Analysis of phenoxyl-type N-methylcarbamate pesticide residues in vegetables by capillary zone electrophoresis with pre-column hydrolysis and amperometric detection

With systematic studies of pre-column thermalhydrolysis, a method is described for the determination of fivephenoxyl-type N-methylcarbamates (NMCs) (viz., propoxur, carbofuran, 3-hydroxy-carbofuran, carbaryl, and bendiocarb) in vegetables by capillary zone electrophoresis with amperometric detection. Effects of hydrolysis parameters such as alkali medium, temperature, and hydrolysis time, as well as separation conditions, are investigated. Under the optimum conditions, baseline separation of five hydrolysates is achieved within 16 min. Good calibration curves of propoxur, carbofuran, 3-hydroxy-carbofuran, carbaryl, and bendiocarb are obtained from 1.20x10(-7) to 5.00x10(-5) mol/L, and their detection limits (S/N=3) are 1.80x10(-8), 1.50x10(-8), 1.80x10(-8), 2.50x10(-8), and 1.80x10(-8) mol/L, respectively, which is approximately 20 approximately 50-fold more sensitive than those previously reported with a UV method. In the application of vegetable samples, the five NMCs are well determined. Average recoveries of 75 approximately 89% and 86 approximately 100% at fortified levels of 0.05 and 0.80 mg/kg are achieved with relative standard deviations of 2 approximately 6%, respectively. The method is sensitive, reproducible, and provides an alternative means for the analysis of phenoxyl-type NMC pesticide residues.     

Stopped-flow spectrophotometric determination of hydrogen peroxide with hemoglobin as catalyst

A rapid and sensitive method was proposed for the determination of hydrogen peroxide based on the catalytic effect of hemoglobin using o-phenylenediamine as the substrate. Stopped-flow spectrophotometric method was used to study the kinetic behavior of the oxidation reaction. The catalytic effectiveness of hemoglobin was compared with other four kinds of catalysts. The initial rate of the formation of the reaction product 2,3-diaminophenazine at the wavelength of 425 nm was monitored, permitting a detection limit of 9.2x10(-9) mol/l H(2)O(2). A linear calibration graph was obtained over the H(2)O(2) concentration range 5.0x10(-8)-3.5x10(-6) mol/l, and the relative standard deviation at a H(2)O(2) concentration of 5.0x10(-7) mol/l was 2.08%. Satisfied results were obtained in the determination of H(2)O(2) in real samples by this method.     

Electrochemical determination of hydrogen peroxide using o -dianisidine as substrate and hemoglobin as catalyst

A new electrochemical method for the determination of microamounts of hydrogen peroxide utilizing o-dianisidine (ODA) as substrate and hemoglobin (Hb) as catalyst is described in this paper. Hb can be used as mimetic peroxidase and it can catalyse the reduction of hydrogen peroxide with the subsequent oxidation of ODA. The oxidative reaction product is an azo compound, which is an electroactive substance and has a sensitive second-order derivative polarographic reductive peak at the potential of -0.58 V (vs. SCE) in pH 80 Britton-Robinson (B-R) buffer solution. The conditions of Hb-catalytic reaction and polarographic detection of the reaction product were carefully studied. By using this polarographic peak and under optimal conditions, the calibration curve for the H₂O₂ was constructed in the linear range of 2.0 x 10^-7 ∼ 10 x 10^-4 mol/l with the detection limit of 5.0 x 10^-8 mol/l. This method can also be used to the determination of Hb content in the range of 20 x 10^-9 ∼ 30 x 10^-7 mol/l with a detection limit of 10 x 10^-9 mol/l. The proposed method was further applied to the determination of the content of H₂O₂ in fresh rainwater with satisfactory results. The catalytic reaction mechanism and the electrode reductive process of the reaction product were carefully studied.     

Synthesis of a novel chemiluminescent reagent for the determination of hydrogen peroxide in snow waters

7-(4,6-Dichloro-1,3,5-triazinylamino)-4-methylcoumarin (DTMC) was synthesized as a completely new chemiluminescent reagent, and with it a novel chemiluminescence method was developed for the determination of hydrogen peroxide in the absence of any added catalyst or co-oxidant. The chemiluminescence intensity of the DTMC-H(2)O(2) system could be enhanced by the addition of cation surfactants. The chemiluminescence intensity was directly proportional to the concentration of H(2)O(2) in the range 1.0 x 10(-7)-4.0x10(-4) mol l(-1), and the detection limit was 4.0 x 10(-8) mol l(-1). The relative S.D. was 4.9% for 1.0 x 10(-6) mol l(-1) of H(2)O(2) (n=10). The selectivity of this method was high, and most of the transition metal ions have no effect on the determination. The proposed method has been applied to the determination of trace amounts of hydrogen peroxide in snow water. A possible mechanism of the chemiluminescence reaction is also discussed.     

