Mechanism of formation of system peaks in ion-exclusion chromatography.

Single or multiple system peaks can be observed in ion-exclusion chromatography (IEC) based on whether the eluent is composed of single or multiple active eluent constituents. It was confirmed experimentally that the number of system peaks is always equal to or lower than (when co-elution occurs) the number of active eluent components. Positive and negative system peaks can be recorded in the IEC systems. Negative system peaks can be expected for each eluent component that is also present in the injected sample plug at a lower concentration than in the eluent. In the opposite case, where the eluent components in the injected sample plug are present at a higher concentration than in the eluent, positive system peaks will be recorded. The retention times of individual system peaks are not dependent on each other, but rather depend on the capacity of the column toward the individual eluent component. This capacity depends on the nature of the eluent component and can be concentration-dependent (e.g. for 2,6-pyridinedicarboxylic acid) or concentration-independent (e.g. for acetic acid). The higher the column capacity, the longer the retention time of the corresponding system peak produced by that eluent component. The positive and negative system peaks are the result of column re-equilibration to an injected sample containing a higher or lower concentration of the eluent component, respectively. In general, positive system peaks have longer retention times than negative system peaks. The larger the concentration-dependent capacity of the IEC stationary phase toward an individual eluent component, the larger the retention time difference between positive and negative system peaks.

Computational method for modeling of gradient separation in ion-exchange chromatography.

A model for the simulation of the gradient separation in ion-exchange chromatography is presented. It is based on discontinuous plate model and simulates the distribution of analytes in the ion-exchange column during the separation process. It enables calculations of chromatograms for the analytes with integer and non-integer effective charges under complex gradient profiles. Equilibrium concentrations of all analytes are calculated using the same mathematical equations and expressions regardless of the effective charge on the analyte. The main parameters required for the simulations have to be determined under isocratic elution. The suitability of the model was tested with different types of gradients. A comparison of retention times and chromatograms shows that reliable predictions for all tested gradients are achieved. The observed average of the absolute values of the relative errors of the retention times obtained for any analyte in the present study from the calculated chromatograms is below 4%, while the average error considering all analytes in the study is below 2%.

Hard modeling of ion chromatography separations on hydroxide-selective stationary phase.

In the present paper the development and testing of the computer software for the simulation of the on-column ion chromatography separation processes are presented. The computer algorithm based exclusively on the selectivity coefficients of the tested analytes has been upgraded to cope with the 2:1 and 1:1 ratios of the analyte-to-eluent ion charge. The analytical solution of the cubic equation, which is needed for calculation of chromatograms for doubly charged analytes in the presence of singly charged eluent, is presented. The developed modeling approach was tested on the data sets produced by the system composed of hydroxide-selective stationary phase in combination with on-line electrolytically generated OH(-)-based eluents. Retention behavior of the selected anions on the AS15 (DIONEX, USA) stationary phase was investigated. The study of the dependence of the peak widths on the number of theoretical column segments considered in the calculated chromatograms enabled us to choose the optimal number of column segments. The average error in the retention time of the calculated chromatograms for the data set used in the study, i.e. seven different ions at eight different eluent concentrations was found to be 1.4%. A good match with the experimental chromatograms allows us to use the information of the intermediate states of calculations to get a detailed insight into the time-dependent on-column analyte distribution.

Analyte-stationary phase interactions in ion-exclusion chromatography.

The currently accepted analyte-stationary phase interactions occurring in ion-exclusion chromatography are re-examined. In particular, the requirement for the existence of a Donnan membrane separating the flowing, interstitial eluent from the static, occluded, liquid acting as the stationary phase is scrutinized, together with the role of hydrophobic adsorption effects in the retention of aromatic analytes. Plots showing the interconversion of the column between the analyte and eluent forms are used to highlight some shortcomings of the currently accepted mechanism for ion-exclusion chromatography. An alternative retention mechanism for ion-exclusion chromatography is proposed, based on the presence of a potential well at the surface of the fully functionalized styrene-divinylbenzene co-polymer stationary phase. Analytes diffuse into the potential well under the effects of concentration gradients, and undergo repulsion effects from the fixed charges inside the pores. The net contributions of these two opposing processes determine the degree to which an analyte is retained on the stationary phase. Negligible hydrophobic adsorption of the analyte onto the polymeric resin supporting the stationary phase is considered to occur.

