Flow electrochemical analyses of zinc by stripping voltammetry on graphite felt electrode.

A flow sensor for trace analysis of zinc, using graphite felt as working electrode is reported here. A flow cell, well-adapted to 3-D porous electrodes and capable to do both the preconcentration step at a cathodic potential and the stripping of the zinc was successfully developed. It was demonstrated that this cell allows to obtain better electrochemical signals for Zn(2+) compared to a standard three-electrodes cell and that the percolation during accumulation increases the kinetics of electrodeposition. The influence on Zn(2+) signal of the deposition potential, the time of deposition and the flow rate was studied. The resulting sensor shows a linear response towards Zn(2+) with a linear range of 10(-6)-10(-4)M and a limit of detection of 5×10(-7) M for an analysis time of 5 min. The interferences study showed that the Cr(3+), Pb(2+), Cd(2+) ions have a small effect on the Zn electrochemical signal, whereas Fe(3+), Cu(2+), Co(2+) and Ni(2+) ions strongly influence it. The electrode was tested on real samples (tap water spiked with Zn(2+), food supplement) with a good recovery by applying the standard addition method.

Electroanalytical properties of a novel biosensor modified with zirconium alcoxide porous gels for the detection of acetaminophen.

The development of composite electrodes for biosensors construction based on HRP and zirconium alcoxide film for acetaminophen detection and finally, acetaminophen determination in pharmaceutical products is described. The enzyme immobilization is performed by retention in a polyetilenimine and zirconium alcoxide porous gel film, technique that offers a good entrapping and in the mean times a "protective" environment for the biocomponent. The operation principle of the biosensor is based on monitoring the amperometric signal generated by reduction at the electrode surface of the enzymatically generated quinoneimine from acetaminophen. The resulting biosensor shows a linear response towards acetaminophen with a linear range of 1.96 x 10(-5)M and 2.55 x 10(-4)M and a limit of detection of 1.17 x 10(-7)M. The proposed biosensor shows long term stability and good reproducibility.

Electrochemical behavior of colchicine using graphite-based screen-printed electrodes.

Miniaturized electrochemical graphite based sensor for the analysis of colchicine, along with its detailed construction was described. The electrochemical behavior of colchicine, both by cyclic and differential pulse voltammetry was investigated, showing an irreversible reduction and oxidation mechanism. The effect of several different electrochemical mediators was studied in order to decrease the oxidation potential of colchicine, but none of them showed a favorable electron transfer between the alkaloid and the electrode's surface. The influences of different biomolecules (calf thymus DNA, bovine serum albumin and beta-tubulin) over colchicine's electrochemical behavior were also presented. For quantitative determinations the anodic differential pulse voltammetric method in H(3)PO(4)/HClO(4) 0.01 M (pH 2.05) was selected, evaluating one of the oxidation peaks of colchicine at +970 mV versus Ag pseudo-reference. The electrochemical scanning parameters were optimized by an experimental design using response surface modelling. The method was validated according to ICH guidelines in the concentration range 85-1200 ng mL(-1) (R(2)=0.9966, n=7) colchicine in terms of linearity, accuracy, limits of detection (LOD) (41 ng mL(-1)), limits of quantification (LOQ) and fidelity and it was successfully applied to the determination of colchicine in tablets, without the interference of the excipients. The studied method showed the same sensitivity as the more complex HPLC procedure and micellar electrokinetic chromatography with spectrophotometric detection.

Fast determination of colchicine by TLC-densitometry from pharmaceuticals and vegetal extracts.

