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Chromatographic scince series - Cazes J.

Cazes J. Chromatographic scince series - Marcel Dekker, 1996. - 1098 p.
ISBN 0-8247-9454-0
Download (direct link): hromatography1996.pdf
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Practically all the stationary phases used in NP- and RP-HPLC are now also available for TLC (37). It is generally accepted that resolution is higher on a thinner layer (0.1 mm), but this effect is also simulated by the detection mode (38). Commercially available analytical thin-layer plates have dimensions of 10 x 10, 10 x 20, or 20 x 20 cm. The silica materials commonly used for precoated plates have an average particle size of ca. 11 ranging from 3 to 18 |im; for self-prepared analytical layers the average particle size is 15 (im and the range of particle sizes is much greater. The average particle size of precoated HPTLC plates is now 56 |im with a very small range of particle sizes.
Precoated analytical layers with preadsorbent zone are also commercially available for linear development. This zone serves to hold the sample until development beings. Compounds soluble in the solvent system pass through the preadsorbent zone and are concentrated in a narrow band before entering the chromatographic layer proper, and this improves their resolution.
B. Vapor Phase Selection
In planar chromatography the separation process occurs in a three-phase system of stationary, mobile, and vapor phases, all of which interact with one another and with the operating parameters. Selection of the chamber type and vapor space is a variable offered only by planar chromatography, as the third dimension of the chromatographic parameters. The role of the vapor phase in TLC is well known, although only little attention is made in practice (39).
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Szepesi and Nyiredy
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Figure 2 Structures of some commercially available surface-modificd silicas. (Reproduced from Ref. 36, with permission.)
1. Conventional Chromatographic Chambers
Basically, one can distinguish between the normal (N) chamber and the sandwich (S) chamber (40). In the common N-chamber there is a distance of more than 3 mm between the layer and the wall (or between the layer and the lid of the chamber in horizontal development) of the chromatographic tank. If this distance is smaller, the chamber is said to have the S-configuration. Both types of chromatographic chambers can be used for unsaturated or saturated systems. Although the chambers used for forced-flow planar chromatography (FFPC) separation can also be assigned to the above two categories.
2. Chambers for Forced-Fiow
The chambers used for overpressured layer chromatography (OPLC) are unsaturated S-chambers, theoretically and practically devoid of any vapor space. This must be considered in the optimization
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of the solvent system, especially in connection with the disturbing zone and multifront effect (41), which are specific features of the absence of a vapor phase.
The main difference between the chamber types used in rotation planar chromatography (RPC) (42-44) lies in the size of the vapor space, which is an essential criterion in RPC (43). Therefore, an additional symbol indicates the vapor space [normal chamber, microchamber, ultramicrochamber, and column RPC (N-RPC, M-RPC, U-RPC, and C-RPC respectively)].
In N-RPC the layer rotates in a stationary N-chamber, where the vapor space is extremely large. Due to extensive evaporation, this chamber is practically unsaturated. The M- and U-chambers in RPC belong to the S-chamber type, the difference between these two chambers being that the former is saturated, while the latter is unsaturated. Since in M-chamber RPC the chromatoplate rotates together with the small chromatographic chamber, where the distance between the layer and the lid of the chamber is smaller than 2 mm, the vapor space is rapidly saturated. In the case of the ultramicrochamber the lid of the rotating chamber is placed directly on the chromatoplate so that practically no vapor space exists.
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