Optimization of the TLC Separation of Seven Amino Acids

JPC – Journal of Planar Chromatography – Modern TLC - A ternary mobile phase for thin-layer chromatographic separation and identification of seven amino acids on microcrystalline...     

TLC Separation and Identification of Heavy Metals Present in Cotton Material

JPC – Journal of Planar Chromatography – Modern TLC - Optimization of the separation and identification of heavy metals present in cotton material has been performed by...     

Determination of terbinafine in pharmaceuticals and dialyzates by capillary electrophoresis

A capillary electrophoresis method has been developed for the separation and determination of terbinafine (TER) in various pharmaceutically relevant matrices. Capillary zone electrophoresis (CZE) separation and UV absorbance photometric detection were carried out in a 160 mm capillary tube with a 300 μm i.d., hydrodynamically (membrane) closed. The influences of pH, carrier cation and counterion on migration parameters of TER were studied and the following conditions were selected: a 20 mmol l⁻¹ glycine running buffer adjusted to pH 2.7 with acetic acid, 0.2% (w/v) methylhydroxyethylcellulose (m-HEC) as an electro-osmotic flow (EOF) suppressor, a 250 μA driving current, and 20 °C. The optimized separation conditions were convenient for the determination of TER in commercial tablets and spray and in dialyzates. Here, the dialysis was used to investigate in vitro permeation of TER through the skin from the gel. The samples of dialyzates were examined with and without simple extraction procedure and the results were compared. A permeation profile of the drug present in the gel of given composition was obtained analyzing pretreated samples. The proposed electrophoretic method was successfully validated. It was suitable for the simple, sensitive, rapid and highly reproducible assay of TER. CZE analysis was completed within 5.5 min. The detection limit of TER was 1.73 μmol l⁻¹ at a 224 nm detection wavelength. The intra- and inter-laboratory precisions over the concentration range 6.0-60.0 μmol l⁻¹ were between 0.32-0.69% and 1.04-1.44% including R.S.D. of migration times and peak areas, respectively. The mean absolute recoveries of drugs from samples were found to be 98.34 (tablets) and 99.47% (spray). It is suggested that there are potentialities to determine TER present in unpretreated complex samples, as CZE in a hydrodynamically closed separation system may be easily on-line combinable with purification and preconcentration CE modes (e.g., isotachophoresis, ITP).     

Towards the development of multisensor for drugs of abuse based on molecular imprinted polymers

The synthetic receptors for cocaine, deoxyephedrine, methadone and morphine were computationally designed and produced using molecular imprinting. The structure and energy of the molecular complexes were analysed by computational techniques. The possible structures of the binding sites in the synthetic receptors have been compared with those of corresponding natural receptors. The composition of imprinted polymers was optimised to allow adequate performance under the same experimental conditions. All selected molecular imprinting polymers (MIPs) demonstrated stronger affinity in comparison with corresponding blank polymers resulting in imprinted factors (I) equal to 1.2 (cocaine), 2.5 (deoxyephedrine), 3.5 (methadone) and 3 (morphine) which suggested that the specific binding site for each molecule was successfully created. The polymers studied possessed good selectivity and affinity towards their templates and could be recommended for the integration with sensor devices. From a practical point of view, especially for multisensor requirements, the synthetic receptors based on imprinted polymers could be superior to natural receptors due to their stability, robustness and compatibility with automation processes required for sensor fabrication.     

2,8'-disubstituted-1,1'-binaphthyls: a new pattern in chiral ligands

The title binaphthyls 19 and 26, which are the positional isomers of 2-methoxy-2'-(diphenylphosphino)-1,1'-binaphthyl (MOP, 19) and 2-amino-2'-hydroxy-1,1'-binaphthyl (NOBIN, 26), have been synthesized by Suzuki coupling as the key step (10 + 15-->18), followed by functional group transformations, involving C-P and C-N bond formation (18-->19 and 18-->23). Racemic intermediate 22 was resolved by co-crystallization with N-benzylcinchonidinium chloride and the absolute configuration determined by X-ray crystallography. These novel binaphthyls are configurationally stable and, as such, potentially usable as chiral ligands in asymmetric reactions. Michael addition of the glycine-derived enolate 40 to methyl acrylate, carried out in the presence of (R)-(-)-27 as the chiral phase-transfer catalyst, afforded L-glutamic acid (S)-(+)-43 of 92% ee (after hydrolysis of the primary product).     

