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  • Author or Editor: Tomasz Tuzimski x
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For very complex multicomponent mixtures, it is preferable to use different variants of multidimensional (planar) chromatography. In multidimensional separations, the modes of planar chromatography commonly applied are comprehensive 2D planar chromatography on mono and bilayers, coupled-layer chromatography, combination of multidimensional planar chromatographic (MDPC) techniques, and also hyphenated methods. Different multidimensional (planar) chromatography techniques and hyphenated methods may be alternative modes for separation and identifcation of pesticides in environmental analysis, especially after coupling with modern detection techniques. Proper identifcation and quantitative analysis of the analytes are possible through the use of sensitive detection techniques such as high-resolution mass spectrometry (HRMS).

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The purpose of the present work is to demonstrate an application of high-performance thin-layer chromatography-diode array scanning densitometry (HPTLC-DAD) after solid-phase extraction (SPE) for the identification and quantitative analysis of azo dyes in samples of drinks and drops. To prepare the samples for SPE, diethylamine was added to a final concentration of 0.025 M. For analysis of dyes, the samples were applied to Bakerbond Octadecyl C18 SPE columns. After being loaded with the samples, the C18 SPE columns were eluted with different volumes and concentrations of mixtures (ν/ν) of methanol-acetate buffer, pH 3.5. The procedure described for the determination of compounds is inexpensive and can be applied to routine analysis of analytes in water-soluble samples of drinks and drops after preliminary cleanup, concentration, and fractionation by SPE. Limits of detection and limits of quatitation values were satisfactory, in the range 31.5–119.4 and 95.3–361.8 ng per zone, respectively, for all determined dyes.

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The purpose of this paper is to report a new procedure for analysis of analytes in complex mixtures by combination of different modes of multidimensional planar chromatography (MDPC) on monolayer or multiphase plates. The procedure described for separation of complex mixtures of compounds is inexpensive and can be applied to routine analysis of analytes in samples of natural origin, e.g., in water or plant extracts, after preliminary clean-up and concentration, e.g., by solid-phase extraction (SPE). Application of multidimensional planar chromatography (MDPC) and modern fiber optical TLC densitometric scanners with DAD is especially useful for correct identification of components of difficult, complicated mixtures, e.g., pesticides in plant extracts (clofentezine in Herba Thymi).

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Mixtures of pesticides have been separated by graft thin-layer chromatography on connected layers — silica and octadecyl silica wettable with water. Separation of pesticide mixtures was achieved by two-dimensional planar chromatography using a non-aqueous mobile phase in the first dimension and an aqueous reversed-phase mobile phase in the second dimension.

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Two mixtures of pesticides have been separated by two-dimensional thin-layer chromatography with adsorbent gradients of the type silica-wettable with water octadecyl silica or silica-cyanopropyl. The pesticides were identified by R F values in both chromatographic systems and by comparison of UV spectra.

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Thin-layer chromatography in combination with fiber optical (diode array) scanning densitometry has been used for identification of fenitrothion in apples and fresh apple juice. The technique enabled parallel recording of chromatograms and in-situ UV-visible spectrometry in the range λ = 191–612 nm.

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Correlation of R F values for pairs of chromatographic systems has been used for practical separation of a mixture of eight cephalosporins by two-dimensional thin-layer chromatography on silica gel layers. Plates were scanned and videoscanned to show the real picture of separation.

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Eighteen pesticides have been separated by two-dimensional thin-layer chromatography on a moderate polarity CN-modified silica gel. CN-silica is widely used as a adsorbent in both normal- and reversed-phase chromatography. Large selectivity differences are obtained by combination of both NP and RP modes on cyanopropyl-bonded polar adsorbents. The greatest spread of points was obtained by combining nonaqueous normal-phase mobile phases (tetrahydrofuran or ethyl acetate in n -heptane) and aqueous reversed phases (a polar solvent (methanol or acetonitrile) in water), both on thin layers of cyanopropyl-bonded polar adsorbent. Correlations of R F values in NP and RP systems were used for practical separation of a mixture of eighteen pesticides by 2D TLC on this adsorbent. Plates were scanned and videoscanned to show the real pictures of the 2D TLC separation.

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A ten-component mixture of pesticides was applied to the edge of the layer in ‘frontal + elution’ mode for preliminary fractionation by zonal micropreparative TLC. The separated, simpler, fractions were applied to an octadecyl silica layer wettable with water (TLC, RP-18W) and re-chromatographed. The plate was scanned and videoscanned, furnishing a real picture of the plate and showing complete separation of the fractions.The simpler fractions were also analyzed by HPLC on an octadecyl silica (LC-18) column. Preparative separation of the complex mixtures by TLC on silica (non-aqueous mobile phase, normal-phase (NP) chromatography) combined with TLC or HPLC (aqueous mobile phase, reversed-phase (RP) chromatography) results in the strong possibility of full separation of the simpler fractions in the second stage by use of the two independent methods, owing to the different selectivity of NP and RP systems (TLC and HPLC).

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The objective of this paper is to report a new procedure for separation of complex mixtures by combining different modes of multidimensional planar chromatography on silica plates. Initially the complex mixture was separated into five groups of compounds. Mobile phases for separation of these different groups were then optimized by regarding each group as an individual separation problem. By use of this new procedure 22 compounds from a complex mixture were separated on 10 cm × 10 cm TLC and HPLC plates.

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