Amperometric detection of hydroxypurines at an electrode modified with a composite based on mixed-valence ruthenium and cobalt oxides in flow injection analysis

A composite material based on mixed-valence ruthenium and cobalt oxides, electrodeposited on the surface of a screen printed electrode, exhibits high catalytic activity in the electrooxidation of uric acid, xanthine, and hypoxanthine. Catalysis manifests itself as a decrease in the substrate oxidation overvoltage and an increase in current at the potential of modifier oxidation. A method is proposed for the simultaneous amperometric detection of two-component systems uric acid–xanthine, xanthine–hypoxanthine, and uric acid–hypoxanthine using a screen printed electrode with two working electrodes modified by this composite. The dependence of the analytical signal on the concentration of analytes is linear in the range 5 × 10^–8 to 5 × 10^–3 M for uric acid and xanthine and from 5 × 10^–7 to 5 × 10^–3 M for hypoxanthine.     

Flow-injection and sequential injection determination of hydroxypurines on an electrode modified with mixed-valence ruthenium and iridium oxides

Mixed-valence ruthenium (RuO _x ) and iridium (IrO _x ) oxides and composites on their basis (RuO _x -IrO _x or IrO _x -RuO _x ) electrodeposited onto the surface of a glassy carbon electrode exhibit a catalytic activity in the electrooxidation of uric acid, xanthine, and hypoxanthine. The transition from metal oxides to composites of two mixed-valence metal oxides leads to an increase in the catalytic effect of the oxidation of hydroxypurines. The IrO _x -RuO _x composite demonstrated the highest catalytic activity. Procedures for the amperometric detection of hydroxypurines on this composite electrode under the conditions of flow-injection analysis (FIA) and sequential injection analysis are proposed. A linear dependence of an analytical signal on the analyte concentration is observed in the range 1 × 10^?6 to 5 × 10^?3 M for uric acid and xanthine and 5 × 10^?7 to 5 × 10^?3 M for hypoxanthine under FIA conditions and from 5 × 10^?7 to 5 × 10^?3 M for uric acid and xanthine and 5 × 10^?9 to 5 × 10^?3 M for hypoxanthine in sequential injection analysis. Under FIA conditions, the sensitivity, rapidity, and performance of the analysis increase compared to the stationary conditions. The advantages of sequential injection analysis over FIA include a lower consumption of the supporting electrolyte; the absence of pumps and connections; and an increase in sensitivity, reproducibility, and rapidity.     

Determination of creatinine and purine derivatives in ruminants' urine by reversed-phase high-performance liquid chromatography.

A procedure is described for the rapid and simultaneous determination of allantoin, creatinine, uric acid, hypoxanthine and xanthine in sheep urine. Separation was achieved on a Novapak C18 column under isocratic conditions. The mobile phase was potassium phosphate buffer (10 mM, pH 4.0). A flow-rate of 0.5 ml/min, detection at 218 nm and a column temperature of 25 degrees C were employed with a total analysis time of less than 15 min. Detection limits for allantoin, creatinine, uric acid, hypoxanthine and xanthine were 1.0, 0.5, 0.5, 0.5 and 0.2 micrograms/ml, respectively, at a signal-to-noise ratio of 3 in a 20-microliters injection volume of tenfold-diluted urine. This sensitivity permits the precise determination of these compounds in ruminants' urine.     

Study on Electrosynthesis of Nonconducting Polyaniline Used as a Permselective Membrane in Glucose Oxidase Electrode*

Nonconducting polyaniline (PANI) is electrosynthesized at the Au electrode in 0.05 mol/l aniline solution (pH 7.0) by the potential-scan methods of 0.0–0.9 V (vs. SCE) and = 0.14 V/s. This polyaniline membrane has permselectivity to H₂O₂ and can prevent uric and ascorbic acids from diffusing to electrode surface. The glucose oxidase (GOD) electrode is prepared with the nonconducting polyaniline as an exclusive membrane. The response time, linear response range, and sensitivity of the PANI–GOD electrode are 30–35 s, 0.02–22.0 mmol/l, and 2.5 nA/mmol/l, respectively. Responses for glucose at GOD and PANI–GOD electrodes are similar. But oxidation currents of uric and ascorbic acids at PANI–GOD are smaller. The response currents of these two interferences will be restrained if the PANI–GOD electrode is used to detect the glucose in human blood.     

