Determination of Melatonin Hormone in Bulk Form,Tablets and Human Serum by Square‐Wave Cathodic Adsorptive Stripping Voltammetry

Melatonin hormone plays an important role in many distinct physiological functions. A fully validated, sensitive and reproducible square‐wave cathodic adsorptive stripping voltammetric procedure was described for determination of melatonin in bulk form, tablets and human serum. The procedure was based on the reduction of the adsorptive hormone onto a hanging mercury drop electrode. Reduction behavior of melatonin was studied in both Britton‐Robinson (pH 2–11) and acetate (4.5–5.5) buffers. Acetate buffer of pH 5.0 was found reasonable as a supporting electrolyte for assay of the drug. The square‐wave cathodic adsorptive stripping voltammogram of melatonin showed a single well‐defined peak at ?1.45 V (vs. Ag/AgCl/KCl_s) using an accumulation potential of ?0.65 V. This peak may be attributed to the reduction of C?O double bond of the amide functional group of the reactant molecule. A mean recovery for 1×10^?8 M melatonin in bulk form followed 30 s accumulation of 98.87%±0.78 and a detection limit of 3.13×10^?10 M were achieved. The proposed procedure was successfully applied for the determination of the drug in tablets and human serum with mean recoveries of 97.68%±0.57 and 101.67%±0.85, respectively. A detection limit of 8.80×10^?10 M was achieved for the determination of the drug in human serum. Results of the proposed method were comparable and coincided with those obtained by reported method. Vitamin B₆ and common excipients, which are co‐formulated with melatonin, did not interfere. Also the effect of some interfering compounds such as serotonin, tryptophan and 5‐hydroxytryptophan on the determination of melatonin in human serum was studied, and all have no significant effect on the assay recovery.     

碳糊电极吸附溶出伏安法测定游离钙

基于Ca(Ⅱ)-茜素红S(ARS)络合物在碳糊电极上的还原波,建立了吸附溶出伏安测定游离钙的新方法。在1.5×10-2mol/LKOH-2.0×10-5mol/LARS介质中,Ca(Ⅱ)-ARS络合物在碳糊电极上于-0.89V处产生一吸附还原波。当富集电位为.0.1V,富集时间90s,扫描速度为100mV/s时,该络合物单扫描阴极溶出峰的二阶导数峰电流与游离钙离子浓度在3.0×10-8~2.0×10-6mol/L范围内呈线性关系;检出限为9.4×10-9mol/L。在0.2mol/LHCl中清洗2min,该电极重现性良好。该方法可用于血清、牛奶和自来水中游离钙的测定。     

Voltammetric Determination of Quercetin in Urine at Carbon Paste Electrode

Abstract

The electrochemical behavior of the quercetin (Q) at a carbon paste electrode (CPE) containing 15% paraffin oil is investigated. Square wave anodic stripping voltammetric (SWASV) was used to determine quercetin in the presence of 0.04 M phosphate buffer (pH ～4) containing 0.1 M KCl with 15 s accumulation time at 25±1°C. Under the optimal experimental conditions, the adsorbed form is oxidized irreversibly. The linear concentration ranged from 67.66 to 338.3 ppb quercetin. The detection limit of 6.77 ppb Q (r=0.9997), with 15 s accumulation time and the relative standard deviation of 0.45 (n=5) was calculated. The interferences of some metal ions and some amino acids were studied. The method was applied to the analysis of spiked urine, with recovery of 99.9±0.5, and the relative standard deviation of 3.2%. Results were compared with the reported methods.     

