Peak-shaped Signals Research Articles

Voltammetric and adsorption study of 4-nitrophenyl-triazole-labeled 2′-deoxycytidine and 7-deazaadenosine nucleosides at boron-doped diamond electrode

Electrochemical properties of 4-nitrophenyl triazole labeled 2′-deoxycytidine (dCTRNO2) and 7-deazaadenosine (dATRNO2) were studied using alumina-polished boron doped diamond (BDD) electrode in acetate buffer pH5.0. Cyclic voltammetry revealed peak-shaped signals for reduction of the nitro group at the potentials of ca −0.6V (vs. AgƖAgClƖ3molL−1 KCl reference electrode) and for the hydroxylamine/nitroso pair exhibiting quasireversible electron transfer at potentials comparable with redox processes of other nitro aromatic compounds at BDD electrodes. All electrochemical processes are directed by adsorption. Moreover, ex situ adsorption of dATRNO2 and transfer to supporting electrolyte enables its quantitation using square wave voltammetry in small (several microliter) sample volumes after optimization of parameters for the hydroxylamine/nitroso redox couple. Competitive adsorption/desorption at the BDD surface was further studied by cyclic voltammetry with dATRNO2 in the presence of electrochemically inactive phenyl triazole 7-deazaadenosine conjugate (dATR), revealing mutual displacement of both compounds at BDD surface. Further, the redox processes of simpler aromatic nitro compounds (4-nitrophenyl acetylene and 2-nitronaphthalene) are strongly affected when the BDD surface is covered by dATR.

Determination of Acid Dissociation Constant of Degradable Tetrabromophenolphthalein Ethyl Ester by Capillary Zone Electrophoresis

An acid dissociation constant of tetrabromophenolphthalein ethyl ester (TBPE) was determined through the measurement of electrophoretic mobility by capillary zone electrophoresis (CZE). Although TBPE is degradable in acidic pH region and it gradually degraded at pH conditions around and below its pKa values in the time scale of the CZE measurement, equilibrium species of interest were detected as a peak-shaped signal with tailing of the degraded species. Changes in electrophoretic mobility of the equilibrium species of TBPE were analyzed at its detectable pH range in the presence of phenol red as a mobility standard. An acid dissociation constant of 3.47 ± 0.06 (pK(a), I = 0.01, 25°C, standard error) was determined for TBPE.

The lipid composition determines the kinetics of adhesion and spreading of liposomes on mercury electrodes

The dependence of membrane properties on their composition was studied by following the adhesion and spreading of unilamellar and multilamellar liposomes on static mercury electrodes with the help of chronoamperometry. The analysis of the peak-shaped signals allows determining the kinetic parameters of the three-step adhesion-spreading process. The presence of cholesterol in the membrane stabilizes the bilayer in the liquid-crystalline phase, and destabilizes the gel phase. The kinetic parameters also show the effect of superlattice formation in the DMPC–cholesterol system. The detergent triton X-100 is only incorporated in the liquid-crystalline DMPC membranes, and it is expelled to the solution when the membrane is transformed to the gel phase. In the liquid-crystalline membrane, it enhances the adhesion-spreading of liposomes on mercury. The lytic peptides mastoparan X and melittin affect the adhesion-spreading in a similar manner. For the rupture-spreading step, their effect is explained by pore formation. The results obtained with lecithins of different length suggest that the bilayer opening process has much in common with flip-flop translocations. For this process the activation energies were found to be independent of the chain length of the lecithin molecules, while the preexponential factor in the Arrhenius equation decreases drastically for longer chains.

A peak structure in isothermal luminescence signals in quartz: Origin and implications

Isothermal heating is commonly used in luminescence dosimetry and trap parameter studies. It is often observed that the isothermal luminescence signal has a peak shape instead of a monotonous decay form. We provide here evidence that this peak shape in quartz may equally result from a ‘thermal lag’ problem in contrast to the earlier propositions of non first-order kinetics. Temperature modelling suggests that the peak-shaped signal contains elements of both the ramped — and isothermal — thermoluminescence (TL). The modelled changes in the peak movement as a function of isothermal temperature and the ramp-rate show an excellent agreement with those obtained experimentally. This understanding of thermal lag is extended to optically stimulated luminescence (OSL) measurements for which the effects of isothermal TL contamination and changing thermal assistance during thermal equilibration are discussed. Appropriate methods are suggested to identify thermal lag on the basis of the peak structure, and to circumvent this effect in isothermal methods.

