Cathodic stripping voltammetry of NADH in presence of Cu2+

The adsorption and accumulation of NADH and the Cu²⁺-NADH system at the mercury electrode surface was examined using differential pulse cathodic stripping voltammetry (DPCSV). The method was developed for analytical trace determination of NADH. Experimental and operational parameters for the quantitative determination of NADH were optimized and the detection limit was found to be 9.960 × 10^–8 mol/L. The effect of some interferences (e.g. purine compounds, amino acids and some metal ions) was considered. Received: 3 January 1997 / Revised: 1 April 1997 / Accepted: 4 April 1997     

Cathodic stripping voltammetry of 2-mercaptoethanol

The behaviour of 2-mercaptoethanol at a hanging mercury drop electrode by cathodic stripping voltammetry (c.s.v.) is studied. The stripping curves are recorded by three scanning modes: rapid-scan direct-current, differential-pulse and fundamental harmonic alternating-current polarography. Under the recommended conditions, pre-electrolysis is done at a potential of 0.0 V vs. Ag/AgCl for 3 min in a medium of pH 6.7 or 8 (Britton-Robinson buffer). Then after 1 min, stripping is done at a scan rate of 6.6 mV s^?1 preferably in the differential-pulse mode. The stripping peak at about ?0.4 V is used to determine 2-mercaptoethanol within the concentration range 3 × 10^?8/2-8 × 10^?7 mol l^?1. Calibration functions are reported; the standard additions method is preferred near the limit of detection. The interferences of several organic compounds are described.     

Cathodic stripping voltammetry and adsorptive stripping voltammetry of selenium(IV)

Cathodic stripping voltammetry of selenium(IV) in 0.1 M hydrochloric acid media yielded a nonlinear calibration graph for the concentration range 10^?9?10^?8 M. In this concentration range, adsorptive stripping voltammetry based on adsorption of the selenium/3,3′-diaminobenzidine complex on the surface of the hanging mercury drop electrode at the deposition potential of +0.05 V (vs. SCE) is more convenient. A linear calibration graph is obtained for selenium concentrations of 3×10^?9?3×10?8 M, with an accumulation time of 300 s.     

Cathodic adsorptive stripping voltammetry of 6-thiopurine in absence and presence of some metal ions

Summary Cathodic stripping voltammetry of an adsorbed 6-thiopurine at HMDE was investigated in solutions of varying pH. A rapid and sensitive differential pulse voltammetric method was selected for its trace determination. A method has also been developed for the determination of 6-thiopurine in presence of Cu(II), due to the strong adsorption of the Cu-6-thiopurine complex at the surface of the HMDE and subsequent reduction of the surface-bound complex. A detection limit of 9.9×10^–9 mol/l was achieved in presence of Cu(II) and the slope of the straight line was seven times the slope in absence of Cu(II). Cathodic adsorptive stripping (CAS) voltammetry of 6-thiopurine in presence of Ni(II), Pb(II), Cd(II), and Fe(III) was also investigated. The influence of several operational parameters has been considered. Statistical analysis of the calibration curve data is included.     

Cathodic stripping voltammetry of the peptide felypressin in trace amounts

Felypressin, a peptide containing eight amino acids including cystine, is studied by cathodic stripping voltammetry (c.s.v.) at a mercury drop electrode at pH 4.6 in the concentration interval 5 × 10^?9-7 × 10^?7 M. Excess of copper(II) ions is required to obtain the c.s.v. activity. The stripping peak potential is ?0.55 to ?0.70 V vs. SCE depending on the excess of copper(II). The accumulated product is adsorbed both in its oxidized and reduced state. Interference from c.s.v.-active substances which desorb in the reduced state can be eliminated by applying a repetitive cyclic scan and evaluating the second or third scan. Lypressin and somatostatin, two other cystine-containing peptides, are also c.s.v.-active.     

