Quantitative analysis of oxytetracycline and its 4-epimer in calf tissues by high-performance liquid chromatography combined with positive electrospray ionization mass spectrometry

Tetracycline antibiotics are commonly used in veterinary medicine because of their broad spectrum activity and cost effectiveness. Oxytetracycline (OTC) is one of the most important members of this antibiotic family. The purpose of this study was to develop and validate a method to determine OTC residues in edible tissues of calf. Extraction of OTC and its 4-epimer (4-epiOTC), in the presence of the internal standard demethylchlortetracycline (DMCTC), was performed using a liquid extraction with sodium succinate solution (pH 4.0), followed by protein removal with trichloroacetic acid and paper filtration. Further solid-phase extraction clean-up on an HLB polymeric reversed phase column was performed to obtain an extract suitable for LC-MS-MS analysis. Chromatographic separation of the internal standard, and especially OTC and its 4-epimer, was achieved on a PLRP-S polymeric reversed phase column, using a mixture of 0.001 M of oxalic acid, 0.5% (v/v) of formic acid and 3% (v/v) of tetrahydrofuran in water (mobile phase A) and tetrahydrofuran (mobile phase B) as the mobile phase, and at a column temperature of 60 degrees C. OTC and its 4-epimer could be identified using the MS-MS detection technique, and were subsequently quantified. The method has been validated according to the requirements of the EC at the MRL (maximum residue limit, 100 ng g(-1) for muscle, 300 ng g(-1) for liver and600 ng g(-1) for kidney), half the MRL and double the MRL levels, as well for OTC as for 4-epiOTC. Calibration graphs were prepared for all tissues and good linearity was achieved over the concentration ranges tested (r > 0.99 and goodness of fit < 10%). Limits of quantification of half the MRLs were obtained for the analysis of OTC and 4-epiOTC in muscle, liver and kidney tissues of calf. Limits of detection ranged for both components between 0.8 and 48.2 ng g(-1). The within-day and between-day precisions, expressed as RSD values, were all below the maximum allowed RSD values calculated according to the Horwitz equation. The results for accuracy fell within the -20% to +10% range. Recoveries were between 47 and 56% for OTC, and between 52 and 62% for 4-epiOTC, depending on the tissue. The method has been successfully used for the quantitative determination of OTC and 4-epiOTC in tissue samples of calves medicated with OTC by intramuscular injection.     

Counterion exchange selectivity coefficients at water-in-oil microemulsion interface

The counterion binding at a water/Aerosol-OT (AOT)/heptane microemulsion interface was treated in the context of the pseudo-phase ion exchange formalism. Two approaches were used to determine the selectivity coefficient for copper/sodium counterion binding at the AOT microemulsion interface: measurements of the Cu(II) concentration taken up by the reverse micelle in a Winsor II microemulsion system and steady-state emission quenching measurements of an anionic water-solubilized probe, the tris-(4,4'-dicarboxy-2,2'-bipyridine) ruthenium (II) ion. In addition, the selectivity coefficient for methyl viologen/sodium at the microemulsion surface was determined by the same photophysical technique. The value for copper (II)/sodium exchange (K(Cu/Na)) is 1.1+/-0.3 and that for methyl viologen/sodium (K(MV/Na)) is 0.9+/-0.3. The results show that the pseudo-phase ion exchange model can be used to obtain the selectivity coefficient in a microemulsion system.     

Optimizing separation conditions for riboflavin,flavin mononucleotide and flavin adenine dinucleotide in capillary zone electrophoresis with laser-induced fluorescence detection

DNA sequencing is performed in a multiplexed capillary electrophoresis system by UV absorption detection. Four individual electropherograms are obtained by simultaneously running the unlabeled DNA products of the four ddNTP-terminated reactions in the capillary array. The sequence of the template used in the cycle-sequencing reaction can be determined by overlaying the four electropherograms. Two internal standards are employed to adjust for the variance in migration times among the capillaries. After applying the correction algorithm, base calling can be done at a high level of confidence.     

Conformational analysis of carbonyl and thiocarbonyl ethyl esters: The HC(X) (X,Y = O or S) internal rotation

The internal rotation in the HC(?X)YCH₂CH₃ (X, Y = O or S) series of molecules was studied by the ab initio SCF-MO method using 3-21G and 3-21G + d(ζ = 0.65S) basis sets. Energies and structures of several conformations of these molecules, determined by gradient geometry refinement, are reported and used to assess the effects of oxygen-by-sulphur substitution on molecular properties. The nature and relative importance of intramolecular interactions involving both the ? CH₂CH₃ and the HC(?X)Y (X, Y = O or S) fragments are also discussed. © 1992 by John Wiley & Sons, Inc.     

The oxygen evolution reaction on cobalt: Part II. Transient measurements

Open-circuit overpotential decays on an aged cobalt electrode in the oxygen evolution range in 6 M KOH show different slopes for two overpotential regions. These slopes are lower than the Tafel slope in the same region: Tafel slopes of ～100 and ～40 mV/dec, at high and low overpotentials, respectively. compared to decay slopes of ～?60 and ～?20 to ?30 mV/dec. For a fresh cobalt electrode a decay slope of ～?40 mV/dec is found at high overpotentials. From impedance measurements during a decay it is concluded that the electrode capacitance cannot account for the decay curves obsered. By means of steady-state potentiostatic impedance measurements (with stabilization times > 24 h) it is found that the differential Tafel slope remains constant at ～40–50 mV/dec and differs considerably from the Tafel slope at high overpotentials, ～100 mV/dec. Galvanostatic pulse experiments give evidence of the presence of CoO₂ in the oxide layer.Two models which may explain the observed experimental results are analysed. Both include a potential-dependent (extra) process which is fixed by the amount of CoO₂ at the surface. In one model, CoO₂ is responsible for partial surface blockage (parallel process); in the other model, CoO₂ controls the conductivity of the top layer of the oxide layer on the electrode.     

