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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): сhromatography1996.pdf
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F. Development Mode Selection
The ascending linear development mode is most frequently used for TLC. Because ascending development has no theoretical advantage over horizontal development, the latter, being more adaptable has become more common in recent years.
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Figure 5 Possibilities of transferring the optimized mobile phase between different planar chromatographic methods, as well as HPLC and fully on-line FFPC techniques.
The advantage of circular development, where the solvent system migrates radially from the center of the plate to the periphery, is well known for the separation of compounds in the lower Rf range (37,52). Working with the same mobile phase the resolution, particularly in the lower /grange, is about 4-5 times greater in circular than in the linear development mode. It can be stated that the separation power of the circular development mode can be better exploited if the samples are spotted near to the center (41,52).
In the anticircular development mode the solvent system enters the layer at a circular line and flows towards the center. Since the solvent flow velocity decreases with the square of the distance, but the area wetted also decreases with the square of the distance traveled, the speed of solvent system migration is practically constant. Therefore this developing mode is the fastest with respect to separation distance. Anticircular development is a widely accepted approach in analytical TLC, if the resolution must be increased in the higher /grange (38).
From the point of view of development distance and mobile phase composition, multiple development (MD) techniques can be classified into four basic categories (64).
UMD (unidimensional MD) means the repeated development of the chromatographic layer on the same development distance (D) in solvent systems with the same composition (STi, SF1) (see Figure 6). The removal of the mobile phase between development steps is performed by careful drying of the chromatoplate. The dried layer is returned to the development chamber for repeated development under the same chromatographic conditions as in earlier development steps.
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Szepesi and Nyiredy
STjSFi STi;SFl STfSF1 STi;SFi STj! SF)
r r.
r1
1 r1
St - Sc St *8c St_;Sp St :Sp St ;Sc
Tr F1 T2 F2 t3 f3 *4 f4 t5 f5
STiSF-
ST5;SF5
Figure 6 Schematic of different multiple development techniques.
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The alternative process of UMD called IMD (incremental MD), where the rechromatography is performed in increasing development distances (D| > D5) with the same composition (5n, SFl) of mobile phase (see Figure 6). Using this multiple development technique the first development length is the shortest, the subsequent development steps are accomplished in a (usually by an equal distance) longer development distance, and the last front migration distance is the longest, corresponding to the useful development length of the chromatoplate and to the feature of the mobile phase.
Gradient MD (GMD) is called the multiple development technique, where the successive chromatographic development steps are performed with a change in solvent strength and selectivity (Sni Sfi'i (Sts, ^fs) the same chromatographic length (D = const.) (see Figure 6).
Bivariate MD (BMD) is the most complex multiple development technique, where the development distance and the mobile phase composition are varying simultaneously (Db ST], 5fl; -> D5, Sn, SF$) during the successive chromatographic runs (see Figure 6).
The advantages of MD techniques can be summarized (61) as follows:
UMD is mostly effective to improve separation in the lower/?yrange.
IMD improves the zone center separation.
GMD increases most significantly the separation capacity of the chromatographic system.
BMD is effective for samples of differing polarity, where the smaller separation capacity is adequate to detect the separated compounds between starting position and final solvent front as a single chromatogram.
G. Selection of Other Operating Parameters
In TLC, the solvent velocity is the parameter which, in principle, cannot be influenced by the chromatographer. The enhanced efficiency of FFPC techniques by comparing TLC where the mobile phase migrate only by capillary action is the constant linear mobile phase velocity. FFPC techniques guarantee the optimal H/u values. In OPLC the upper limit of velocity depends on the applied external pressure, besides the viscosity. In RPC, the higher the rotational speed, the faster the migration of the mobile phase.
The local mobile-phase velocity can be influenced by the selection of the development mode.
In TLC the separation distance improves with the square root of the separation distance. However, the optimum depends on the quality of the plate (average particle size and size distribution of the stationary phase), the vapor space, the development mode, and the properties of the compounds to be separated.
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