Anthony H. Harakuwe

Separation of inorganic and organic anionic components of Bayer liquor by capillary zone electrophoresis I. Optimisation of resolution with electrolyte-containing surfactant mixtures

The simultaneous separation of chloride, sulfate, oxalate, malonate, fluoride, formate, phosphate, carbonate and acetate in Bayer liquor using capillary zone electrophoresis with indirect detection is demonstrated using electrolytes comprising binary mixtures of the surfactants tetradecyltrimethylammonium bromide (TTAB) and dodecyltrimethylammonium bromide (DTAB). Two optimal electrolyte compositions were identified, namely 3 mM TTAB, 3 mM DTAB and 7.5 mM chromate at pH 9 (optimum 1) and 5 mM TTAB, 1 mM DTAB and 5.5 mM chromate at pH 9 (optimum 2). The separation selectivities of these electrolytes differ and the choice between them rests on consideration of the relative concentrations of the ionic species in the sample. Best results were obtained when the Bayer liquor sample was diluted by a factor of 500 before analysis. Linear calibrations were achieved in the working concentration range (1–10 μg/ml) and detection limits fell in the range 0.09–0.34 μg/ml for optimum 1 and 0.16–0.88 μg/ml for optimum 2. Recoveries of ions added to the diluted sample were close to quantitative, except for phosphate which showed low and variable recovery, and carbonate which was also variable due to absorption of carbon dioxide by the sample. Tartrate and succinate could not be resolved with either of the optimal electrolyte compositions.

Optimisation of separation selectivity in capillary zone electrophoresis of inorganic anions using binary cationic surfactant mixtures

Mixtures of different cationic surfactants such as dodecyltrimethylammonium bromide and tetrade-cyltrimethylammonium bromide can be used as carrier electrolyte additives in order to manipulate separation selectivity and resolution of inorganic anions in capillary zone electrophoresis. The migration order is dependent on both the total surfactant concentration and the ratio of the two surfactants. The optimisation of separation selectivity is demonstrated for a nine-anion standard mixture as well as for complex matrices such as Bayer liquor.

Manipulation of separation selectivity for inorganic anions in capillary zone electrophoresis using control of electrolyte pH

The influence of the pH of chromate-tetradecyltrimethylammonium bromide-based electrolytes on the separation selectivity of anions using capillary zone electrophoresis was studied. Changes in migration order over the pH range 8–13 were marked for weak acid anions at or near their pKa due primarily to changes in effective charge. Precipitation problems encountered for electrolyte pH values less than 8 were prevented by the incorporation of small (less-than-or-equals, slant7.5% v/v) amounts of 1-butanol as additive. Using this approach the pH could be lowered to 7. Manipulation of electrolyte pH was used to increase the resolution between fluoride and phosphate, making possible the fully resolved separation of trace fluoride (1 μmg ml−1) in the presence of excess phosphate (> 800 μg ml−1). The separation of fluoride from phosphate in toothpaste is also shown.

Control of separation selectivity in capillary zone electrophoresis of inorganic anions

The control of the separation selectivity of anions in capillary zone electrophoresis is critically reviewed. Selectivity effects arising from the chemical environment of the analyte (that is, the pH, surfactant content and organic modifier content of the background electrolyte and the presence of complexing species) are shown to be more significant than those arising from physical effects, such as the sampling mode and the separation conditions. In general, the effects on separation selectivity exerted by these parameters are quite subtle, so that control of selectivity in capillary electrophoresis of inorganic anions is often quite difficult. Some practical limitations to selectivity control are highlighted and possible areas that can be studied in the future for selectivity variation are suggested.

Quantitative determination of oxalate in Bayer liquor by capillary zone electrophoresis : a validative study

The applicability of capillary zone electrophoresis (CZE) for the routine determination of oxalate in Bayer liquors was investigated. The effects of electrolyte composition, capillary conditioning, temperature and applied voltage were studied, with the use of temperature control, isomigration mode and an appropriate capillary conditioning regime proving critical in minimising variations in peak migration time. The CZE method, which utilised a chromate-based electrolyte (containing an osmotic flow modifier) and indirect photometry, demonstrated excellent within-batch precision, giving a migration time precision of less than 0.30% R.S.D. and an analysis precision of less than 1.2% R.S.D. The method also demonstrated excellent between-batch precision, with the overall analysis precision, obtained for five alumina refinery samples run over five days, being 2.3, 1.1, 1.7, 1.5 and 1.3% R.S.D., respectively. The between-batch migration time precision for the oxalate peak was 0.89% R.S.D., while the absolute oxalate peak area precision was 2.5% R.S.D. The method within- and between-batch precision values were comparable to those expected from other instrumental separation techniques, demonstrating that CZE can be an appropriate technique for routine analysis, even for complicated sample matrices, such as Bayer liquors.