Determination of bisphosphonates by ion chromatography-inductively coupled plasma mass spectrometry.

An ion chromatography-inductively coupled plasma mass spectrometry (IC-ICP-MS) was introduced in the analysis of bisphosphonates. Two compounds (alendronic acid and etidronic acid) were separated on a Dionex AS-7 anion-exchange column with dilute nitric acid employed as the mobile phase. The analytes were detected at m/z 31, as they contain phosphorus. The detection limits achieved were 0.20 mg l(-1) for alendronic acid and 0.05 mg l(-1) for etidronic acid. Since the determination of phosphorus by ICP-MS is difficult due to polyatomic interferences at m/z 31 (15N16O+, 14N16O1H+, and 12C1H(3)16O+), a detailed study of the influence of plasma parameters on phosphorus and background signal was performed.

Determination of epichlorohydrin by sulfite derivatization and ion chromatography: characterization of the sulfite derivatives by ion chromatography-mass spectrometry.

This work is an upgrade of a previously developed method (J. Chromatogr. A 884 (2000) 251] for epichlorohydrin determination by ion chromatography (IC) and conductivity detection. Here, an ion chromatography-mass spectrometry (IC-MS) coupling has been employed for the separation and the identification of products of epichlorohydrin when reacted with the nucleophilic agent SO3(2-). The high capacity column (IonPac AS11-HC) used for separation provided good resolution. This allowed evaluation of the IC behavior and mass spectrometric identification of epichlorohydrin sulfite derivatives. By using atmospheric pressure interfaces (ESI and APCI) the following species were tentatively identified: 2,3-dihydroxy-1-propanesulfonic, 2,3-epoxy-1-propanesulfonic,1,3-dihydroxy-2-propanesulfonic and 3-oxetanesulfonic acids and 2-hydroxy-1,3-propanedisulfonic acid (or its isomer 3-hydroxy-1,2-propanedisulfonic acid). The study showed that chlorine atoms are displaced from epichlorohydrin during the reaction, while mass spectrometry confirmed that none of the products formed contains chlorine atoms.

Interference of some matrix ions in cation-exchange chromatography.

Ion chromatographic analysis of ions in samples containing complex matrix composition strongly depends on the on-column co-processes caused by sample matrix components. In the present publication studies of different separation phenomena in cation-exchange chromatography are described. The studies were performed at 'non-linear' chromatographic conditions, when the concentration of matrix (interfering) ions significantly exceeded the concentration of the eluent ions. During the research work, the processes already identified in anion-exchange chromatography, i.e. self-elution, on-column change of the eluent and sample-induced micro-gradient elution were used to explain the chromatographic behavior of alkaline and earth-alkaline cations when samples with high matrix cation concentration were analyzed. When present at higher concentrations, the ammonium cation was found to cause self-elution (NH4+ fraction) as well as on-column eluent neutralization due to its ability to diffuse into/back from the porous core of the stationary phase (NH3 fraction). Co-elution of a matrix component and analytes of interest caused spectroscopic interferences that additionally influenced the peak shape of each individual analyte.

Efficiency and characteristics of solid-phase (ion-exchange) extraction for removal of Cl- matrix.

Certain types of samples contain chloride in concentrations that are too high to accurately determine other anions by ion chromatography without any pretreatment. One of the most widely used approaches for such samples is matrix elimination using disposable cartridges containing a cation-exchange resin in the Ag+ form. The efficiency and characteristics of the commercially available cartridge for Cl- removal were tested by the on-line connection of the cartridge effluent to an inductively coupled plasma mass spectrometer. Displacement efficiency of Ag+ ions and loading capacity of the cartridges were studied at different flow-rates. Significant amounts of silver were detected in the effluent, which were attributed to colloidal AgCl as well as dissolved Ag+ ions. Because silver ions can cause irreversible damage to the analytical column, an Ag cartridge followed by an on line filter (pore size 0.22 microm) and cartridge in the H3O+ form were proposed for improvement of this sample pretreatment technique for chloride removal.

