Silvia Corona-Avendaño

Spectrophotometric study on the stability of dopamine and the determination of its acidity constants

The interest in determining the acidity constants of the catecholamines stems from the fact that they play rather an important biological role. The present work reveals the effect of different parameters such as oxygen, light, analysis time and pH on the dopamine oxidation process, where oxygen has an effect on the dopamine oxidation of 40% and up to 20% is attributed to exposure to light as a function of the pH. The application of adequate control on the said parameters (which ensured stability of the dopamine) facilitated the determination of the corresponding three acidity constants, 9.046+/-0.147, 10.579+/-0.148 and 12.071+/-0.069.

Simultaneous Electrochemical Determination of Adrenaline and Ascorbic Acid: Influence of [CTAB]

A carbon paste electrode modified with the cationic surfactant cetyltrimethyl ammonium bromide (CTAB) was used to carry out an electrochemical study at pH 3 in 0.1 M NaCl by differential pulse voltammetry to discern the effect of the surfactant on the electrochemical signals of adrenaline (AD) and ascorbic acid (AA). It was shown that in the absence of CTAB, AA and AD voltammetric signals overlap in the 500-600 mV potential interval. However, in the presence of CTAB, the AD oxidation process was not favored, whereas the opposite is true for the AA, which leads to an effective peak separation. This method permitted to obtain reliable analytical parameters to undertake the determination of AD in the presence of AA as the detection limit was 7.20 ± 0.04 μM,the quantification limit was 24.32 ± 0.08 μM,the sensitivity was 2.478 ± 0.067 μA mM -1 , and the linearity range was 0.01-0.27 mM. This approach was used effectively to determine AD in real pharmaceuticals obtaining satisfactory results. Moreover, it is shown that the simultaneous determination of AD and AA may be performed and that AD can be adequately determined even in the presence of both AA and uric acid.

Guest-host complex formed between ascorbic acid and β-cyclodextrin immobilized on the surface of an electrode.

This work deals with the formation of supramolecular complexes between ascorbic acid (AA), the guest, and β-cyclodextrin (β-CD), the host, that was first potentiodynamically immobilized on the surface of a carbon paste electrode (CPE) throughout the formation of a β-CD-based conducting polymer (poly-β-CD). With the bare CPE and the β-CD-modified CPE, an electrochemical study was performed to understand the effect of such surface modification on the electrochemical response of the AA. From this study it was shown that on the modified-CPE, the AA was surface-immobilized through formation of an inclusion complex with β-CD, which provoked the adsorption of AA in such a way that this stage became the limiting step for the electrochemical oxidation of AA. Moreover, from the analysis of the experimental voltammetric plots recorded during AA oxidation on the CPE/poly-β-CD electrode surfaces, the Gibbs’ standard free energy of the inclusion complex formed by the oxidation product of AA and β-CD has been determined for the first time, ∆G0inclus = −36.4 kJ/mol.

Dispersive solid-phase extraction based on butylamide silica for the determination of sulfamethoxazole in milk samples by capillary electrophoresis

ABSTRACT A new method based on the combination of dispersive solid-phase extraction and capillary electrophoresis is proposed for the determination of sulfamethoxazole in milk samples. Butylamide silica was synthesized and used as extractant. Factors involved in sample treatment method such as: butylamide silica amount, NaOH concentration in methanol, sample volume, and dispersion time were evaluated using a Taguchi parameter design. Under optimal conditions, average recoveries ranged from 73 to 85% with a limit of detection of 0.05 mg L−1 were achieved. The proposed method is a useful technique for cleanup milk samples. GRAPHICAL ABSTRACT

DEVELOPMENT OF A SECOND TYPE ELECTRODE BASED ON THE SILVER/SILVER IBUPROFENATE PAIR FOR IBUPROFEN QUANTIFICATION IN PHARMACEUTICAL SAMPLES

Ibuprofen is a widely used pharmaceutical because of its therapeutic properties; it is considered a safe medicament, thus it does not require medical prescription to be sold. However, in order to ensure consumers health it is indispensable that the pharmaceutical industry relies on analytic methods for its quantification. Potentiometry has proven to be a successful technique using second type electrodes, which in agreement with Nernsts equation can detect anions activity. On consideration of this, this research work presents the development of a second type electrode based on the silver/silver ibuprofenate pair. This involved modifying a pure silver wire using a sodium ibuprofenate solution, to obtain the redox pair. The following analytic parameters were obtained with the aid of the modified electrode: a sensitivity of -0.049 V decade [ibuprofenate]-1, 8 µmol L-1 for the detection limit and a quantification limit of 1.2 µmol L-1. The repeatability value in terms of the relative standard deviation was 5.9%. After performing an interferences analysis using some ions and excipients, it was corroborated that there were none, thus allowing appropriate quantification on a real sample.

Analysis of the Copper Electrodeposition Current Transients in Nitrates Media

This work deals with the kinetic changes originated by nitrate presence on copper nucleation. From analysis of experimental potentiostatic current transients recorded during copper electrodeposition from aqueous solutions, with and without nitrate ions, it is shown that when NO3 - are present, the copper nucleation rate increases while the number density of active site almost remains constant for every applied potential considered.

Taking advantage of CTAB micelles for the simultaneous electrochemical quantification of diclofenac and acetaminophen in aqueous media

From spectrophotometric and electrochemical techniques, it was shown that cetyltrimethylammonium bromide, CTAB, hemimicelles, formed on the surfaces of a carbon paste electrode (CPE), selectively adsorbed diclofenac, DCF, molecules from a neutral aqueous solution of DCF and acetaminophen, ACT. This CTAB–DCF interaction is so important that it modified the DCF electrochemical oxidation mechanism from a mass-transferred controlled one (in the absence of CTAB) to an adsorption controlled mechanism which allows the simultaneous quantification of both drugs. This novel methodology was used for the individual and simultaneous quantification of DCF and ACT in both aqueous media and synthetic urine. Moreover, it is shown that the analytical method proposed in this work is similar or even better than other more sophisticated and expensive techniques available.

Voltammetric determination of ibuprofen using a carbon paste – multiwalled carbon nanotube composite electrode

ABSTRACT An optimized voltammetric method is reported for the quantification of ibuprofen in aqueous solution using a carbon paste–multiwalled carbon nanotube composite as the working electrode. The electrode preparation and the experimental conditions were optimized; a sensitivity of 228.08 mA mol−1 L was obtained. The limits of detection and quantification were 9.1 µM and 25.5 µM, respectively, and the linear dynamic range extended from 2.36–242 µM. The repeatability was 5.35% expressed as the relative standard deviation. Using statistical analysis for the lower concentrations, the detection limit was reduced to 4.13 µM. The method was employed for the analysis of ibuprofen tablets.

Simultaneous electrochemical quantification of naproxen, acetaminophen and diclofenac using a bare carbon paste electrode

It is shown, for the first time, that a bare carbon paste electrode can be used for the simultaneous quantification of anti-inflammatory drugs: naproxen (NPX), diclofenac (DCF) and acetaminophen (ACT) in aqueous media. The analytical features are: 19 ± 7, 4.1 ± 1.9 and 3.7 ± 1.5 μM as detection limits of DCF, NPX and ACT, respectively, while the sensitivity of this quantification was 93.0 ± 0.7, 40.0 ± 0.3, and 54.0 ± 1.8 μA mM−1 for ACT, NPX and DCF, respectively. The analytical performance of this electrode is compared with that reported for the quantification of these drugs using HPLC, diffuse reflectance photometry, and other electrodes.