Determination of selenium, zinc and cadmium in antidandruff shampoos by atomic spectrometry after microwave assisted sample digestion

Microwave assisted pre-treatments for atomic spectrometric determination (inductive coupled plasma-optical emission spectrometry, ICP-OES or flame atomic absorption spectrometry, FAAS) of metallic elements, usually present in antidandruff shampoos, are proposed. They are based on the digestion of the sample with HNO₃ into a closed reactor, which is irradiated at 800 W for a few minutes. Selenium was determined by ICP-OES. The limit of detection was 0.11 mg l⁻¹; the relative standard deviation (R.S.D.) for the selenium content in the samples was in the 0.6–3.6% range. The results obtained were in agreement with the label contents and the recovery of the proposed method was in the 100–106% range. Zinc and cadmium were determined by FAAS. The limit of detection for zinc determination was 0.078 mg l⁻¹; the R.S.D. for zinc contents was in the 0.8–8.6% range. A limit of detection of 0.09 mg l⁻¹ was obtained for cadmium determination; the R.S.D. for cadmium contents was in the 0.7–2.7% range. The determinations were performed after two different sample mineralization pre-treatments — dry ashing (in an electric furnace) and wet mineralization (in a microwave oven). Both methodologies provided comparable results for zinc and cadmium determination in shampoos. The proposed microwave assisted digestion procedures allow a precise and accurate determination of selenium, zinc and cadmium in commercial antidandruff shampoos, and the sample pre-treatment is less time-consuming than the classic methods.     

Cyphostemmins A-B, two new antifungal oligostilbenes from Cyphostemma crotalarioides (Vitaceae)

Two new antifungal resveratrol dimers, cyphostemmins A-B (1–2), have isolated from the roots of Cyphostemma crotalarioides planch (Vitaceae) together with resveratrol 3 and previously known reseveratrol dimers (4–7). Structures of these new compounds have been established on the basis of their MS and ¹H and ¹³C NMR spectroscopic data.     

Fused heterocycles. 8 . An efficient procedure for the stereoselective synthesis of trans-2,3,3a,4-tetrahydro-3-aryl-2-phenyl[1]benzopyrano[4,3-c]pyrazoles and their [1]benzothiopyrano analogues

Stereoselective synthesis of trans-2,3,3a,4-tetrahydro-3-aryl-2-phenyl[1]benzopyrano[4,3-c]pyrazoles 13–18 and their [1]benzothiopyrano analogues 19–24 has been performed by the reaction of 3-arylidenechromanones 1–6 and 3-arylidene-1-thiochromanones 7–12 with phenylhydrazine in hot pyridine. The structure and stereochemistry of the compounds prepared have been elucidated by ir, ^lH and ¹³C nmr measurements.     

A novel sequential aminodiene Diels-Alder strategy for the rapid construction of substituted analogues of Kornfeld's ketone

Through a novel sequence of aminodiene Diels-Alder reactions, amidofurans 18a-c were converted to tricyclic ketones 21a-c in moderate to good yields. Ketone 21a could be converted to Uhlé's ketone (6) by cleaving the tert-butyl carbamate and oxidatively removing the methyl ester. Tricycle 21a readily underwent bromination to give 22. Formation of the corresponding enol triflate 25 followed by carbonylation gave ester 27, which was then coupled with N-methyl propriolamide to furnish 26. [reaction: see text]     

Influence of crystallite size on acetone hydrogenation over copper catalysts

Acetone hydrogenation was studied over a family of Cu/SiO2 catalysts as well as UHP Cu powder and a Cu chromite catalyst. Oxygen chemisorption via dissociative N2O adsorption was used to count surface Cu atoms and calculate crystallite sizes, and a microwave absorption technique was used to measure the electrical conductivity of these Cu particles. Under differential reaction conditions at 423 K and 1 atm, all catalysts exhibited deactivation on stream and activities were typically 10-20% of their initial values after 3-4 h on stream. However, initial turnover frequencies (TOFs) varied from 0.056 s(-1) on the most highly dispersed Cu catalyst to 0.50 s(-1) on Cu powder, with the highest TOF of 2.4 s(-1) occurring on 110 nm crystallites. A similar trend with a broader (80-fold) variation was observed in the "steady-state" TOF values. Apparent activation energies varied between 11 and 14 kcal/mol. These initial TOF values are in good agreement with previous results, and a correlation exists between TOF and Cu crystallite size in this reaction, which appears to be structure sensitive. In addition, the electrical conductivity of these dispersed Cu nanoparticles, which was always less than that of bulk Cu, also increased with increasing Cu crystallite size; consequently, the change in this parameter may offer a possible explanation for the increase in TOF.     

Correction to: Preface

Foundations of Computational Mathematics -