Electrochemical properties and square-wave voltammetric determination of pravastatin

The electrochemical reduction and adsorptive voltammetric behaviour of pravastatin have been studied by means of cyclic and square-wave voltammetry at a hanging mercury-drop electrode in electrolytes of different pH. Within the entire pH range (2.0–9.0) in Britton–Robinson buffer, pravastatin gave rise to a single voltammetric peak in the potential interval from −1.22 to −1.44 V, depending on pravastatin concentration. It was found that the reduction of pravastatin proceeds via a relatively stable intermediate, which is transformed to the final electroinactive product by a coupled chemical reaction or can be re-oxidized back to pravastatin. The rate of chemical transformation is controlled by the proton concentration. The electrode mechanism has the properties of a surface redox reaction. A sensitive analytical method for trace analysis of pravastatin based on the adsorptive stripping technique has been developed. The calibration plot was linear in the range 8×10⁻⁸–5×10⁻⁷ mol L⁻¹. Application of the square-wave voltammetric method to determination of pravastatin in a pharmaceutical dosage form, without sample pretreatment, resulted in acceptable deviation from the stated concentration.     

Osteryoung square wave voltammetric determination of lactose in food samples by a derivative procedure

A method for determination of lactose in food samples by Osteryoung square wave voltammetry （OSWV） was developed. It was based on the nucleophilic addition reaction between lactose and aqua ammonia. The carbonyl group of lactose can be changed into imido group, and this increases the electrochemical activity in reduction and the sensitivity. The optimal condition for the nucleophilic addition reaction was investigated and it was found that in NH4C1-NH3 buffer of pH 10.1, the linear range between the peak current and the concentration of lactose was 0.6-8.4 mg L-l, and the detection limits was 0.44 mg L- 1. The proposed method was applied to the ＇determination of lactose in food samples and satisfactory results were obtained.     

Simultaneous square-wave voltammetric determination of acetazolamide and theophylline in pharmaceutical formulations

Simple, rapid, sensitive and low cost voltammetric method for simultaneous determination of acetazolamide and theophylline in pharmaceutical formulations, was developed using a static mercury drop electrode (SMDE). Well-defined voltammetric peaks were obtained at–0.87 and–1.33 V for acetazolamide and–0.21 V (vs. Ag/AgCl) for theophylline in Britton–Robinson (B–R) buffer (pH 2.4). The reduction peak currents are found to be linearly dependent on the concentration for the both drugs. Calibration graphs were obtained over the concentration range 1.98 × 10^–6 to 5.94 × 10^–5 M and 2.0 × 10^–5 to 5.6 × 10^–4 M for acetazolamide and theophylline, respectively. The limits of detection (LOD) and quantitation (LOQ) of the procedure were also presented. Factors such as, pH of supporting electrolyte, equilibrium time, frequency, scan rate and pulse height were optimized. The validated voltammetric method was successfully applied for simultaneous determinations of the two drugs. The procedure does not require any sample pretreatment or timeconsuming separation steps.     

Electrochemical studies and square-wave voltammetric determination of fenofibrate in pharmaceutical formulations

The electrochemical reduction of fenofibrate at a hanging mercury drop electrode (HMDE) was investigated by cyclic voltammetry, square-wave voltammetry, and chronoamperometry. Different buffer solutions were used over a wide pH range (3.0–10.0). The best definition of the analytical signals was found in borate buffer (pH 9.0)–tetrabutylammonium iodide mixture containing 12.5% (v/v) methanol at –1.2 V (versus Ag/AgCl). According to cyclic voltammetric studies, the reduction was irreversible and diffusion controlled. The diffusion coefficient was 2.38×10^–6 cm² s^–1 as determined by chronoamperometry. Under optimized conditions of square-wave voltammetry, a linear relationship was obtained between 0.146–4.96 g mL^–1 of fenofibrate with a limit of detection of 0.025 g mL^–1. Validation parameters such as sensitivity, accuracy, precision, and recovery were evaluated. The proposed method was applied to the determination of fenofibrate in pharmaceutical formulations. The results were compared with those obtained by a published high-performance liquid chromatography method. No difference was found statistically.     