Cathodic stripping voltammetry of 2-thioorotic acid at the hanging mercury drop electrode

Controlled adsorptive accumulation of 2-thioorotic acid (6-carboxy-2-thiouracil) on the hanging mercury drop electrode provides the basis for the direct stripping measurement of that compound in the nanomolar concentration level. Differential pulse voltammetry, following 3 min preconcentration, yields a detection limit of 5.0×10^-10 M 2-thioorotic acid. The cathodic stripping response is evaluated with respect to experimental parameters such as preconcentration time and potential, bulk concentration and others. Best results are obtained using a 0.001 M NaOH electrolyte.Two different methods of cathodic stripping voltammetry can be proposed for the determination of 2-thioorotic acid and the reproducibility of these methods is studied.     

Cathodic adsorptive stripping square-wave voltammetry of the anti-inflammatory drug meloxicam

The adsorptive behavior of the anti-inflammatory drug meloxicam was studied by cyclic, differentia-pulse and square-wave voltammetry on a hanging mercury drop electrode (HMDE). The drug was accumulated at HMDE and a well-defined stripping peak current was obtained at -1.42 V vs. Ag/AgCl (saturated KCl) electrode in acetate buffer solution (pH 5.0). A voltammetric procedure was developed for the determination of meloxicam using square-wave cathodic adsorptive stripping voltammetry (SW-CASV). The optimum working conditions for the determination of the drug were established. The analysis of meloxicam in human plasma was carried out satisfactorily.     

Cathodic stripping voltammetry of the anticancer agent 5-fluorouracil and determination in urine

Cathodic stripping voltammetry was used to determine 5-fluorouracil (5-FU) in the presence of traces of Cu(II). It was found that the addition of 5 x 10(-9) mol dm(-3) Cu(II) to the measurement cell greatly enhanced the peak current of the adsorbed molecule. Different parameters were tested to optimize the conditions for the determination of 5-FU. The adsorbed form is reduced irreversibly. It was observed that by controlling the deposition potential, the technique could be directed to the determination of Cu(II) or the drug. The linear range was from 5 x 10(-9) to 6 x 10(-8) mol dm(-3) for 5-FU and from 6 x 10(-9) to 5 x 10(-8) mol dm(-3) for Cu(II). Detection limits of 4.6 x 10(-10) and 5 x 10(-10) mol dm(-3) were obtained for 5-FU and Cu(II), respectively. The method was applied to urine and molecules or ions which may interfere were studied.     

Cathodic stripping voltammetry of cysteine using silver and copper solid amalgam electrodes

Silver and copper solid amalgam electrodes (modified with mercury meniscus and based on amalgamation of fine metallic powder) have been successfully tested for cathodic stripping voltammetry of cysteine. In the case of the silver solid amalgam electrode AgSAE the relative standard deviation (RSD) and the detection limit (3 SD) reached +/-2.3% and 3x10(-9) mol l(-1) cysteine, respectively.     

Cathodic stripping voltammetry of selenocystine,cystine, and cysteine in dilute aqueous acid

Selenocystine, cystine, and cysteine can be determined by cathodic stripping voltammetry (c.s.v.) in 0.1 M HClO₄ or 0.1 M H₂SO₄. The amino acids are accumulated at potentials more positive than —0.35 V, —0.20 V, and —0.10 V vs. s.c.e., respectively, and the stripping peak potentials are —0.45 V, —0.38 V and —0.15 V, respectively. Limiting coverage of the mercury electrode surface is observed for cysteine and selenocystine, but not for cystine. The detection limit for selenocystine is 5 × 10^-10 M in the presence of 100-fold amounts of cystine and cysteine. The detection limits for cystine and cysteine are 1 × 10^-5 M and 1 × 10^-9 M, respectively.     

Cathodic stripping voltammetry of trace Mn(II) at carbon film electrodes

A sensitive voltammetric method is presented for the determination of tract levels of Mn (II) using carbon film electrodes fabricated from carbon resistors of 2 Ω. Determination of manganese was made by square wave cathodic stripping voltammetry (CSV), with deposition of manganese as manganese dioxide. Chronoamperometric experiments were made to study MnO₂ nucleation and growth. As a result, it was found to be necessary to perform electrode conditioning at a more positive potential to initiate MnO₂ nucleation. Under optimised conditions the detection limit obtained was 4 nM and the relative standard deviation for eight measurements of 0.22 nM was 5.3%. Interferences from various metal ions on the response CSV of Mn(II) were investigated, namely Cd(II), Ni(II), Cu(II), Cr(VI), Pb(II), Zn(II) and Fe(II). Application to environmental samples was demonstrated.     

