Selection of Appropriate PPC Method

All types of PPC can be used for purification and isolation in the micropreparative and preparative range. As a rule of thumb, if the sample contains more than five substances, up to 10 mg of sample can be separated by a micropreparative method and up to 500 mg by a preparative method. If the sample contains fewer than five substances, these values can be increased to 50 and 1000 mg.

Conventional PPC can be used successfully, if: (1) no more than five compounds must be separated, (2) the compounds to be isolated are distributed over the whole Rf range and are present in more or less the same amounts, and (3) the total amount of sample does exceed 150 mg. This is the simplest and therefore the most widely used method.

Online OPLC can be used for the separation of five to seven compounds in amounts up to 300 mg. The use of centrifugal force for online purification and isolation is the oldest forced flow method. Generally, a 15 |im particle size stationary phase is used with all the advantages of the free selection of the size of the vapour space and the development mode. Up to ten compounds in amounts up to 500 mg can be isolated using the appropriate RPC method.

Table 2 Comparison of PPC methods at their present state of development

Viewpoint

CPPC

OPLC

Migration of mobile phase

Layer/column

Stationary phase

Particle size of sationary phase

Layer thickness

Volume of stationary phase

Vapour space

Separation distance

Separation mode

Isolation

Typical amount of sample Number of compounds

Capillary action Pre-coated Silica, RP-2, RP-18 5 |im < x < 40 |im 0.5, 1, 2 mm Constant Normal tank 18 cm

Linear (circular) Offline 50-150 mg 2-5

Pressure

Pre-coated

Silica

5 |im < x < 40 |im 0.5, 1, 2 mm Constant None 18 cm

Linear (circular) Online (offline) 50-300 mg 2-7

Centrifugal force/capillary action Self-prepared/filled All available 5 |im < x < 15 |im

Increasing (constant) Variable (N, M, UM, none) 12 cm (unlimited) Circular (circular-anticircular) Online 50-500 mg

2-12

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