Simultaneous square-wave voltammetric determination of riboflavin and folic acid in pharmaceutical preparations

The square-wave voltammetric (SWV) behaviour of riboflavin and folic acid was studied at a static mercury drop electrode by square wave voltammetry. In 0.05M KCl (pH 5.89) a cathodic scan gave peaks at — 0.56 and — 0.87 V vs. Ag/AgCl for riboflavin and folic acid, respectively. The reduction peak currents are linearly dependent on the concentration of vitamins. Both vitamins can be simultaneously determined from the same voltammogram. The method proposed for the determination of riboflavin and folic acid in multivitamin tablets is very simple, rapid and does not involve time-consuming separation steps. The average contents of riboflavin and folic acid were found to be 14.8 ± 1.26% and 1.46 ± 2.66%, for tablet A and 9.86 ±1.40% and 1.47 ± 2.0% for tablet B, respectively.     

Simultaneous square-wave voltammetric determination of aspartame and cyclamate using a boron-doped diamond electrode

A simple and highly selective electrochemical method was developed for the simultaneous determination of aspartame and cyclamate in dietary products at a boron-doped diamond (BDD) electrode. In square-wave voltammetric (SWV) measurements, the BDD electrode was able to separate the oxidation peak potentials of aspartame and cyclamate present in binary mixtures by about 400 mV. The detection limit for aspartame in the presence of 3.0x10(-4) mol L(-1) cyclamate was 4.7x10(-7) mol L(-1), and the detection limit for cyclamate in the presence of 1.0x10(-4) mol L(-1) aspartame was 4.2x10(-6) mol L(-1). When simultaneously changing the concentration of both aspartame and cyclamate in a 0.5 mol L(-1) sulfuric acid solution, the corresponding detection limits were 3.5x10(-7) and 4.5x10(-6) mol L(-1), respectively. The relative standard deviation (R.S.D.) obtained was 1.3% for the 1.0x10(-4) mol L(-1) aspartame solution (n=5) and 1.1% for the 3.0x10(-3) mol L(-1) cyclamate solution. The proposed method was successfully applied in the determination of aspartame in several dietary products with results similar to those obtained using an HPLC method at 95% confidence level.     

Adsorptive stripping square-wave voltammetric behavior of insulin

The adsorptive stripping voltammetric behavior of bovine insulin has been studied by both differential pulse and square-wave techniques, leading to analytical methodologies for its determination in aqueous samples and pharmaceutical preparations. The application of a square-wave mode for the stripping of adsorbed bovine insulin has proved to be much more sensitive, yielding signals 20 times larger than those obtained by applying a differential pulse scan. The precision obtained for the voltammetric method, at a concentration level as low as 0.77 mg L^–1 of the protein, was found to be 1.45% (RSD., n = 8) by SWS and 2.20% (RSD., n = 8) by DPS. The pharmaceutical product Actrapid containing biosynthetic human insulin was analyzed by both methods. Received: 25 February 1999 / Revised: 6 April 1999 / Accepted: 12 April 1999     

Adsorptive stripping square-wave voltammetric behavior of rofecoxib

Adsorption and reduction of rofecoxib were investigated by cyclic and square-wave voltammetry on a hanging mercury drop electrode in electrolytes of various pH values. The reduction process on hanging mercury drop electrodes gave rise to a single peak within the entire pH range (2.0-11.5). In alkaline solutions, rofecoxib gave a sensitive adsorptive reductive peak; approximately 10 times larger than those obtained by applying a square-wave scan without prior accumulation. Application of the method to the determination of rofecoxib in two pharmaceutical products (Vioxx 12.5 and 25 mg), without sample pretreatment, resulted in acceptable deviation from the stated concentrations.     

