Preparation of the Layer

The preparation of layers in general is described on pp. 52—59. Activation of the layer is undesirable, at least for the compound classes treated here. 

When the layer is heated even to a little over 100° C, the gypsum in the silica gel G ceases to fulfil properly its function as a binder and the layer becomes soft and almost powdery. Layers activated at high temperatures (e. g., 2 h at 140° C according to Cherbtjliez et al. [4] or even 4 h at 140° C according to Nicolaus [5]) necessarily become more sensitive to atmospheric moisture it is not surprising that larger fiuctuations of P/-values occur on such layers. 

In contrast to the general procedure (p. 60) we air-dry moist layers by leaving overnight. The separations described below (amino acids, peptides, DNP-amino acids and PTH-amino acids) have all been obtained on air-dried layers [7—9]. Narrow plates (50 x 200 mm) are suitable only for orientation experiments since layers on such plates are as a rule uneven. 

Further pre-treatment of the layers may be needed when certain solvents are used. An example of this is referred to later, in the section on chromatography of DNP-amino acids (p. 768). 

We have carried out preliminary experiments with Alox for TLC (Firm 60) (a suspension of 20 g Alox in 60 ml water was spread with the usual spreader and the layer thus obtained was dried as for silica gel) they were unsatisfactory because of the slow rate of flow of the mobile phase, a mixture of n-butanol-acetic acid-water (80 + 20 + 20) 4 hours were required for a 10 cm run The amino acid separation was, however, comparable to that achieved on silica gel. 

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Suppose further that at first we have suspension dip-coating in mind for the preparation of the layer to be obtained. The coating should be suitable as a substrate for a microfiltration membrane (layer 3) with a pore diameter of 200 nm. Which coating compoimd material is most appropriate This depends on the application and on the substrate material. When there is no reason not to use alumina, this is the best choice because thermal shock cracks can then be avoided during heat treatment (sintering) of the coating. 

The separation efficiency obtained in TLC is essentially determined by the mean particle size and the size distribution of the sorption agent used in the preparation of the layer. As can be seen from Fig. 4, the mean particle size of silica gel of a quality suitable for HPTLC is 5 pm, that of TLC quality ca. 11 pm and that of PSC quality (for preparative work) over 20 pm. In a very recent development, spherical particles with... 

Zhang et al demonstrated a general approach to the preparation of the layered sandwiched composites of GO and conducting polymers of different morphologies, on the basis of the electrostatic interactions between negatively charged GO sheets and positively charged surfactant micelles [43]. The composite exhibited an excellent electro-capacitive performance... 

Preparation of the Layers. An aquieous siLspension containing about 15% cellulose powder, rendered homogeneous by 30—60 secs in an electric mixer, yields layers with the optimum uniformity, smoothness etc. The layers adhere excellently and can even he wiped in the dried state. Cellulose layers are best air dried. It is unnecessary to activate them at higher temperatures. In fact, the separating properties of cellulose layers improve on long exposure to air. 

Preparation of the layers according to [46] 30 g kieselguhr G (Firm 88) + 0.05 g sodium diethyldithiocarbamate are carefully stirred into a mixture of 45 ml water and 15 g polyethylene glycol and the mixture spread on to 5 plates using the standard method these are... 

The circular separation mode has the advantage that no preparation of the layer is necessary, and excellent separation may be achieved in the lower /iyrange (60). No prerun is, furthermore, necessary when working with lower mobile phase velocities, because of the reduced effect of the disturbing zone. Especially high resolution can be achieved when the point of sample application is directly below the mobile phase inlet, i.e., when the sample is applied in the exact center of the plate. 

Reproducibility in RPC depends upon the preparation of the layer (planar column), the vapor space, and sample application. If the same amount of stationary phase is used, layer preparation does not exert a significant effect on reproducibility. Production of a highly reproducible planar column requires use of exactly the same amount of stationary phase under the same conditions of compression. 

Shimazaki, Y., Mitsuishi, M., Ito, S., Yamamoto, M. Preparation of the layer-by-layer deposited ultrathin film based on the charge-transfer interaction. Langmuir 13, 1385-1387 (1997)... 

Impregnation of layers with a chiral selector for use in enantiomer separations has usually been carried out by mixing the compound with silica gel to prepare a slurry for in-house preparation of the layer. For example, Bhushan and Martens [18] reported use of this method for preparing layers containing selectors such as (-F)-tartaric acid or (-F)-ascorbic acid, erythromycin, vancomycin, L-lysine and L-arginine, and (—)-brucine. 

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