Vasile David

Multi-insertion of small controlled volumes of solutions in a flow assembly for determination of nitrate (photoreduction) and nitrite with proflavin sulfate

A simple, maintenance-free, non-contaminating and completely automated flow system was developed for non- simultaneous determinations of nitrite and nitrate, in which spectrophotometric measurements at 328 nm were performed on the nitrite reaction with proflavin sulfate [bis(3,6-diaminoacridinium)sulfate]. The sample solutions were automatically inserted into phosphate buffer (pH 6.8) containing 10 mM EDTA by using a programmable solenoid valve; then they flowed through a 6.4 m × 0.5 mm id PTFE coil, wrapped around an 8 W low pressure mercury lamp, and finally they merged with a hydrochloric acid solution of proflavin. With the lamp off the nitrite in the sample was selectively measured with a sensitivity of 0.75 × 104 absorbance (linearity range up to 0.6 absorbance; detection limit of 1.9 × 10–7M). With the lamp on, to determine the sum of nitrite + nitrate there was a decrease in sensitivity of 41% versus that of the selective variant for nitrite determination. The sample throughput was 15 h–1 and the RSD at a 6.4 × 10–5M level of nitrate was 0.4% (n = 15). Many interferences were studied and simple methods of correcting for them were applied to analyse samples such as waters, soils, fertilisers and meat.

Retention Phenomena Induced by Large Volume Injection of Solvents Non‐Miscible with the Mobile Phase in Reversed‐Phase Liquid Chromatography

Abstract Enhancement of sensitivity for spectrometric detection in liquid chromatography is obtained when increased sample volumes are loaded into the chromatographic column. A new approach of injecting large volumes of solvents non‐miscible with the mobile phase is proposed. The paper focuses on the retention study of butylated hydroxyanisole (BHA) loaded into the chromatographic column dissolved in liquid alkanes (n‐hexane, n‐heptane, and iso‐octane, respectively) at volumes up to 600 µL. A simple model based on adsorption for explaining the experimental results is proposed. A clear insight of the process was possible by studying the migration of the solvent zone within the column, at different mobile phase compositions and different loaded volumes, by means of refractive index detection. The model allowed calculation of the number of solvent molecules saturating a C8 alkyl chain site within the stationary phase layer. Competition between the analyte and solvent molecules, together with the homogeneity of the “saturation” realized by solvent over the adsorption sites, could explain the experimental results. It was demonstrated that large volume injection of solvents non‐miscible with the mobile phase is possible in RP‐LC with a gradual reduction of retention and a relatively small loss in terms of peak efficiency.

Voltammetric determination of B1 and B6 vitamins using a pencil graphite electrode

Due to the importance of B1 and B6 vitamins for human health it is useful to develop new cheap and rapid methods for their determination. Voltammetric behavior of these vitamins on a pencil graphite electrode was investigated using cyclic voltammetry in different media. Direct quantitative determination of the two vitamins, one in the presence of the other, was done by differential pulse voltammetry. Vitamin B1 was electroactive only in a NaOH solution generating two irreversible oxidation peaks; the first peak obtained at 250 mV is well-defined and was used in quantitative determinations. In case of vitamin B6, a well-defined oxidation peak was observed in all investigated supporting electrolytes except for HCl. The linear concentration ranges were 10−5–10−3 M for vitamin B1 in a NaOH solution and 5 × 10−6–10−3 M for vitamin B6 in an acetate buffer solution. The obtained detection limits were 5.34 × 10−6 M and 2.81 × 10−6 M for vitamin B1 and vitamin B6, respectively. The developed method is simple and rapid and it was successfully applied in the determination of the two vitamins in pharmaceuticals.

SELECTIVE DPV METHOD OF DOPAMINE DETERMINATION IN BIOLOGICAL SAMPLES CONTAINING ASCORBIC ACID

A modified carbon paste electrode that incorporates the anionic surfactant sodium dodecyl sulfate (SDS) in the paste (SDS-CP) is reported. It is shown that the SDS-CP electrode discriminates effectively between the cationic form of dopamine and the anionic electroactive species existing in biological fluids at the physiological pH. Therefore, a differential pulse voltammetry method has been developed. It has the detection limit comprised in the submicromolar range and the capability to remove the interference of the ascorbic acid and to diminish the interference of the uric acid. The method has been tested with good results on real samples of deproteinized serum.