核黄素丁酸酯在汞电极上的吸附伏安特性及其反应速率常数的测定

循环伏安法表明,在ＮａＯＨ底液中核黄素丁酸酯（ｒｉｂｏｆｌａｖｉｎｅ　ｔｅｔｒａｂｕｔｙｒａｔｅ,ＲＴ）在汞电极上有一对氧化还原峰,其峰电位Ｅｐｃ＝Ｅｐａ＝－０．６４Ｖ（ｖｓ．Ａｇ／ＡｇＣｌ）。本文用多种电化学手段对ＲＴ的吸附特性进行了较详细的研究。吸附粒子为ＲＴ中性分子,测得ＲＴ在汞电极上的饱和吸附量为５．４２×１０－１１ｍｏｌ／ｃｍ２,每个ＲＴ分子所占电极面积为３．０６ｎｍ２,ＲＴ在悬汞电极上的吸附符合Ｆｒｕｍｋｉｎ等温式。测得吸附系数β＝４．６×１０５,吸引因素γ＝ １． １０,吸附自由能△Ｇ°＝－ ３２． ３０ ｋＪ／ｍｏｌ。本文还用循环伏安法研究了ＲＴ的电极反应动力学性质,测定了其在汞电极上的反应速率常数。     

Validated Voltammetric Method for the Simultaneous Determination of Anti‐diabetic Drugs,Linagliptin and Empagliflozin in Bulk,Pharmaceutical Dosage Forms and Biological Fluids

A cobalt oxide nanoparticles (Co₃O₄NPs) and multi walled carbon nanotubes (MWCNTs) modified carbon paste electrodes were used to study the electrochemical behavior of linagliptin and empagliflozin in Britton Robinson buffer solution of pH 8.0 using cyclic and square wave voltammetry. The above mentioned modified electrodes showed highly sensitive sensing and gave an excellent anodic response for both drugs. The peak current varied linearly over the concentration ranges: 3.98×10^?5–1.53×10^?3 mol L^?1 (18.82–723.00 μg/mL) and 7.94×10^?6–1.07×10^?4 mol L^?1 (3.65–48.25 μg/mL) with determination coefficients of 0.9999 and 0.9998 for linagliptin and empagliflozin, respectively. The recoveries and relative standard deviations were found in the following ranges: 98.80 %–102.00 % and 0.23 %–1.90 % for linagliptin and 98.30 %–101.80 % and 0.11 %–1.86 % for empagliflozin. The detection and quantification limits were 1.13×10^?5 and 3.76×10^?5 mol L^?1 (5.34and17.77 μg/mL) for linagliptin, 1.71×10^?6and 5.68×10^?6 mol L^?1 (0.77 and 2.56 μg/mL) for empagliflozin. The proposed sensors have been successfully applied for the determination of the drugs in bulk, pharmaceutical formulations and biological fluids.     

Voltammetric quantitation at the mercury electrode of the anticholinergic drug flavoxate hydrochloride in bulk and in a pharmaceutical formulation

Flavoxate hydrochloride, 2-piperidinoethyl 3-methyl-4-oxo-2-phenyl-4-H-chromene-8-carboxylate, is a smooth muscle antispasmodic. Its electrochemical behavior was studied at the mercury electrode in buffered solutions containing 30% (v/v) methanol using dc-polarography, differential-pulse polarography, cyclic voltammetry, and linear sweep-and square-wave adsorptive stripping voltammetry. Sensitive and precise procedures were developed for determination of bulk flavoxate hydrochloride and in the pharmaceutical formulation Genurin® S.F, without sample pretreatment or extraction. Limits of quantitation (LOQ) of 1 × 10^?5, 5 × 10^?6, 1 × 10^?8 and 1 × 10^?9 M flavoxate hydrochloride were achieved by dc-polarography, differential-pulse polarography, linear sweep and square-wave adsorptive stripping voltammetric, respectively.      

Electrochemical Behavior of Dopamine at a Mercury Electrode in the Presence of Citrate: Analytical Applications

Abstract

Dopamine can be determined by voltammetric methods using a mercury electrode, previously oxidized at +0.30 V. The oxidation product formed is stabilized in the presence of citrate and undergoes reduction at ?0.31 V. This work describes the electrochemical behavior of dopamine at a mercury electrode in the presence of citrate and its application in the development of a square‐wave voltammetric method for the dopamine determination in pharmaceutical formulations. The method was in‐house validated for determination of dopamine in injectable formulations. The detectability of the method was 0.02 µg ml^?1.     