Adsorptive stripping square-wave voltammetric behavior of insulin

The adsorptive stripping voltammetric behavior of bovine insulin has been studied by both differential pulse and square-wave techniques, leading to analytical methodologies for its determination in aqueous samples and pharmaceutical preparations. The application of a square-wave mode for the stripping of adsorbed bovine insulin has proved to be much more sensitive, yielding signals 20 times larger than those obtained by applying a differential pulse scan. The precision obtained for the voltammetric method, at a concentration level as low as 0.77 mg L^–1 of the protein, was found to be 1.45% (RSD., n = 8) by SWS and 2.20% (RSD., n = 8) by DPS. The pharmaceutical product Actrapid containing biosynthetic human insulin was analyzed by both methods. Received: 25 February 1999 / Revised: 6 April 1999 / Accepted: 12 April 1999     

Electroanalytical applications of cationic self-assembled monolayers: square-wave voltammetric determination of dopamine and ascorbate

Gold electrodes modified with cationic self-assembled monolayers (SAMs) of 2,2'-dithiobisethaneamine (CYST) and 6,6'-dithiobishexaneamine (DTH) were used for the simultaneous determination of dopamine (DA) and ascorbate (AA). The cationic SAM modified electrodes have several advantages over the bare electrode for the oxidation of AA. A very large (approximately 450 mV) decrease in the overpotential for the oxidation of AA when compared with the bare electrode has been observed at the cationic monolayer-modified electrode. The electrostatic interaction of negatively charged AA with the monolayer shift the oxidation peak potential of AA to less positive potential and enhances the peak current. On the other hand, the positively charged DA is repelled from the monolayer and the oxidation potential shifts to more positive potential when compared to the bare electrode. The electrochemical oxidation of AA at the mixed monolayer of CYST and diethyl disulfide (DEDS) supports the influence of cationic terminal group of the monolayer on the oxidation of AA. Since the oxidation of AA occurs well before the oxidation potential of DA is reached, the homogeneous catalytic oxidation of AA by the oxidized DA has been advantageously eliminated at the monolayer-modified electrode. The cationic self-assembled monolayers successfully detect DA in the presence of high concentration of AA. The sensitivity of the electrode modified with CYST monolayer was found to be 0.036 and 0.021 microA/microM towards AA and DA, respectively.     

Determination of furazolidone in urine by square-wave voltammetric method

A fast, simple and sensitive square-wave voltammetric (SWV) method for the determination of trace amounts of furazolidone (FZ) in urine is reported. A three-electrode system containing stationary mercury dropping (SMDE) working electrode, Pt auxiliary electrode and Ag/AgCl reference electrode was used throughout. Briton-Rabinson buffer solution is used as both pH adjusting agent and supporting electrolyte. The calibration graph showed good linearity in the concentration range of 20–900 ng ml^?1 of furazolidone with a regression coefficient of 0.9996. The equation Δ(i) = 0.0095C_FZ + 0.234 was used for calculation of furazolidone concentration in the sample solution, where C_FZ is the concentration of furazolidone in ng ml^?1 and Δ(i) is the difference between voltammogram peak currents of sample and blank solution. The RSD for 8 replicate measurements of a 60 ng ml^?1 solution and LOD of the proposed method were found to be 2.2% and 5.2 ng ml^?1, respectively. The procedure was successfully applied to the determination of furazolidone in urine samples.     

Liquid chromatographic determination of nicarbazin in feeds

A new liquid chromatographic method has been developed for determination of nicarbazin in feeds. Approximately 40 g feed is extracted with 200 mL acetonitrile-water (80 + 20, v/v). An aliquot of the extract is filtered and assayed using a reversed-phase isocratic method that measures the 4,4'-dinitrocarbanilide moiety of nicarbazin at a wavelength of 340 nm. For medicated feeds, the method uses a standard linear range of 5 to 100 microg/mL. For lower levels, a linear range of 50 to 150 ng/mL can be used. The method has a limit of detection of 250 ng/g and a limit of quantitation of 500 ng/g in a 40 g feed sample. Recovery was 99.1%, with a range of 95.2 to 101.8%. In the typical U.S. dosing range of 27 to 113.5 g/ton, the precision of the method based on one analyst, one day, and 2 weighings ranged from 2.8% (113.5 g/ton) to 4.7% (27 g/ton).     

