Evaldas Naujalis

On-line preconcentration and determination of chromium(VI) in waters by high-performance liquid chromatography using pre-column complexation with 1,5-diphenylcarbazide

A method for on-line chromatographic preconcentration and determination of chromium(VI) traces has been developed. Chromate was preconcentrated on a C18 column (50 x 6 mm I.D.) after complexation with diphenylcarbazide (DPC). Following the preconcentration step, analysis of the sample was performed using a C18 column (100 x 6 mm I.D.) with an eluent containing 6.10(-3) mol/l sulphuric acid and 20% (v/v) acetonitrile. Direct spectrophotometric detection at 546 nm was used. Experimental parameters such as mobile phase pH, DPC concentration, preconcentration flow-rate, sample volume were optimized for preconcentration and detection of Cr(VI)-DPC complex. Under the optimum conditions, most metal ions [Cr(III), Fe(III), Cu(II), Hg(II), Mo(VI), V(V)] and inorganic anions did not interfere. A detection limit of 0.02 ng/ml Cr(VI) can be attained when a sample volume of 100 ml is used. The technique has been applied successfully to the determination of Cr(VI) traces in drinking, surface and groundwater samples and the recoveries of added chromium were in the range 94-104%.

Matrix solid-phase dispersion extraction of carbadox and olaquindox in feed followed by hydrophilic interaction ultra-high-pressure liquid chromatographic analysis

A new method involving matrix solid-phase dispersion (MSPD) extraction and hydrophilic interaction ultra-high-pressure liquid chromatography (HILIC-UHPLC) with photodiode array detection was developed for the determination of carbadox and olaquindox in feed. Separation of carbadox and olaquindox was achieved within 1 min on the 1.7 microm Acquity UPLC BEH HILIC column by using isocratic elution with a mobile phase consisting of 10 mmol L(-1) ammonium acetate in acetonitrile-water (95:5, v/v) at a flow rate of 0.5 mL min(-1). Optimization of MSPD extraction parameters, such as type of solid sorbent and elution solvent were carried out. Optimal conditions selected for MSPD extraction were: 0.25 g of feed sample, 0.5 g of octadecylsilica as solid sorbent and 10 mL of acetonitrile-methanol (8:2, v/v) as eluting solvent. Both analytes provided average recoveries from spiked feed samples ranging from 89.1 to 98.4% with relative standard deviations less than 10%. Obtained performance characteristics are comparable to those achieved by liquid-liquid extraction-HPLC with the advantages of being simpler and significantly faster.

Development of capillary electrophoresis for the determination of metal ions using mixed partial and complete complexation techniques.

A new capillary electrophoretic (CE) method was developed for the selective and sensitive determination of common metal ions. The proposed method is based on conventional CE separation of metal cations followed by complete complexation of separated analytes with 1,10-phenanthroline using the zone-passing technique. This approach combines both partial and complete complexation modes and, thus, enables rapid, selective, efficient separation together with sensitive direct UV detection of metal species. The optimal conditions for the separation and derivatization reaction were established by varying type of electrolyte, electrolyte pH, introduction time and concentration of 1,10-phenanthroline. The optimized separations were carried out in 50 mmol l(-1) glycolic acid electrolyte (pH 6.0 with imidazole) using direct UV detection at 254 nm. Five common metal cations (Fe2+, Co2+, Ni2+, Cu2+ and Zn2+) were separated in less than 4 min. The proposed system was applied to the determination of Fe(II) and Zn(II) in snow samples. The recovery tests established for snow samples were within the range 100+/-12%.

Preparation and characterization of quercetin-loaded lipid liquid crystalline systems.

The aim of the present study was to investigate mixtures of soy phosphatidylcholine (SPC) and glycerol dioleate (GDO) as encapsulation matrices for antioxidant quercetin. The effects of quercetin loading into non-aqueous formulations, non-lamellar liquid crystalline phases and their colloidal dispersions were studied by using synchrotron small angle X-ray diffraction, dynamic light scattering, cryogenic electron microscopy and high performance liquid chromatography. Quercetin incorporation is discussed in the context of lipid aggregation behavior, self-assembled nanostructure and chemical stability. The obtained results show that SPC/GDO-based formulations can incorporate relatively high amounts of quercetin and serve as liquid crystalline delivery vehicles in the form of bulk phases or colloidal dispersions.

Capillary electrophoretic speciation of Cu(II) and Co(III) in the electroless copper plating baths

SummaryA capillary electrophoretic method for the determination of Cu(II) and Co(III) chelates with ethylenediamine in electroless copper plating baths has been developed. The influence of carrier electrolyte parameters such as nature of counter-ion and pH were studied and discussed. The optimised separations were carried out in a fused silica capillary (57 cm × 75 μm I.D.) filled with an ethylenediamine sulfate electrolyte (20 mol L−1 ethylendiamine, pH7.0 with H2SO4; applied voltage, +25 kV) using direct UV detection at 214 nm. The detection limits for a signalto-noise ratio of 3 and 10s hydrodynamic injection were 5×10−6 mol L−1 for Cu(II) and 1×10−6 mol L−1 for Co(III). The relative standard deviations of the peak areas for Cu(II) and Co(III) were found to be 1.5% and 2.4%, respectively, with five consecutive injections of standard solution containing 5×10−5 mol L−1 of each metal ion. Application of the method to the speciation of Cu(II) and Co(III) complexes in copper plating bath samples is also demonstrated.

Influence of MOCVD Growth Pressure on Magnetoresistance of Nanostructured La-Ca-Mn-O Films Used for Magnetic Field Sensors

The results of structure and magnetoresistance (MR) of nanostructured La_{1–<italic>x</italic>}Ca_{<italic>x</italic>}Mn_{<italic>y</italic>}O₃ (LCMO) films, grown at different gas pressure (from 3 to 7 Torr) by pulse injection metal–organic chemical vapor deposition (MOCVD) technique, are presented. The MR was investigated in pulsed magnetic fields up to 60 T in the temperature range 1.5–294 K. The results were analyzed from the perspective of using these films for magnetic field sensors operating at low temperatures. It was demonstrated that with the increase of Ar + O₂ gas pressure, the surface morphology of the films becomes rougher and grain size increases. Also, the ratio of Mn/(La + Ca) increases with the increase of the pressure. Large MR of the films was observed in a wide temperature range below the ferromagnetic–paramagnetic phase transition. It was shown that at cryogenic temperatures, the films grown at gas pressure of 3 Torr have higher sensitivity in lower magnetic field range (<10 T), while in high magnetic fields (20–60 T), the properties of films grown at higher gas pressure 5–7 Torr are favored. The obtained results allow evaluating the influence of MOCVD growth pressure on MR and sensitivity to the magnetic field of LCMO manganites used for pulsed magnetic field sensors.