V. Concialini

Electrochemical detection in the high-performance liquid chromatographic analysis of plant phenolics.

The electrochemical behaviour of several phenolic compounds of biological interest in vegetable materials was studied. Oxidation potentials of the examined compounds and voltammetric curves of some representative phenols are reported. As a practical application of electrochemical detection in HPLC, the analysis of monomeric units emanating from nitrobenzene oxidative hydrolysis of wheat straw lignin is described. Electrochemical detection simplifies the analysis, permitting the direct injection of the reaction mixture without interference from nitrobenzene. Aniline formed by the reduction of nitrobenzene is also detectable and may be used as an oxidisability index of the vegetable matter.

Differential pulse polarographic determination of the herbicides atrazine, prometrine and simazine.

A differential pulse polarographic method, using the dropping mercury electrode for the determination of the herbicides atrazine, prometrine and simazine is described. The optimum pH is 2. The limit of detection is 8 x 10(-8)M, corresponding to about 15 mu/l. The electrochemical behaviour of the compounds on glassy-carbon and mercury-coated glassy-carbon electrodes was also examined with a view to its use for electrochemical detection of the herbicides after their separation by HPLC.

Optimization of electrochemical detection in the high-performance liquid chromatography of lignin phenolics from lignocellulosic by-products.

Free phenolic acids and aldehydes (p- and o-hydroxyphenylacetic acid, p-hydroxybenzoic acid, p-hydroxybenzaldehyde, vanillic acid, vanillin, syringic acid, p-coumaric acid, syringaldehyde and ferulic acid) were detected in wheat straw extracts by high-performance liquid chromatography with a dual-cell electrochemical detector operated in the redox mode. Phenolics were oxidized with coulometric efficiency in the first cell (+ 1.00 V), then detected by reduction in the second cell (-0.20 V). Compared with the oxidative mode, the reductive detection mode has the advantage of being unaffected by large amounts of interferents eluting at the front of the chromatogram that interfere with the detection of small and early eluting compounds. Hydrodynamic voltammograms in the oxidative, reductive and screen-out modes are presented and the corresponding detection limits for real sample are discussed. Perfect linearity of response was found in the range 5 x 10(-7) - 5 x 10(-5) M and detection limits were of the order of 50-500 fmol injected.

HPLC characterization of phenolics in lignocellulosic materials

SummaryThe application of HPLC with an electrochemical detector for the determination of phenolics in lignocellulosic materials is reported. The separation of phenolic acids and aldehydes (gallic acid, p-hydroxybenzoic acid, vanillic acid, p-coumaric acid, syringic acid, ferulic acid, vanillin, syringaldehyde and p-hydroxybenzaldehyde) on two different columns (reversed phase C6 and styrene-divinylbenzene PLRP-S) is shown. Chromatograms of phenolic compounds in neutral, basic and oxidative extracts of wheat straw treated with NaOH and white rot fungusStropharia rugosoannulata are reported along with quantitative results.

Self-protonation mechanism in the electroreduction of quinolinols

Abstract The electroreduction of 8-, 5- and 2-hydroxyquinoline (8-, 5- and 2-QOH, respectively) was studied in DMF. Cyclic voltammetry, polarography, controlled potential coulometry and UV-visible spectroscopy were used to provide information on the stoichiometry and kinetics of the process. The electroreduction of 8- and 5-QOMe, i.e. the aprotic analogues of 8- and 5-QOH, was also investigated for comparison. The main feature of the reduction process is a fast proton transfer from the parent compounds to the electrogenerated basic intermediates (self-protonation mechanism), with the formation of the conjugate base of the former, QO − , together with the two-electron reduction product QH 2 OH. The rate constant of the self-protonation step for the primary radical anion was calculated to be 1.0x10 5 M −1 s −1 for 2-QOH, whose reduction becomes reversible at high scan rates. For 8- and 5-QOH the corresponding rate constants were evaluated to be almost diffusional, in agreement with the higher acidity of these quinolinols with respect to 2-QOH. The electrode reaction mechanism of the conjugate base QO − , reducible at very negative potentials, is discussed.

Preliminary studies for the differential-pulse polarographic determination of a new class of herbicides: sulphonylureas

The electrochemical behaviour of four sulphonylureas (chlorsulfuron, metsulfuron methyl, DPX-M6316 and chlorimuron ethyl) used as active compounds in commercial herbicides was studied. Methanol-water solutions of the compounds were analysed by differential-pulse polarography. The dependence of peak potentials and peak height on the pH is reported. Detection limits of the order of 10–8M were obtained, working at a pH of between 2 and 3. Attempts to use a mercury-coated glassy carbon electrode, in view of the possible applications to high-performance liquid chromatography with electrochemical detection, gave unsatisfactory results due to the interference from hydrogen ion reduction.

Photocurrents produced by sulphur hexafluoride in aqueous solutions

Summary The photocurrent produced by u.v. irradiation of mercury in contact with aqueous solutions containing SF 6 has been studied. At potentials more negative than −10. V vs . SCE the electron transfer number at neutral pH equals 8 pointing to the reduction of SF 6 to S 2− and F − ions. The photocurrent mechanism parallels the mechanism in pulse radiolysis in its first two main steps but in the electrochemical case the intermediate SF 4 is destroyed by the electrode at negative potentials. The formation of OH radicals by reaction of the electron adduct with water has been confirmed by studying the influence of ethanol on the photocurrent. At pH>7 the photocurrent falls due to hydrolysis of SF 4 but the data suggest some complication such as the formation of OH prior to the formation of SF 4 in the multi-step reaction SF 5 ⊸ + 2 H 2 O → OH + H 3 O + + SF 4 + F −

Separation and determination of traces of metallic impurities in silver halides

Abstract A method for the enrichment and determination of elements present in trace amounts in high-purity silver halides is described. Concentrated solutions of potassium iodide are used as solvents for the samples. The elements determined are concentrated by solvent extraction: zinc, cadmium, lead, copper, cobalt and nickel as dithizonates, and iron, aluminium and manganese as oxinates. The complexes formed are then determined spectrophotometrically. The relative lower limits of detection are of the order of 1 p.p.m. with an accuracy better than 10%.

The use of phosphate to generate H-atoms at pH 5–8 as determined by photocurrents: electrochemical properties of H-atoms

The electro-chemical properties of H-atoms at pH 5–8 have been studied based upon photocurrents following the interaction of hydrated electrons with phosphate in aqueous solution. The rate constants for the interaction of eaq− with H2PO4− and HPO42− were determined to be 1.9 × 107 dm3 mol−1 s−1 and <5 × 105 dm3 mol−1 s−1 in agreement with the values determined by pulse radiolysis. From the dependence of the electron transfer number on applied potential, the half-wave potentials for reduction of H-atoms were determined at various pH values. At pH 7.0, the half-wave potential was determined to be −1.26 V using SCE as reference. These findings emphasise the use of phosphate as a potentially useful method of generating H-atoms at neutral pH.

Photocurrents produced by chlorate, bromate and iodate ions in aqueous solutions

Abstract The photocurrents produced by UV irradiation of mercury in contact with aqueous solutions of chlorate, bromate and iodate ions have been studied. The photocurrent data agree fairly well with previous pulse radiolysis data, except for chlorate ions for which we found no reaction with hydrated electrons. For iodate and bromate ions possible mechanisms following the electron capture are given.