Abdelkader Hilmi

Development of Electrokinetic Capillary Electrophoresis Equipped with Amperometric Detection for Analysis of Explosive Compounds

Cyclic voltammograms of trinitrotoluene (TNT) and other related explosive compounds obtained by using glassy carbon, platinum, nickel, gold, and silver electrodes revealed the applicability of gold and silver in capillary electrophoresis (CE) amperometric detection. The selected electrode, gold or silver, was inserted into a specially designed detection cell that was easily adapted to a commercial CE apparatus. The electrochemical reduction of TNT, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and 10 other explosives could be reliably monitored at −700 mV vs Ag/AgCl. CE analyses were performed with a borate buffer (15 mM, pH 8.7) containing 25 mM sodium dodecyl sulfate (SDS) to attain baseline resolution of the selected compounds. A bimetal electrode, prepared by depositing silver on gold, offered a superior performance by exploiting the sensitivity of gold while suppressing its response toward acetonitrile to achieve a 10-fold lower detection limit...

Determination of explosives in soil and ground water by liquid chromatography–amperometric detection

Electrochemical reduction of trinitrotoluene (TNT) and several nitroaromatics has been exploited toward the development of an amperometric detector for liquid chromatography (LC). Up to a ten-fold increase in sensitivity was accomplished for the explosives using amperometric detection instead of conventional UV measurement. A working glassy carbon electrode (poised at -0.80 V vs. Ag/AgCl) offered a detection limit of 9, 44 and 550 nM for trinitrobenzene, TNT and 1,4-dinitrobenzene, respectively. Separation of eleven TNT-related compounds in a mixture was achieved within 15 min using a C18 column and a mobile phase consisting of acetonitrile-50 mM phosphate buffer pH 5 (1:2, v/v) and 18 mM sodium dodecylsulfate. The LC-amperometric detection system was applicable for analyzing soil extracts and ground water and the results obtained agreed well with that of the US Environmental Protection Agency recommended procedure. Extension to analysis of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) and RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) was accomplished with a silver working electrode instead of a glassy carbon electrode installed in a thin channel cell.

In-line coupling capillary electrochromatography with amperometric detection for analysis of explosive compounds.

Amperometric detection at a bare gold electrode has been in‐line coupled with capillary electrochromatography (CEC) for analysis of nitroaromatic and nitroamine explosives in contaminated soils and ground water. The CEC column packed with 3u2005μm C18 particles performed best using a mobile phase containing 70—80% methanol, 30 or 20% water, 5u2005mM sodium dodecyl sulfate (SDS) and 10mM 2‐(N‐morpholino)ethanesulfonic acid (MES). In contrast, the separation column packed with 1.5u2005μm C18 particles exhibited the best separation when only 30% methanol was added to a mobile phase containing 70% water, 7u2005mM SDS, and 10u2005mM MES. The detection, based on electrochemical reduction of the explosives (−0.7 or −1u2005V vs. Ag/AgCl, depending upon the level of methanol in the mobile phase), was compatible with such mobile phases. The detection limits for 13 explosives ranged from 100 to 200 ppb, i.e., about twofold better than those obtained with electrokinetic chromatography (EKC)/amperometric detection. From an operational viewpoint, exhaustive column conditioning was a prerequisite and care should be taken to prevent bubble formation and current breakdown during the course of separation. The CEC column equipped with amperometric detection successfully measured explosives in ground water and extracts prepared from contaminated soils and the results obtained agreed well with those of the U.S. Environmental protection Agency (EPA) method.

Utilization of TiO2 deposited on glass plates for removal of metals from aqueous wastes

Glass plates coated with TiO2 were used in a photocatalytic process to collect mercury, lead, copper and cadmium from aqueous solutions containing individual metals and mixtures. Stripping voltammetry, verified to achieve 1-10 ppb detection limits, was used to show that individual metals at concentrations of 1000 to 5200 ppb were reduced to undetectable levels in 3 to 55 min. Capillary electrophoresis (CE) with 8-hydroxyquinoline-5-sulfonic acid as complexing agent was used when appropriate, since it could quantitate all four metals under study in one run although it was less sensitive. It was demonstrated that 100 mL solutions containing 10 ppm of each of the four metals could be treated with a 10 cm2 TiO2-coated plate to leave undetectable metal concentrations in one hour. Stripping voltammetry using carbon electrodes coated with mercury films was estimated to generate daily about 1.1 L of aqueous waste containing 0.1 ppm of each metal. The results indicate the feasibility of assembling an apparatus capable of treating the waste generated by stripping voltammetry to render the latter suitable for routine on-site analyses without environmental concern. Data were also obtained to show the effectiveness in treating silver containing solutions, indicating suitability of the photocatalytic process in treating photographic processing wastes.

