Descending development

The end of the paper nearest the origins is placed in a trough supported in the upper part of a well-sealed tank. The paper is anchored by a glass rod and is folded over an antisiphon rod supported above the trough (Figs. 5.7 and 8). The solvent used as the 

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Fig. 5.7. Panglas all-glass Chromatanlc Model 300 (Shandon Southern, Sewickley, PA) with troughs and antisiphon rods supported at the top for simultaneous descending development of two paper sheets.

Tanks similar to those for descending development can be used, with paper sheets, suspended from the lid, a glass rod, or a frame, dipping into solvent held in a tray on the floor of the tank. Alternatively, the paper can be stood in solvent by rolling it into a cylinder held together by staples or plastic clips [7]. This allows wider sheets to be developed in low volume cylinders or tanks. Special frames can accomodate several sheets for simultaneous development [39] (Fig. 5.9). 

The usual two-dimensional chromatography is a multiple development method in which the sample is spotted in the corner of a square of paper and ascending or descending development is made successively with two different solvents in two directions perpendicular to each other. Between developments, the paper is thoroughly dried to remove all traces of the first solvent. The best separations of complex mixtures are obtained if different types of solvents are employed. 

Figure 21.10. Developing chambers for paper chromatography. A Ascending development B Descending development. Note that similar chambers are used in TLC. From D. Abbott and R. S. Andrews, An Introduction to C3iromatography, Boston Houghton Mifflin, 1965, by permission of the publisher.

Operation. After activation, the sample application and developing procedures are carried out almost exactly as in PC. Normally, sample sizes range from 10-100 pg per spot for analytical TLC, but in preparative TLC when samples are applied as bands, up to 100 mg can be used with a 20 x 20-cm plate. Spots should be 2-5 mm in diameter. Ascending or descending development and one- or multi-dimensional techniques can be used. Because of differences in capillary action and possibly in solvent heat of adsorption, development times are usually faster in TLC than in PC. Depending on the mobile phase and the particle size of the adsorbent, a typical time... 

Descending development is frequently used in paper chromatography, yet seldom in TLC. It neither shortens the migration time nor improves separation and continuous development is more easily achieved in another way (p. 69, 75). 

Fig. 28. Large test tube with side arm for descending development of a narrow TLC-strip (according to [688])...

A more or less horizontal plate is essential in the TLC on loose layers. A uniform supply of solvent must be ensured here also, as in descending development. The following procedures can be distinguished according to the nature of solvent supply and the technique of application ... 

Other development techniques also can be carried out with the BN-chamber, e. g., an iterating , a polyzonal and the descending development already described (see p. 72). 

The use of descending development has also been reported for preparative separations (e.g., 34). It has no significant advantages with regard to resolution, however, and is therefore rarely used. 

T.J. lints, L.M. Parsons, L. Hartley, et al., Nkx- 2.5 a novel murine homeobox gene expressed in early heart progenitor cells and their myogenic descendants, Development 1993, vol. 119, pp. 419 31. 

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