Electrocatalytic oxidation of reduced nicotinamide coenzymes by graphite electrodes modified with an adsorbed phenoxazinium salt,meldola blue

Meldola Blue (7-dimethylamino-1,2-benzophenoxazine) can be adsorbed on graphite to give chemically modified electrodes. The electrochemical redox reactions of the phenoxazine are fairly reversible at low coverages with an E′^o of ?175 mV vs. SCE at pH 7.0. The electrode was most stable in acid solutions, at pH 6.0 its electrochemical activity decreased by 15% during 2h. The adsorbed compound mediated electron transfer in the electrocatalytic oxidation of the nicotinamide coenzymes (NADH and NADPH). The formation of a charge transfer complex between Meldola Blue and the coenzyme is demonstrated by experiments with a rotating disk electrode. The complex decomposes in a rate limiting step (k₊₂=30 s^?1) to the oxidized coenzyme and the reduced Meldola Blue. The latter can be reoxidized in a fast electrochemical step. The overall result is an electrocatalytic oxidation at a voltage which is about 500 mV lower than at an unmodified electrode.     

Controlled potential lodometric determination of nitrate after reduction to nitrite by coppered cadmium

A controlled-potential coulometric iodometric method previously developed for the accurate determination of small amounts of nitrite has been extended for the determination of nitrate after its reduction on a coppered cadmium reductor. The conditions for quantitative reduction have been investigated with respect to type of reductor and pH. Nitrate-nitrogen in the range 0.01-100 mug ml may be determined with high accuracy in less than 10 min, including the reduction step. The method has been applied with good results to a large variety of samples such as meat products, juices and waste waters.     

Trace metal ion preconcentration for Flame Atomic Absorption by an Immobilized N,N,N′-tri-(2-pyridyl-methyl)ethylene diamine (TriPEN) chelate ion exchanger in a flow injection system

Summary Trace Metal Ion P reconcentration for Flame Atomic Absorption by an Immobilized N,N,N-tri-(2-pyridylmetkyl)ethylene diamine (TriPEN) Chelate Ion Exchanger in a Flow Injection System Chelating groups of N,N,N-tri-(2-pyridyl-methyl)ethyIene diamine (TriPEN) were synthesized on the surface of porous glass. A 100l column of the chelate ion exchanger was incorporated into a flow injection manifold with a flame atomic absorption spectrometer. Metal ions from samples (<1 ml to>80 ml) were retained on the chelate ion exchanger until an injection of acid displaced them into the flame. A peak-shaped absorbance curve was recorded. The use of immobilized TriPEN for preconcentration of metals from dilute solution and for matrix modifications was demonstrated.     

On a free-electron-laser in a uniform magnetic field. A solution for arbitrarily strong electromagnetic radiation fields

Exact expressions are derived for the gain of a free-electron laser based on a uniform longitudinal magnetic field configuration operating in the single-particle, low-gain regime. The gain is calculated for different parameters of the system. For strong enough a field, the gain decreases with an increase of the amplitude, becoming negative when passing a threshold value, which depends on the system parameters. Implications regarding the saturation of the lasing process are discussed.     

Controlled-potential iodometric titration of nitrite. Application to the determination of nitrite in meat products

A controlled-potential coulometric method using iodine as an intermediate has been devised for the determination of nitrite. Nitrite is reduced by iodide and the iodine formed is then reduced coulometrically. The reduction of nitrite in the pH range 0-6 has been studied and the optimum conditions for an accurate determination are stated. The time of analysis for a determination in the range 0.005-5 mg of nitrite is about 2-5 min and the error +/- 0.1%. The method is applied to the determination of nitrite in some meat products.     

Controlled-potential back-titration with electrogenerated iodine as an intermediate: application to the determination of thiols

A controlled-potential electroanalytical method has been devised for the accurate determination of small amounts of thiols. An adequate excess of iodine is generated, the sample is added and the unreacted iodine is then electrolytically reduced. The iodine consumed is calculated from the measured number of coulombs. Amounts of thiols in the range 0·2–5 mg may be determined with an error of ± 0.05 %. A coulometric cell has been constructed which permits very high electrolysis rates, owing to efficient stirring by a large rotating platinum electrode combined with a high electrode-area to solution-volume ratio. A constant of 0·11 sec⁻¹ is found for the reduction of iodine. The necessity of careful control of background currents and current efficiency in coulometric determinations is discussed.     

Coulometric determination of dissolved oxygen in water

The oxygen content of air-saturated distilled water has been determined at between 10 and 40 degrees by using a controlled-potential coulometric method based on an earlier published method for the iodometric determination of nitrite. The maximum error for the determinations was +/- 0.3% over the whole range, and the time of analysis about 3 min. An equation is given for the solubility in the measured range, and some thermodynamic functions are calculated.     

Immobilisation and evaluation of a vancomycin chiral stationary phase for capillary electrochromatography

The macrocyclic antibiotic, vancomycin, is covalently bonded to LiChrospher diol silica packed columns and evaluated in capillary electrochromatography (CEC) both in the reversed-phase and polar organic mode. Initially, capillaries were packed with 5 microm LiChrospher 100 A diol silica and evaluated in CEC with a reversed-phase biphenyl-pyrene achiral test resulting in resolution and efficiency values of ca. 2.5 and 100000 plates meter(-1), respectively. Repeatability for this test (resolution and efficiency) was also examined and found to be acceptable for both run-to-run (n=5, 0.74% and 1.5%) and column-to-column (n=5, 3.4% and 9.0%), respectively. Similar results were obtained when the 10 microm LiChrospher 1000 A diol silica was examined with the exception of efficiency, where a reduced plate height value of four times lower was obtained compared to the 100 A material. A simple three step in-situ vancomycin immobilisation procedure was subsequently carried out on these packed diol columns. Selectivity was obtained for thalidomide enantiomers on this vancomycin chiral stationary phase in reversed-phase CEC with resolution and efficiency values of ca. 2.5 and 80000 plates meter(-1), with acceptable repeatability (n=8) 0.9% and 3.0%, respectively. Selectivity was also obtained for thalidomide enantiomers on this phase in the polar organic mode with resolution and efficiency values of ca. 2.5 and 120000 plates meter(-1), with acceptable repeatability (n=7) 0.9% and 2.0%, respectively. It was possible to deduce from a chemometric design carried out for evaluating the mobile phase component effects that organic modifier ratio, MeOH/MeCN, played a significant role in controlling both resolution and efficiency. It was also possible to separate a number of basic analytes including four beta-adrenergic blocking agents in the polar organic mode albeit with lower resolution and efficiency values, ca. 1.5 and 45000 plates meter(-1), respectively.