V. Zelenská

Determination of inorganic anions in river water by column-coupling capillary isotachophoresis

Abstract Optimal operating conditions for the simultaneous determination of Cl−, NO−3, SO2−4, NO−2, F− and PO3−4 in river water by column-coupling capillary isotachophoresis were found. The time for the complete analysis was ca. 25 min and no sample pre-treatment was needed. Detection limits for constituents that are present in river water at lower concentrations (NO−2, F− and PO3−4) were in the range 30–60 pmoles. Decomposition of NO−2 during the separation was observed and a possible mechanism is proposed.

Contactless conductivity detection in capillary zone electrophoresis

Abstract Some basic performance parameters of a high-frequency contactless conductivity detection technique originally designed for capillary isotachophoresis (ITP) were evaluated from the point of view of its use in capillary zone electrophoresis (CZE). These parameters included the response characteristic (a response of the detector to changes of the specific conductivity in the on-column detection cell), short-term noise and the detection sensitivity. A series of seven detectors, provided with capillary tubes (300 μm I.D.) that were made either of PTFE (six detectors) or fused-silica (one detector), was taken into the evaluation. A certain scatter of the noise/detection sensitivity ratios of the tested detectors (attributable to differences in the on-column detection cells of the same construction) was found. These differences were also associated with different limits of detection (LODs) attainable for the test analytes (inorganic anions) by the evaluated detectors. For the detector characterized by minimum values of noise/detection sensitivity ratios for the test anions, the LOD (a 200-nl sample injection volume) ranged from 0.3 μmol/l (sulfate) to 0.7 μmol/l (fluoride) in the carrier electrolyte solution, which had a pH value of 3.5. These LOD values were two–eight times higher in comparison to those attainable by an on-column contact conductivity detector under otherwise identical working conditions. A lower sensitivity of the contactless conductivity detector can be more than compensated for in practical situations by its operational robustness. These features were proved effective in the CZE analyses of aqueous extracts of soil samples of high humic acid content (these samples are known to contaminate surfaces of the detection electrodes of the contact conductivity detectors). Here, 1–3% RSD values in the determinations of chloride, nitrate and sulfate could be achieved in large series of CZE runs, with no disturbances linked with the performance of the contactless conductivity detector even in situations when the sample matrix precipitated in the capillary.

Capillary zone electrophoresis in a hydrodynamically closed separation system with enhanced sample loadability

Some phenomena linked with capillary zone electrophoresis (CZE) performed in a hydrodynamically closed separation system with enhanced sample load capacity via the use of capillary tubes of larger I.D.s were studied. Calculations of the plate heights for varying amounts of the analytes loaded onto 50 and 300 μm I.D. columns under identical CZE separating conditions revealed that the analytical advantages of using columns of larger I.D. include significantly reduced contributions of electromigration dispersion. At higher concentrations of the carrier electrolytes this gain, however, can be partially lost due to increased thermal dispersive effects. Calculated resolutions for a varying ratio of a pair of the analytes loaded onto 50 and 300 μm I.D. columns favoured the use of the latter I.D. in situations when the ratio of the analytes was higher than ca. 102:1. CZE experiments were carried out in a 300 μm I.D. capillary tube made of fluorinated ethylene–propylene copolymer (FEP) with a porous cellophane membrane serving as a hydrodynamic barrier to prevent a flow of the solution in the separation compartment due to a pressure difference between the electrode vessels. Movement of the membrane was found to be a source of undesired flows in the separation compartment, which adversely affected both the reproducibilities of migration times of the analytes and their separation efficiencies. Mechanical support eliminated these problems. Dispersive phenomena associated with electroosmosis in the closed separation compartment were effectively suppressed by using high molecular weight derivatives of water soluble polymers (methylhydroxyethylcellulose and polyethyleneglycol) in the carrier electrolyte solutions. Examples from the separations of various groups of analytes (synthetic food colourants, some inorganic anions, alkali and alkaline earth metal cations and glycoforms of erythropoietin) are given to illustrate a practical utility of the studied CZE approach.

Isotachophoretic separation of alkali and alkaline earth metal cations in water—polyethylene glycol mixtures

Abstract The influence of polyethylene glycol (PEG) on the effective mobilities of alkali and alkaline earth metal cations was investigated. It is shown that appropriate proportions of water and PEG in the leading electrolyte enable a complete separation of this group of cations to be achieved by capillary isotachophoresis (ITP). The high dynamic concentration ranges of the ITP determinations achieved in the operational systems based on water—PEG mixtures are illustrated by practical examples (water samples, urine, aqueous extract of apple flesh).

Photometric detection at 405 nm in trace analysis by capillary isotachophoresis

The possibilities of photometric detection at 405 nm were investigated for trace isotachophoretic analysis of various ionic constituents. Increased selectivity of the detection enabled, for example, the detection and determination of nitrophenols at 10(-8) M in a 25-microliter sample. 5-Nitrofurylacrylic acid, a wine stabilizer, could be detected with confidence at ca. 0.1 ppm in wine without any sample pretreatment. Approximately 250-fmol amounts of amino acids, labelled with 2,4-dinitrophenyl, were detectable with the technique employed. This is almost three decades lower in comparison to what is achievable with an high resolution universal detector. The adsorption of the analytes in the sample handling devices as well as in the separation compartment were problems in the analysis at this concentration level. The detection limit given above was achieved when these phenomena were suppressed by the addition of naphthalene-1,3,6-trisulphonate or pyrophosphate to the sample solution.

Isotachophoretic determination of chromium(VI) at low parts per billion concentrations.

Factors important in the isotachophoretic determination of CrVI at low ppb concentrations were studied. To increase the selectivity of the analysis, the use of photometric detection at a 405 wavelength was preferred. Losses of CrVI due to adsorption were found to be the main potential source of analytical errors at the concentration levels of interest. The presence of sulphate in the sample solution at ca. 10(-4) M concentration eliminated the losses due to adsorption on the walls of the sample handling glassware. In analysis with a low pH of the leading electrolyte, adsorption of CrVI on the walls of the separation compartment also played a role; addition of naphthalene-1,3,6-trisulphonate to the sample solution considerably reduced this disturbance. When the precautions concerning adsorption were taken, the detection limits for CrVI were in the range 4-5 ppb (depending on the pH of the leading electrolyte) for a 30-microliters sample volume. The calibration graphs were linear over the concentration range 10(-7)-5 X 10(-6) M with good correlation coefficients. The reproducibilities of the determinations within this concentration range were 2-3% or better. The practical utility of capillary isotachophoresis in the trace determination of CrVI in drinking water and wastewater samples seems promising.

Solid phase extraction for sample preparation in trace analysis of ionogenic compounds by capillary isotachophoresis

Various sorbents recommended for solid phase extraction (SPE) in sample preparation procedures were studied for use in combination with capillary isotachophoresis (ITP). They were very efficient in achieving trace concentration levels (low ppb, i.e., low parts per 109) for different types of ITP analytes present in environmental and biological matrices. A macroporous carbon sorbent was convenient for sample preparation in ITP analysis of short chain fatty acids (C4−C9) in drinking water. Chelating sorbents based on hydroxyalkyl methacrylate matrix with salicylate, thioglycolate and 8-hydroxyquinolinate functionalities were found to be very suitable for preconcentration of heavy metals with an inherent sample clean-up. An octadecyl-bonded silica sorbent enabled in ITP a photometric detection of γ-aminobutyrate (labeled with a 2,4,6-trinitrophenyl group) at concentrations considerably lower than required for the determination of this amino acid in cerebrospinal fluid (∼5·10−8 mol/l).