Determination of morphine in process streams by sequential injection analysis with chemiluminescence detection

A procedure for the determination of morphine in process streams by sequential injection analysis based on the chemiluminescence reaction of morphine with acidic potassium permanganate in the presence of sodium hexametaphosphate is presented. The chemiluminescence emission has been monitored using an in-house detection system which consisted of a fibre optic flowthrough cell and a sensitive, low dark current, photomultiplier tube. The calibration graph (range 2 x 10(-8) to 1 x 10(-4) mol/l) was not linear over the entire range of concentration, with a polynomial equation of best fit of y = 1.0 x 10(15) x(3) - 2.2 x 10(11) x(2) + 1.3 x 10(7) x - 8.3. The calibration function approximates linearity over the concentration range 2.5 x 10(-6) to 3.0 x 10(-5) mol/l where the slope of the log-log plot is 1.09 +/- 0.16. The detection limit was estimated at about 10(-8) mol/l from the response of the lowest calibration standard (2.5 x 10(-8) mol/l) which gave a signal to noise ratio of 3 : 1. Although the structurally related codeine did not interfere significantly the results suggest that this method may be susceptible to matrix effects, dependent on the location of sampling from the process stream.     

Determination of diclofenac sodium by capillary zone electrophoresis with electrochemical detection

Capillary zone electrophoresis was employed for the determination of diclofenac sodium using an end-column amperometric detection with a carbon fiber microelectrode, at a constant potential of 0.83 V vs. saturated calomel electrode. The optimum conditions of separation and detection are 4.90 x 10(-3) mol/l Na2HPO4-3.10 x 10(-3) mol/l NaH2PO4 (pH 7.0) for the buffer solution, 10 kV for the separation voltage, 5 kV and 10 s for the injection voltage and the injection time, respectively. The limit of detection is 2.5 x 10(-6) mol/l or 5.2 fmol (S/N=2). The relative standard deviation is 0.8% for the migration time and 4.7% for the electrophoretic peak current. The method was applied to the determination of diclofenac sodium in human urine.     

Chemiluminescence flow injection analysis of 1,3-dichloro-5,5-dimethylhydantoin in swimming pool water

A new chemiluminescence (CL) flow-injection method is proposed for the determination of 1,3-dichloro-5,5-dimethylhydantoin (DDH). The method is based on the chemiluminescent reaction of DDH and luminol-H(2)O(2) in an alkaline medium (pH 12.0-12.5). The concentration of the analyte shows a good linear relationship with the produced luminescence intensity in the range of 3.0x10(-8) to 8.0x10(-6) mol l(-1). The detection limit of the proposed method is 1.0x10(-8) mol l(-1) and the relative standard deviation (R.S.D.) is 4.7% (n=5) at 5.0x10(-7) mol l(-1). This method was successfully applied to the determination of trace amounts of this disinfectant in water samples obtained from five different swimming pools. Satisfying recovery values were also obtained.     

A fluorescence quenching method for the determination of nitrite with Rhodamine 110

A simple and sensitive fluorescence quenching method for the determination of trace nitrite has been developed. The method is based on the reaction of Rhodamine 110 with nitrite in acidic medium to form a new compound, which has much lower fluorescence. The optimum experimental conditions were studied. The linear range was obtained at a nitrite concentration of 1.0 x 10(-8)-3.0 x 10(-7)mol l(-1) with a detection limit of 7.0 x 10(-10) mol l(-1) (S/N=3). The proposed method has been successfully applied to the determination of nitrite in tap water and lake water without extraction.     