Electrochemical determination of xanthine and hypoxanthine in rat striatum with an acetylene black-dihexadecyl hydrogen phosphate composite film modified electrode by HPLC coupled with in vivo microdialysis

The fabrication and application of a new electrochemical detector for use in HPLC is presented. The detector consists of an electrode modified with a composite film composed of acetylene black and dihexadecyl hydrogen phosphate. The electrochemistry of xanthine and hypoxanthine at this chemically modified electrode (CME) was investigated by cyclic voltammetry. It is found that the CME exhibits efficient electrocatalytic activity towards xanthine and hypoxanthine, with good sensitivity, stability, and lifetime. The linear ranges cover three orders of magnitude, and the detection limits are 6.0 × 10^?8 mol L^?1 for xanthine and 2.5 × 10^?7 mol L^?1 for hypoxanthine (at an S/N ratio of 3). The method was coupled to in-vivo microdialysis sampling and successfully applied to quantify xanthine and hypoxanthine in rat striatal microdialysates of freely moving rats.     

Superoxide sensor based on cytochrome c immobilized on mixed-thiol SAM with a new calibration method

Cytochrome c was immobilized on a mixed-thiol (mercaptoundecanoic acid/mercaptoundecanol) modified gold electrode (MUA:MU/cyt c electrode). Characterization of the cyt c electrode showed a quasi-reversible, electrochemical redox behavior with a formal potential of −13±5 mV (versus Ag/AgCl) for the surface adsorbed protein and 3±5 mV for covalently immobilized cyt c. The heterogeneous electron transfer rate constants were determined to be about 70 and 40 s⁻¹ for both states of the protein, respectively. They were found to be significantly higher than those of pure MUA-modified cyt c electrodes (MUA/cyt c electrodes). The interaction of superoxide radicals (O₂⁻) with the (MUA:MU)/cyt c electrode was characterized and used for an amperometric O₂⁻ detection. The influence of H₂O₂ and uric acid on the sensor signal was investigated. The sensitivity of the (MUA:MU)/cyt c electrode to O₂⁻ was significantly improved compared with that of the MUA/cyt c electrode. Based on a kinetic model for the superoxide detection system, a new calibration method was established. This simple and fast method used the spontaneous dismutation of KO₂ and was compared with the enzymatic superoxide generation system using xanthine oxidase.     

Extraction of palladium from nitric acid medium and its spectrophotometric determination using arsenazo III

Summary Di-octyl sulphoxide in xylene was successfully employed for extracting palladium from 0.5 to 1.5 mol/l nitric acid medium. The extracted species was found to be Pd(NO₃)₂·2DOSO. The palladium was back extracted into a mixture of 2 mol/l sodium carbonate and 0.05 mol/l ammonia. The recovery was found to be quantitative. A spectrophotometric method using Arsenazo III was developed in nitric acid medium for the determination of palladium. The colour development was found to be maximum in the acid range of 2.5 to 5 mol/l. Beer's law was found to be obeyed in 1 to 100 g range of palladium. The molar extinction coefficient was found to be 2.26×10⁴ l/mol/cm. The RSD obtained at 16 g of palladium was 5%.     

Enzyme sensor for L-lactate using differential pulse amperometric detection

An enzyme sensor using differential pulse (DP) amperometric detection has been developed based on the measurement of the reduction current of the oxidized form of -nicotinamide adenine dinucleotide (NAD⁺) consumed by an enzyme reaction. This biosensor has the definite advantage to prevent interference caused by electrooxidative species such as ascorbate and uric acid and exhibits higher sensitivity and selectivity in comparison to the classical DC amperometric detector. The linear detection range of this biosensor was 5.0×10^–5 — 5.0×10^–4 mol/l and the relative standard deviation (R.S.D.) at 2.5×10^–4 mol/l was 5.0%.     

Immobilization of xanthine oxidase and its use in the quantitation of hypoxanthine in fish muscle tissue extracts using a flow injection method

Fish muscle extracts (Scomberomorus— brasiliensis- carite) were analyzed for their hypoxanthine content using a flow injection system incorporating an immobilized xanthine oxidase bioreactor. The xanthine oxidase was immobilized under mild conditions to a 2-fluoro-1-methylpyridinium Fractogel support. The uric acid produced from the oxidation of hypoxanthine by the immobilized xanthine oxidase at pH 7.0 and 35‡C was monitored at 290 ran. Hypoxanthine concentrations as low as 4.4 Μmol/L can be detected. Up to 30 samples per hour can be analyzed at a flow rate of 1 mL/min, using 150 ΜL sample volumes and a bioreactor dimension of 1.0 cm x 2.0 mm id. Recovery yields were between 92 and 99%. Both within day and between day precisions gave CVs < 5.00% (n = 30). Good correlation (r = 0.998) is obtained when 78 fish samples were analyzed for their hypoxanthine content both by this FI method and a reference HPLC method.     

