Elucidation of the vicarious nucleophilic substitution of hydrogen mechanism via studies of competition between substitution of hydrogen, deuterium, and fluorine.

Relations of rates of the vicarious nucleophilic substitution of hydrogen (VNS) and S(N)Ar substitution of fluorine in 2-fluoronitrobenzenes with chloroalkyl aryl sulfone carbanions were determined from competitive experiments carried out at various concentrations of base. The observed dependence of the VNS/S(N)Ar rate ratio on the base concentration confirmed the two-step mechanism of the VNS, which consists of reversible formation of sigma(H) adducts of the alpha-chlorocarbanion to nitroarene, followed by base-induced beta-elimination of HCl. It was also evidenced that both of these processes can be the rate-limiting steps: the beta-elimination at low base concentration and the nucleophilic addition at high base concentration. Consistent with that conclusion is the finding that the kinetic isotope effect in the VNS reaction decreases from 4.2 (a value typical of a primary KIE) to 0.8 (a value typical of a secondary KIE) with increasing base concentration. Also reported is our discovery that the S(N)Ar substitution of the 2-fluoronitrobenzenes studied in this work was subject to base catalysis under some of the experimental conditions employed in our competitive experiments.     

Decomposition of Toluene and Acetone in Packed Dielectric Barrier Discharge Reactors

The influences of TiO₂ catalytic material and glass pellet packing on the decomposition efficiency of toluene and acetone in air by dielectric barrier discharge (DBD) reactors were experimentally investigated in this study. The effects of both packing materials on the formation of byproducts such as CO and CO₂ were also evaluated. Experimental results indicate that the introduction of glass materials into the plasma zone of a wire-tube reactor would improve the decomposition efficiency of toluene and acetone compared to a nonpacked reactor. The apparent decomposition rate constant of a glass packed-bed reactor was 4.5–4.8 times greater than that of a nonpacked reactor. The results also indicate that the decomposition rate constant of toluene was approximately 2.6 times higher than that of acetone no matter which type reactor was utilized. The application of TiO₂ coated pellets in DBD reactors will enforce the hydrocarbon byproducts to further be oxidized to CO₂, notwithstanding, it will not significantly improve the performance of the reactors in the decomposition of toluene and acetone, and in the formation of CO. The results show that the best selectivity of CO₂ for acetone decomposition in a TiO₂ coated pellets packed-bed reactor was approximately 40% higher than that in a glass packed-bed reactor.     

Development and application of a simple routine method for the determination of selenium in serum by octopole reaction system ICPMS

The aim of the study was to develop an inductively coupled plasma mass spectrometry (ICPMS) method for robust and simple routine determination of selenium in serum. Polyatomic interferences on ⁷⁶Se, ⁷⁷Se, and ⁷⁸Se were removed by applying an octopole reaction system ICPMS with the reaction cell pressurized with H₂ gas. We developed a novel simple optimization routine for the H₂ gas flow based on a signal-to-noise ratio (SNR) calculation of the selenium signal measured in a single selenium standard. The optimum H₂ flow was 2.9 mL min^–1. The selenium content in serum was determined after a 50-fold dilution with 0.16 M HNO₃ and quantified by using addition calibration and gallium as an internal standard. The method detection limit was 0.10 g L^–1 for ⁷⁶Se and ⁷⁸Se and 0.13 g L^–1 for ⁷⁷Se. Human serum samples from a case-control study investigating if selenium was associated with risk of colorectal adenoma were analyzed. The average selenium concentration for the control group (n=768) was 137.1 g L^–1 and the range was 73.4–305.5 g L^–1. The within-batch repeatability (a batch is ten samples) estimated from 182 replicate analyses was 6.3% coefficient of variation (CV), whereas the between-batch repeatability was 7.4% CV estimated from 361 replicates between batches. The method accuracy was evaluated by analysis of a human serum certified reference material (Seronorm Serum level II, Sero A/S, Norway). There was a fairly good agreement between the measured average of 145±3 g L^–1 (n=36) and the certified value of 136±9 g L^–1. In addition the method was successfully applied for analysis of zinc serum concentrations without further optimization. For the Seronorm certified reference material a value of 911±75 g L^–1 (n=31) for zinc was obtained, which corresponds well to the certified zinc value of 920±60 g L^–1.     

High-resolution solid-state MAS 73Ge NMR spectra of some organogermanes

High-resolution solid-state magic angle spinning (73)Ge NMR spectra of some organogermanium compounds were measured. Most tetrasubstituted germanes with identical substituents exhibited signals except for one case. Tetrasubstituted germanes with two kinds of different but somewhat similar substituents exhibited broad peaks. Trisubstituted germanes failed to show signals, indicating the importance of symmetry around germanium.     

Determination of trace metals by ICP-OES in plant materials after preconcentration of 1,10-phenanthroline complexes on activated carbon