Determination of the herbicide desmetryne in organised media by adsorptive stripping voltammetry

An electroanalytical method for the determination of the herbicide desmetryne at nanomolar levels in dispersed media, based on adsorptive stripping voltammetry, is reported. The adsorption of desmetryne at the hanging mercury drop electrode was checked both in micellar solutions, where the anionic surfactant sodium pentanesulphonate was chosen as the most suitable surfactant agent, and in oil-in-water emulsions prepared with ethyl acetate as the organic solvent. In a micellar medium formed with 0.02% sodium pentanesulphonate and with 0.1 mol l(-1) Britton-Robinson buffer (pH 1.5), the herbicide could be determined over the 1.0 x 10(-8)-4.0 x 10(-7) mol l(-1) concentration range, when an accumulation potential of -0.70 V was applied for 50 s. On the other hand, in an oil-in-water emulsion formed with 2% ethyl acetate and 0.04% sodium pentanesulphonate as emulsifying agent in 0.1 mol l(-1) HClO(4), desmetryne could be determined over the 2.0 x 10(-9)-1.0 x 10(-7) mol l(-1) concentration range. The limits of detection were 2.4 x 10(-9) and 4.2 x 10(-10) mol l(-1) in micellar and emulsified media, respectively, with R.S.D.s (n=10) 3.6 and 3.7%. The degree of interference from some other s-triazines on the desmetryne differential pulse response was also evaluated. Finally, the method developed in emulsified medium was applied to the determination of desmetryne in spiked apple juice.     

Evaluation of luminol-H(2)O(2)-KIO(4) chemiluminescence system and its application to hydrogen peroxide, glucose and ascorbic acid assays

A novel chemiluminescence (CL) system was evaluated for the determination of hydrogen peroxide, glucose and ascorbic acid based on hydrogen peroxide, which has a catalytic-cooxidative effect on the oxidation of luminol by KIO(4). Hydrogen peroxide can be directly determined by luminol-KIO(4)-H(2)O(2) CL system. The detection limit was 3.0x10(-8) mol l(-1) and the calibration graph was linear over the range of 2.0x10(-7)-6.0x10(-4) mol l(-1). The relative standard deviation of H(2)O(2) was 1.1% for 2.0x10(-6) mol l(-1) (N=11). Glucose was indirectly determined through measuring the H(2)O(2) generated by the oxidation of glucose in the presence of glucose oxidase at pH 7.6. The present method provides a source for H(2)O(2), which, in turn, coupled with the luminol-KIO(4)-H(2)O(2) CL reaction system. The CL was linearly correlated with glucose concentration of 0.6-110 mug ml(-1). The relative standard deviation was 2.1% for 10 mug ml(-1) (N=11). Detection limit of glucose was 0.08 mug ml(-1). Ascorbic acid was also indirectly determined by the suppression of luminol-KIO(4)-H(2)O(2) CL system. The calibration curve was linear over the range of 1.0x10(-7)-1.0x10(-5) mol l(-1) of ascorbic acid. The relative standard deviation was 1.0% for 8.0x10(-7) mol l(-1) (N=11). Detection limit of ascorbic acid was 6.0x10(-8) mol l(-1). These proposed methods have been applied to determine glucose, ascorbic acid in tablets and injection.     

Electrochemical determination of 6-mercaptopurine at silver microdisk electrodes

The electrochemical behaviour of 6-mercaptopurine (6-MP) at a microdisk electrode is investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results indicate that 6-MP can be strongly adsorbed on the surface of the static mercury drop electrode (SMDE) and reacts with Ag+ ions which are produced at positive potentials. 6-MP yields a well-defined cathodic stripping signal during the negative scan at about -0.812 V (vs. SCE) in pH 9.0 phosphate buffer solution. The electrode has hence been used for the determination of 6-MP by differential pulse voltammetry (DPV). The linear range is between 2.0x10(-7) and 5.0x10(-5) mol/l, with the calculated detection limit (S/N=3) of 8.0x10(-8) mol/l. The relative standard deviation is 3.0% for eight successive determinations of 4.0x10(-5) mol/l 6-MP. The determination of 6-MP in tablets has also been carried out and satisfactory results have been obtained.     

Flotation-spectrophotometric determination of mercury in water samples using iodide and ferroin.

This paper describes a simple and highly selective method for separation, preconcentration and spectrophotometric determination of trace amounts of mercury. The method is based on the flotation of an ion-associate of HgI4(2-) and ferroin between aqueous and n-heptane interface at pH 5. The ion-associate was then separated and dissolved in acetonitrile to measure its absorbance. Quantitative flotation of the ion-associate was achieved when the volume of the water sample containing Hg(II) was varied over 50 - 800 ml. Beer's law was obeyed over the concentration range of 3.2 x 10(-8) - 9.5 x 10(-7) mol l(-1) with an apparent molar absorptivity of 1 x 10(6) l mol(-1) cm(-1) for a 500 ml aliquot of the water sample. The detection limit (n = 25) was 6.2 x 10(-9) mol l(-1), and the RSD (n = 5) for 3.19 x 10(-7) mol l(-1) of Hg(II) was 1.9%. A notable advantage of the method is that the determination of Hg(II) is free from the interference of the almost all cations and anions found in the environmental and waste water samples. The determination of Hg(II) in tap, synthetic waste, and seawater samples was carried out by the present method and a well-established method of extraction with dithizone. The results were satisfactorily comparable so that the applicability of the proposed method was confirmed in encountering with real samples.     

