Browse

You are looking at 51 - 60 of 33,787 items for :

  • Chemistry and Chemical Engineering x
  • Refine by Access: All Content x
Clear All
JPC - Journal of Planar Chromatography - Modern TLC
Authors: Irina V. Marutsenko, Tatyana V. Polenova, Eduard D. Virus, Andrew G. Borzenko, and Igor A. Revelsky

A method developed for quantitative determination of the total amount of polyaromatic hydrocarbons (PAH) in human hair has been applied to the analysis of smokers’ hair. The procedure involves washing the hair with dichloromethane, acid decomposition of the hair matrix, micro-liquid extraction, and HPTLC determination of total PAH concentrations in the whole extract. The concentrations of PAH in human hair were in the range 0.005–0.3 μg g–1 hair.

Restricted access

High-performance thin-layer chromatography (HPTLC) on silica plates, with two successive mobile phases, has been used for analysis of seven amine azo dye isomers prohibited under a German ban. Dichloromethane was used for the first development, to a distance of 4.5 cm. After drying, the second development was performed with toluene–tetrahydrofuran, 10 + 1 (v/v), to a distance of 8.5 cm. Spots were visualized at λ = 254 nm. Eighteen amines have been studied by use of the method. The limit of determination (X) and the correlation coefficient in the range X to 5X are reported for each amine. To increase the sensitivity of detection, the UV spectrum was acquired for each amine and the wavelength of maximum absorbance (λmax) was used to establish the limit of determination. Synthetic mixtures and dye samples have been resolved and quantified.

Restricted access

This paper reports a recently developed technique, coupling of optimum-performance laminar chromatography (OPLC) with online radioactivity detection (OPLC–RD). Sample preparation for purification of urinary metabolites of a 14C-labeled drug candidate was performed on octadecyl-modified silica solid-phase extraction columns. Optimum-performance laminar chromatography was conducted on aluminum-backed silica gel layers with butanol– acetic acid–water as mobile phase. Eluate radioactivity was detected by use of a flow-cell radioactivity detector equipped with solid scintillator, and eluate fractions containing radioactivity were collected. Strongly retained radioactive components were detected off-line after development by use of digital autoradiography (DAR).

The isolated fractions containing metabolites were further purified by reversed-phase high-performance liquid chromatography coupled with radioactivity detection; this provided a second dimension for additional selectivity. Purified metabolite fractions were submitted to structure elucidation by spectroscopic methods.

The novel on-line optimum-performance layer chromatography with radioactivity detection, combined with high-performance liquid chromatography with radioactivity detection and digital autoradiography techniques, the advantages and disadvantages of which are discussed in detail in this paper, provides a rapid, effective, and economic tool which can be applied to advantage in metabolism research.

Restricted access

In this paper we report our most recent experimental results relating to the thermal instability of selected alkyl chemically bonded stationary phases. The phenomenon of the thermal instability of such phases, as a result of their irradiation with a high-power neodymium laser, the light source of a Raman spectrometer, has been demonstrated in a series of our earlier publications. We even established a novel method of estimating the density of coverage of the silica matrix with alkyl ligands by utilizing the thermal instability of the phases discussed. The mechanism of the thermal transformation of alkyl chemically bonded ligands remained rather obscure, however, vague speculations only were made about the possible aromatization of these ligands.

In this paper we provide additional experimental evidence in support of our earlier hypothesis about possible aromatization of alkyl ligands on the surface of the acidic/basic silica matrix under the influence of high temperature. This evidence was obtained by use of instrumental techniques such as Raman spectroscopy, differential scanning calorimetry (DSC), and UV absorption spectroscopy. Although all these techniques supply indirect evidence only, the results reported furnish a convincing confirmation of our earlier intuitions.

Restricted access

The effect of cold-stress (2°C, 7 days cold-hardening) on the level of polyamines in different varieties of wheat and genetically modified wheat samples (chromosome substitution lines, deletion lines, recombinant lines) has been investigated. Biogenic amines (spermine, spermidine, putrescine, and agmatine) were determined as the dansyl derivatives by stepwise gradient elution with the Personal OPLC BS 50 Chromatograph. Short-term cold-hardening caused characteristic changes in free polyamine content of wheat genotypes. The total free polyamine content was significantly lower in the cold-hardened samples than in the control samples. It seems that accumulation of spermine and agmatine might be good markers of cold sensitivity.

