Thin mobile phases

One of the most important advances in column construction has been the development of open tubular, or capillary columns that contain no packing material (dp = 0). Instead, the interior wall of a capillary column is coated with a thin film of the stationary phase. The absence of packing material means that the mobile phase... 

Immobilization. The abiUty of cyclodextrins to form inclusion complexes selectively with a wide variety of guest molecules or ions is well known (1,2) (see INCLUSION COMPOUNDS). Cyclodextrins immobilized on appropriate supports are used in high performance Hquid chromatography (hplc) to separate optical isomers. Immobilization of cyclodextrin on a soHd support offers several advantages over use as a mobile-phase modifier. For example, as a mobile-phase additive, P-cyclodextrin has a relatively low solubiUty. The cost of y- or a-cyclodextrin is high. Furthermore, when employed in thin-layer chromatography (tic) and hplc, cyclodextrin mobile phases usually produce relatively poor efficiencies. 

THIN-LAYER CHROMATOGRAPHY OF BENZOIC ACIDS WITH EXTERNAL CONTROL PROPERTIES OF THE MOBILE PHASE... 

One of trends of development of thin-layer chromatography implies that replacement of aqueous-organic eluents by micellar surfactants solution. This is reduces the toxicity, flammability, environmental contamination and cost of the mobile phases, reduce sample prepar ation in some cases. 

The new way of quantitative determination of the ascorbic acid (AC) by means of ion-pair thin layer chromatography (TLC) in organo-aqueous mobile phases containing cetyltrimethylammonium bromide (CTAB) has been alaborated. 

SERS has also been applied as a sensitive, molecule-specific detection method in chromatography, e.g. thin layer, liquid, and gas chromatography. SERS-active colloids were deposited on the thin layer plates or mixed continuously with the liquid mobile phases. After adsorption of the analytes, characteristic spectra of the fractions were obtained and enabled unambiguous identification of very small amounts of substance. 

This is an oversimplified treatment of the concentration effect that can occur on a thin layer plate when using mixed solvents. Nevertheless, despite the complex nature of the surface that is considered, the treatment is sufficiently representative to disclose that a concentration effect does, indeed, take place. The concentration effect arises from the frontal analysis of the mobile phase which not only provides unique and complex modes of solute interaction and, thus, enhanced selectivity, but also causes the solutes to be concentrated as they pass along the TLC plate. This concentration process will oppose the dilution that results from band dispersion and thus, provides greater sensitivity to the spots close to the solvent front. This concealed concentration process, often not recognized, is another property of TLC development that helps make it so practical and generally useful and often provides unexpected sensitivities. 

The multipath dispersion on a thin layer plate is the process most likely to be described by a function similar to that in the van Deemter equation. However, the actual mobile phase velocity is likely to enter that range where the Giddings function (3) applies. In addition, as the solvent composition is continually changing (at least in the vast majority of practical applications) the solute diffusivity is also altered and thus, the mobile phase velocity at which the Giddings function applies will vary. 

There is no other facet where thin-layer chromatography reveals its paper-chromatographic ancestry more clearly than in the question of development chambers (Fig. 56). Scaled-down paper-chromatographic chambers are still used for development to this day. From the beginning these possessed a vapor space, to allow an equilibration of the whole system for partition-chromatographic separations. The organic mobile phase was placed in the upper trough after the internal space of the chamber and, hence, the paper had been saturated, via the vapor phase, with the hydrophilic lower phase on the base of the chamber. 

In the case of thin-layer chromatography there is frequently no wait to establish complete equilibrium in the chamber before starting the development. The chamber is usually lined with a U-shaped piece of filter paper and tipped to each side after adding the mobile phase so that the filter paper is soaked with mobile phase and adheres to the wall of the chamber. As time goes on the mobile phase evaporates from the paper and would eventually saturate the inside of the chamber. 

TLC has been traditionally regarded as a simple, rapid, and inexpensive separation method, currently used mainly for preliminary examinations to give an indication of the number and variety of pigments present and help in the selection of suitable separation and purification procedures for further analysis. To avoid epoxy-furanoid rearrangements caused by inherent silica gel acidify, one pellet of a strong alkali such as KOH or NaOH should be added to the water used to make the thin layer, or in case of ready commercial plates, 0.1% triethylamine (TEA) should be added to the mobile phase. 

Chromatographic characterisation of hydrolysis products Hydrolysis products from sodium polypectate were analysed by thin-layer chromatography on silica gel G-60, using ethyl acetate / acetic acid / formic acid / water (9 3 1 4, by volume) as the mobile phase system. Sugars were detected with 0,2% orcinol in sulphuric add-methanol (10 90ml) [14]. 

---