Optimization of artificial neural networks used for retention modelling in ion chromatography.

The aim of this work is the development of an artificial neural network model, which can be generalized and used in a variety of applications for retention modelling in ion chromatography. Influences of eluent flow-rate and concentration of eluent anion (OH-) on separation of seven inorganic anions (fluoride, chloride, nitrite, sulfate, bromide, nitrate, and phosphate) were investigated. Parallel prediction of retention times of seven inorganic anions by using one artificial neural network was applied. MATLAB Neural Networks ToolBox was not adequate for application to retention modelling in this particular case. Therefore the authors adopted it for retention modelling by programming in MATLAB metalanguage. The following routines were written; the division of experimental data set on training and test set; selection of data for training and test set; Dixon's outlier test; retraining procedure routine; calculations of relative error. A three-layer feed forward neural network trained with a Levenberg-Marquardt batch error back propagation algorithm has been used to model ion chromatographic retention mechanisms. The advantage of applied batch training methodology is the significant increase in speed of calculation of algorithms in comparison with delta rule training methodology. The technique of experimental data selection for training set was used allowing improvement of artificial neural network prediction power. Experimental design space was divided into 8-32 subspaces depending on number of experimental data points used for training set. The number of hidden layer nodes, the number of iteration steps and the number of experimental data points used for training set were optimized. This study presents the very fast (300 iteration steps) and very accurate (relative error of 0.88%) retention model, obtained by using a small amount of experimental data (16 experimental data points in training set). This indicates that the method of choice for retention modelling in ion chromatography is the artificial neural network.

On-line pH modification of carbonate eluents using an electrolytic potassium hydroxide generator for ion chromatography.

Classical gradient elution, based on the application of a gradient pump used for mixing two or more prepared eluent components in pre-determined concentrations, was replaced by a chromatography system equipped with an isocratic pump and an electrolytic KOH generator. The isocratic pump delivered a constant concentration eluent composed of pure hydrogencarbonate solution. Carbonate ions, the main component of carbonate/hydrogencarbonate-based eluents, were formed by titration of hydrogencarbonate with KOH formed on-line in the electrolytic KOH generator. By changing the concentration of electrolytically-generated KOH, the eluent composition could be changed from pure hydrogencarbonate to a carbonate/hydrogencarbonate buffer, and finally to a carbonate/hydroxide-based eluent. The described system was tested to achieve pH-based changes of retention behavior of phosphate under constant inflow eluent composition conditions.

Simple and rapid determination of iodide in table salt by stripping potentiometry at a carbon-paste electrode.

A simple and rapid procedure, utilising constant-current stripping analysis (CCSA) at a carbon-paste electrode containing tricresyl phosphate as a pasting liquid (TCP-CPE), has been developed for the determination of iodide in table salt. Because of a synergistic accumulation mechanism based on ion-pairing and extraction of iodide in combination with electrolytic pretreatment of the TCP-CPE, the method is selective for iodide and enables direct determination of iodide in samples of table salt containing anti-caking agents such as K(4)[Fe(CN)(6)] (food additive "E 536") or MgO. The iodide content (calculated as KI) can be determined in a concentration range of 2 to 100 mg kg(-1) salt, with a detection limit (S/N=3) of 1 mg kg(-1), and a recovery from 90 to 115%. The proposed method has been used to determine iodide in several types of artificially iodised table salt and in one sample of natural sea salt. The results obtained agreed well with those obtained by use of three independent reference methods (titration, spectrophotometry, and ICP-MS) used to validate the CCSA method, indicating that the developed method is applicable as a routine procedure for rapid testing in salt production process control and in the analysis of marketed table salts.