Cathodic adsorptive stripping square-wave voltammetric determination of nifedipine drug in bulk,pharmaceutical formulation and human serum

Nifedipine is a calcium-channel antagonist drug used in the management of angina pectoris and hypertension through inhibition of calcium influx. A fully validated sensitive cathodic adsorptive stripping square-wave voltammetry procedure was optimized for the determination of the drug at trace levels. The procedure was based on the reduction of the nitrophenyl group after the interfacial accumulation of the drug onto a hanging mercury drop electrode in Britton-Robinson buffer of pH 11.0. The optimal conditions of the procedure were found to be: accumulation potential=-0.9 V vs. Ag/AgCl/KCl(s)), accumulation time=30 s, scan increment=10 mV, pulse amplitude=50 mV and frequency=120 Hz. Under these conditions, a well-defined peak was obtained; its peak current showed a linear dependence on drug concentration in the range of 2x10(-9)-2x10(-7) mol L(-1) bulk nifedipine. The mean recoveries based on eight replicate measurements for 1x10(-8) and 5x10(-8) mol L(-1) bulk nifedipine solutions were 98.46+/-0.86% and 98.23+/-0.92%, respectively. A detection limit of 3.42x10(-10) mol L(-1) bulk nifedipine was achieved. The procedure was successfully applied for assay of the drug in tablets and spiked human serum with mean recoveries of 101.95+/-1.42% and 98.70+/-0.63%, respectively. The limit of detection of the drug in spiked human serum was found to be 3.90x10(-10) mol L(-1).     

A square-wave adsorptive stripping voltammetric method for the determination of Amaranth, a food additive dye

Square-wave adsorptive stripping voltammetric (AdSV) determinations of trace concentrations of the azo coloring agent Amaranth are described. The analytical methodology used was based on the adsorptive preconcentration of the dye on the hanging mercury drop electrode, followed by initiation of a negative sweep. In a pH 10 carbonate supporting electrolyte, Amaranth gave a well-defined and sensitive AdSV peak at -518 mV. The electroanalytical determination of this azo dye was found to be optimal in carbonate buffer (pH 10) under the following experimental conditions: accumulation time, 120 s; accumulation potential, 0.0 V; scan rate, 600 mV/s; pulse amplitude, 90 mV; and frequency, 50 Hz. Under these optimized conditions the AdSV peak current was proportional over the concentration range 1 x 10(-8)-1.1 x 10(-7) mol/L (r = 0.999) with a detection limit of 1.7 x 10(-9) mol/L (1.03 ppb). This analytical approach possessed enhanced sensitivity, compared with conventional liquid chromatography or spectrophotometry and it was simple and fast. The precision of the method, expressed as the relative standard deviation, was 0.23%, whereas the accuracy, expressed as the mean recovery, was 104%. Possible interferences by several substances usually present as food additive azo dyes (E110, E102), gelatin, natural and artificial sweeteners, preservatives, and antioxidants were also investigated. The developed electroanalyticals method was applied to the determination of Amaranth in soft drink samples, and the results were compared with those obtained by a reference spectrophotometric method. Statistical analysis (paired t-test) of these data showed that the results of the 2 methods compared favorably.     

Electrochemical behavior of thiamine on a self-assembled gold electrode and its square-wave voltammetric determination in pharmaceutical preparations.

The electrochemical behavior of thiamine on a self-assembled electrode of L-cysteine (Cys/SAM/Au) has been investigated and Cys/SAM/Au can be used to detect thiamine using square-wave voltammetry (SWV). At pH 11.40 Britton-Robinson buffer, thiamine exhibits a well-defined anodic peak on Cys/SAM/Au. Under the optimized conditions, the anodic peak current of SWV was linear with the content of thiamine in the range of 1.1 x 10(-8) - 2.2 x 10(-6) mol/L; the detection limit was 5.5 x 10(-9) mol/L. The method was successfully applied to the determination of thiamine in pharmaceutical preparations.     

