A. Zapardiel

Simultaneous UV and electrochemical determination of the herbicide asulam in tap water samples by micellar electrokinetic capillary chromatography

Abstract A simple end-column electrochemical detector was designed and attached to an available commercial capillary electrophoresis instrument with UV detection to detect different kind of herbicides and to determinate methyl-4-aminophenyl-sulfonylcarbamate (asulam) in water samples. The designed cell is very easy to assemble and disassemble in a short period of time; the working electrode positioning is also quickly achieved without micropositioners. The alignment between working electrode and capillary outlet was very reproducible for the all checked electrodes; the R.S.D. obtained was lower than 6.0% for 100xa0μm gap distance. In this mode, the non-electroactive and electroactive compounds could be detected by UV and electrochemical detection, respectively at the same time. The electrochemical determination of asulam using micellar electrokinetic capillary chromatography (MEKC) is the first time that is reported. In both detection systems, a linear range was obtained for asulam concentrations lower than 25.0xa0mgxa0l−1, in boric acid 0.020xa0molxa0l−1 at pH 8.20 and containing 0.025xa0molxa0l−1 of sodium dodecyl sulfate, to obtain selectivity additional separation by the micellar distribution process. Under these conditions, an experimental detection limit of 0.4xa0mgxa0l−1 was achieved. A new experimental scheme is also described for asulam determination in tap waters with a previous preconcentration step. Using both, UV and electrochemical detection, with a previous extraction procedure, the detection limits of asulam in tap water samples were of 1.0 and 0.8xa0μgxa0l−1, respectively.

Electrochemical determination of carbaryl oxidation in natural water and soil samples

An electrochemical method for the determination of carbaryl, after prior oxidation to 1,4-naphthoquinone in natural water and soils is reported. The coulometric oxidation of carbaryl at a platinum electrode was studied using 0.024 mol/L Britton-Robinson buffer (pH 7.0). The reduction of the oxidation product 1,4-naphthoquinone at a dropping mercury electrode was used for the indirect determination of carbaryl after separation on C18 Sep-pak cartridges by differential pulse polarography (detection limits: 0.41 mg L−1 of water and 0.47 mg kg−1 of soil) and directly without separation by adsorptive stripping voltammetry (detection limits: 5 μg L−1 of water and 7 μg kg−1 of soil, for 75 s preconcentration time). Relative errors were lower than 3.7% and relative standard deviations smaller than 4.5%.

Ephedrine Determination in Human Urine Using a Carbon Paste Electrode Modified with C18 Bonded Silica Gel

Abstract An electrochemical method by adsorptive stripping voltammetry with a modified carbon paste electrode with C18 bonded silica gel is described for the determination of ephedrine in human urine samples. The preconcentration step is performed at open circuit and constant stirring (1000 rpm) in 5×10−4 M sodium hydroxide and the measurement step in ammonia-ammonium chloride buffer (pH=10.0; 0.2 M) by linear scan voltammetry to 150 mVs−1 between 0.20 and 1.10 V. With the proposed method, ephedrine can be directly determined in urine, a relative standard deviation of 3.4% (n=12) and a detection limit of 270 μgmL−1 for a 2 minute preconcentration time, were obtained.

Application of matrix solid-phase dispersion to the determination of amitrole and urazole residues in apples by capillary electrophoresis with electrochemical detection

A new method based on matrix solid-phase dispersion (MSPD) extraction was studied for the extraction of amitrole (3-amino-1,2,4-triazole), and its metabolite urazole (3,5-dihydroxy-1,2,4-triazole), in apple samples. The influence of experimental conditions on the yield of the extraction process and on the efficiency of the cleanup step was evaluated. Determination was carried out by capillary electrophoresis (CE) with electrochemical detection, demonstrating the compatibility between MSPD and CE techniques. The method has been successfully applied to different apple varieties. Recoveries in samples spiked at 1.6 and 1.7xa0μg g−1 for amitrole and urazole were 88 and 82%, respectively. The limits of detection were 0.4xa0μg g−1 for both compounds using electrochemical detection.

Electrochemical study of mefexamide at glassy-carbon electrodes and its determination in urine by differential pulse voltammetry

Abstract The electrochemical oxidation of mefexamide N-[2-(diethylamino)ethyl]-2-(4-methoxyphenoxy)acetamide was investigated using cyclic, linear scan and rotating disk voltammetry at glassy-carbon electrodes. The value of pKa (9.01) was determined by the potentiometric method. In cyclic voltammetry, in neutral media, the compound shows two electrochemical irreversible oxidation peaks (both 2e−), Ox1 and Ox2. A new redox couple Red3/Ox3, formed as a result of the oxidation Ox1 peak, followed for an irreversible chemical reaction, appears on the reverse negative sweep. In acidic media, only the Ox1 peak was observed. The most defined peaks were obtained in 0.040 M Britton–Robinson buffer (pH 6.0) and 0.010 M sulfuric acid with 0.10 M sodium sulfate. The Ox1 and Ox2 peak currents were diffusion-controlled, showing an adsorption effect for low mefexamide concentrations (1.0×10−4 M) and calibration plots at 20 mV s−1, being linear in the range 5.0×10−5–5.0×10−4 M. The limiting currents in a rotating disk electrode were mass transport controlled for rotation speeds lower than 3000 rpm. The anodic charge transfer coefficient, the mass-transport rate constant, the diffusion coefficient and the charge-transfer conditional constant were determined. Also, a method for the electrochemical determination of mefexamide in human urine was developed using differential pulse voltammetry, in 0.040 M Britton–Robinson buffer (pH 6.0), being extracted with dichloromethane. The standard addition method was applied. The detection limit was 0.8 μg of mefexamide per milliliter of urine. The statistical validation reveals that the method is free from significant systematic errors.