Three-way partial least-squares regression for the simultaneous kinetic-enzymatic determination of xanthine and hypoxanthine in human urine

The performance of three-way principal component analysis and three-way partial least-squares regression when applied to a complex kinetic-enzymatic system is studied, in order to investigate the analytical potential of the combined use of these chemometric technologies for non-selective enzymatic systems. A enzymatic-kinetic procedure for the simultaneous determination of hypoxanthine and xanthine in spiked samples of human urine is proposed. The chemical system involves two consecutive reactions catalyzed by xanthine oxidase (EC 1.17.3.2). This enzyme catalyzes the oxidation of hypoxanthine, first to xanthine and then to uric acid, a competitive inhibitor of the reactions. The influence of uric acid during quantitative determination was considered in the design of the calibration set. The sample and enzyme solution were mixed in a stopped-flow module and the reaction was monitored using a diode array spectrophotometer. The recorded data have an intrinsical three-component structure (samples, time and wavelength). This data array was studied via three-way principal component analysis and was modeled for quantitative purposes using a three-way partial least-squares calibration procedure. Results are compared with those obtained by applying classical bilinear PLS to the previously unfolded data matrix.     

Interaction between ozone and the chloride ion in sulfuric acid solutions up to 6-M concentration

The effect of sulfuric acid concentration on Cl₂ evolution in the reaction between O₃ and Cl^? has been investigated. The catalytic effects of metal ions in this reaction have been studied as a function of solution acidity. The chlorine evolution rate increases markedly with increasing acid concentration. At acid concentrations below 4 mol/l, the most effective catalyst is Co²⁺. The catalytic activities of Fe³⁺ and Cu²⁺ peak at $C_{H_2 SO_4 } $ = 4.8 mol/l. In passing to highly acidic solutions ( $C_{H_2 SO_4 } $ > 5 mol/l), the catalytic activity of the metal ions decreases, but the chlorine evolution rate remains high owing to the high acidity. Kinetics of VO²⁺ oxidation with ozone in acid media have been studied, and the ozone solubility in aqueous sulfuric acid has been measured.     

Separation of six purine bases by capillary electrophoresis with electrochemical detection

Capillary zone electrophoresis with electrochemical detection (ED) has been employed for the separation and determination of adenine (A), guanine (G), theophylline (Thp), hypoxanthine (HX), xanthine (Xan) and uric acid (UA). Effects of several important factors such as the acidity and concentration of running buffer, separation voltage, injection time and detection potential were investigated to acquire the optimum conditions. The detection electrode was a 300 μm carbon disc electrode at a working potential of +0.95 V (versus saturated calomel electrode (SCE)). The six purine bases can be well separated within 14 min in a 40 cm length fused-silica capillary at a separation voltage of 10 kV in a 100 mmol/l borate buffer (BB, pH 10.0). The current response was linear over about three orders of magnitude with detection limits (S/N=3) ranging from 0.157×10⁻⁶ to 0.767×10⁻⁶ mol/l for all compounds. The proposed method was successfully applied to determine Thp in tea and aminophylline tablets, UA in human urine, and two purine bases in DNA.     

A novel anti-interference and pH-modulation device: application to enzyme-free glucose detection

We report on a novel anti-interference and pH-modulation device (herein after referred to as ??device??). It is based on electrodialysis and can continuously increase the pH value of the carrier solution and - at the same time - remove interfering analytical signals obtained for ascorbic acid (AA) and uric acid (UA). The ??device?? was coupled to the FIA-amperometric detection of glucose. The linear range is from 1???mol?L^?1 to 0.4?mmol?L^?1, with a sensitivity of 213???A?cm^?2?mM^?1 and a detection limit of 1???mol?L^?1 at a signal-to-noise ratio of 3. The method was used to sucessfully determine glucose in serum. This study represents a novel technique for overcoming analytical interference and is expected to find applications in liquid chromatography, for example in on-line pH-modulation if different pH values are needed for separation and detection.

Reactions of halogenated organic peroxyl radicals with various purine derivatives,tyrosine, and thymine: A pulse radiolysis study

Absolute rate constants for the one electron oxidation of guanine, guanosine, uric acid, xanthine, hypoxanthine, tyrosine, and thymine by various halogenated peroxyl radicals in aqueous solutions have been determined using the technique of pulse radiolysis. Roughly, linear correlations have been observed between the logarithm of these rate constants and Taft's inductive parameter (σ*) for the radicals. However, the rate constants for the radical CBr₃O are slightly higher than those for CCl₃O for most of these compounds. © John Wiley & Sons, Inc.     

Analysis of seven purines and pyrimidines in pork meat products by ultra high performance liquid chromatography–tandem mass spectrometry

An ultra high performance liquid chromatography–tandem mass spectrometry method (UPLC–MS/MS) is proposed for the simultaneous quantification of inosine, adenosine, guanosine, uridine, hypoxanthine, xanthine and uric acid in pork meat, dry-cured and cooked ham. Samples were added with ¹⁵N₂-xanthine (internal standard) and extracted with boiling water for 30 min. Supernatants were washed with hexane, added with formic acid 10% in water, methanol:acetone (1:1, v/v), evaporated to dryness under N₂, and finally re-dissolved in water prior to injection. Chromatographic separation was carried out with a HSS T3 column with a total time of analysis of 15 min. Two specific transitions for each compound were used for identification and quantification (with matrix matched calibration curves). Linearity, limit of detection, repeatability and accuracy were evaluated. The method was used to quantify the seven purines and pyrimidines in 15 commercial samples.     