Cathodic stripping voltammetry of pipemidic acid and ofloxacin in pharmaceutical dosages and human urine

Sensitive cathodic stripping voltammetric methods have been developed for two quinolone antibacterial drugs, pipemidic acid (PIP) and ofloxacin (OFL) using hanging mercury drop electrode as working electrode vs. Ag/AgCl reference electrode. The methods were developed for the determination of drugs individually as well as simultaneously. 0.1 M and 0.01 M hydrochloric acid was used as medium for PIP and OFL, respectively, 0.1 M potassium chloride was used as base electrolyte. Reduction waves were observed for PIP within ?700 mV to ?800 mV and for OFL within ?1100 mV to ?1200 mV. Linear calibration ranges for PIP and OFL were observed within 10–100 μg ml^?1 with detection limits of 50 ng ml^?1 and 1 μg ml^?1, respectively. Relative standard deviations (RSD) for the analysis of 10 gµg ml^?1 of PIP and OFL (n = 6) were 0.5% and 1.4%, respectively. The presence of glucose, lactose, sorbitol, gum arabic, starch, magnesium stearate, methylparaben and propylparaben did not affect the determinations of both PIP and OFL. The methods were used for the analysis of pharmaceutical preparations and the results indicated relative deviation of 0.5–5.5% from labeled values with RSD within 0.49–2.5%. PIP and OFL could also be determined simultaneously, and were determined from spiked human urine.     

Determination of cystine and cysteine in seawater using cathodic stripping voltammetry in the presence of Cu(II)

A method is described for the determination of cystine and cysteine in seawater and freshwater using cathodic stripping voltammetry in the presence of added copper(II). The optimized conditions include a copper concentration of 150 nM, a pH of 8.5, and a collection potential of −0.15 V; the cathodic reduction peak is located at −0.55 V. The detection limit is 0.1 nM after a collection period of 4 min. The sensitivity is diminished by surfactants similar to Triton X-100 in natural waters; the sensitivity therefore needs to be calibrated by internal standard additions of amino acids. It is possible to differentiate between cystine and cysteine by employing a solution pH of 6.2, where the peak due to cystine is absent. The response in seawater is different from that previously reported in buffer solutions. It is shown that the amino acid reduction peak is due to Cu(I) reduction of the adsorbed Cu(I)-cysteinate complex; this complex is formed with Cu(I) generated from dissolved copper(II) at the electrode surface at −0.15 V in the presence of cysteine; cystine is reduced to cysteine at the electrode surface during the collection process.     

Determination of molybdenum in the presence of 2-(2-benzothiazolylazo)-p-cresol by catalytic-adsorptive stripping voltammetry

The adsorptive collection of the molybdenum (VI) complexed with 2-(2-benzothiazolylazo)-p-cresol (BTAC) coupled with the catalytic current of the adsorbed complex at a static mercury drop electrode yields an ultrasensitive voltammetric procedure for the determination of molybdenum. Optimal experimental conditions were: a stirred acetate buffer 0.2 M (pH 3.5) as supporting electrolyte, a BTAC concentration of 1.0 x 10(-6) M as ligand, and a concentration of 0.1 M potassium nitrate as the oxidizing agent. In addition, a preconcentration potential of -0.080 V vs Ag/AgCl (3 M KCl), equilibration time of 15 s, a frequency of 30 Hz, a scan increment of 2 mV, a pulse amplitude of 0.050 mV, and a drop area of 0.032 cm2 were used. The cyclic voltammogram was recorded using a staircase wave with a scan rate of 100 mV/s. The forward scan starts at the initial potential of -0.080 V and is reversed at -0.90 V. Using the catalytic current at approximately -0.55 V the response to the Mo(VI) was found to be linear over a concentration range of 1.0-10.0 microg/L. The limit of detection is as low as 6.2 x 10(-10) M with 4 min of preconcentration time. The possible interference of other trace ions was investigated. The merits of this procedure are demonstrated using of reference samples.     

