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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.

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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.

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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.

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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.

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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.

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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.

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The thin-layer chromatography of eighteen anions has been performed on silica layers with mixed aqueous–organic mobile phases. Binary mobile phases consisting of distilled water and acetone or acetonitrile were found suitable for resolving hexacyanoferrate(II) from hexacyanoferrate(III) or thiocyanate whereas ternary solvent systems consisting of distilled water, carbon tetrachloride, and acetonitrile was found most suitable for separating ternary mixtures of hexacyanoferrate(II), hexacyanoferrate(III), and thiocyanate. Distilled water–acetonitrile–carbon tetrachloride, 10 + 80 + 5, was the best mobile phase for separation of three-component mixtures of these anions in the presence of heavy metal cations and amino acids. The proposed method works well for the separation of Fe(CN)6 4–, Fe(CN)6 3–, and SCN from industrial waste water, saline water, hard water and fixer and bleach samples.

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Combined technology for the synthesis, separation, screening, and analysis of combinatorial libraries is described. The technique enables a rapid route from synthesis to the testing of chemical compounds. Chemistry can be rapidly optimized and vital information obtained by testing by-products and reagents simultaneously if desired. Screening can be performed without need for reaction work-up and without the need for undesired chemical manipulation, or further handling. Emphasis has been given to bioautographical/agar overlay screening methods for the testing of antimicrobial agents.

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