Richard M. Cassidy

Separation of metal ions by capillary electrophoresis

Abstract A number of experimental parameters have been optimized for the separation of 26 metal ions, including alkali, alkaline earth, transition and lanthanide metal ions. Experimental parameters that were evaluated included nature of indirect-detection reagent, pH of electrolyte, concentration of complexing agent and nature of the surface of the capillary; unbonded and C1 and C18 bonded phases were studied. In addition the effect of internal diameter on linearity and signal-to-noise ratio was examined, and separation efficiency was determined for a variety of experimental conditions. Detection limits (signal-to-noise ratio = 3) were ca. 1 μg/ml for the lanthanides, ca. 0.6 μg/ml for transition and alkaline earth ions and ca. 0.1–0.8 μg/ml for alkali metal ions. The average relative standard deviations of were 3.7, 5.1 and 2.5% on unbonded, C1 and C18 capillaries, respectively. Whereas conventional regression analysis suggested that the calibration curves were linear over the range of 1·10−5 to 4·10−4 mol/l, sensitivity plots showed that the results were actually linear to within 6% only over the range of 2.5·10−5 to 4·10−4 mol/l.

End-column electrochemical detection for inorganic and organic species in high-voltage capillary electrophoresis

The electronics and construction for an end-column ultramicroelectrode (3–10 μm) detection system that permits the use of medium-sized capillaries (25 μm I.D.) without appreciable effects from the potential field at the end of the capillary. Normal peak-to-peak noise over 10 s was 0.01–0.1 pA. The background noise observed for a 200 × μm carbon-fiber electrode placed either 180 μm within a 25-μm capillary or at a point 500 μm away from the capillary was essentially the same. A study of detector response as a function of the position of the electrode has shown that accurate location of the electrode is important for sensitive and reproducible detection. These studies also showed that differences between the density of the electrolyte existing the capillary and the electrolyte in the detection cell could cause anomalous electrode response depending on the location of the electrode relative to the end of the capillary. Application of a carbon fiber or an Hg film electrode gave detection limits (twice the peak-to-peak noise over 10 s) of 2 · 10−8 mol/l for Pb2+, 1 · 10− 5 mol/l for NO2− and 5 · 10−10 mol/l for catechol.

Bonded-phase capillaries and the separation of inorganic ions by high-voltage capillary electrophoresis

Abstract Both a C 1 and a C 18 saturated hydrocarbon have been bonded to 75-μm capillaries for high-voltage capillary electrophoresis separations. The performance of these bonded phases has been compared with unbonded capillaries under a variety of experimental conditions. The bonded phases were prepared by a flow-through procedure at room temperature, and the reproducibilities of the electroosmotic flow for the C 1 and C 18 capillaries were 6 and 3%, respectively. Both of the bonded phases reduced interactions between the silica surface and positively charged ions, but for larger hydrophobic ions sorption and peak tailing were observed on the C 18 phase. In the presence of sodium dodecylsulfate and sodium decanesulfonate it was possible to control the electroosmotic flow over a wide range. The separation of lanthanide metal ions is illustrated, and improved resolution and reduced surface interactions are shown for the bonded phases.

Effect of electrolyte composition in the capillary electrophoretic separation of inorganic/organic anions in the presence of cationic polymers

Abstract Electrolyte modification with two cationic polyelectrolytes has been investigated for the separation of inorganic anions, aliphatic and aromatic carboxylic acids, and sulphonic acids. Changes in migration rates and separation selectivity, arising from hydrophobic interactions, ion exchange, and changes in electroosmotic flow were studied for benzoate, chromate and phosphate + chloride counter-ions as a function of pH, and for the addition of methanol and acetonitrile. Large changes in separation selectivity, for aliphatic and sulphonic acids, were observed with benzoate electrolytes as compared to chromate electrolytes. In addition, electroosmotic flow was faster in the former separation electrolytes by about 65%. Changes in selectivity as a result of hydrophobic interactions were investigated with a series of aromatic acids where shifts in migration order were observed as function of polyelectrolyte concentration. For inorganic anions, in chromate electrolytes at a constant concentration of polyelectrolyte, the addition of methanol or acetonitrile decreased migration rates by as much as 67%. The analytical merits of the methodology were examined for a potash (concentrated KCl) sample and a simulated decontamination solution containing EDTA, Fe 3+ (EDTA) and oxalate.

Control of separation selectivity and electroosmotic flow in nonaqueous capillary electrophoretic separations of alkali and alkaline earth metal ions

Abstract Factors affecting the separation selectivity and electroosmotic flow (EOF) for the separation of alkali, alkaline earth cations, and ammonium ion in nonaqueous systems were evaluated. The nature of electrolyte anion (chloride, perchlorate, and acetate) and solvent (methanol-acetonitrile mixtures) had important effects on the EOF in acidic conditions. Evidence was observed for ion adsorption (protons and anions) on the silica surface and for ion-interactions in the electrolyte: both effects could be used to control the direction and magnitude of the EOF. Relative to aqueous systems, unique separation selectivity was achieved for alkali and alkaline earth cations; ammonium and alkali cations were resolved from alkaline earth cations and potassium and ammonium ions were well separated. Calibration of all the cations gave linear curves in the range of 5 × 10 −4 −5 × 10 −5 mol/l; detection limits (indirect with imidazole) were in the range of 7 × 10 −6 –2 × 10 −5 mol/l. To evaluate this system, the separation of metal ions was examined for a brine solution and an aqueous soil extract sample.

