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  • Author or Editor: Abdul Moheman x
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A new spray reagent has been used for TLC detection and spectrophotometric quantification of dichlorvos after extraction with ethanol from bluish tinged maize grains. Silica gel G as stationary phase with cyclohexane-acetone-methanol 8:3:0.5 (ν/ν) as mobile phase was identified as the best TLC system for detection and migration (R F) of dichlorvos. On alkaline hydrolysis dichlorvos forms dimethylphosphoric acid and dichloroacetaldehyde; the latter reacts with 2-thiobarbituric acid to give a sharp pink spot. The reagent is selective for dichlorvos, and does not react with other organophosphorus, organochlorine, carbamate, and synthetic pyrethroid insecticides. The constituents of viscera (amino acids, peptides, proteins, etc.) and grain do not interfere with the test. The lower limit of detection on silica TLC plates was 18 μg. The absorbance maximum (λ max) of the pink color formed by dichlorvos was 500 nm. A plot of absorbance against concentration was a straight line passing through the origin and obeying the Beer-Lambert law in the concentration range 50–350 μg mL−1. The method has been used for identification of dichlorvos in cereals, pulses, vegetables, and fruit.

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Adsorption of Zn(II) and Cd(II) ions on soil TLC plates was studied using aqueous solutions of surfactants as mobile phases. The effects of decomposition of soil organic matter, cation saturation, soil pH, and sewage sludge on the hR F values (or adsorption) of Zn(II) and Cd(II) were investigated. Among these conditions, soil pH was the most important factor affecting adsorption of Zn(II) and Cd(II) ions by soil layers. Aqueous solutions of cationic (cetyltrimethylammonium bromide, CTAB), nonionic ( t -octylphenoxydecaethoxyethanol, Triton X-100), and anionic (sodium dodecyl sulfate, SDS) surfactants at different concentrations (below, near, and above their critical micelle concentrations, CMC) were tested as mobile phases to examine their effect on the efficiency of adsorption of zinc(II) and cadmium(II) by soil stationary phases. Furthermore, the combined effect of surfactants and fertilizers on the adsorption characteristics of Cd(II) and Zn(II) was also investigated. Fertilizers such as KCl and CaCl 2 in SDS remain insoluble whereas MgCl 2 (1.0 m ) in SDS was highly viscous and causes difficulty during development of soil TLC plates. Addition of Cl-fertilizers to aqueous surfactant mobile phases led to increases in the hR F values (or reduced adsorption) of Zn(II) and Cd(II) ions on soil layers. Among the mobile phases examined, 1.0 M MgCl 2 in aqueous CTAB (at concentrations above the CMC) was found to be the best mobile phase for separation of Cd and Zn ions on soil layers. RSD of hR F values were calculated for both Zn(II) and Cd(II). Cd(II) was found to migrate through the soil layer more quickly than Zn(II). Thus, Zn(II) is strongly retained by the soil surface and Cd(II) is able to pass through the soil layer. The limit of detection of these metal ions was also determined.

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Planar thin-layer chromatography of heavy metals has been performed on soil layers with aqueous solutions of amino acids as mobile phases. Several amino acids at different concentrations (0.5 to 5.0%) were tested to examine their effect on the mobility of the heavy metals. Increasing the concentration of the amino acids in the mobile phase resulted in increased mobility of most of the heavy metal ions studied. Neutral amino acids were capable of promoting differential migration among the heavy metals. Important separations of heavy metal ions from their mixtures were obtained with 2% aqueous solutions of neutral amino acids (alanine, serine, proline, valine, and methionine). When aqueous solutions of neutral amino acids were used as mobile phases, better separation of the heavy metals was achieved by use of a 2% solution of proline. Separations of heavy metals achieved experimentally on soil layers are listed.

