Comparison and Outlook of PPC Methods

The principal differences between classical PPC, OPLC, and RPC are summarized in Table 2 which lists the generally accepted characteristics of the methods. As is apparent, the major difference between the methods is the nature of mobile phase migration. Better resolution can always be achieved by use of forced-flow techniques (OPLC, RPC) because the mobile phase velocity is nearer to the optimum; the use of online separation eliminates the need to scrape the separated compounds from the plate and means that all the compounds migrate over the whole separation distance. This enables connec tion of a flow detector, recorder, and fraction collector. These techniques require more sophisticated instrumentation. Unfortunately, the particle size and size distribution of pre-coated plates for OPLC are at present inadequate for this preparative technique.

Modern online forced flow methods enable not only micropreparative (OPLC) and preparative (RPC) separations, but - using appropriate split systems - also the hyphenation of these methods with different spectroscopic techniques like diode-array detection (DAD), FTIR, MS, and NMR, as is apparent from Figure 9. In this way not only isolation but also structure elucidation can be carried out in a single operation process.

The greatest flexibility with regard to choice of stationary phase, particle size, layer thickness, and chamber type is provided by RPC. Because of the availability of suitable vapour phases and combination of development modes, RPC offers the greatest separating power both in terms of the amount of sample and number of compounds to be separated.

It can be stated that PPC covers a special range of preparative separations. PPC does not compete with column liquid chromatography for purification and isolation of compounds from a complex matrix. Instead, the two approaches are complementary and together they enable successful and rapid separation. It is expected that as a result of development of modern forced-flow and multiple-development techniques, PPC will further expand its importance in the isolation and purification of natural and synthetic products.

See also: N/Chromatography: Liquid: Large-Scale Liquid Chromatography. Chromatography: Thin-Layer (Planar):

Densitometry and Image Analysis; Instrumentation; Modes of Development: Conventional; Modes of Development: Forced Flow, Overpressured Layer Chromatography and Centrifugal; Spray Reagents.

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