Restricted access

A quantitative thin layer chromatographic (TLC) method on silica gel, with ethyl acetate–acetone–alcohol, 62 + 40 + 3 (v/v) as mobile phase, has been developed for the determination of olaquindox at ppm levels in animal feed. Preliminary purification consists of extraction with water, adsorption on XAD-4, washing, and elution with acetonitrile. Before the elution step acetylolaquindox is added as internal standard. The residue obtained after evaporation of acetonitrile is dissolved in a small volume of methanol. Quantitation is performed by densitometry at λ = 360 nm.

Restricted access

Abstract

A modified QuEChERS method using a GC-ECD to determine the multiple residues of pyraclostrobin, difenoconazole, dimethomorph and azoxystrobin and to indirectly determine the total residues of maneb, mancozeb and propineb by a GC-FPD (with an S filter) was established and validated. Meanwhile, field trials were conducted in accordance with good agricultural practice (GAP) to study their characteristics of residue degradation under the agricultural climate and cropping system of Guangxi Province. The separation effect of each target peak was good with a linearity range of 0.01–5 mg L−1, a limit of detection (LOD) of 0.003–0.015 mg kg−1 and a limit of quantification (LOQ) of 0.01–0.05 mg kg−1. The average recovery ranges of vegetable tomatoes and cherry tomatoes were 70.5–120.0% and 70.8–119.8%, respectively, with relative standard deviations (RSDs) of less than 7.1%. Field trials of seven fungicides in vegetable and cherry tomatoes showed that the half-lives (t 1/2) of the dithiocarbamate fungicides (metiram, mancozeb, and propineb, defined as total residues determined as CS2), pyraclostrobin, difenoconazole, dimethomorph, and azoxystrobin were in the ranges of 5.2, 12.7–17.8, 7.6–7.9, 6.6–6.9, and 6.3–6.6 d in vegetable tomatoes, respectively. The cherry tomatoes presented ranges of 4.3–4.5, 10.8–11.8, 6.7–7.0, 5.4–5.5, and 5.9–6.2 d, respectively. Combined with the final residue and market monitoring results, the results show that cherry tomatoes have significantly higher terminal residues, initial deposits, and maximum residues of seven fungicides than vegetable tomatoes, and these seven pesticides can be detected in cherry tomatoes purchased from three markets. Therefore, cherry tomatoes may be regarded as representative varieties of tomatoes in realizing residual extrapolation for the establishment of the maximum residue limit (MRL) value of fungicides in tomatoes and for conducting market monitoring.

Open access

The chromatographic behavior of six macrocyclic antibiotics (erythromycin, troleandomycin, tylosin, vancomycin, rifamycin B, and rifampicin) has been examined on Polyamide 11F254 TLC plates as stationary phase with five binary solvent mixtures (methanol–water, ethanol–water, propanol–water, acetonitrile–water, and tetrahydrofuran–water) as mobile phases in which the concentration of the organic modifier was from 0 to 100% (v/v). Chromatographic retention data and a possible retention mechanism are discussed.

Restricted access

The retention behavior of twenty-four alkaloids has been investigated in normal-phase systems on thin layers of aminopropylsilica and bare silica developed with a variety of binary mobile phases prepared from n-hexane and polar modifiers – 2-propanol, ethyl acetate, methyl ethyl ketone, tetrahydrofuran, dioxane – in different concentrations. The retention factors obtained were fitted to the logarithmic Snyder–Soczewiński equation; the results are presented as the equation parameters and the statistical coefficients which prove the goodness of fit of the results with the displacement theory of retention.

The retention behavior of the alkaloids was also investigated in reversed-phase systems with buffered aqueous–acetonitrile mobile phases (pH 9.15) and the cyanopropyl- and octadecylsilica as stationary phases. Retention factors obtained for use of different concentrations of acetonitrile in the mobile phase were fitted to the semi-logarithmic equation or to quadratic polynomial equation selected for RP systems and are also presented as equation parameters and statistical coefficients which prove the goodness of fit of the results obtained with the theories derived for RP systems.

Separation selectivity was presented as the RF spectrum for the aminopropyl phase, comparing the separation selectivity obtained by use of different mobile phases, and as RF1 vs RF2 correlation diagrams for different systems, enabling selection of the most selective systems for separation of particular pairs or groups of alkaloids. Such correlation diagrams are presented for the aminopropyl phase and silica with four non-aqueous mobile phases and also for CN-silica and C18 phases, and for CN-silica and diol phases with aqueous mobile phases.

Restricted access