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Micellar thin-layer chromatography has been used to investigate the separation and migration behavior of metal ions. Thin layers (0.25 mm) of silica gel G on glass plates (5 cm × 20 cm) were used as adsorbent. Ascending development of the plates was performed with aqueous sodium dodecyl sulfate (SDS) and two-component mixtures of aqueous SDS (0.2 m ) and 0.04 m , 0.08 m , or 0.1 m carboxylic acids in different ratios (1:9, 1:1, and 9:1) as mobile phases. The R F values of metal ions were measured. The mobile phase 0.2 m SDS–0.08 m tartaric acid 1:1 was best for separation and identification of Pb 2+ , Zn 2+ , and Co 2+ . The order of mobility ( R F ) was Pb 2+ < Zn 2+ < Co 2+ . On-plate identification of Pb 2+ , Zn 2+ , and Co 2+ in the presence of impurities was achieved. The method was successfully applied to identification and separation of Pb 2+ , Zn 2+ , and Co 2+ in drugs, river water, sea water, and tap water. TLC-spectrophotometry was used for quantitative analysis of Zn 2+ in spiked drug samples with preliminary separation from Pb 2+ , and Co 2+ . The detection limits for Pb 2+ , Zn 2+ , and Co 2+ were also evaluated.

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The chromatographic behavior of eight pesticides has been examined on cationic-micelles impregnated silica layers using mixed organic solvent (different combinations of hexane-acetone, v/v) systems. The chromatographic system constituting 0.01% CTAB (N-cetyl-N,N,N-trimethyl ammonium bromide) impregnated silica gel as stationary phase, and hexane-acetone in 1:1 ratio (v/v) as mobile phase was most favorable for on-plate identification of pesticides with preliminary separation. Surface modification of silica gel on impregnation, as indicated by SEM and FTIR studies was responsible for improved chromatographic performance. The results obtained on 0.01% CTAB impregnated silica layers were compared with those achieved on 0.01% CTAB impregnated kieselguhr, cellulose, or alumina layers. With selected chromatographic system, fivecomponent mixtures of pesticides (glyphosate, acephate, chlorpyrifos, malathion/methyl parathion, and isoproturon) were successfully resolved. The interference of metal cations as impurities on separation of pesticides from their mixtures was also examined. The developed method was successfully applied to the identification of pesticides in cereals, vegetables, and fruit grains. The applicability of the proposed method for the identification of five-component mixture of pesticides present in maize grains was also tested after separation on TLC plates. The limit of detection of glyphosate, acephate, chlorpyrifos, malathion, methyl parathion, and isoproturon was ≈20 μg per zone. For validation and reproducibility of the developed method, standard deviation (SD), ΔR F, and separation factor (α) were calculated.

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A thin-layer chromatographic system comprising of silica gel as stationary phase and 1.0% aqueous urea solution as mobile phase (pH 7.44) has been developed for the mutual separation of five-component mixture of amino acids [lysine (R F = 0.38), histidine (R F = 0.59), leucine (R F = 0.78), alanine (R F = 0.87), and glutamic acid (0.98)]. The presence of foreign substances such as metal cations, anions, and vitamins as impurities in the sample on the separation was also examined. Thin-layer chromatographic parameters such as standard deviation (SD), ΔR F, separation factor (α), and resolution (R S) values of separated components of the mixture of these five amino acids were calculated. The limits of detection for lysine, leucine, and alanine were found to be 1.5 μg spot−1, whereas for histidine and glutamic acid, these were 1.2 μg and 3.0 μg spot−1, respectively. The proposed TLC system is applicable for the identification and separation of amino acids in pharmaceutical formulations.

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A mobile phase system comprising of ethyl acetate and propionic acid in 1:1 (v/v) ratio was identified as the most suitable green mobile phase for selective separation of maltose from fructose, dextrose, galactose, or mannose on precoated silica gel 60 HPTLC plates. The effect of presence of inorganic cations as impurities in the sample was examined for the separation of sugars. The chromatographic parameters like ΔR F, separation factor (α), and resolution (Rs) of separated components of the mixture of maltose-fructose, maltose-dextrose, maltose-galactose, and maltose-mannose were calculated. The limits of detection for fructose, dextrose, and mannose were 7.5 ± 0.41 μg spot−1 and for maltose and galactose were 1.5 ± 0.09 μg spot-1. The proposed method is rapid, sensitive, and free from the use of toxic organic solvents and is therefore environmentally safe. Maltose has been selectively identified in the presence of fructose, dextrose, galactose, and mannose using scanning densitometry.

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