. From the slope and y-intercept data, the major retention mechanism was interpreted as ion-exchange with solvophobic interactions.

A comparison of gradient capacity anion chromatography using macrocycles D-2.2.2 and D-2.2.1 in constant or variable temperature mode

The ability of macrocyclic ligands to complex alkali metal cations has been exploited to perform chromatographic separations of anions. Macrocycles adsorbed to reversed phase columns can complex eluent cations, thus generating anion exchange sites. Gradient separations of anions can be performed by changing the column capacity during the course of the separation, either by changing the eluent cation or by changing the column temperature. Gradient anion separations are performed by changing the eluent from sodium hydroxide to lithium hydroxide with the cryptand D-2.2.2, while similar anion separations are achieved with D-2.2.1 by a KOH-LiOH gradient. Since the complexation of cations by macrocycles is exothermic, increasing the column temperature decreases the anion column capacity, allowing temperature gradient separations. The experimentally measured Δ H values for D-2.2.1 are higher than for D-2.2.2, leading to steeper gradients and thus better separations with D-2.2.1.

Separation of nucleotides and nucleosides by gradient macrocycle-based ion chromatography

Adsorption of the macrocyclic cryptand n-decyl-2.2.2 (D2.2.2) to the matrix of the reversed-phase polystyrene Dionex MPIC column generates a novel mixed-mode chromatographic column on which both ionic and hydrophobic interactions can occur. To the aqueous eluent are added cations that bind dynamically to the adsorbed macrocycle, forming positively charged ion-exchange sites. The hydrophobic tail and the MPIC column matrix provide the basis for hydrophobic interactions. Experiments have been carried out to characterize the use of this column in separating both nucleotides and nucleosides. The influence of eluent cation concentration, type of eluent cation and anion, eluent pH and organic solvent are demonstrated. Excellent resolution of nucleotides and nucleosides was achieved under different conditions, due to the respective differences in the mode of retention between the two compound types. A chromatographic gradient was designed which facilities the simultaneous determination of both species.

Chemically suppressed anion chromatography based on macrocycle—cation complexation

A novel ion chromatographic technique is described using macrocyclic ligand—cation complexes as anion-exchange sites, yielding columns of variable capacity and chromatograms independent of sample cation. Hydrophobic macrocycles are coated onto commercially available C 18-derivatized silica or polystyrene columns. The aqueous eluent contains cations that bind dynamically to the macrocycle, forming positively charged exchange sites. The nature of the ion-exchange sites, and thus the column characteristics, can be altered simply by changing the eluent cation.

Exchange of the cation and anion of the sample (sodium or potassium chloride) with the cation and anion of the eluent (sodium or potassium phosphate buffer) and their elution, from a sephadex g-15 column, in separate fractions

Various concentrations of sodium and potassium chloride were eluted with sodium or potassium phosphate buffer (0.025 M, pH 7.0) in various sample—buffer combinations from a Sephadex G-15 column. A more acidic buffer (pH 6.0) or a more concentrated buffer (0.125 M) were also used as the eluent. By observing the elution behaviour of all ions in the eluate, it was found that the cation from the sample accompanied by phosphate ion from the eluent was eluted in earlier fractions, and the chloride ion from the sample accompanied by the eluent's cation was eluted in later fractions in all homocationic and heterocationic sample–eluent combinations employed. The mechanism assumed was that chloride ion from the sample was eluted slowly and the phosphate ion from the eluent rapidly, resulting in the occurrence of cation-exchange reactions between the sample and the eluent.

Metal ion selectivity on sulfonated cation-exchange resins of low capacity

Although ion chromatography has become a popular analytical method, quantitative data for retention of metal cations by modern ion-exchange resins are rather limited. In the present work, the selectivity of low-capacity, sulfonated resins is determined. Electron micrographs have shown the functional groups to be concentrated near the outer perimeter of these resin beads. A column packed with a 12% cross-linked polystyrene cation exchanger, 6.1 μequiv./g capacity, was used to obtain retention data for 36 metal cations. Adjusted retentin times are given for these cations for 8 different concentrations of perchloric acid eluents, ranging from 0.10 to 1.00 M. Linear plots were obtained for log adjusted retention times vs. log eluent ion activity. In almost every case the slope was close to the theoretical slope of −2 for divalent metal ions and −3 for trivalent metal ions. The adjusted retention times of metal cations were also obtained with eluents containing Na +, Mg 2+, [(−CH 2NH 3) 2] 2+, and [C 6H 5NH 2CH 2CH 2NH 3] 2+ perchlorates. These data permit comparison of various selectivity coefficients as a function of the eluent cation. finally, the adjusted times are reported for 32 metal cations using resins of 3 different capacities, with a 0.75 M perchloric acid eluent.

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.

Prediction of the pH dependence of the separation of weak electrolytes by ion-exchange: Amino acid chromatography with competitive buffer ion eluents

In cation exchange when the sample cations are weak electrolytes, several benefits arise from choosing the eluent cation to be a weak electrolyte with a similar p K. The entire pH buffering range of the eluent can then be practically utilized to effect a modulation of the sample elution volumes. For elution of aliphatic amino acids from sulfonated polystyrene resin using a pyrazolium chloride eluent (pH 4.0 to 1.5) we obtained selectivity coefficients that are independent of pH and ionic strength as well as only weakly dependent on temperature and sample concentration. As a result the sample elution behavior is accurately predictable. The analytical-scale data were used to develop a preparative procedure capable of resolving a sample to resin load 10 times larger than previously reported for amino acid chromatography on laboratory-scale systems.