Chemically modified electrode for the determination of gold — an electrochemical and spectrophotometric study

The voltammetric determination of trace amounts of gold has been studied using a chemically modified electrode. The role played by the host molecule attached to a glassy carbon electrode (GCE), i.e. 8,9,17,18-dibenzo-1,7-dioxa-10,13,16-triazacyclooctadecane (DDTC), was examined by electrochemical and spectrochemical techniques. Cathodic stripping voltammetry (CSV) was applied to study the various parameters affecting the affinity of gold ions by a DDTC modified electrode as well as the interference by other ions. Finally, the method has been successfully applied to determine gold traces in a geological sample. In addition, the formation of an AuCl ₄ ^– -DDTC complex was followed and its formation constant was determined by spectrophotometry.     

A new approach to micropatterning: application of potential-assisted ion transfer at the liquid-liquid interface for the local metal deposition

A new approach to micropatterning is demonstrated. The approach is based on driving an electrochemical process at the solid-liquid interface through the formation of a flux of ions from a micropipet that is held in close proximity to the surface. The flux of ions is generated by the so-called potential assisted ion transfer at the interface between two immiscible electrolyte solutions (ITIES). As a model system, the local deposition of silver was examined. Specifically, a constant potential, which was applied to a micropipet filled with an aqueous solution of silver ions, caused the transfer of Ag(+) into the outer nitrobenzene (NB) solution that consisted of an electrolyte, tetrabutylammonium tetrakis[4-chlorophenyl]borate (TBATPBCl). To facilitate the transfer of silver ions a macrocyclic ligand, that is, dibenzo-24-crown-8 (DB24C8), was added to the organic phase. The Faradaic current of this micro-ITIES was used as a means of controlling the tip-surface distance in scanning electrochemical microscopy (SECM) and depositing silver microstructures on a gold substrate.     

The immunosensors for measurement of 2,4-dichlorophenoxyacetic acid based on electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was evaluated for the direct determination of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Specific antibody against 2,4-D was immobilised onto different gold electrodes. Several methods of antibody immobilisation by covalent linkage to modified surface were studied. Self-assembled monolayers formed using thiocompounds as cystamine, 4-aminothiophenol (ATPh), 3,3'-dithiopropionic acid di-(N-succinimidyl ester) (DTSP) and 11-mercaptoundecanoic acid (MUA) were chosen for the sensing surface activation. Three different sensor types were tested: screen-printed disc and finger-like structures and interdigitated array (IDA) electrodes produced by lithography. The measurements were carried out in a stationary arrangement, and the reaction between hapten and the immobilised antibody was observed online. Changes of impedance parameters were evaluated, and the best immobilisation technique (using 4-aminothiophenol) was chosen for further measurements. Impedance changes due to immunocomplex formation were evaluated, and the possibility of direct monitoring of 2,4-D binding to the antibody was demonstrated at a fixed frequency. For the strip sensor, the calibration curves were constructed in concentration range from 45 nmol l(-1) to 0.45 mmol l(-1) of 2,4-D.     

Nucleation and growth of Cu onto polycrystalline Pt electrode from acidic CuSO4 solution in the presence of H2SeO3

The early stages of Cu electrodeposition onto Pt(poly) have been investigated in 0.5 M H₂SO₄ + 0.01 M CuSO₄ solution without or with H₂SeO₃ when a molar concentration ratio [Cu(II)]/[Se(IV)] ≥ 2×10² using electrochemical and ex situ AFM techniques. The overpotential deposition of Cu has been performed onto a Pt surface precovered independently with Cu in amount close to an equivalent monolayer. Chronoamperometric results have been shown to follow an instantaneous 3D nucleation and diffusion-controlled growth model. The values of diffusion coefficient D for Cu²⁺, number of nuclei N and average nuclei radius r _av have been calculated. In the local regions of the surface, the separate large agglomerates composed of the different diameter clusters have been revealed in both cases, but, in the presence of the H₂SeO₃, they attained a distinct chain-like configuration. Some morphological characteristics have been reported.