Colchicine, (S)-N-(5,6,7,9-tetrahydro-1,2,3,10-tetramethoxy-9-oxobenzo-(a(-heptalen-7-yl)-acetamide, is the main alkaloid contained in Colchicum autumnale (meadow saffron). There are known colorimetric, spectrophotometric, volumetric, potentiometric, voltametric, gravimetric and various chromatographic methods for quantitative determination of colchicine, each of them presenting a series of advantages and disadvantages. As an alternative, we proposed the use of a densitometric determination for colchicine allowing the determination of this alkaloid from pharmaceutical products, as well from seeds of meadow saffron. The total alkaloid extract was separated by Thin-Layer Chromatography using Silicagel 60F(254) layers and a mixture of chloroform:acetone:diethylamine (5:4:1) as mobile phase. The same conditions were used for the determination from pharmaceutical products. Densitometric measurements were carried out at the absorption maximum (350 nm) of colchicine, the determinations being made by reflectance and by fluorescence. The peaks were optimized regarding to their area and shape by varying four scanning parameters (slit width and height, number of measurements and scanning speed). We established the calibration plot using pure colchicine in the range 50-600 ng mL(-1). The proposed method could be widely used in the pharmaceutical industry for the quick and accurate quantitative determination of colchicine because it eliminates the interferences given by other bioactive or degradation compounds. The method was characterized by validation parameters (linearity, accuracy, fidelity, sensitivity) and it was established its performances in comparison with an HPLC method and an official quantitative determination from the Romanian Pharmacopoeia X edition respectively.

Essential oils analysis. II. Mass spectra identification of terpene and phenylpropane derivatives.

Mass spectra are widely used in order to identify the peaks resulting from a chromatographic separation. The most common approach to solve the problem for unknowns on whom very little other structural information is available is the use of a retrieval algorithm and a reference mass spectra database. The wide variety of mass spectra recorded with different instruments under various experimental conditions can lead to erroneous results. In order to improve the accuracy of the results, we proposed earlier an identification algorithm, which combines the information obtained from both GC and MS fingerprints. This paper presents a new algorithm based on the comparison of the unknown mass spectra with several libraries (including Wiley and NIST) by using reverse and direct search algorithms respectively. The results of the comparisons were quantified with respect to the match quality and the interference compounds. A global match index for the comparison using all the above information was computed and the results were presented as the match probability. This index expresses more accurately the matches between unknown and all the available libraries mass spectra. In order to verify our algorithm, we tried to identify the compounds separated by GC-MSD from different species of Acorus calamus L. (Araceae) essential oils. The probability of the matches increases compared with the quality of matches resulting from Wiley and NIST libraries.

The development of spectrophotometric and electroanalytical methods for ascorbic acid and acetaminophen and their applications in the analysis of effervescent dosage forms.

The electroanalytical study of ascorbic acid, acetaminophen and of several mixtures of these compounds in different ratios has been made by using a carbon paste electrode (CPE-graphite:solid paraffin 2:1) as working electrode and an Ag/AgCl reference electrode. The potential curves were recorded using different concentrations of ascorbic acid and acetaminophen by measuring samples between 10 and 50 microl. The oxidation reactions were studied in a potential range from -0.1 to +1.3 V with different sweep rates, at different current sensitivities, in stationary working conditions and stirring before each replicate. The oxidation of ascorbic acid occurs at +0.31 +/- 0.02 V and the oxidation of acetaminophen at +0.60 +/- 0.05 V; meanwhile, the current has a linear variation for the following concentration ranges: 10(-3)-10(-2) M for the ascorbic acid and 3 x 10(-6)-7.5 x 10(-3) M for acetaminophen (r2 = 0.999 for both ascorbic acid and acetaminophen). The mixtures of ascorbic acid and acetaminophen were made as follows: 1:1, 1:2, 1:3, 2:1, and 3:1. The studies revealed the alteration of the voltammograms processed according to the validation methodology. The best potential variation range for different current sensitivities, the influence of the sweep rate, of the solvent volume and of the pH were studied. The mutual interferences of the compounds in the mixtures and the electroactive compounds in the pharmaceutical dosage forms, especially effervescent ones, also made the object of the research. The same mixtures were studied using the direct spectrophotometric method that revealed a lot of spectral interferences. In order to solve this problem, an appropriate separation or an indirect spectrophotometric method (the apparent content curves method) were used. The spectrophotometric and voltammetric methods developed were used to determine ascorbic acid and acetaminophen in different dosage forms (vials, tablets, suppositories and effervescent dosage forms). The results were compared with those obtained by other techniques.

