Determination of lead in seawater by flow-injection on-line preconcentration-electrothermal atomic absorption spectrometry after coprecipitation with iron(III) hydroxide.

A flow-injection on-line preconcentration-electrothermal atomic absorption spectrometric (ETAAS) method coupled with a coprecipitation method has been developed for the determination of lead in seawater. The combination of two preconcentration procedures, coprecipitation with iron(II) hydroxide and solid-phase extraction with a lead-selective resin, Pb-Spec, allowed the determination of lead at the ng kg(-1) level. Lead in 250 g of a sample solution was collected by coprecipitation with 10 mg of iron. The precipitate was dissolved in 25 ml of 1 mol l(-1) nitric acid; then, a 4-ml aliquot of the sample solution was introduced into the flow-injection system to preconcentrate and separate lead from iron on a Pb.Spec microcolumn. The sorbed lead was eluted with a 1.0 x 10(-4) mol l(-1) EDTA solution. The 30-microl portion of the eluate corresponding to the highest analyte concentration zone was injected into a graphite furnace. The overall enhancement factor was about 200 for 250 g of the sample. The average and standard deviation of ten blank values obtained were 1.7 ng and 0.38 ng, respectively. The recovery was 93.7 +/- 5.0% for seawater spiked with 20 ng kg(-1) lead. The proposed method is applicable to the analysis of seawater for lead at slightly higher levels.     

Zincate-type enolate for radical α-trifluoromethylation

Ketone zincate-type enolates can be applied to radical trifluoromethylation for the general synthesis of α-CF₃-ketones, cyclopentanones in particular. The addition of diethylzinc to lithium enolates is the key in the preparation of the zincate-type enolates for efficient radical trifluoromethylation.     

Structure of protonated thioformaldehyde

It is proposed, from an ab initio (4–31G) SCF calculation, that the most stable forms of protonated thioformaldehyde may be the C-protonated complexes H₃CS⁺. The energy-gradient technique was used for geometry optimization, and an energy decomposition analysis was carried out to elucidate the origin of this abnormal behavior of the C=S group.     

Electronic structure of C84 and KxC84: Comparison to C60 and graphite

X ray photoemission spectra show ～10 valence band features in C₆₀ and C₈₄ films, although they are less distinct in C₈₄ because of the increased number of valence electrons and the decreased molecular symmetry. The presence of inequivalent carbon atoms in C₈₄ is reflected by the fact that the C 1s main line is ～0.2 eV wider than in C₆₀. The C 1s satellite region of C₈₄ exhibits four features within 15 eV of the main line and shows that the HOMO-LUMO π-π* excitation energy of C₈₄ is ～0.5 eV smaller than in C₆₀. Potassium 2p spectra for K-doped C₈₄ films suggest the formation of a phase with composition K₃C₈₄ with two spectroscopically-distinguishable K sites. A new spectral feature emerges when the K content is increased beyond K₃C₈₄, suggesting a phase transition and a saturated composition of K₆C₈₄, as for K_xC₆₀. These similarities are intriguing since K_xC₈₄ is insulating for all x while K₃C₆₀ is metallic and superconducting.     

Synthesis of a versatile functionalized tetraquinane. Convenient access to 1,3-bishomopentaprismanedione

The tetracyclic diene dione 1 has been synthesized by a short oxidative decarboxylation route starting with the domino Diels-Alder adduct 2. Photocyclization of the bisketal of 1 was used to arrive expediently at 1,3-bishomopentaprismanedione (7).