Nebojsa Avdalovic

Applications of ion chromatography with electrospray mass spectrometric detection to the determination of environmental contaminants in water

Ion chromatography (IC) is widely used for the compliance monitoring of common inorganic anions in drinking water. However, there has recently been considerable interest in the development of IC methods to meet regulatory requirements for analytes other than common inorganic anions, including disinfection byproduct anions, perchlorate, and haloacetic acids. Many of these new methods require the use of large injection volumes, high capacity columns and analyte specific detection schemes, such as inductively coupled plasma mass spectrometry or postcolumn reaction with UV-Vis detection, in order to meet current regulatory objectives. Electrospray ionization mass spectrometry (ESI-MS) is a detection technique that is particularly suitable for the analysis of permanently ionized or polar, ionizable compounds. The combination of IC with MS detection is emerging as an important tool for the analysis of ionic compounds in drinking water, as it provides increased specificity and sensitivity compared to conductivity detection. This paper reports on the application of IC-ESI-MS for the confirmation and quantitation of environmentally significant contaminants, i.e. compounds with adverse health effects which are either regulated or being considered for regulation, such as bromate, perchlorate, haloacetic acids, and selenium species, in various water samples.

Protein variant separations by cation-exchange chromatography on tentacle-type polymeric stationary phases

We developed a set of prototype cation-exchange column packings that are based on a hydrophilic coated, pellicular polymeric support with a grafted tentacular surface chemistry that is highly suited to resolving closely related protein variants. These column packings (1) afford minimal band spreading in conjunction with extremely high selectivity, (2) exhibit a very hydrophilic character and (3) have moderate loading capacity. Cytochrome c variants (bovine, horse, rabbit) were baseline-separated, as was native ribonuclease A and its two deamidation products, the Asp67 and isoAsp67 forms. Humanized monoclonal antibody variants differing in the presence of lysine at the C terminus of the heavy chains were baseline-resolved. Finally, the separation of hemoglobin variants found in a sample containing elevated levels of glycated hemoglobin was also demonstrated.

Capillary ion chromatography

This review summarizes progress in capillary ion chromatography. Theoretical aspects and practical limitations of packed and open tubular capillary columns are considered. Applications of packed and open tubular capillary IC are described. Emerging technologies such chip-scale IC and the use of monolithic columns are discussed.

Prediction of retention times for anions in linear gradient elution ion chromatography with hydroxide eluents using artificial neural networks.

The feasibility of using an artificial neural network (ANN) to predict the retention times of anions when eluted from a Dionex AS11 column with linear hydroxide gradients of varying slope was investigated. The purpose of this study was to determine whether an ANN could be used as the basis of a computer-assisted optimisation method for the selection of optimal gradient conditions for anion separations. Using an ANN with a (1, 10, 19) architecture and a training set comprising retention data obtained with three gradient slopes (1.67, 2.50 and 4.00 mM/min) between starting and finishing conditions of 0.5 and 40.0 mM hydroxide, respectively, retention times for 19 analyte anions were predicted for four different gradient slopes. Predicted and experimental retention times for 133 data points agreed to within 0.08 min and percentage normalised differences between the predicted and experimental data averaged 0.29% with a standard deviation of 0.29%. ANNs appear to be a rapid and accurate method for predicting retention times in ion chromatography using linear hydroxide gradients.

Critical comparison of retention models for optimisation of the separation of anions in ion chromatography III. Anion chromatography using hydroxide eluents on a Dionex AS11 stationary phase

Three ion chromatography (IC) retention models, namely the linear solvent strength model (LSSM), empirical end points model (EEPM) and three-point curve fitting using DryLab from LC Resources were evaluated in terms of their ability to predict retention factors for inorganic anions separated on a Dionex AS11 column using electrolytically generated hydroxide eluents. Extensive experimental retention data were gathered for 21 anions (fluoride, acetate, formate, bromate, chloride, nitrite, methanesulfonate, bromide, chlorate, nitrate, iodide, thiocyanate, succinate, sulfate, tartrate, oxalate, tungstate, phthalate, chromate, thiosulfate and phosphate) using hydroxide eluents of varying concentration. Although the purely theoretical LSSM was found to give adequate performance, the EEPM (in which a linear relationship is assumed between the logarithm of retention factor and the logarithm of eluent strength, but the slope is determined empirically) and DryLab performed better, with DryLab giving the best accuracy and precision of the three models. The EEPM and DryLab were also shown to have advantages in terms of their low knowledge requirements and ease of solution. Compared with IC using dual eluent species, the retention behaviour in IC using single eluent species was found to be easier to model by both theoretical and empirical approaches.