Capillary electrophoresis of some tetracycline antibiotics coupled with reductive fast cyclic voltammetric detection.

The separation and quantitative performance parameters for tetracycline, chlortetracycline and oxytetracycline antibiotics were investigated by capillary zone electrophoresis coupled with fast cyclic voltammetric detection. Optimization of pH and complexation with a boric acid-sodium tetraborate buffer provided good resolution of all compounds. Detection by electrochemical reduction using fast on-line cyclic voltammetric detection with a Hg-film-microm electrode gave detection limits (2 x peak-to-peak baseline noise) of 7 x 10(-7) mol/l for tetracycline and chlortetracycline, and 1.5 x 10(-6) mol/l for oxytetracycline. The influence of electrode material, potential range and scan rate was examined and discussed. Optimal electrochemical detection was obtained at a Hg-film electrode with a waveform that consisted of an initial constant potential of -0.6 V for 200 ms followed by a cyclic voltammetry (CV) scan at 300 V/s from - 0.6 V to a vertex potential of 1.7 V. The analytical signal was obtained by plotting the integrated values of the CV current from each applied waveform as a function of time. The calibration plot (peak areas) for each separated peak was found to be linear over three-orders of magnitude.

Selectivity control in the non-aqueous capillary electrophoretic separation of amino acids

Abstract The separation of dansylated amino acids and underivatized amino acids in non-aqueous electrolytes was evaluated with direct and indirect UV detection. Different migration orders were achieved for dansylated amino acids in methanol compared to aqueous electrolyte systems. A reversed migration order was observed for some dansylated amino acids. Separation selectivity was different under acidic and basic conditions and was also a function of the solvation properties of the solvent. Underivatized amino acids were separated in basic and acidic electrolytes in methanol; different separation selectivities and, for some amino acids, a reversed migration order were also observed in these electrolyte systems. Analytical merits of the separation of both derivatized and underivatized amino acids were briefly evaluated; detection limits for dansylated amino acids were in the range of 2·10 −7 –4·10 −7 mol/l and, for underivatized amino acids, were 2·10 −6 –4·10 −5 mol/l.

Capillary electrophoretic separation of metal ions in the presence of polyethylene glycols

The capillary electrophoretic separation of the metal ions lithium, potassium, magnesium, barium, zinc, lead, lanthanum, samarium, europium and dysprosium was examined in the presence of polyethylene glycol additives (0.01–10%) having relative molecular masses between 200 and 20 000 000 g mol l–1. Changes observed in electrophoretic mobilities, separation selectivity, resolution and electroosmotic flow depended on the relative molecular mass and concentration of the polyethylene glycol. Decreases in electrophoretic mobility (e.g., 45% for lithium) were attributed to polyethylene glycol–metal ion interactions. For the higher molecular mass polymers ( 20 000 g mol l–1), concentrations greater than about 1% m/v were found to have a detrimental effect on detection sensitivity and separation current stability, presumably owing to aggregate formation. Efficiencies were in the range 120 000–260 000 theoretical plates, and applications to the analysis of a fermentation broth and an aqueous soil extract sample were examined.

Experimental factors in pulsed electrochemical detection in capillary electrophoresis

Abstract A number of experimental approaches that should offer improved S/N in electrochemical detection were evaluated and compared. In addition, to evaluate and optimize the electrochemical response behavior of analytes under actual CE conditions, an on-line cyclic voltammetry (CV) system was developed. The experimental parameters examined included waveform shape, waveform frequency and various signal treatments, using a lock-in amplifier and Fourier analysis. A multiple-step pulse waveform provided the maximum S/N enhancement [up to 10-fold relative to pulse amperometric detection (PAD)], with detection limits in the range of 2·10−8 to 2·10−7 mol/l. The use of the second harmonic from Fourier analysis offered the best improvement in baseline stability. Other approaches such as high pulse frequency (100 to 200 Hz), data collection over selected time windows, digital filters and average smoothing also enhanced S/N 2- to 5-fold relative to PAD. On-line CV studies showed that the adsorption of organic electrolytes on electrode surfaces can inhibit O2 reactions, and thus give low and stable background currents without O2 removal. The CV studies also showed that detection of Pb2+, Cu2+ and Ag+ affected subsequent electrode response, and that H+ evolution contributed to the cathodic signals of the analytes Ni2+, Co2+ and Zn2+.

Capillary electrophoresis with end‐capillary potentiometric detection using a copper electrode

Potentiometric end‒capillary detection in capillary electrophoresis has the advantage of relatively easy miniaturisation without having to compromise the concentration sensitivity. Potentiometric end‒capillary detection using a copper electrode is also attractive because of the sensitive detection of many inorganic and organic UV‒transparent ions and the ability to work in both direct and indirect mode. In this work, detection of a number of common anions in a tartrate electrolyte at pH 3 was studied. The influence of the end‒capillary detection geometry on the detection performance was investigated. An end‒capillary detection cell allowing the separation capillary to be changed without the need to realign the detection electrode was constructed and fitted into a commercial CE apparatus. Under the optimal configuration, which was a 25 μm diameter copper electrode aligned coaxially with a 25 μm capillary and positioned at a distance of about 25 μm from the capillary end, excellent peak shapes were achieved and comparison with simultaneous on‒capillary photometric detection showed no additional peak broadening. Good sensitivity was obtained, resulting in concentration limits of detection (LODs) in the low μM range and mass LODs in the low amol range. Examples of separations of inorganic and organic anions are presented and the analytical potential of the detection method is assessed.