Adsorption of Cl? ions on electrodeposited rhodium black layer (rhodized electrode) at low concentrations

The adsorption of Cl^– ions on rhodium black layer (rhodized electrodes) was studied by radiotracer technique at low Cl^– ion concentrations (c10^–5 mol dm^–3) in 1 mol dm^–3 H₂SO₄ supporting electrolyte. The specific adsorption of Cl^– ions was treated in terms of partition between solution phase and electrodeposited Rh black layer. The potential dependence of the partition coefficient is determined.     

Combined effect of organic modifier concentration and column temperature on retention in reversed-phase ion-pair liquid chromatography

Summary The retention behavior of phenylamine and naphthylamine sulphonic acid was evaluated in reversed-phase ion-pair liquid chromatography as a function of organic modifier concentration and column temperature. It has been observed that the logarithm of capacity factors decrease linearly with organic modifier concentration, and there is a good linear relationship between the intercept and slope for this relationship. Phenylamine and naphthylamine sulphonic acid retention decreases with increase in column temperature. A linear dependence of lnk _ip on the reciprocal of the absolute temperature, the Van't Hoff plot, was observed over the column temperature range studied, and the standard enhalpic change (H^o) for these sulphonic acid transfers from the mobile phaser to the stationary phase was determined. H^o was dependent on the solute structure and in the range from 2.5 Kcal/mol to 5.5 Kcal/mol, which is close to that observed in RP-HPLC. The enthalpy/entropy compensation effect was evaluated by plotting lnk _ip(T) vs. –H^o, and the apparent differences in retention mechanisms between the analytes were observed, which may arise from the significant differences in their configuration, hydrophobicity and the charges of the solutes as well as the complex retention processes of RP-IPC.     

New books

Summary Micromolar analyses of the nitrogen species NH₃, NO ₂ ^– , and NO ₃ ^– in soil and other samples are usually accomplished by extracting several samples and testing each for a different species. This procedure is not viable when the quantity of the initial sample is limited. An improved method of separating and analysing for ammonia NH₃(aq), nitrite NO ₂ ^– (aq), and nitrate NO ₃ ^– (aq) from a single small sample with concentrations of 0–50 mol/l is reported. No interferences or carryovers among the three nitrogen containing species were found. Uncertainties were ±2–5 mol/l and accuracies with respect to standards were ±3 mol/l.     

The electrochemistry of uric acid at a gold electrode modified with L-cysteine, and its application to sensing uric urine

The electrochemistry of uric acid at a gold electrode modified with a self-assembled film of L-cysteine was studied by cyclic voltammetry and differential pulse voltammetry. Compared to the bare gold electrode, uric acid showed better electrochemical response in that the anodic peak current is stronger and the peak potential is negatively shifted by about 100 mV. The effects of experimental conditions on the oxidation of uric acid were tested and a calibration plot was established. The differential pulse response to uric acid is linear in the concentration range from 1.0?×?10^?6 to ~?1.0?×?10^?4 mol?L^?1 (r?=?0.9995) and from 1.0?×?10^?4 to ~?5.0?×?10^?4 mol?L^?1 (r?=?0.9990), the detection limit being 1.0?×?10^?7 mol?L^?1 (at S/N?=?3). The high sensitivity and good selectivity of the electrode was demonstrated by its practical application to the determination of uric acid in urine samples.

Simultaneous radiochemical determination of237Np and239Np with235Np as a tracer,and application to environmental samples

Neptunium is sorbed, together with plutonium, uranium and iron on Bio Rad AG 1×4 anion exchange resin from 9 mol/1 HCl, eluted with 7 mol/l HNO₃ and 1.2 mol/l HCl, purified on a second, identical column and electrodeposited on stainless steel discs.²³⁷Np is determined by -spectrometry,²³⁹Np by -spectrometry with a Ge detector, or via its 14.3 keV LX-rays in a Si(Li) measurement. The neptunium yield is determined from the count rate of the 13.6 keV LX rays of the tracer²³⁵Np in a later Si(Li) measurement. The average chemical yield is 78±11%. The detection limit for²³⁷Np is 1 mBq. The procedure allows the sequential determination of neptunium, together with plutonium, strontium, uranium and iron from one sample. The method was applied to air and total deposition samples from Munich-Neuherberg and to sediment samples from the Irish sea.