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51.
The flame expansion factor (ratio of volumes of flame gases before and after burning) has been determined by mercury vapour introduction. The method is based on the determination of atomic concentration ratio in the flame and the flameless gas jet using the same sample introduction rate. A good agreement has been found between the expansion factors determined by the proposed method and those calculated by the gas law.  相似文献   
52.
53.
A software-controlled flow-through optical fiber diffuse reflectance sensor capitalized on the implementation of disk-based solid-phase pre-concentration schemes in a multisyringe flow injection analysis (MSFIA) set-up is proposed for the trace determination of sulfide in environmental waters and wastewaters. The fully automated flowing methodology is based on Fischer's coupling reaction of sulfide with N,N-dimethyl-p-phenylenediamine (DMPD) in the presence of Fe(iii) as oxidizing reagent in a 0.5 M HCl medium. The on-line generated methylene blue dye is subsequently delivered downstream to a dedicated optode cell furnished with an octadecyl-chemically modified (C(18)) disk, while continuously recording the diffuse reflectance spectrum of the pre-concentrated compound. A double regeneration protocol is finally executed to warrant minimum background noise and negligible baseline. Under the optimized chemical and hydrodynamic conditions, the optosensing MSFIA method features coefficients of variation better than 0.7%(n= 10) at 50 microg l(-1) concentration, a linear working range of 20-200 microg l(-1) sulfide, a 3sigma(blank) detection limit of 2.9 microg l(-1) sulfide and an injection throughput of 8 h(-1) for a pre-concentration sample volume of 2.9 ml. The interfacing of the robust and versatile multisyringe flow injection-based optode with a plug-in spectrophotometer furnished with a light emitting diode assures the miniaturization of the overall flow analyzer, which is, thus, readily adaptable to real-time monitoring schemes. The potential of the multisyringe flow method was assessed via the determination of sulfide traces in water samples of different complexity (namely, freshwater, seawater and wastewater).  相似文献   
54.
The first syntheses of the alkaloids (-)-mehranine (3), (+)-voaphylline/conoflorine (4), (+)-N(a)-methylvoaphylline/hecubine (5), and (-)-lochnericine (2) were achieved by stereoselective epoxidation starting from (-)-tabersonine (1), through intermediates with the aspidospermane and quebrachamine skeleton.  相似文献   
55.
Juan-García A  Font G  Picó Y 《Electrophoresis》2005,26(7-8):1550-1561
A method to identify and quantify six pesticide residues - dinoseb, pirimicarb, procymidone, pyrifenox, pyrimethanil, and thiabendazole - in peaches and nectarines using capillary electrophoresis-electrospray ionization-quadrupole ion trap-tandem mass spectrometry (CE-ESI-MS/MS) is described. Separation was carried out using a buffer of 0.3 M ammonium acetate at pH 4 with 10% methanol. Pesticide residues present in peach and nectarine samples were preconcentrated by solid-phase extraction using C(18), eluted with CH(2)Cl(2), concentrated to dryness, and redissolved in buffer to obtain lower detection limits. The recoveries of the analytes ranged from 58 to 99% and the relative standard deviations were 9 to 19%. Under optimized CE-MS/MS conditions the minimum detectable levels for the six pesticides in spiked peach samples were between 0.01 mg/kg for pirimicarb and 0.05 mg/kg for procymidone with pressure injection of 50 mbar for 5 s (5 nL) at a signal-to-noise ratio of 3, which constitutes a severalfold increase in sensitivity compared to CE-MS, using a single quadrupole, and to conventional CE-UV. The potential of the method was demonstrated by analyzing different samples taken from regional agricultural cooperatives. The pesticides most often detected were thiabendazole and procymidone.  相似文献   
56.
