Chromatogram multiple development

Figure 7.13 Separation of a test eixture using automated multiple development with a universal mobile phase gradient from acetonitrile through dlchloromethane to carbon disulfide on a silica gel HPTLC plate. The chromatogram was scanned at different wavelengths to enhance the chromatographic information.

The AMD method of Burger is also based on vertical TLC. This system for automated multiple development of thin-layer chromatograms is discussed in Chapter 11 Special Methods in TLC . This equipment falls into the top price bracket and is included here only for completeness. 

If certain separation problems carmot be solved by a single development, multiple developments can be the answer. Here, the chromatogram is developed two or more times with intermediate drying of the layer. There are five types of repeated development ... 

E. Multiple (manual) In multiple (manual) development, following a single development in the ascending mode, the chromatogram is removed from the chamber, air-dried and then placed in the same solvent and redeveloped in the same direction. This process, which may be repeated numerous times, increases the resolution of components with Rp values below 0.5. The theory of unidimensional multiple development has been reviewed by Perry et al. [47]. 

Optimum conditions for isocratic multiple development are summarized in Table 6.10. The outcome of separations by multiple chromatography is the most predictable (see section 6.3.7) but is rarely the best approach for distributing sample zones throughout the whole chromatogram, and incremental multiple development is generally preferred. Incrementing the solvent entry position while simultaneously increasing... 

At the same time, with diversification of the stationary phases, various new apparatuses, like the apparatus for automatic application of spots, the chromatographic chambers for circular and anti-circular development, automatic multiple development, or development at high pressure, chambers with gradients, equipment for registering in situ chromatograms have appeared. 

Automated multiple development (AMD), providing automatic chromatogram development and drying, is a novel form of the PMD technique. Automated multiple development as an instrumental technique can be used to perform normal-phase chromatography with solvent gradients on HPTLC plates. Most of the AMD applications use typical gradients Starting with a very polar solvent, the polarity is varied by means of base solvent of medium polarity to a... 

The Revalue is the fundamental parameter in planar chromatography to describe the position of a spot on a developed chromatogram. values in linear, circular, and anticircular chromatography were defined. Correlations between these types of R were evidenced for conversion of linear R values in circular and anticircular and unidimensional multiple development. Definition of thermodynamic and relative Revalues were also reported and discussed. In addition, the importance of Rm value, which has a linear relationship with structural elements of the solute and can be used to characterize molecular hydrophobicity in reversed planar chromatography, was evidenced. 

When the substances to be separated are in the lowest third of the chromatogram after a single development, continuous or multiple development usually brings about a better separation. Reference has already been made xmder Separation Chambers and Development to various devices for continuous development (see p. 69, 72, 76) the BN-chamber was made specially for this purpose (Fig. 34). Since there is no front in this technique, a suitable reference substance is chosen which is chromatographed at the same time and R -values are quoted. 

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