Direct Determination of Ephedrine Alkaloids and Epinephrine in Human Urine by Capillary Zone Electrophoresis

Abstract The complete separation of a mixture of six phenylamines and epinephrine in human urine was achieved by Capillary Zone Electrophoresis (CZE) in 15 min. For the CZE separation of the compounds, electrophoretic media with phosphate-borate and phosphate-acetonitrile buffer at different pH were used. A buffer solution that contained 40 mM phosphoric acid and 10 mM boric acid adjusted to pH 9.7 with NaOH 1 N, was found to be the most suitable electrolyte forepinephrine separation. The results successfully demonstrated the use of CZE with UV detection for screening and quantification of phenylamines and epinephrine in human urine without previous treatment, in concentration lower than 35.0 μg/ml, and quantification limit of 2.0±0.1 μg per millilitre of urine.

Determination of ephedrine in human urine using a glassy carbon electrode

Abstract The electrochemical behaviour of ephedrine at a glassy carbon electrode was studied using linear-sweep and differential-pulse voltammetry at a stationary electrode and a rotating disc electrode. The studies were performed using 0.04 M Britton-Robinson buffer (pH 10.0), which allowed the evaluation of the diffusion constant, the charge-transfer coefficient and the conditional charge-transfer rate constant. A method was also devised for the electrochemical determination of ephedrine in human urine samples using differential- pulse voltammetry with prior drug separation. If this separation method is applied, either calibration graphs or standard additions can be used to determine ephedrine at concentrations between 7 and 20 g ml −1 , with relative errors lower than 6.0% and relative standard deviations lower than 5.8%. The presence of norephedrine at levels 20% lower than those of ephedrine has no influence on ephedrine determination.

Determination of phenylephrine with a modified carbon paste electrode

An anodic stripping voltammetric method for determination of phenylephrine with a modified carbon paste electrode is developed. The modified carbon paste electrode was prepared with carbon paste mixed with a cation exchange resin containing the functional group CO 2 Na. Linear scan voltammetry was used for the electrochemical measurements. The experimental conditions for preconcentration and measurement steps have been optimized in order to attain the optimum analytical signal. In 0.1 M hydrochloric acid the drug shows a well defined oxidation peak at 1.07 V (vs. Ag/AgCl/3 M KCl). A procedure for determining phenylephrine in urine is also described

Electrochemical studies of ethamivan at glassy-carbon and platinum electrodes and its determination in urine by differential pulse voltammetry

Abstract The electrochemical oxidation of ethamivan (N,N-diethyl-4-hydroxy-3-methoxy-benzamide) was investigated using cyclic, linear scan and differential pulse voltammetry at glassy-carbon and platinum stationary electrodes, and rotating disk voltammetry. The values of pKa, 8.98 and 9.03, were determinated by potentiometric and spectrophotometric methods. The compound showed an electrochemically reversible two electrons oxidation peak followed by an irreversible chemical reaction, and produced a new redox couple. The best defined peaks were obtained in 0.05 M sulphuric acid or 0.1 M hydrochloric acid at 0.87, 0.55 and 0.61 V (vs. Ag/AgCl/3 M KCl). The peak currents in cyclic voltammetry and the limiting current at a rotating disk electrode are diffusion-controlled. Using rotating disk voltammetry the mass-transport rate constant was 5.26 × 10−3cms−1 for a rotation frequency of 20.34 Hz, the diffusion coefficient was 6.5 × 10−6 cm2 s−1, the charge-transfer conditional rate constant was 1.2 × 10−3cm s−1 and the charge transfer coefficient (β) was 0.91. By linear scan voltammetry calibration plots were linear in the range 7.0 × 10−5−8.0 × 10−4M at the Pt electrode and in the range 4.0 × 10−5 − 1.0 × 10−3M at the GCE in 0.05 M sulphuric acid at 50 mV s−1. A method was also developed for the electrochemical determination of ethamivan in human urine using differential pulse voltammetry with prior drug separation with C18 cartridges (Sep-Pak, Waters). The mean recovery was 92 ± 6% (n = 8). The standard addition method was applied. The relative standard deviation was lower than 9% (samples of 15 μg ml−1 ethamivan). The detection limit was 4 μg per milliliter of urine.

Application of adsorptive stripping voltammetry to the determination of the psychoactive drug temazepam in urine

SummaryThe influence of various operational parameters on the stripping response for temazepam is discussed. Interfacial and redox behaviour was also studied. 205 Å2 was the average surface area per adsorbed molecule.The determination of temazepam in urine can be performed by using adsorption as a preconcentration step previous to its measurement at the hanging mercury drop electrode by differential pulse voltammetry in 0.01 mol/l Britton-Robinson buffer at pH 3.0 with a −0.50 V accumulation potential. The detection limit was 17 ng temazepam per ml of urine (15 s accumulation time) and the relative standard deviation was lower than 6.0% for 500 ng ml−1 samples (10 s). The effects of various urine components on the voltammetric response have also been studied.