Fluorescence study of the interaction between metal ions and methyl methacrylate–methacrylic acid copolymers in aqueous solutions: thallium(I), calcium(II), and terbium(III)

 The binding of the cations thallium(I), calcium(II) and terbium(III) to methyl methacrylate– methacrylic acid copolymers with different fractions of acid groups (x) has been studied in aqueous solution at various pH values using the fluorescence of covalently bonded 9-vinyl anthracene as a probe. In all cases, the extent of binding increases as a function of the charge of the polymer with either increasing fraction of carboxylic acids or of pH. However, differences are observed in the behavior of the three cations. With Tl(I), quenching of the anthracene group fluorescence is observed, indicating that the thallium(I) approaches the probe and suggesting that the alkylanthracene is probably in a relatively polar region. Binding constants have been determined from anthracene quenching data and from studies with the fluo-rescent-probe sodium pyrenetetra-sulfonate. Good agreement is obtained between the two methods, and values for the binding constants increase from 250 to 950 M^-1 as x increases from 0.39 to 1. It is suggested that the cation is held in the polyelectrolyte domain, partly by Debye–Hückel effects and partly by more specific interactions. Stronger binding is found with calcium(II) and terbium(III), and in this case increases in fluorescence intensity are observed on complexation due to the anthracene group being in a more hydrophobic region, probably as a result of conformational changes in the polymer chain. In the former case the stoichiometry of the interaction was determined from the fluorescence data to involve two carboxylate groups bound per calcium. Association constants were found using murexide as an indicator of free calcium to vary from 8400 to 37 000 M^-1 as x increases from 0.39 to 1. It is suggested that in this case specific calcium(II)–carboxylate interactions contribute to the binding. With terbium(III), a greater increase in the probe fluorescence intensity was observed than with calcium, and it is suggested that the interaction with the polymer is even stronger, leading to a more pronounced conformational change in the polymer. It is proposed that the terbium(III) interacts with six carboxylic groups on the polymer chain, with three being coordinated and three attracted by electrostatic interactions. Received: 10 June 1997 Accepted: 24 October 1997     

运用NMR和DFT研究Michael加成反应的动力学行为及反应机理：(CO)5W=C(OEt)C2Ph作为底物

利用核磁共振方法研究了取代吡唑对炔基Fischer卡宾化合物的Michael加成的动力学行为，该反应为典型的二级反应。当吡唑的3，5-位由较大基团取代时，反应速率常数变小，而活化焓和活化熵明显增大。利用密度泛函理论研究了炔基钨卡宾为底物的Michael加成反应机理，发现吡唑上取代基团的增大可以导致第三步反应的活化能大于第一步，从而使反应的决速步骤由原来的第一步转变为第三步。     

Sonic spray ionization applied to liquid chromatography/mass spectrometry analysis of endocrine-disrupting chemicals in environmental water samples

A sonic spray ionization liquid chromatography/mass spectrometry (LC/SSI-MS) procedure combined with off-line solid-phase extraction was optimized for the analysis of 20 endocrine-disrupting chemicals (EDCs) in water samples. Method development included a comparison of the novel sonic spray ionization (SSI) with more traditional ion sources, i.e. pneumatically assisted electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). It was demonstrated that SSI and ESI spectra were very similar, but were more prone to the formation of solvent cluster ions as compared with APCI spectra. This phenomenon was most prominent for SSI and resulted in an increased chemical background in full-scan mass spectra. However, this chemical noise did not affect the overall sensitivity of SSI and ESI. After optimization of LC and MS parameters, the LC/SSI-MS method was validated. Recoveries ranged from 76.3 up to 113.4% for all compounds. Limits of detection (LOD) and quantitation (LOQ) were established between 3.0 and 11.5 ng/L and 9.9 and 38.0 ng/L, respectively. Within-day (n = 5) and between-day (n = 5) reproducibility were investigated at three levels and ranged from 3.3-16.5% and 7.6-19.2%, respectively. Eight-point calibration curves were established and showed linearity for all compounds (r(2) > 0.987) over a linear dynamic range of 10-10 000 ng/L.     

Extension of the Coulomb–Hole–Hartree–Fock theory to molecules

The Coulomb–Hole–Hartree–Fock method introduced by E. Clementi in the early 1960s and reparametrized more recently by S. Chakraworty and E. Clementi to compute the correlated electronic energy in atomic systems, is here extended to compute molecules. The new parametrization is obtained empirically by fitting first and second atomic ionization potentials from He to Ca and a few diatomic molecules. The present formulation makes use of either one or more determinants in order to ensure proper dissociation products, following the early proposal of G.C. Lie and E. Clementi in the context of density functional computations for molecular systems. The new formulation is tested against the dissociation energies of a large number of molecules and it is found satisfactory. © 1995 John Wiley & Sons, Inc.