A Low-cost Paper-based Diamond Electrode for Trace Copper Analysis at On-site Environmental Area

In the present study, an alternative platform for trace copper ions (Cu(II)) determination using a diamond electrode in combination with a disposable paper-based analytical device (d-PAD) has been proposed. First, the complexation between Cu(II) and 1,10-phenanthroline ligand was adsorptively accumulated onto a paper that is directly in contact with the diamond electrode and measured via square wave anodic stripping voltammetry. Under the optimal experimental conditions, the peak current was proportional to the concentration of Cu(II) in the range of 0.4–70 ng mL⁻¹, and the detection limit was found to be 0.1 ng mL⁻¹. Most importantly, this platform can be successfully applied to detect trace Cu(II) in different water samples (drinking water, tap water, groundwater and river water) with satisfactory results. Thus, the proposed d-PAD could be a highly efficient platform for trace Cu(II) determination in environmental samples.     

Oligonucleotide Functionalized Microporous Gold Electrode for the Selective and Sensitive Determination of Mercury by Differential Pulse Adsorptive Stripping Voltammetry (DPAdSV)

Abstract

A novel electrode modified with oligonucleotide and microporous gold was fabricated for the determination of mercury by differential pulse adsorptive stripping voltammetry (DPAdSV). Microporous gold was synthesized by electrochemical reduction using dynamic hydrogen bubble template. The oligonucleotide was immobilized on microporous gold by self-assembly. The prepared electrode exhibited an improved electrochemical response for mercury(II) ion because of the large surface area and excellent electron transfer capacity provided by microporous gold and the specific coordination between mercury ion and thymine bases in oligonucleotides. Under the optimal experiment conditions, the oligonucleotide functionalized microporous gold electrode had a linear relationship between the stripping current and mercury ion concentration in the range from 0.5 to 30?µg/L with a detection limit of 0.021?µg/L. Moreover, the prepared electrode exhibited good selectivity, reproducibility, repeatability and stability. Furthermore, the prepared electrode was applied to detect mercury in tap water with satisfactory results.     

Measuring the Electrode Kinetics of Vitamin B2 at a Constant Time Window of a Square Wave Voltammetric Experiment

The electrode kinetics of vitamin B2 was measured at a constant scan rate of a square‐wave (SW) voltammetric experiment by recently introduced method based on alteration of the height (amplitude) of the SW potential pulses. The electrode reaction mechanism was analyzed in the light of the simplified model considering two‐electron one step surface electrode mechanism, as well as to more appropriate EE surface reaction pathway consisting of two successive one‐electron quasireversible reactions. Theoretical analysis indicated that the evolution of the voltammetric response under variation of the SW amplitude enables estimation of the rate determining step of the EE mechanism.     

Imprinted Polymer – Modified Hanging Mercury Drop Electrode for Differential Pulse Adsorptive Stripping Voltammetric Analysis of a Diquat Herbicide

An imprinted polymer modified hanging mercury drop electrode (HMDE) in Model 303A system in conjunction with a PAR Model 264A Polarographic Analyzer/Stripping Voltammeter has been used for the selective analysis of a diquat herbicide viz., 5,6‐dihydropyrazino[1,2,3,4‐[lmn]‐1,10‐phenanthrolinium dichlorides in differential pulse cathodic stripping voltammetry mode. Complex aqueous samples (drinking water and agricultural soil suspension), spiked with a diquat herbicide, were directly analyzed by the adsorptive accumulation of the analyte over the working electrode (accumulation potential ?0.8 V (vs. Ag/AgCl), accumulation time 120 s, pH 7.0, supporting electrolyte 0.1 M KCl, scan rate 10 mV s^?1, pulse amplitude 25 mV). The limit of detection for diquat herbicide was found to be 0.34 nmol L^?1 (0.1 ppb, RSD 2%, S/N=2).     