Calcium Pulstrodes with 10-Fold Enhanced Sensitivity for Measurements in the Physiological Concentration Range

Ion-selective electrodes ideally operate on the basis of the Nernst equation, which predicts less than 60- and 30-mV potential change for a 10-fold activity change of monovalent and divalent ions measured at room temperature, respectively. Typical concentration ranges in extracellular fluids are quite narrow for the electrolytes of key importance. A range of 2.2-2.6 mM for calcium ions, for instance, translates into just a 2.2-mV potential change. The direct potentiometric measurement of physiological electrolytes is certainly possible with direct potentiometry and is done routinely in clinical analyzers and handheld measuring devices. It places, however, strong demands on the precision of the reference electrode and requires careful temperature control and frequent calibration runs. In this paper, a robust 10-20-fold sensitivity enhancement for calcium measurements is attained by departing from the classical response mechanism and operating in a non-Nernstian response mode. Stable and reproducible super-Nernstian responses of these so-called pulstrodes in a narrow calcium activity range can be controlled by instrumental means in good agreement with theory. The potentials may be measured during a galvanostatic excitation pulse (mode I) or immediately after it (mode II), under open-circuit conditions. Subtraction of the potentials, sampled at different times during a single pulse, allows one to obtain a sensitive differential peak-shaped signal at a critical and fully adjustable analyte activity range. Calcium pulstrodes based on the diamide ionophore AU-1 were characterized and applied to the measurement in model physiological liquids. Super-Nernstian responses exceeding 700 mV/decade were observed in a physiological range of calcium concentration. Such remarkable sensitivity of the pulstrodes, complemented with the well-documented high selectivity of these potentiometric sensors, may provide a significant increase in the accuracy and precision of electrolyte measurements in clinical analysis.

Stochastic Study on the Sales Pattern of Influenza Anti-Viral Agents at Pharmacies

This paper focuses on the stochastic analysis of the daily variations in formulations dispensed at pharmacies. A recent study showed that the cross-correlation functions between the time variations can be used for estimating the route and speed of influenza propagation process in society. In reality, however, the peak-shaped signals of the time variations and cross-correlation function are so complicated and noisy that it is almost impossible to know the original shape of the signals. The aim of this paper is to give an answer to this problem by considering the mathematical relationship between the time variations and cross-correlation function. Influenza anti-viral agents for adults and children are taken as an example.

Ranitidine hydrochloride X-ray assay using a neural network

A simple X-ray powder diffractometric (XRD) method with artificial neural networks (ANNs) for data modelling was developed to recognize and quantify two crystal modifications of ranitidine–HCl in mixtures and thus, provide information about the solid state of the bulk drug. The method was also used to quantify ranitidine–HCl from tablets in the presence of other components. An ANN consisting of three layers of neurons was trained by using a back-propagation learning rule. A sigmoid output function was used in the hidden layer to facilitate non-linear fitting. Unlike other techniques the ANN method described here employed pattern recognition on the entire XRD pattern. Correct classification was mainly influenced by the XRD pattern resolution. It was shown that data transformations improved the quantitative performance when the XRD patterns were not contaminated by other components. Only smoothed X-ray diffractograms were required to distinguish between the two crystalline forms in a mixture. In the case of ranitidine–HCl quantification from tablets, where significant interference with tablet excipients was present, better results were obtained without data transformations. The trained ANN perfectly quantified ranitidine–HCl polymorphic forms from mixtures (mean sum of squared error was less than 0.02%) and ranitidine–HCl form 1 from tablets (recovery=98.65). Excellent quantification performance of the ANN analysis, demonstrated in this study, serves as an indication of the broad potential of neural networks in pattern analysis. While the system described has been developed to interpret XRD patterns, peak detection has implications in every chemical application where the recognition of peak-shaped signals in analytical data is important.

The “Presodium” Catalysis of Electroreduction of Hydrogen Ions on Mercury Electrodes by Metallothionein. An Investigation by Constant Current Derivative Stripping Chronopotentiometry

Metallothionein (MT) yields on mercury electrodes a “presodium” catalysis of the evolution of hydrogen, which is, as a peak-shaped signal, particularly well measurable in chronopotentiometry, and which is, unlike presodium catalytic effects of other biopolymers, strongly dependent on the presence of cobalt ions in the solution. The behavior of the “presodium” derivative chronopotentiometric peak under various experimental conditions was tested.

Flow injection determination of cationic polyelectrolytes using a tetraphenylborate-selective electrode detector.

A potentiometric flow injection determination method for cationic polyelectrolytes utilizing a flow-through-type tetraphenylborate-selective electrode detector is described. The method is based on detecting any concentration decrease of the tetraphenylborate ion by the formation of an ion associate between cationic polyelectrolyte, poly(diallyldimethylammonium chloride) (Cat-floc) and tetraphenylborate ion. The response of the electrode detector, as a peak-shaped signal, was obtained for injected cationic polyelectrolyte samples. A linear relationship was found to exist between the peak height and the logarithmic concentration of Cat-floc with a slope of 17 mV/decade over a concentration range of 5 × 10 -5 to 1 × 10 -3 mol l-1. The detection limit for Cat-floc was 1 × 10 -5 mol l-1. The sampling rate was ca. 12 samples/h.