Electropolymerized molecular imprinting on glassy carbon electrode for voltammetric detection of dopamine in biological samples.

A simple and reliable method for preparing a selective dopamine (DA) sensor based on a molecularly imprinted polymer of ethacridine was proposed. The molecularly imprinted polymer electrode was prepared through electrodepositing polyethacridine-dopamine film on the glassy carbon electrode and then removing DA from the film via chemical induced elution. The molecular imprinted sensor was tested by cyclic voltammetry as well as by differential pulse voltammetry (DPV) to verify the changes in oxidative currents of DA. In optimized DPV conditions the oxidation peak current was well-proportional to the concentration of DA in the range from 2.0×10(-8)M up to 1×10(-6)M. The limit of detection (3σ) of DA was found to be as low as 4.4nM, by the proposed sensor that could be considered a sensitive marker of DA depletion in Parkinsons disease. Good reproducibility with relative standard deviation of 1.4% and long term stability within two weeks were also observed. The modified sensor was validated for the analysis of DA in deproteinized human serum samples using differential pulse voltammetric technique.

DNA-wrapped multi-walled carbon nanotube modified electrochemical biosensor for the detection of Escherichia coli from real samples

This paper introduces DNA-wrapped multi-walled carbon nanotube (MWCNT)-modified genosensor for the detection of Escherichia coli (E. coli) from polymerase chain reaction (PCR)-amplified real samples while Staphylococcus aureus (S. aureus) was used to investigate the selectivity of the biosensor. The capture probe specifically recognizing E. coli DNA and it was firstly interacted with MWCNTs for wrapping of single-stranded DNA (ssDNA) onto the nanomaterial. DNA-wrapped MWCNTs were then immobilised on the surface of disposable pencil graphite electrode (PGE) for the detection of DNA hybridization. Electrochemical behaviors of the modified PGEs were investigated using Raman spectroscopy and differential pulse voltammetry (DPV). The sequence selective DNA hybridization was determined and evaluated by changes in the intrinsic guanine oxidation signal at about 1.0V by DPV. Numerous factors affecting the hybridization were optimized such as target concentration, hybridization time, etc. The designed DNA sensor can well detect E. coli DNA in 20min detection time with 0.5pmole of detection limit in 30µL of sample volume.

Development of a New HPLC Method for Determination of Papaverine in Presence of Its Photooxidation Products

A new HPLC method was developed for determination of papaverine in a papaverine naturally photooxidized solution using a Duet C18/SCX column. The mobile phase used was a mixture of phosphate buffer (pH = 3.80) and acetonitrile in 40:60 ratio. The compounds identified were: papaverinol, papaveraldine, papaverine, and pyrrocolonium ion. Very good resolutions, peak shapes, and asymmetries were obtained. An ion exchange experiment was developed in order to identify and separate the pyrrocolonium ion. This was retained completely by a cation exchange resin in its Na+ form, while papaverine, papaverinol, and papaveraldine were retained more or less, due to a retention explained by a combined ion exchange-solvent extraction mechanism. Voltammetric studies showed that natural photooxidation of papaverine occurs in three irreversible steps and yields to stable compounds in solution.

Flow Injection Biamperometric Determination of Nitrite and Nitrate

A simple and efficient FIA method was used with good results to determine nitrite in residual waters and nitrate in natural waters. Nitrite determination is based on the reaction with iodide occurring in acidic medium and biamperometric detection of the formed iodine at two platinum electrodes polarised at a potential of 100 mV. Nitrate is similarly determined after its previous reduction to nitrite in a cadmium column. The method does not need the solution deaeration. However, the calibration graphs present two regions of linearity owing to the catalytic effect of the dissolved oxygen on the iodide oxidation by nitrite.

Signal processing algorithm for Schlieren effect correction in flow analysis

Due to the radial refractive index gradients that sometimes develop in flow cells during the travelling of the samples through them, the absorbance records obtained in such conditions have a strange shape, particularly at low analyte levels. To correct this phenomenon, called Schlieren effect, a simple and reliable signal processing algorithm is described in correlation to the detector characteristics of noise. The algorithm is illustrated in nitrite and nitrate determinations with proflavin from more real samples. It has a solid theoretical support and involves the subtraction from the sample records of a smoothed signal, acquired by working with a blank solution.