L. K. Shpigun

Shipboard flow injection method for the determination of manganese in sea-water using in-valve preconcentration and catalytic spectrophotometric detection

A flow injection method for the determination of manganese at trace level concentrations in sea-water is proposed. The in-valve ion-exchange microcolumn for preconcentration was directly coupled to the spectrophotometer. The spectrophotometric detection was based on the catalytic effect of MnII on the oxidation of N,N-diethylaniline by potassium periodate in a neutral medium. Variation of the preconcentration time from 10 s to 10 min allowed the determination of manganese in the concentration range 20 µg l–1–10 ng l–1. A sampling frequency of up to 15 h–1 and a relative standard deviation of 5–8% were achieved. The proposed method was successfully applied to the direct shipboard measurement of the manganese content in deep sea-water samples for the purpose of indicating the presence of active hydrothermal vents on the sea floor.

A flow-injection amperometric method for determining antiviral guanine derivatives

A flow-injection method is proposed for determining antiviral guanine derivatives, acyclovir, valacyclovir, ganciclovir, and famciclovir, based on their amperometric detection on a carbositall electrode with a preactivated surface. The method is characterized by a wide analytical range, relatively low detection limits (0.6–1.2 μg/mL), and high performance (75 h–1). The results of examination of the accuracy and reproducibility of the analysis of solutions of different pharmaceutical forms containing the test substances are presented. The potentials of the developed flow-injection method in the implementation of the “Dissolution” test in an automated mode are demonstrated.

Development of simple pocket test tools for fast determination of dissolved sulfate in waters

A rapid field method for routine checks on dissolved sulfate in surface, running, and potable waters is presented. The method uses reagent indicator paper strips and a thermometric unit. The RIS-Sulfate-Test was developed by immobilization of an Arsenazo III-barium complex together with buffer and masking reagents on cellulose paper. It was adapted to sulfate-ion determination with the use of visual, densitometric, and photometric techniques, the lower detection limit being 0.05– 0.1 g L−1. The sensing principle is based on the reaction, which gives a colorless barium sulfate precipitate with a heat of formation of ΔH = 4.6 kcal M−1. After the strip is immersed into a sample solution for 1 s, the color changes from black–blue to pink–violet because of sulfate-induced complex decomposition. Color changes are monitored with the use of a standard color scale and a miniaturized reflectometer with 660-nm light diode. A miniaturized calorimeter was used for determining high concentrations of sulfate. A 0.3-ml aliquot of sample solution is placed on the bottom of a 5-ml polyethylene vessel, and 0.1–0.2 ml of 0.3-M barium chloride is introduced into the vessel with a 1-ml syringe. The lower part of the syringe with the reagent is immersed into the sample solution for a quicker thermal equilibration. After 10 min, the reagent is injected into the solution and readings are taken with a small voltmeter. A linear dependence of the instrument readings on sulfate concentration was observed in the 0.2 to 5-g L−1 range (P = 0.95, n = 3, RSD = 20%). Both methods were checked by standard addition and dilution procedures, and their reliability was confirmed by flow-injection analysis of seawater with the use of spectrophotometric detection of a turbidimetric indicator reaction.

Electroanalytical study of guanine synthetic derivatives with antiviral activity

Experimental information on the electrochemical behavior of guanine synthetic derivatives (acyclovir, valacyclovir, ganciclovir, and famciclovir) with the strong antiviral activity on presynthesized activated carbosital electrode (CSE) in aqueous solutions of different acidity is obtained. The mechanism of irreversible oxidation of substances containing guanidine group (–NH–C(NH–)=N–) is discussed and the dependences of current and potential of experimental anodic peaks on the potential scan rate, the nature and acidity of supporting electrolyte solution, and also on the concentration of depolarizer and the time of its accumulation on the electrode surface are found. It is shown that all other factors being equal (0.1 М phosphate buffer, рН 6), the position of the observed anodic peak depends substantially on the nature of acyclic substituent at N(9) of imidazole ring in the aminopurine molecule: the susceptibility to oxidation decreases in the series Gua (Epa = 0.84 V), ACV (Epa = 1.00 V), VACV (Epa = 1.04 V), GCV (Epa = 1.07 V), FCV (Epa = 1.20 V). It is shown that the electrocatalytic activity and the high absorbability of the activated CSE with respect to substances tested make it possible to reach their lower detection limits (20–40 nM) in multicomponent solutions.

Voltammetric sensors based on gel composites containing carbon nanotubes and an ionic liquid

Possibilities of using electrode coatings based on a gel of carboxylated multiwall carbon nanotubes (MWCNTs) in an ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [BMIm]PF6) for the creation of a voltammetric sensor with electrocatalytic properties with respect to the pharmacological group of catecholamines—levodopa, methyldopa, and carbidopa—are considered. Using cyclic voltammetry, it was found that a glassy carbon electrode coated with a thin layer of an MWCNT–[BMIm]PF6 gel or an MWCNT–[BMIm]PF6–Nafion gel-composite induced a decrease in overvoltage (~60 mV), improved the reversibility of the redox reaction, and increased oxidation currents of the studied substances in comparison with an unmodified glassy carbon electrode. The concentration dependence of the analytical signal was linear in the ranges of 1–250, 2–350, and 5–400 mM for carbidopa, levodopa, and methyldopa, respectively. In the determination of the specified substances in diluted urine samples and tableted drugs, the accuracy index was 98–102% and the relative standard deviation, 0.3–5% (n = 5, P = 0.95).

The anodic behavior of purine alkaloids on the carbositall electrode in acid solutions

The anodic behavior of natural purine alkaloids pertaining to the group of xanthine N-methyl derivatives is studied on the carbositall electrode in aqueous solutions of dilute acids. The conditions favorable for irreversible electrooxidation of these substances in the acidic medium are considered and the mechanism is discussed. The possibility of measuring defined oxidation peaks of these alkaloids by the background of ascorbic acid and paracetamol, which are the electrochemically active compounds often accompanying the studied substances in various drugs. The results of determination of theophylline and caffeine in certain drug formulations are given.

Spectrophotometric determination of purine alkaloids by flow injection and sequential injection analysis

Procedures are developed for the flow-injection and sequential-injection determination of purine (xanthine) alkaloids (caffeine, theophylline, theobromine, and aminophylline) based on their spectrophoto-metric detection by the oxidative coupling reaction with 3-methyl-2-benzothiazolinone hydrazone hydrochloride under the action of potassium periodate in a weakly acidic aqueous medium (pH 5–6). The analytical range is 3–500 μg/mL, and throughput is 120–180 h–1. It is demonstrated that the developed procedures can be used for the quality control of pharmaceutical preparations.