Flow injection determination of anionic polyelectrolytes using an anionic surfactant-selective plasticized poly(vinyl chloride) membrane electrode detector

A potentiometric flow injection method for the determination of anionic polyelectrolytes utilizing a flow-through type surfactant-selective electrode detector is described. The method is based on the detection of the concentration increase of anionic surfactant liberated from a reagent stream containing an ion associate between cationic polyelectrolyte, poly(diallyldimethylammonium chloride) and anionic surfactant, dodecylbenzenesulfonate, which is caused by the formation of a polyion complex between cationic and anionic polyelectrolytes. The response of the electrode detector as a peak-shaped signal was obtained for injected anionic polyelectrolyte samples. A linear relationship was found to exist between peak height and the logarithmic concentration of potassium poly (vinyl sulfate) (PVSK) with a slope of 30 mV decade-1 in a concentration range of 1.0×10-4 to 1.0×10-3 mol/L. Identical relationships were obtained for sodium alginate and carageenan (also anionic polyelectrolytes) as for PVSK but with a lower sensitivity. The detection limit for PVSK was 2.5×10-5 mol/L. The relative standard deviation for 5 injections of a 2.5×10-4 mol/L PVSK solution was 1.3% and the sampling rate was ca. 10 samples h-1.

Application of Multivariate Curve Resolution to Voltammetric Data: II. Study of Metal-Binding Properties of the Peptides

The complexation of Cd2+by glutathione (GSH), in 0.13mborate buffer at pH 9.5, was studied by differential pulse polarography (DPP) and multivariate curve resolution. The Cd–GSH system has been chosen as a model to check the possibilities of this new polarographic approach to the study of metal ion complexation by peptides. Experimental data obtained by DPP for different Cd2+-to-GSH concentration ratios have been analyzed by a procedure which consists of using several chemometrical techniques based on factor analysis: principal component analysis, evolving factor analysis, and multivariate curve resolution with alternating least-squares (ALS) optimization. The use of different constraints during the ALS optimization process, such as nonnegativity and unimodality constraints, yields the optimal sought solution from a chemical point of view. In the present work, a new constraint has been implemented during ALS optimization to take into account the expected peak-shaped signal of DPP. This data treatment allows us to detect simultaneously the formation of 1:1 and 1:2 Cd:GSH complexes which were very difficult to detect by univariate analysis of DPP data. It is concluded that the described multivariate curve resolution method could be a reliable tool for studying metal-binding properties of peptides.

Determination of Gaseous and Dissolved Oxygen in a Closed Fat Cell Culture System

A novel method facilitates the determination of O2 content in the gas phase and in the culture medium of closed cell culture systems: Total O2 was swept out for 3 min by a stream of N2 under atmospheric pressure and carried to the measuring unit of a PBI Dansensor TIA-111-LV oxygen sensor, creating a peak-shaped signal, which was integrated over time. From the peak area, gas flow (determined by a Rotameter flow meter), and actual gas pressure and temperature, O2 content was calculated. The precision of the method was 1.1% (CV), recovery was 97.7%. O2 uptake was measured in white adipocytes, isolated by the collagenase method and cultured in a 3D matrix of agarose gel. As a basis of reference, DNA content of the cultures (9.5 pg/cell) was determined by a fluorimetric method. The viability of the cells was not affected by O2 determination, as shown by the maintenance of cellular adenine nucleotide content during the analytic procedure. The decline of O2 content in the cultures was constant over 72 h. The mean rate of cellular O2 consumption was 34.1 μmol/μg DNA·h.

Determination of saponification value of oils by FIA.

A flow injection method for rapid and simple determination of saponification value of oils was investigated using neutralization reactions in the nonaqueous solvent system. The flow injection system consisted of three stream channels. A sample solution of an ester (200 μl) was injected into a carrier stream (ethanol) and was merged with an ethanol stream containing 0.1 M tetrabutylammonium hydroxide. Hydrolysis reaction of the ester was completed during flowing through a 50 m reaction coil at 100°C. The resulting stream was subsequently merged with a stream 0.1 M butyric acid-ethanol solution containing 0.1 M lithium chloride and 2 × 10-5 M Methyl Red. The pH or absorbance change of the final mixed stream was measured by two detectors (a glass electrode and/or a spectrophotometer). Absorbance was measured at 490 nm, corresponding to the maximum absorbance of Methyl Red in the acidic form. The changes of the electrode potential and of the absorbance were observed as peak-shaped signals. Peak heights provide concentration of esters. The linear relationship between peak heights and concentrations of lauric acid ethyl ester was observed from 0.02 M to 0.1 M. Other esters of saturated and unsaturated aliphatic carboxylic acids were determined with almost the same sensitivity as lauric acid ethyl ester. The sampling rate and the relative standard deviation were ca. 20 samples/h and 1.3 %, respectively. The proposed method was successfully applied to the determination of saponification value of butter samples.

Spectral peak verification and recognition using a multilayered neural network

The verification and recognition of peak-shaped signals in analytical data are ubiquitous scientific problems. Experimental data contain overlapping signals and noise, which make sensitive and reliable peak recognition difficult. A peak detection system based on a class of neural networks known as "multilayered perceptrons" has been created. The network was trained and evaluated with use of vapor-phase infrared spectral data. The results of varying the network architecture on system training and prediction performance along with refinement of the form of the input pattern are presented.