Several triterpenic derivatives, with the A-ring functionalized, were semisynthesized from oleanolic and maslinic acids. The reactivities of sulfites, sulfate, and epoxides in these triterpene compounds were investigated under different reaction conditions. Moreover, contracted A-ring triterpenes (five-membered rings) were obtained, by different treatments of the sulfate 7. From the epoxide 8, deoxygenated and halohydrin derivatives were semisynthesized with several nucleophiles. Ozonolysis and Beckmann reactions were used to yield 4-aza compounds, from five-membered ring olanediene triterpenes. The X-ray structure of sulfate 7 is given and compared with density functional theory geometries. Theoretical (13)C and (1)H chemical shifts (gauge-invariant atomic orbital method at the B3LYP/6-31G*//B3LYP/6-31G* level) and (3)J(H,H) coupling constants were calculated for compounds 5-9 and 34-36, identifying the (R)- or (S)-sulfur and alpha- or beta-epoxide configurations together with 4-aza or 3-aza structures.  相似文献   
57.
Palladium-catalyzed electrophilic allylic substitution of functionalized allyl chlorides and allyl acetates can be achieved in the presence of hexamethylditin under mild and neutral reaction conditions. This efficient one-pot procedure involves palladium-catalyzed formation of transient allylstannanes followed by generation of a bis-allylpalladium intermediate, which subsequently reacts with electrophiles. Using this catalytic transformation, various aldehydes and imines can be allylated providing highly functionalized homoallyl alcohols and amines. Furthermore, tandem bis-allylation reactions could be performed by employing tosyl isocyanate and benzylidenemalonitrile as substrates. A particularly interesting mechanistic feature of this reaction is that palladium catalyzes up to three different transformations in each catalytic cycle. Various allylic functionalities, including COOEt, CONH(2), COCH(3), CN, Ph, and CH(3), are tolerated in the catalytic reactions due to the application of neutral and mild reaction conditions. The substitution reaction occurs with very high regioselectivity at the branched allylic terminus. Moreover, in several reactions, a high stereoselectivity was observed indicating that this new catalytic process has a high potential for stereoselective synthesis. The regioselectivity of the reaction can be explained on the basis of DFT calculations. These studies indicate that the allylic substituent prefers the gamma-position of the eta(1)-allyl moiety of the reaction intermediate.  相似文献   
58.
A new screening method for the analysis of enrofloxacin and ciprofloxacin in edible animal tissues is described. The method is based on the application of principal component regression to luminescence measurements after reaction of quinolones with terbium(III) in a micellar medium. The method was used, first, to discriminate between quinolone-containing or quinolone-not-containing samples (concentration below the detection limit) and, then, to quantify the sum of both analytes. Standards in a pure-water matrix, using the first three principal components, were used for the determination. RRMSE range from 4 to 10% depending on the analyte. Calibration was successfully applied to the analysis of spiked chicken and trout muscle at concentrations between 10 and 50 micrograms kg-1.  相似文献   
59.
Determination of macrolide antibiotics by liquid chromatography   总被引:7,自引:0,他引:7  
The liquid chromatographic separation of seven macrolides used in food producing animals in the European Union has been studied. Separation was performed by using an end-capped high-purity silica-based C18 column and mobile phases consisting of phosphate buffer (pH 2.5)–acetonitrile mixtures. The effect of pH and acetonitrile percentage on the separation was examined. Two UV-based detection systems, wavelength programming and diode array, were assayed. Detection limits were in the range 6–33 μg l−1 for spiramycin, tilmicosin, tylosin, kitasamicin and josamicin and about 400 μg l−1 for erythromycin and oleandomycin. The suitability of the method for multiresidue determination of the five macrolides is demonstrated by the analysis of spiked samples of chicken muscle.  相似文献   
60.
Lima MJ  Tóth IV  Rangel AO 《Talanta》2005,68(2):207-213
A sequential injection system based on the ABTS (2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulphonic-acid) methodology was developed. The proposed method, incorporating a mixing chamber in the side port of the selection valve, was evaluated to measure the total antioxidant activity of several beverages and foods.The ABTS+ is generated by oxidation of ABTS with potassium persulfate and is reduced in presence of hydrogen-donating antioxidants converting into a colourless product. The applicability of the developed method was tested by measurement of the antioxidant activity of pure compounds as well as by analysing complex food and beverage samples. The antioxidant activity was presented as l(+) ascorbic acid equivalence. The values obtained by this methodology were not significantly different from the results obtained by the original spectrophotometric ABTS assay. For most of the studied antioxidants, antioxidant activity varied with pH and dilution. The proposed SIA system is suitable for screening direct or diluted total antioxidant activity of pure compounds or food samples.  相似文献   
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