Voltammetric determination of lactate dehydrogenase using a carbon paste electrode.

A voltammetric study was performed by linear sweep voltammetry using a carbon paste electrode, in -0.1 to +1.3 V potential range, with the view of elaborating an assay for lactate dehydrogenase (LDH) in different enzymatic preparations and biological fluids. There have been performed studies concerning pH influence upon the enzymatic reaction, as well as the electrochemical behavior of LDH in the presence of modified carbon paste electrodes saturated with sodium pyruvate and/or NADH.

The voltammetric study of 2-mercapto-5-phenylammino-1,3,4-thiadiazole using carbon paste electrodes.

A rapid and accurate voltammetric method for the quantitative determination of 2-mercapto-5-phenylammino-1,3,4-thiadiazole (MPATD) with carbon paste electrodes (CPE) has been developed. The study was made by cyclic voltammetry between -0.4 and +0.6 V with 50 mV s(-1) sweep rate in aqueous solution. After successive oxidation/reduction cycles we found a total oxidation of MPATD at +0.45 V. As the compound is oxidated, the reduction current peak increases at +0.13 V, indicating an irreversible process. Following only the oxidation process in the -0.1 to +0.6 V range, we investigated the optimum scan rates at different current densities and pH values (realised with buffers, pH between 1.0 and 10.0) with CPE versus Ag/AgCl reference electrode using linear sweep voltammetry. We found a good linear relation between the current peak height and concentration in a 2.5 x 10(-9)-1.25 x 10(-7) mol ml(-1). This method allows the quantitative detection of the MPATD as it or from dosage forms and biological media.

Electrochemical study of some 2-mercapto-5-R-ammino-1,3,4-thiadiazole derivatives using carbon paste electrodes.

The electrochemical study of some 2-mercapto-5-R-ammino-1,3,4-thiadiazole derivatives was made by cyclic and linear sweep voltammetry using a carbon paste electrode (CPE, graphite/solid paraffin ratio 2:1) as working electrode and an Ag/AgCl reference electrode. The current-potential curves were recorded in anodic polarisation in -0.1 and +1.3 V range using aqueous solutions and different buffers (between pH 1.2 and 10.0), with 20 or 50 mV s(-1) sweep rate. The oxidation peak appears between +0.65 and +0.70 V due to disulphides formation. The 5-phenyl derivative has two oxidation peaks, the first at +0.45 +/- 0.03 V and the second at +0.65 +/- 0.03 V. The oxidation potentials are pH dependent, decreasing from 0.9 +/- 0.1 V at pH 1.2 to 0.6 +/- 0.1 V at a pH between 8.0 and 10.0. In some potential ranges depending on pKa of molecules the oxidation potential and oxidation current are pH independent. Simple, precise and accurate voltammetric methods for the determination of these compounds were developed and validated in 2.5 x 10(-6)-7.5 x 10(-4) mol l(-1) concentration ranges. The detection limits were 2.3 micromol l(-1) for 5-ammino-, 12.3 micromol l(-1) for 5-acetylammino-, 11.6 micromol l(-1) for 5-allylammino-, and 1.2 micromol l(-1) for 5-phenylammino-2-mercapto-1,3,4 thiadiazole derivatives.

Essential oils analysis. I. Evaluation of essential oils composition using both GC and MS fingerprints.

