Cation-exchange Stationary Phase Research Articles

Simple approach for reversal of the epinephrine—norepinephrine elution order in ion-pair reversed-phase liquid chromatography

Liquid chromatography with electrochemical detection is the most commonly used technique for catecholamine (CA) assay [l]. Nearly all separations are performed on reversed-phase (RP) silica with Cl8 or Cs alkyl groups that enable the insertion into the sorbent structure of an anion surfactant, such as octyl sulphate (OS) or sodium dodecyl sulphate (SDS). These sulphates, dissolved in the mobile phase and coated onto the sorbent structure dynamically, act as ion-exchangers and result in increased retention of CAs trough ion-pair formation. Retention of the biogenic amines increases on addition of the negatively charged ion-pairing agent and becomes inversely proportional to the concentration of the positively charged counter-ion and the amount of organic modifier in the mobile phase. In these separation systems CAs are eluted in the following order: norepinephrine (NE), then epinephrine (E), then dopamine (DA); dihydroxybenzylamine (DHBA), which is used mostly as an internal standard, is eluted between E and DA. This elution order may be disadvantageous when the concentration of late eluting compounds is low, as in case of free E and DA m plasma. The same is true for urinary E from an 8-h sleep period because there is a decrease to less than 10% of the total daily secretion and this fraction seems to be diagnostically important [2]. It is obvious that compounds with a low abundance should be eluted first so as to give as narrow peaks as possible. Although an ideal system, which would allow E and DA to be eluted prior to NE, has not yet been published, solutions for reversed elution order of E and NE have been described. Successful results were obtained by substituting RP silica for a weak cationexchange stationary phase [3], by using a special surfactant N-methyloleoyl taurate with a Cl8 support [4,5] or by coating tri-n-butylphosphate on Cs-alkyl silica r&71. In addition, crown ethers, which are known to form complexes with cations, have been used [8]. It was found that 18-crown-6 ether forms a complex with

Routine measurement of plasma catecholamines in clinical pharmacology by high-performance liquid chromatography with electrochemical detection.

A method for the simultaneous determination of epinephrine, norepinephrine and dopamine in human plasma is described, which combines the advantages of liquid-liquid extraction sample preparation, high-performance liquid chromatography on weak cation-exchange stationary phases and dual-electrode coulometric detection. The limits of quantification are less than 5 pg/ml (at a signal-to-noise ratio greater than 5) for each analyte. The influence of various experimental parameters (e.g., composition of the mobile phase, pretreatment of the assay buffer, components of the re-extraction system) on the performance of the assay is reported in detail. A number of applications are presented, which demonstrate the quality of the data obtained in terms of sensitivity, reproducibility and significance.

High-performance liquid chromatography of plant-derived oligosaccharides on a new cation-exchange resin stationary phase: HPX-22H

High-performance liquid chromatography of plant-derived oligosaccharides on a new cation-exchange resin stationary phase: HPX-22H

Simultaneous determination of mono- and bivalent cations in environmental chemistry using isocratic ion chromatography

The influence of the pH of the mobile phase ethylenediamine (EDA)/citrate on the retention behaviour of mono- and bivalent cations on a silica based cationexchange stationary phase was studied. Variation of the pH-value was accomplished either by changing the concentration of one component of the eluent or by adding potassium hydroxide or hydrochloric acid to an eluent of fixed composition. The pH range between 2.8 and 3.0 proved to be most suitable for the separation of mono- and bivalent cations. Alkali metals, alkaline earths and transition elements could be separated in a single run. Extending the pH-range up to 5.2 showed the possibility of an additional determination of more strongly retained ions, such as Pb2+ or Sr2+.

Metal Ion Chromatography with Fluorescence Detection

Abstract Nonpolar, agglomerated anion exchanger, and surface-sulfonated cation exchanger stationary phases have been used in conjunction with 8-hydroxyquinoline-5-sulfonic acid (HQS) in the eluent or as a postcolumn reagent for the separation and detection of a number of metals that form fluorescent HQS complexes. Several metals, notably those classified as transition metals, form nonfluorescent HQS chelates and quenches the fluorescence of other metal-HQS metal chelates. Such transition metals have been detected by introducing the fluorescent Al-HQS chelate postcolumn. Cation exchange stationary phases are the most useful for chromatographic applications involving HQS and are able to provide a variety of useful separations by tailoring elution conditions. Although not sensitive to Ba, the approach may be particularly good for the determination of the other alkaline earth metals. Fluorescence quenching resulting from Fe and Ni leaching from stainless steel chromatographic systems present a problem for tra...

Polymer-coated cation-exchange stationary phases on the basis of silica

The procedure of polymer coating of preferably inorganic porous particle support materials for LC has been applied to the preparation of a new type of weak cationexchange phases. A special copolymer of butadiene and maleic acid could be immobilized on silica by cross-linking effected with radical starters such as peroxides or γ-radiation. The chromatographic properties of such materials proved to be excellent regarding efficiency, ion-exchange capacity and selectivity, as well as chemical stability also in comparison to other, commercially available, materials. Test measurements were successfully performed with ionic or ionizable inorganic and organic solutes over the entire applicable pH-range of the mobile phase which also contained organic modifiers. A special feature of the new type of cation-exchange phase is the minor contribution of hydrophobic (lipophilic) interaction to the retention mechanism besides the actual ion-exchange process.

Synthesis of cation-exchange stationary phases using an adsorbed polymeric coating

We have prepared several silica-based cation-exchange materials that were suitable for the high-performance liquid chromatography of basic proteins. Two synthetic routes were examined. Central to both procedures was the adsorption of a low molecular weight polyamine. One method crosslinks the adsorbed polyamine with a multifunctional oxirane, which is then extensively derivatized with a monomeric cyclic anhydride. The second involves an adsorbed uncrosslinked polyethyleneimine layer which is reacted with polyacrylic anhydride, thereby crosslinking and imparting an- ionic character simultaneously. The resulting media prepared by either of these methods bound more than 40 mg of hemoglobin per gram of support depending on the reaction conditions. These cation-exchange stationary phases also exhibited good chromatographic performance, successfully resolving (horse heart) cytochrome c and lysozyme. Two of the more promising support materials were effectively used to isolate cytochrome c 553 from a crude extract of cyanobacteria.

High-performance liquid chromatographic elution behaviour of alcohols, aldehydes, ketones, organic acids and carbohydrates on a strong cation-exchange stationary phase

The high-performance liquid chromatographic separation of alcohols, aldehydes, ketones, carboxylic acids and carbohydrates on a polystyrene-based strong cation-exchange resin is described. The column temperature was found to be a very important parameter for optimizing separations of these substances. The effect of different functional groups on the elution behaviour is discussed.