Adsorptive Stripping Voltammetric Determination of Phenol at an Electrochemically Pretreated Carbon-Paste Electrode with Solid Paraffin as a Binder

An adsorptive stripping voltammetric method for determination of phenol at an electrochemically pretreated carbon-paste electrode has been developed. Solid paraffin was used as the binder of the carbon-paste electrode. The carbon-paste electrode was pretreated in the solution of 0.001 mol L⁻¹NaOH by holding it at +1.8 V (versus an Ag/AgCl electrode) for 5 min. On the pretreated electrode, the adsorption of phenol was greatly enhanced. Phenol was accumulated in NH₃–NH₄Cl (pH 9.25) medium at the potential of +0.1 V (versus Ag/AgCl electrode) for a certain time and then determined by second order differential anodic stripping voltammetry. An oxidative peak was observed at about +0.66 V. The relationship between second order peak current and phenol concentration was linear in the range of 2.5 × 10⁻⁷–5.0 × 10⁻⁶mol L⁻¹phenol, and the detection limit was 5.0 × 10⁻⁸mol L⁻¹. The method has been applied to the determination of phenol in tap water and waste water. The relative standard deviation (six determinations) was less than 3.5%.     

Electroanalysis of the Anthelmintic Drug Bithionol at Edge Plane Pyrolytic Graphite Electrode

An electrochemical study of the anthelmintic drug bithionol using edge plane pyrolytic graphite electrode (EPPGE) is presented for the first time by applying different electrochemical techniques, such as cyclic voltammetry (CV), square‐wave voltammetry (SWV), square‐wave adsorptive stripping voltammetry (SWAdSV), and alternating current (AC) impedance spectroscopy. Mechanistic aspects of the electrode reaction were studied implying a quasireversible electrode reaction from an adsorbed state of the reactant, coupled with a follow‐up chemical reaction to a final electroinactive product. The overall mechanism appears totally irreversible under conditions of CV at moderate scan rate, while being quasireversible under conditions of the fast SWV. Furthermore, an optimisation of the analytical procedure for quantitative determination of bithionol was conducted by applying SWV in an adsorptive stripping mode. The calibration curve was constructed in the concentration range of 0.1–1.0 μmol L^?1 (R²=0.9984) with a sensitivity of 3.6 μA L μmol^?1 and LOD of 26.7 nmol L^?1. The simple and sensitive SWAdSV procedure was proved to be suitable for the analysis of spiked urine samples.     

Sensitive Detection of Capsaicin by Adsorptive Stripping Voltammetry at a Boron‐Doped Diamond Electrode in the Presence of Sodium Dodecylsulfate

In the present paper, a sensitive electroanalytical methodology for the determination of capsaicin using adsorptive stripping voltammetry (AdSV) at a boron‐doped diamond (BDD) electrode is presented. The voltammetric results indicate that in the presence of sodium dodecylsulfate (SDS) the BDD electrode remarkably enhances the oxidation of capsaicin which leads to an improvement of the peak current with a shift of the peak potential to less negative values. A linear working range of 0.05 to 6.0 µg mL^?1 (0.16–20 µM) with a detection limit of 0.012 µg mL^?1 (0.034 µM) has been obtained using BDD electrode by AdSV.     

Behavior and Quantification Studies of Phoxim Pesticide Using Cyclic and Linear Sweep Stripping Voltammetry at a Mercury Electrode

Phoxim, benzoyl cyanide-O-(diethoxyphosphinothioyl)oxime, is an organophosphorusinsecticide of moderate to low mammalian toxicity. Phoxim was individuallydetermined in fruit and vegetable samples, utilizing chromatography1 and single sweeppolarography2.Ex…     

Highly Sensitive and Selective Adsorptive Stripping Voltammetric Method Employing a Lead Film Screen‐printed Electrode for Determination of Cobalt as its Nioximate Complex

A carbon screen‐printed electrode modified in‐situ with lead film (PbF‐SPCE) was applied for the adsorptive stripping voltammetric determination of Co(II) in the form of a complex with 1,2‐cyclohexanedione dioxime. Lead film was electrochemically deposited in situ on SPCE from a 0.2 M ammonia buffer solution (pH 8.7) containing 5 ? 10^?5 M Pb(NO₃)₂ and 5 ? 10^?5 M nioxime. Due to the very low LOD (0.003 µgL^?1, i.e., 0.05 nmol L^?1 Co(II); t_acc=120s), the developed procedure could be rated among the most sensitive methods employing SPEs. The Ni(II) signal was significantly lower than the Co(II) one and the separation of Ni(II) and Co(II) peaks was even better at the PbF‐SPCE than at the hanging mercury drop electrode.     