Capillary electrophoresis of inorganic anions and organic acids using suppressed conductivity detection strategies for selectivity control

The use of suppressed conductivity as a detection scheme for capillary electrophoresis (CE) is described. A comparison is made between several electrolytes for CE with suppressed conductivity detection (CESC) in terms of efficiency of separation and peak shape. The ability to modify electrophoretic mobility and selectivity as a function of temperature and electrolyte ionic strength is demonstrated. The separation of a variety of low-molecular-mass organic acids is optimized using the addition of metal ions to the separation electrolyte.

Analyzing mixtures of amino acids and carbohydrates using bi-modal integrated amperometric detection

Described in this work is a new detection methodology - bi-modal integrated amperometric detection - for identifying peaks and as a tool for solving difficult separation problems. Bi-modal integrated amperometry makes it possible to selectively detect amino acids, amino sugars, and carbohydrates following their separation by anion-exchange. Selectivity is gained by two different methods of integrating anodic current on an otherwise identical waveform. As with the single-mode integrated amperometry reported previously, the limits of detection are in the femtomole range and linear calibration plots are possible over three orders of magnitude. This new detection method does not require analyte derivatization. The practical utility of this new technique is demonstrated in the analysis of amino acids and sugars in a recombinant mammalian cell culture medium.

On-capillary ion-exchange preconcentration of inorganic anions using open-tubular capillaries followed by elution with a transient isotachophoretic gradient

A new method is described for the elution of inorganic anions in open-tubular capillary electrochromatography (in which an anion-exchange stationary phase is adsorbed to the capillary wall) after preconcentration via ion-exchange interactions. The method utilises a transient isotachophoretic gradient, established using two electrolytes. The first contains a low mobility, weak ion-exchange competing anion, with the second containing a higher mobility strong ion-exchange competing anion. Analytes are adsorbed onto the stationary phase in the weakly eluting electrolyte, but are efficiently desorbed in the stronger eluting electrolyte. By selecting the appropriate mobilities and concentrations of the competing anions, it is possible to obtain highly efficient elution of the analytes from the concentrator column, resulting in a peak compression/focusing effect. The principle of this method for peak compression and elution from the column was demonstrated for iodide, which could be eluted as a peak of width less than 0.1 s, correlating to an elution volume of approximately 40 nL. The potential of this method was demonstrated using bromide and nitrate, which were preconcentrated on a coated section of the capillary, eluted with an isotachophoretic gradient, and separated by capillary zone electrophoresis. Detection limits obtained for iodide, bromide and nitrate using this method were 2.24 nM, 0.86 μM, and 0.32 μM, respectively.

Adjustable-capacity anion-exchange separator

A cryptand-based anion exchanger has been developed in which the capacity and to a lesser degree, selectivity are adjustable simply by the choice of the mobile phase. Although much work has been done in the past using cryptand-based anion exchangers, these stationary phases were based on adsorbed cryptands rather than covalently bound cryptands. These phases suffered from the usual problems associated with adsorbed systems. A novel styrene-based cryptand has been synthesized which can be covalently attached to a solid support. A brief review of cryptands and binding constants as well as comparisons of adsorbed phases versus covalently bound phases will be discussed. Some of the unique chromatographic properties of this prototype column will be illustrated as well.

Determination of anions at trace levels in power plant water samples by ion chromatography with electrolytic eluent generation and suppression

An ion chromatographic method was developed for the determination of nine inorganic and organic acid anions at sub- to low-microg/l levels in power plant water samples. In this method, samples were injected using a large-volume direct injection technique, the analyte anions were separated on a hydroxide-selective anion-exchange column using high-purity hydroxide eluents generated by an on-line electrolytic eluent generator and detected using the suppressed conductivity detection method. The method performance was evaluated by analyzing synthetic water samples containing additives encountered in the power plant water samples and four water samples from a fossil fuel power plant. The relative standard deviations of retention times of analyte ions separated on the hydroxide-selective anion-exchange column were less than 0.4%. The recoveries of analyte ions spiked into the synthetic water samples at concentrations of 0.13-1.0 microg/l were in the range of 70-120%. The method detection limits for analyte ions in deionized water were 0.0099, 0.0056, 0.019, 0.057, 0.0084, 0.023, 0.067, 0.037, and 0.079 microg/l for fluoride, acetate, formate, chloride, nitrite, sulfate, bromide, nitrate, and phosphate, respectively.