In this work, 1,10-phenanthroline was used as a complexing agent for the separation and preconcentration of Cd(II), Co(II), Ni(II), Cu(II) and Pb(II) on activated carbon. The metals were adsorbed on activated carbon by two methods: static (1) and dynamic (2). The optimal condition for separation and quantitative preconcentration of metal ions with activated carbon for the proposed methods was for (1) in the static methods in the pH range 7-9. The desorption was found quantitative with 8 mol dm⁻³ HNO₃ for Cd(II) (92.6%), Co(II) (95.6%), Pb(II) (91.0%), and with 3 mol dm⁻³ HNO₃ for Cd(II) (95.4%), Pb(II) (100.2%). The preconcentration factor was 100 with R.S.D. values between 1.0 and 2.9%. For (2), the dynamic method (SPE), the pH range for the quantitative sorption was 7-9. The desorption was found quantitative with 8 mol dm⁻³ HNO₃ for Cd(II) (100.6%), Pb(II) (94.4%), and reasonably high recovery for Co(II) (83%), Cu(II) (88%). The optimum flow rate of metal ions solution for quantitative sorption of metals with 1,10-phenanthroline was 1-2 cm³ min⁻¹ whereas for desorption it was 1 cm³ min⁻¹. The preconcentration factor was 50 for all the ions of the metals with R.S.D. values between 2.9 and 9.8%.The samples of the activated carbon with the adsorbed trace metals can be determined by ICP-OES after mineralization by means of a high-pressure microwave mineralizer. The proposed method provides recovery for Cd (100.8%), Co (97.2%), Cu (94.6%), Ni (99.6%) and Pb (100.0%) with R.S.D. values between 1.2 and 3.2%.The preconcentration procedure showed a linear calibration curve within the concentration range 0.1-1.5 μg cm⁻³. The limits of detection values (defined as “blank + 3s” where s is standard deviation of the blank determination) are 5.8, 70.8, 6.7, 24.6, and 10.8 μg dm⁻³ for Cd(II), Pb(II), Co(II), Ni(II) and Cu(II), respectively, and corresponding limit of quantification (blank + 10s) values were 13.5, 151.3, 20.0, 58.9 and 33.2 μg dm⁻³, respectively.As a result, these simple methods were applied for the determination of the above-mentioned metals in reference materials and in samples of plant material.     

Investigation of Cd(II), Pb(II) and Cu(I) complexation by glutathione and its component amino acids by ESI-MS and size exclusion chromatography coupled to ICP-MS and ESI-MS

The elucidation of structures of glutathione (GSH) complexes play an important role in the fundamental understanding of biochemical pathways of metal ion deactivation in plants. This article attempts to feature key studies for stoichiometry of metal complexes with glutathione and its constituent amino acids to obtain a better understanding of the different metal affinities of the complexation sites of glutathione. The SEC-ICP-MS experiments have indicated that oxidation process of glutathione was accelerated by metal ion presence in following order Cu⁺, Pb²⁺ and Cd²⁺. The redox activity of metal ions was confirmed by ESI-MS experiments, which allowed to observe formation of glutathione disulphide (GSSG) in time. The stoichiometry of Cd²⁺, Cu⁺ and Pb²⁺ complexes with GSH was defined by observing the isotope pattern of investigated metals and hydrogen loss or transfer during binding. The complexes with metal bound to sulphur of 1:1 and 1:2 stoichiometry were found in case of cadmium and lead. The number of hydrogen atoms lost during metal binding and the SEC-ESI-MS results allowed to elucidate that copper is bound by GSSG in ratio 1:1 and 1:2. Additionally, size exclusion chromatography coupled to electrospray MS allowed to differentiate more stable complexes from weak ones that could be created in the gas phase.     

Selective degradation of lignin and elimination of HO radicals in pulps by O<Subscript>3</Subscript> and UV laser flash irradiation

HO^. radical is an aggressive reagent to abstract hydrogen from diverse substitutes and lead them to degradation, however, in reaction of active oxygen species with lignins, complex phenolic polymers, in dispersed lignocellulose such as pulp for environment-benign delignification, HO^. radicals should be eliminated as more as possible to prevent cellulose from unfavorably concomitant degradation. A reaction system of O₃ is constructed under UV laser flash irradiation, and HO^. radicals are controlled efficiently by it. A new mechanism is proposed, for the first time, that O^. radicals generated from reaction of O₃ with UV laser flash irradiation might be the contributor to scavenge HO^. radicals.     

High-performance liquid chromatographic determination of cocaine and benzoylecgonine by direct injection of human blood plasma sample into an alkyl-diol-silica (ADS) precolumn

A column-switching high-performance liquid chromatographic method with UV detection for the determination of cocaine (COC) and benzoylecgonine (BZE) in human blood plasma samples is described. The method uses an alkyl-diol-silica ADS-C18 extraction precolumn. A 50- micro L plasma sample was introduced to the ADS precolumn in order to separate the analytes from proteins and endogenous compounds. The fraction containing COC and BZE was back-flushed and transferred to an Alltech mixed-mode C(18)/cation-exchange analytical column for final separation. The validation of the method revealed quantitative recoveries from 95.0 to 99.0% for COC at three different concentrations (0.5, 1.0 and 2.0 micro g mL(-1)), and from 96.0 to 99.0% for BZE at the same concentration levels with coefficients of variation <4.00% (n=5). The detection limit (signal to noise ratio (S/N)>3) was 0.03 micro g mL(-1) for all the compounds with an injection volume of 50 micro L. However, it was possible to enhance the sensitivity further by injecting larger plasma volumes, up to 200 micro L, at the same optimal conditions. The overlap of sample preparation, analysis and reconditioning of the extraction column, increase the overall sample throughput to 5 samples h(-1). The developed method has been applied to human blood plasma samples from subjects suspected of cocaine abuse.     

Thermal decomposition of surface compounds for the generation of small quantities of acetaldehyde: Calibration of a thermal desorption-gas chromatography-flame ionization detection

The paper presents a new method for the generation of acetaldehyde in the amounts that allow to calibrate the measuring system. The method is based on thermal decomposition of surface compounds at the range 130-190 °C, in which an acetaldehyde as the decomposition product is released. The synthetic pathway leading to the desired surface compounds is described. Investigations demonstrated that the synthesis of the compounds on the surface of the support material (silica gel) was carried out homogeneously. The technique proposed was used for the generation of gaseous standard mixtures containing acetaldehyde for the calibration of a thermal desorber-gas chromatograph-flame ionization detector system.