Electroanalytical Studies of Biologically Active Azosalicylic Acid at a Hanging Mercury Drop Electrode

The electrochemical behavior of disodium 3,3′‐azobis‐(6‐hydroxy‐)benzoate (olsalazine sodium), 2‐hydroxy‐5‐[(4‐sulfophenyl)azo]benzoic acid and 2‐hydroxy‐5‐azo‐benzoic acid was studied as a simple model of the metabolic reduction of azosalicylic acids using cyclic and square‐wave voltammetry. The stability of the hydrazo intermediate generated in the electroreduction process is strongly dependent on pH and the effect of different substituents in the aromatic ring adjacent to the azo bridge. The electrode mechanism of azosalicylic acids at hanging mercury drop electrode exhibits characteristic properties of a surface redox reaction. The kinetic parameters of azo/hydrazo reduction, such as the standard redox reaction rate constant and the electron transfer coefficient were estimated applying the methods of a split SW peak and a quasireversible maximum. A sensitive analytical method for the determination of trace level of biologically active azosalicylic acids by square‐wave voltammetry based on adsorptive stripping technique is described.     

Cathodic Stripping Voltammetric Behavior of Cytosine in Presence of Acridine Orange at the Static Mercury Drop Electrode

Abstract

Cathodic adsorptive electrochemical behavior of cytosine in the presence of acridine orange (DNA‐intercalating dye) at the static mercury drop electrode was investigated. A 0.001 M NaOH solution containing 2 ppm of acridine orange was found to be the optimal supporting electrolyte. Other optimum experimental conditions were found to be as follows: the initial potential at 0.00 V, linear n, and the potential scan rate at 50 mV s^?1. The analytical response was found to be linear in a range of cystosine concentrations from 0.30 ppm to 1.1 ppm. For a direct response, the detection limit was 1.2 ppb. The presence of ssDNA and the interference of some trace metals were also investigated.     

Study on the Electrochemical Behavior of Melatonin with an Activated Electrode

It was found that melatonin could be incorporated and accumulated on the surface of the glassy carbon electrode which was activated electrochemically by pretreatment in sodium hydroxide solution by means of cycling the potential well into the positive limit of the solvent. In Britton‐Robinson buffer solution (pH 6.7), melatonin gave a sensitive oxidation wave at a potential of +0.65 V (vs.Ag/AgCl) by using Osteryoung square‐wave stripping voltammetry (OSWSV). The oxidation process has been shown to be irreversible and adsorption‐controlled at this electrode by means of cyclic voltammetry and linear sweep voltammetry. A chronocoulometric characterization of the adsorption characteristics of melatonin at this electrode is also presented. The factors affecting the peak current were optimized, and the dependence of peak currents on the concentration of melatonin was found to be linear in the range 8.0×10^?7?1.0×10^?5 mol L^?1. A detection limit of 5.0×10^?8 mol L^?1 was obtained (signal‐to‐noise ratio of 3). This method was applied to the assay of melatonin in tablets and capsules with good recoveries (98–100%).     

阴极溶出伏安法测定异烟肼的研究

报道了吸附阴极溶出伏安法测定异烟肼。探讨了电极反应机理。在０．０１ｍｇｏｌ／Ｌ硼砂底液中，峰电位为－１．４０Ｖ。峰电流与异烟肼浓度在４．０＊１０＾－７－１．５＊１０＾－５ｍｏｌ／Ｌ范围内呈良好的线性关系，相关系为０．９９１３。该方法用于异烟肼片剂的测定，获得满意结果。