Spectrophotometric determination of fluoxetine by batch and flow injection methods

A rapid, simple, and accurate spectrophotometric method is presented for the determination of fluoxetine by batch and flow injection analysis methods. The method is based on fluoxetine competitive complexation reaction with phenolphthalein-beta-cyclodextrin (PHP-beta-CD) inclusion complex. The increase in the absorbance of the solution at 554 nm by the addition of fluoxetine was measured. The formation constant for fluoxetin-beta-CD was calculated by non-linear least squares fitting. Fluoxetine can be determined in the range 7.0 x 10(-6)-2.4 x 10(-4) mol l(-1) and 5.0 x 10(-5)-1.0 x 10(-2) mol l(-1) by batch and flow methods, respectively. The limit of detection and limit of quantification were respectively 4.13 x 10(-6) mol l(-1) and 1.38 x 10(-5) mol l(-1) for batch and 2.46 x 10(-5) mol l(-1) and 8.22 x 10(-5) mol l(-1) for flow method. The sampling rate in flow injection analysis method was 80+/-5 samples h(-1). The method was applied to the determination of fluoxetine in pharmaceutical formulations and after addition to human urine samples.     

Catalytic spectrofluorimetric determination of superoxide anion radical and superoxide dismutase activity using N,N-dimethylaniline as the substrate for horseradish peroxidase (HRP)

The coupled reaction of N,N-dimethylaniline (DMA) with 4-aminoantipyrine (4-AAP) using superoxide anion radical (O2-) as oxidizing agent under the catalysis of horseradish peroxidase (HRP) was studied. Based on the reaction, O2- produced by irradiating Vitamin B2, (VB2) was spectrophotometricly determined at 554 nm. The linear range of this method was 1.8 x 10(-6)-1.2 x 10(-4) mol l(-1) with a detection limit of 5.3 x 10(-7) mol l(-1). The effect of interferences on the determination of O2- was investigated. The proposed method was successfully applied to the determination of superoxide dismutase (SOD) activity in human blood and mouse blood.     

Voltammetric copper(II) determination with a montmorillonite-modified carbon paste electrode

The clay mineral montmorillonite has been tested as modifier for the carbon paste electrode with a novel electrode modification technique. The differential pulse voltammetric determination of copper(II) by means of this modified carbon paste electrode has been studied. A detection limit of 4x10(-8) mol/l has been achieved after 10 min preconcentration under open circuit conditions with subsequent anodic stripping voltammetry. The calibration curve for Cu(II) is linear in the range of 4x10(-8)-8x10(-7) mol/l. Pb interferes in a 10-fold molar and Cd and Hg in a 100-fold molar excess. The interference by humic ligands is significant.     

Flow-injection chemiluminometric determination of some thioxanthene derivatives in pharmaceutical formulations and biological fluids using the [Ru(dipy)3(2+)]-Ce(IV) system.

A flow-injection (FI) methodology using tris(2,2'-dipyridyl)ruthenium(II), [Ru(dipy)3(2+)], chemiluminescence (CL) was developed for the rapid and sensitive determination of three thioxanthene derivatives, namely zuclopenthixol hydrochloride, flupentixol hydrochloride and thiothixene. The method is based on the CL reaction of the studied thioxanthenes with [Ru(dipy)3(2+)] and Ce(IV) in a sulfuric acid medium. Under the optimum conditions, calibration graphs were obtained over the concentration ranges 0.002-6 migrograms/ml for zuclopenthixol hydrochloride, 0.5-15 micrograms/ml for flupentixol hydrochloride and 0.05-7.5 micrograms/ml for thiothixene. The limits of detection (s/n = 3) were 4.2 x 10(-9) mol/l zuclopenthixol hydrochloride, 2 x 10(-8) mol/l flupentixol hydrochloride and 4.5 x 10(-8) mol/l thiothixene. The method was successfully applied to the determination of these compounds in dosage forms and biological fluids.     

