Determination of Phenolic Acids in Sugarcane Vinasse by HPLC with Pulse Amperometry.

A reversed-phase liquid chromatographic separation with pulsed amperometric detection of phenolic acids at a glassy carbon electrode is described. Chromatographic separation was carried out in isocratic conditions using 0.20 mol·L-1 acetic acid (pH 5.0)/water (80 : 20, v/v) as mobile phase under constant working potential mode of 0.80 V. Chromatographic peaks presented high resolution and separation. Calibration curves exhibited excellent correlation coefficients, above 0.995. Linear ranges of the analytes, in mg L-1, were of 0.018-18 (gallic acid), 0.146-19 (vanillic acid), 0.13-17 (caffeic acid), 0.016-16 (ferulic acid), and 0.008-17 (p-coumaric acid), respectively. Limits of detection ranged from 1.6 to 97 µg·L-1 and precision varied in 1.73-3.78% interval. Concentrations of 19 ± 0.51 mg·L-1 and 7.8 ± 2.5 mg·L-1 were found for vanillic and caffeic acids, respectively, in a sugarcane vinasse sample. Gallic, ferulic, and p-coumaric acids were not detected. Recovery results demonstrated that the proposed method is accurate, and it can be used to detect and quantify phenolic acids in sugarcane vinasse without any influence of interferents.

Flow injection amperometric detection of ascorbic acid using a Prussian Blue film-modified electrode.

The PB film-modified electrode was used as an amperometric detector for flow injection analysis of ascorbic acid. The modified electrode detector showed good sensitivity, stability and reproducibility. The calibration curve for ascorbic acid was linear over the concentration range from 5.0x10(-6) to 1.0x10(-3) mol l(-1) with a slope of 19.9 mA mol(-1) per litre and a correlation coefficient of 0.999. The detection limit of this method was 2.49x10(-6) mol l(-1). The relative standard deviation of six replicate injections of 2.5x10(-4) mol l(-1) ascorbic acid was 2.5%. The results obtained for ascorbic acid determination in pharmaceutical products are in good agreement with those obtained by using the procedure involving the reaction between triiodide and ascorbic acid.

Potentiometric determination of potassium cations using a nickel(II) hexacyanoferrate-modified electrode.

Electroactive nickel(II) hexacyanoferrate (NiHCF) thin film modified electrodes are effective potentiometric sensors for the determination of potassium ions. The NiHCF films are deposited onto glassy carbon electrodes by repetitive potential cycling in K(3)Fe(CN)(6)/NaNO(3)/Ni(NO(3))(2) solution. The modified electrodes exhibit a linear response to potassium ions in the concentration range 1x10(-3) to 2.0 mol dm(-3), with a near-Nernstian slope (45-49 mV per decade) at 25 degrees C. In the determination of potassium ion in syrups used for treatment of potassium deficiency, the NiHCF-modified electrode gave comparable results to those obtained using flame emission spectrophotometry.

Simultaneous determination of trace amounts of zinc, lead and copper in rum by anodic stripping voltammetry.

The determination of traces of Zn, Pb and Cu in rum samples by anodic stripping voltammetry without previous treatment or addition of a supporting electrolyte, using a hanging mercury drop electrode, is described. The choice of an appropriate stripping voltammetric method and deposition potential minimizes the influence of the organic content and ensures good reproductibility of the measurements. The reliability of the method was tested by comparing the results with those given by absorption spectrometry, the differences being about 10%. The method allows heavy metal ions to be determined in the mug l(-1) concentration range.

Determination of cinnamic acid in human urine by differential-pulse polarography.

The differential-pulse polarographic behaviour of cinnamic acid was studied in acetate and phosphate buffer solutions (pH 3.5-7.5). The reduction mechanism is discussed. The drug can be determined at pH 5.0 over the concentration range 5 x 10(-5)-1 x 10(-3) mol l-1. The effect of tetraalkylammonium salts on the electroanalytical determination of cinnamic acid was investigated. The direct determination of the drug (0.7-5.5 micrograms ml-1) in urine samples diluted with acetate buffer (pH 5.0) can be effected in the presence of 1 x 10(-3) mol l-1 cetyldimethylethylammonium bromide solution. The detection limit was found to be 0.1 microgram ml-1. The relative standard deviation from six determinations at the 5.5 micrograms ml-1 level was 1%.

Cyclopropamitosenes, novel bioreductive anticancer agents. Synthesis, electrochemistry, and biological activity of 7-substituted cyclopropamitosenes and related indolequinones.

The synthesis of the indolequinones 8 and 9 starting from methyl 4-(benzyloxy)-5-methoxy-indole-2-carboxylate (10) is described. The methoxy group in the indolequinones 1, 2, 4, 5, and 7-9 can be displaced by various nitrogen nucleophiles (ammonia, 2-methoxyethylamine, aziridine, 2-methylaziridine, pyrrolidine) in 22-88% yield. The resulting amino-substituted quinones, together with their methoxy precursors, were studied by cyclic voltammetry to determine their reduction potentials, which, in DMF solution, lie in the range -1.355 to -1.597 V (vs ferrocene). The cytotoxicity of the compounds towards aerobic and hypoxic mammalian cells was also determined; in general, under aerobic conditions, the cyclopropamitosenes are more toxic than the corresponding pyrrolo[1,2-a]indolequinones, which are in turn more toxic than the simple 1,2-dimethylindolequinones, with many of the compounds in each series showing greater toxicity toward hypoxic cells.