The chemical nature of essential oils makes them suitable for analysis using a gas chromatography-mass selective detector (GC-MSD). Mass spectra (MS) libraries can not be used as unique and absolute criteria for the identification of chromatogram peaks. The wide variety of MS of the libraries, recorded in different conditions, can lead us to erroneous results. In order to increase the reliability of the analytical results, we used as identity criteria, both GC fingerprints resulted from the relative retention indices (RRI) and the recorded MS of the separated compounds. The two criteria have been quantified by their correlation with the standards. A new parameter called global composition evaluation index (I(GCMS)), which resulted from a well-balanced average of the two criteria, has been defined. Because the comparison of the results of the MS with databases is more accurate than the RRI, we considered that the ratio of the two criteria must be at least GC:MS 1:2. A database containing RRI of about 600 components, widely found in essential oils composition and separated on HP-5 column, was created. Two macros based on the Microsoft Excel spreadsheet were also created. The program offers the best 20 matches of each compound with the combined MS and RRI library. The composition of Romanian Acorus calamus L. essential oil was established and the results were compared with those obtained by 'classical' methods.

Spectrophotometric determination of fluoride in dosage forms and dental preparations.

The method is based upon the reaction between fluoride ions and the coloured complex of Fe(III) with methyl salicylate to form the stable, colourless hexaflouride complex of iron. The conditions of the method (pH, time and combination ratio) were studied and a standard curve was obtained for 0.01-0.08 mg NaF ml-1, at 525 nm. A study was conducted on interference with complexing anions of Fe(III), cations that react with fluoride ions and with common ingredients of dosage forms and dental preparations. The method was validated and the results showed good precision (100.16 +/- = 2.33%) comparable with that of other analytical methods. Good results were obtained in the spectrophotometric determination of fluoride ions in a stomatological gel and in a toothpaste.

Preconcentration of Pb(II), Cd(II), Cu(II) and Hg(II) with 2-mercapto-5-phenylamino-1,3,4-thiadiazole impregnated on silica gel.

Silica gel modified by the addition of 2-mercapto-5-phenylamino-1,3,4-thiadiazole (MPATD) was developed for the preconcentration of Pb(II), Cd(II), Cu(II) and Hg(II) at trace levels. A "series" experimental procedure permitted the determination of the optimal working conditions (time and pH) and an "on-column" experimental method permitted the determination of the quantity of MPATD retained on silica gel, the uptake capacity, the most suitable eluent and the minimum volume of eluent for which the maximum quantity of metal cations is recovered. The maximal retention capacity on the impregnated silica gel columns was obtained at pH 7.5-8 and was 1500 micrograms g-1 for Pb(II) and about 1000 micrograms g-1 for Cd(II), Cu(II) and Hg(II). The effect of preconcentration is relevant; on passing 1 l of solution containing 1 microgram ml-1 of cation through a column containing 5 g of impregnated silicagel, after elution with 30-50 ml of the appropriate eluent the cation concentration increased about 15-20-fold. The determination of cations by flame AAS was statistically evaluated and showed recoveries higher than 96% for Pb(II), 98.5% for Cd(II) and 97.95% for Cu(II) after elution with 4 M HNO3 and about 99.15% for Hg(II) after elution with thiourea.

A modified McLafferty rearrangement in the electron impact mass spectra of dansylated amino-acid methyl esters.

Electron impact mass spectra of dansylated amino-acids and their methyl esters show a very time intense peak at m/z 171. A systematic study failed to show the genesis and origin of this ion. The present work is devoted to the electron impact mass spectrometric (EIMS) analysis of the appearance of this ion m/z 171 in the spectra of dansylated amino-acid methyl esters and its diagnostic importance in the analysis of dansyl derivatives. A structural model for this study is offered by a series of N-alkylated aromatic sulphonamides. The EIMS spectra of these compounds indicate that the classic fragmentation is accompanied by hydrogen migration from aliphatic radicals to the aromatic ring in a manner analogous with dansylated amino-acids. A modified McLafferty rearrangement mechanism is proposed for this EI reaction. The metastable transitions and the deuterium-labelled compounds confirm this mechanism and the origin of the itinerant hydrogen.