Flow injection turbidimetric determination of thiamine in pharmaceutical formulations using silicotungstic acid as precipitant reagent

A simple flow injection system is proposed for the determination of thiamine in pharmaceutical formulations. The determination is based on the precipitation reaction of thiamine with silicotungstic acid in acidic medium to form a thiamine silicotungstate suspension that is measured at 420 nm. Adding 0.05% (w/v) poly(ethyleneglycol) in the carrier solution (0.5 mol l(-1) hydrochloric acid), an improvement in the sensitivity, repeatability and baseline stability of the flow injection system was obtained. The calibration graph was linear in the thiamine concentration range from 5.0x10(-5) to 3.0x10(-4) mol l(-1) with a detection limit of 1.0x10(-5) mol l(-1). The relative standard deviations for ten successive measurements of 1.0x10(-4) mol l(-1) and 2.5x10(-4) mol l(-1) thiamine were less than 1% and an analytical frequency of 90 h(-1) was obtained.     

Carbon paste electrode for trace zirconium(IV) determination by adsorption voltammetry.

A very sensitive and selective procedure was developed for trace measurement of zirconium based on the cathodic adsorptive stripping voltammetry of the zirconium-alizarin red S(ARS) complex at a carbon paste electrode (CPE). The 2nd-order derivative linear scan voltammograms of the zirconium-ARS complex were recorded by a model JP-303 polarographic analyzer from 0.0 to -1.0 V (vs. SCE). Optimal analytical conditions were found to be: an acetic acid (0.1 mol l(-1))-potassium biphthalate (0.08 mol l(-1)) buffer solution (pH 4.8) containing 4.0 x 10-6 mol l(-1) ARS; accumulation potential, 0.0 V; accumulation time, 180 or 90 s; rest time, 10 s; scan rate, 250 mV s(-1). The results showed that the complex can be adsorbed on the surface of the CPE, yielding one peak at -0.51 V, corresponding to the reduction of ARS in the complex at the electrode. The detection limit was found to be 1.0 x 10(-10) mol l(-1) (S/N = 3) for 240 s accumulation. The linear range was 2.0 x 10(-10)-4.0 x 10(-7) mol l(-1). The developed method was applied to the determination of trace zirconium in the ore samples with satisfactory results.     

Determination of liquiritigenin and isoliquiritigenin in Glycyrrhiza uralensis and its medicinal preparations by capillary electrophoresis with electrochemical detection

A simple, reliable, reproducible and sensitive method, based on capillary electrophoresis with electrochemical detection (ED), for the determination of liquiritigenin and isoliquiritigenin in Glycyrrhiza uralensis and its medicinal preparations was described. Operated in a wall-jet configuration, a 300 microm diameter carbon-disk electrode was used as the working electrode, which exhibits good responses at + 1000 mV (versus SCE) for the two analytes. Under the optimum conditions, the analytes were base-line separated within 8 min, and excellent linearity was obtained in the concentration range from 5.0 x 10(-4) to 1.0 x 10(-6) mol/l. The detection limit (S/N = 3) was 4.7 x 10(-7) and 2.9 x 10(-7) mol/l for liquiritigenin and isoliquiritigenin, respectively. This work provides a useful method for the analysis of traditional Chinese medicines.     

Electrochemiluminescence of terbium (III)-two fluoroquinolones-sodium sulfite system in aqueous solution

The electrochemiluminescence (ECL) of Tb3+-enoxacin-Na2SO3 system (ENX system) and Tb3+-ofloxacin-Na2SO3 system (OFLX system) in aqueous solution is reported. ECL is generated by the oxidation of Na2SO3, which is enhanced by Tb3+-fluoroquinolone (FQ) complex. The ECL intensity peak versus potential corresponds to oxidation of Na2SO3, and the ECL emission spectra (the peaks are at 490, 545, 585 and 620 nm) match the characteristic emission spectrum of Tb3+, indicating that the emission is from the excited state of Tb3+. The mechanism of ECL is proposed and the difference of ECL intensity between ENX system and OFLX system is explained. Conditions for ECL emission were optimized. The linear range of ECL intensity versus concentrations of pharmaceuticals is 2.0 x 10(-10) -8.0 x 10(-7)mol l(-1) for ENX and 6.0 x 10(-10) -6.0 x 10(-7)mol l(-1) for OFLX, respectively. A theoretical limit of detection is 5.4 x 10(-11)mol l(-1) for ENX and 1.6 x 10(-10)mol l(-1) for OFLX, respectively. The ECL was satisfactorily applied to the determination of the two FQs in dosage form and urine sample.