Nonaqueous capillary electrophoresis equipped with amperometric detection for analysis of chlorinated phenolic compounds.

Nonaqueous capillary electrophoresis (NACE) equipped with amperometric detection has been developed for separation and detection of an 11-member model mixture of chlorinated phenolic compounds. With triacetyl-beta-cyclodextrin (TACD) as a novel selectivity selector, acetonitrile proved to be an excellent solvent for this water-insoluble cyclodextrin derivative. Resolution of the analytes was achieved by using an optimized acetonitrile medium consisting of 500 mM acetic acid, 10 mM sodium acetate, 12 mM TACD and 50 mM tetrabutylammonium perchlorate. Separation of analytes was attributed to differential electrostatic and/or inductive interactions of the analytes with the TACD/TBA+ complex and charged tetrabutylammonium phases. A simple end-column amperometric detector (Pt vs. Ag/AgCl, poised at +1.6 V) in conjunction with NACE was used to analyze chlorophenols. Amperometric detection of such target compounds in acetonitrile-based media offers high sensitivity and alleviates electrode fouling compared to aqueous buffers. The detection limits obtained, ranging from 30 nM to 500 nM, are 3-8-fold lower than those obtained with aqueous buffers.

Capillary electrophoretic separation of chlorophenols using amperometric detection

A simple end-column amperometric detector, without a porous junction, was designed and attached to a capillary electrophoresis instrument to analyze eight mono-, di- and tri-chlorophenols. The platinum detecting electrode was modified by electrodeposition of tin, to enhance sensitivity as well as reduce electrode fouling owing to phenol oxidation. The modified electrode (0.127 mm in diameter) in combination with a 20 μm I.D. separation capillary yielded sharp peaks and enabled us to detect phenol at levels as low as 0.10 μM. Satisfactory separation of the eight chlorophenols was achieved by using a mixed buffer of phosphate-borate.

Capillary electrophoresis applied to kinetic studies of photocatalytic oxidation of substituted anilines

Abstract Cyclodextrin modified capillary electrophoresis (CMCE) and micellar electrokinetic chromatography (MEKC) were tested for their ability to analyze mixtures containing substituted anilines. CMCE separated analytes by their partitioning preference between the two substituted cyclodextrins while MEKC exploited the analyte partitioning between the micelles and the bulk aqueous phase. MEKC was selected to analyze samples that were subjected to photocatalytic oxidation using TiO 2 and/or SnO 2 as the semiconductor catalyst. The rate constant for each compound was determined when it was degraded individually or in the presence of other derivatives in the same family, indicating a significant degree of analyte competition and synergism offered by the mixed catalyst. The results demonstrated that CE, with its minimal sample preparation and requirement, is a valuable tool, particularly suitable for studying highly toxic pollutants, without generating a high volume of waste.

Applicability of micellar electrokinetic chromatography to kinetic studies of photocatalytic oxidation of dibenzo-p-dioxin

Micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate was tested for its ability to analyze mixtures containing three dioxin derivatives. Dioxin-containing samples, subjected to photocatalytic oxidation using TiO2 and/or SnO2 as the semiconductor catalyst were analyzed by MEKC to obtain the rate constant for each dioxin when it was degraded individually or in the presence of the other two dioxins. A significant degree of analyze competition was observed and the mixed catalyst offered important synergism.

Photocatalysis for Pretreatment of Metal-Containing Samples and for Removing Metals from the Waste

Biotreatment is under consideration as a feasible alternative in remediating mixed-waste sites that are contaminated with organic chemicals as well as metals. Monitoring the bioremediation of such sites requires techniques for analyzing metals in the presence of interfering organics. Glass plates coated with TiO2 were used in a photocatalytic process to collect mercury, lead, copper and cadmium from aqueous solutions. The collected metals were reconstituted in an acidic solution to allow accurate determination by stripping voltammetry. The titania-coated glass plates were also used to remove all metals in the analysis waste to leave metal-free solutions, as asserted with stripping voltammetry. The double-purpose utilization of the photocatalytic process provides a convenient and sensitive, interference-free method for on-site metal analyses, while producing metal-free waste.