Specific adsorption of azide ions on mercury electrodes from constant ionic strength solutions

The adsorption of azide ions on mercury from aqueous solutions of NaN₃+NaF at constant ionic strength has been studied by measurements of the double layer capacity. Taking into account changes in the structure of the diffuse layer, it was possible to show that the properties of the inner layer are similar to those observed previously in solutions of pure NaN₃, and that the same adsorption isotherm applies to both systems.     

Initial evaluation of quantitative performance of chromatographic methods using replicates at multiple concentrations

The introduction of a novel analytical method must be supported by consistent information about its quantitative potentialities; this is critical for whoever considers its utilization for an specific application. Unfortunately, literature abounds in papers proposing excellent chromatographic methods of analysis that have been subjected to comparatively poor quantitative evaluation. The methodology proposed in the present work makes use of some of the performance characteristics whose measurement is recommended in validation protocols; pertinent to this stage of method development are the detection and quantitation limits, the linear range and the repeatability. All this information can be calculated from the results of a calibration with several replicates at each analyte level. Replicates enable the calculation of reproducibility at several analyte levels and the estimation of the linear range; more important, replicates are necessary to detect changes in peak area standard deviation with analyte amount. Regression of calibration data by means of unweighted least-squares (ULSR) can only be performed in those cases in which homoscedasticity has been previously verified; heteroscedastic calibration data demand regression by means of weighted least-squares (WLSR), since ULSR results in gross overestimation of prediction limits at low analyte concentration. The proposal is used for the preliminary quantitative evaluation of a method for the determination of nine biogenic amines by means of pre-column derivatization with dabsyl chloride and separation of derivatives by RPLC. Limits of detection are calculated by a regression approach and by the classical signal-to-noise ratio method (S/N approach). No significant difference was detected for the amines limits of detection estimated by WLSR and by the S/N approach; ULSR estimated limits of detection are between 7 and 78 times larger than those obtained by the other two methods, as a consequence of the heteroscedasticity of calibration data.     

Photoreactivity of tiaprofenic acid and suprofen using pig skin as an ex vivo model

The skin is repeatedly exposed to solar ultraviolet radiation. Photoreaction of drugs in the body may result in phototoxic or photoallergic side effects. Non-steroidal anti-inflammatory drugs, such as tiaprofenic acid (TPA) and the closely related isomer suprofen (SUP) are frequently associated with photosensitive disorders; they may mediate photosensitised damage to lipids, proteins and nucleic acids. Using ex vivo pig skin as a model, we investigated the photodegradation of TPA and SUP, and photobinding of these drugs to protein by means of HPLC analysis and drug-directed antibodies. Both with keratinocytes, which were first isolated from the pig skin and thereafter exposed to UVA and with keratinocytes which were isolated from pig skin after the skin was UVA exposed, time-dependent photodegradation of TPA and SUP was found, beside photoadduct formation to protein. The results of this work show that: (a) TPA and SUP were photodecomposed with similar efficiency; major photoproducts detected were decarboxytiaprofenic acid (DTPA) and decarboxysuprofen (DSUP), respectively. (b) Both drugs form photoadducts, as concluded from recognition by drug-specific antibodies. Pig skin appears to be a good model for studying the skin photosensitising potential of drugs.     

A new selective cleavage of n,n-dicarbamoyl-protected amines using lithium bromide

A mild and new procedure for the selective cleavage of an alkoxycarbonyl group (Boc, CBz) in N,N-dicarbamoyl-protected amino compounds is described. The method is based on the use of lithium bromide in acetonitrile and is compatible with a large range of other functionalities present in the substrates. Compared with other reported methodologies, the procedure is particularly useful for the Cbz-selective cleavage in N,N-Ts,Cbz-diprotected amines. A rationalization of the selectivity supported by ab initio calculations is also presented.     

Direct versus resonances mediated F+OH collisions on a new 3A" potential energy surface

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.     

Voltammetric studies on the interactions between camazepam metabolic series and human serum albumin. Determination of oxazepam using adsorptive stripping voltammetry

The behaviour of oxazepam in adsorptive stripping voltammetry was studied taking into account those conditions which have an influence on the accumulation step (electrolyte, pH, time, potential, drop size and stirring rate), rest time and stripping step (pulse amplitude and scan rate). Oxazepam can be determined at a hanging mercury drop electrode by differential-pulse voltammetry in 0.008 M Britton-Robinson buffer at pH 2.0 with a ?0.50 V accumulation potential. Its detection limit was found to be 3.6 × 10^?10 M (30-s accumulation) and the relative standard deviation for oxazepam concentrations in the range 2.8 × 10^?8?4.0 × 10^?7 M is lower than 2.8% (80-s accumulation). In addition, a procedure using adsorptive stripping voltammetry was developed to study the interactions occurring between human albumin and the camazepam metabolic series (camazepam, temazepam and oxazepam). The interactions decreased in the order temazepam ? oxazepam ? camazepam and the groups and structural modifications favouring interaction were determined.     

Density estimation and adaptive control of markov processes: Average and discounted criteria

We consider a class of discrete-time Markov control processes with Borel state and action spaces, and ^d i.i.d. disturbances with unknown distribution . Under mild semi-continuity and compactness conditions, and assuming that is absolutely continuous with respect to Lebesgue measure, we establish the existence of adaptive control policies which are (1) optimal for the average-reward criterion, and (2) asymptotically optimal in the discounted case. Our results are obtained by taking advantage of some well-known facts in the theory of density estimation. This approach allows us to avoid restrictive conditions on the state space and/or on the system's transition law imposed in recent works, and on the other hand, it clearly shows the way to other applications of nonparametric (density) estimation to adaptive control.Research partially supported by The Third World Academy of Sciences under Research Grant No. MP 898-152.     

A nonlinear programming algorithm based on non-coercive penalty functions

 We consider first the differentiable nonlinear programming problem and study the asymptotic behavior of methods based on a family of penalty functions that approximate asymptotically the usual exact penalty function. We associate two parameters to these functions: one is used to control the slope and the other controls the deviation from the exact penalty. We propose a method that does not change the slope for feasible iterates and show that for problems satisfying the Mangasarian-Fromovitz constraint qualification all iterates will remain feasible after a finite number of iterations. The same results are obtained for non-smooth convex problems under a Slater qualification condition. Received: September 2000 / Accepted: June 2002 Published online: March 21, 2003 Research partially supported by CAPES, Brazil Research partially supported by CNPq, Brazil, and CONICIT, Venezuela. Mathematics Subject Classification (1991): 20E28, 20G40, 20C20