Manipulation of separation selectivity in capillary zone electrophoresis of anionic solutes

This article discusses the main approaches to the manipulation of the separation selectivity of inorganic and low-molecular-mass anions in capillary zone electrophoresis (CZE). Physical or instrumental effects such as the detection mode, the sampling mode, the separation voltage, and the temperature are easy to control but their influence on selectivity is generally minimal, except for the use of selective detection. Selectivity effects arising from chemical parameters (i.e. effective size and charge, and structure of analyte; the pH, surfactant type and content, polyelectrolyte content, organic solvent content of the electrolyte; capillary treatment; and complexing agents) are much more significant than those resulting from physical effects. The effects on separation selectivity exerted by some of the above parameters can be complex, so that manipulation of selectivity in CZE of anionic solutes is often difficult. Nonetheless, many practical applications can be performed through the judicious control of parameters noted in this review. Some practical limitations to selectivity manipulation are highlighted and possible areas that can be studied in the future for selectivity control are noted.

Analysis of difficult samples by capillary zone electrophoresis I. Simultaneous separation of detrimental anions in concrete digested with concentrated nitric acid

The analysis of detrimental anions (chloride and sulfate) in concrete digested with concentrated nitric acid is extremely difficult due to the high acidity, ionic strength and nitrate background of the digest. Employing capillary zone electrophoresis, chloride and sulfate in a diluted digest were separated simultaneously with full resolution in less than 6 min. An electrolyte comprising 10 mM lithium nitrate, 1 mM thiocyanate and 0.8% (v/v) 1-butanol at pH 6 was used. The development of this electrolyte and the analytical performance are discussed.

Analytical performance characteristics of the separation of fluoride in toothpaste by capillary zone electrophoresis

The analytical performance characteristics of the separation by CZE of fluoride in toothpaste using a chromate-based electrolyte is discussed. Good precisions (n = 5) were recorded for absolute migration time (1.7% RSD), peak area (0.8% RSD) and peak height (0.4% RSD). Fluoride was separated in under 4 min without matrix interference or co-migration with phosphate (resolution ca. 8). The peak area calibration curve was linear between 5–120 µg ml - 1 of fluoride and using this curve, one toothpaste sample was determined to have about 12 µg ml - 1 (≈0.1% m/m undried basis) of fluoride. The limit of detection was 0.5 µg ml - 1 (3 × baseline noise) and the mean recovery was 106% (n = 10).

Optimization of the response of phosphate separated by capillary zone electrophoresis

The response of phosphate, which is highly variable in capillary zone electrophoresis (CZE) using silica capillaries, was optimized by monitoring the absorption of phosphate onto the capillary surface. A capillary conditioning regime is described which facilitates maximum response for phosphate without significantly extending the total analysis time. The described regime also allowed excellent precision (⩽0.36%sr, n= 10) to be achieved with regard to absolute migration time for phosphate as well as chloride, bromide, sulfate and hydrogencarbonate. Using this regime, the precisions for the phosphate peak area and height were 9 and 5.5%sr, respectively (n= 10).

THE EFFECT OF BUTAN-1-OL ON THE SEPARATION OF INORGANIC ANIONS USING CAPILLARY ZONE ELECTROPHORESIS WITH REVERSED ELECTROOSMOTIC FLOW

The separation selectivity of inorganic anions in capillary zone electrophoresis with reversed electroosmotic flow was altered by incorporating ≤7.5% v/v butan-1-ol in a chromate-based background electrolyte (BGE). Pronounced selectivity changes occurred for lipophilic anions, whereas, for the common inorganic anions, the changes were minor. The likely causes for these selectivity changes are discussed. Furthermore, BGEs with butan-1-ol yielded separations with (i) short times for lipophilic anions, (ii) smoother baselines, (iii) better resolutions, (iv) increased peak heights for iodide and thiocyanate, and (v) good reproducibility (≤4% RSD, n = 11) for absolute migration time, peak area, and peak height.