Catalytic square-wave voltammetric detection of DNA with reversible metallopolymer-coated electrodes

Pyrolytic graphite electrodes with adsorbed poly(4-vinylpyridine) (PVP) with attached Ru(bpy)₂²⁺ gave reversible voltammetry, facilitating reusable DNA sensors. These electrodes gave catalytic voltammetric responses to poly(guanylic) acid and DNA at about 0.75 V vs SCE caused by catalytic oxidation of guanine moieties in these polynucleotides. ss-DNA gave twice the square-wave voltammetric catalytic current compared to an equivalent amount of ds-DNA, suggesting that the [Ru(bpy)₂–PVP]²⁺ polymer may be useful for detection of DNA hybridization and damage.     

Application of carbon nanoparticle/chitosan modified electrode for the square-wave adsorptive anodic striping voltammetric determination of Niclosamide

A new modified electrode formed by carbon nanoparticle/chitosan film (CNP/CS) was used for electrocatalytic reduction of Niclosamide (NA). The electrochemical behavior of NA at the CNP/CS modified electrode was investigated in detail by the means of cyclic voltammetry. The reduction mechanism of NA, corresponds to the redox chemistry of nitro group, was thoroughly investigated. The effect of the experimental parameters e.g. potential and time of accumulation, pH of the buffered solutions and potential sweep rate on the response of the electrode was studied. The prepared electrode showed high stability and uniformity in the composite film, short response time, good reproducibility and an excellent catalytic activity toward the electro-reduction of NA leading to a significant improvement in response sensitivity. The square-wave adsorptive anodic stripping voltammetry (SWAASV) was used as an efficient method for the determination of NA. Under the optimal conditions, the modified electrode showed a wide linear response to the concentration of NA in the range of 0.01–2 μM with a detection limit of 7.7 nM. The prepared electrode was successfully applied for the determination of NA in pharmaceutical and clinical samples and satisfactory results are obtained.     

Anodic square-wave stripping voltammetric analysis of epinephrine using carbon fiber microelectrode

The behavior of epinephrine on a carbon fiber microelectrode (CFME) was studied with cyclic voltammetry and square-wave (SW) stripping voltammetry in order to find optimum conditions for its analysis using the CFME. An anodic stripping peak at about 0.20 V (vs. Ag/AgCl) was utilized. Under optimum conditions for anodic stripping (a pH value of 7.4, a deposition potential of − 0.5 V, a deposition time of 90 s, a final potential of 0.8 V, a SW frequency of 20 Hz, a step potential of 5 mV, and an amplitude of 45 mV), the calibration was linear in the concentration range of 0.0∼0.4 mg/L, and the detection limit was found to be 0.009 mg/L with a relative standard deviation 0.18% (n = 12) at 0.2 mg/L. The analytical performance of the CMFE, which is unmodified, is comparable to or better than other voltammetric results with various modified electrodes.     

Electrooxidation of the antiviral drug valacyclovir and its square-wave and differential pulse voltammetric determination in pharmaceuticals and human biological fluids

The electrochemical properties of valacyclovir, an antiviral drug, were investigated in pH range 1.8-12.0 by cyclic, differential pulse and square-wave voltammetry. The drug was irreversibly oxidized at a glassy carbon electrode in one or two oxidation steps, which are pH-dependent. For analytical purposes, a very resolved diffusion controlled voltammetric peak was obtained in Britton-Robinson buffer at pH 10.0 using differential pulse and square-wave modes. Limits of detection were 1.04 × 10⁻⁷ and 4.60 × 10⁻⁸ M for differential pulse and square-wave voltammetry, respectively. The applicability to direct assays of tablets, spiked human serum and simulated gastric fluid, was described.     

Adsorptive stripping square-wave voltammetric study of the degradation of lansoprazole in aqueous solutions

The adsorption and reduction behavior of lansoprazole has been studied by square-wave voltammetry at a hanging mercury drop electrode. A study of the variation in the reduction signal with solution variables, such as pH and concentration of lansoprazole, and instrumental variables, such as accumulation time and potential, frequency, pulse height and pulse amplitude, has resulted in optimization of the reduction signal for analytical purposes. The voltammetric procedure was successfully applied for rapid analysis of lansoprazole in stability studies without interference from the degradation products. It has also been used for the precise determination of lansoprazole in a pharmaceutical preparation.