Determination of molybdenum in the presence of 2-(2-benzothiazolylazo)-p-cresol by catalytic-adsorptive stripping voltammetry

The adsorptive collection of the molybdenum (VI) complexed with 2-(2-benzothiazolylazo)-p-cresol (BTAC) coupled with the catalytic current of the adsorbed complex at a static mercury drop electrode yields an ultrasensitive voltammetric procedure for the determination of molybdenum. Optimal experimental conditions were: a stirred acetate buffer ¶0.2 M (pH 3.5) as supporting electrolyte, a BTAC concentration of 1.0 × 10^–6 M as ligand, and a concentration of 0.1 M potassium nitrate as the oxidizing agent. In addition, a preconcentration potential of –0.080 V vs Ag/AgCl (3 M KCl), equilibration time of 15 s, a frequency of 30 Hz, a scan increment of 2 mV, a pulse amplitude of 0.050 mV, and a drop area of 0.032 cm² were used. The cyclic voltammogram was recorded using a staircase wave with a scan rate of 100 mV/s. The forward scan starts at the initial potential of –0.080 V and is reversed at –0.90 V. Using the catalytic current at ～–0.55 V the response to the Mo(VI) was found to be linear over a concentration range of 1.0–10.0 μg/L. The limit of detection is as low as 6.2 × 10^–10 M with 4 min of preconcentration time. The possible interference of other trace ions was investigated. The merits of this procedure are demonstrated using of reference samples.     

Determination of methylmercury in the presence of inorganic mercury by anodic stripping voltammetry

Methylmercury is determined in a non-complexing nitrate medium by differential pulse anodic stripping voltammetry at a gold film electrode, with a detection limit of 2 × 10^-8 mol l^-1 for 5-min plating times. A method of double standard additions is proposed for determining methylmercury in the presence of mercury(II) ions.     

Ultratrace determination of adenine in the presence of copper by adsorptive stripping voltammetry

An electrochemical stripping procedure for ultra-trace measurements of adenine is described based on the adsorption of the adenine-copper complex on a static mercury drop electrode. Cyclic voltammetry was used to characterize the interfacial and redox behavior. Optimum experimental conditions were found when using a 0.005 M NaOH solution containing 0.4 ppm of copper, an accumulation potential of -0.30 V, a scan rate of 100 mV s(-1) and a linear scan mode. There is a linear response to adenine concentration in the range of 0.1-1.0 ppb and the detection limit for 6 min accumulation time was of 4.0 ppt (3.0x10(-11) M). Proper conditions for measuring the adenine in presence of guanine, thymine and cytosine were also investigated. The method was applied for the determination of adenine in a sample of single-stranded calf thymus DNA.     

Catalytic-adsorptive stripping voltammetry of cobalt in the presence of 2,2'-bipyridine and nitrite

A highly sensitive and selective procedure is presented for the voltammetric determination of cobalt. The procedure involves an adsorptive accumulation of cobalt-2,2'-bipyridine complex on a hanging mercury drop electrode, followed by a stripping voltammetric measurement of the catalytic reduction current of nitrite at - 1.25 V (vs. Ag/AgCl). The optimum conditions for the analysis of cobalt include 0.1 M ammonium buffer (pH 8.55-9.25), 2.0-5.0 muM 2,2'-bipyridine, 0.20 M sodium nitrite and an accumulation potential between -0.75 and - 0.90 V (vs. Ag/AgCl). An accumulation time of 30 s results in a very low detection limit of 9.5 pM (0.56 p.p.t.) and a linear current-concentration relationship up to 2.0 nM. The relative standard deviation at 0.10 nM is 4.9%. Possible interferences from co-existing ions are also investigated.