Extraction Technologies For Medicinal And Aromatic Plants - Unido

11 PROCESS-SCALE HIGH PERFORMANCE LIQUID CHROMATOGRAPHY FOR MEDICINAL AND AROMATIC PLANTS
11.4.2 Separation Time
In preparative separations, the stationary phase is usually recovered
and used again to purify the next batch of the same substance. Often
the major operating cost in preparative LC is the solvent rather than the
packing material. Therefore, the choice of solvent is important in method
development and scale-up. As per the need for separation, isocratic mixture
or gradient elution of water with organic solvent (methanol or acetonitrile)
is used. Gradient elution has a shorter run time than isocratic elution, but
sometimes purity of the isolates is compromised.
11.4.3 Solvent Composition
Methanol is often used in preparative separations. It is an inexpensive
and strongly polar solvent commonly used in combination with water
as a mobile phase in RP separations. Methanol can be used for fl ushing
normal-phase silica columns to remove adsorbed polar contaminants. It can
also be recovered easily from many mixtures. In RP applications, acetonitrile
yields better peaks but is too expensive in most situations for process-scale
separations.
An initial goal of the scale-up process is to fi nd an acceptable
separation. If analysts fi nd more than one set of valid conditions, then
the cost of solvents becomes a major criterion. Solvent selection is usually
determined during the initial method development with the 4-mm i.d.
analytical-scale columns. Sometimes, when the overall costs of the goods is
important to a fi nal product, one can perform a systematic solvent selection
even at later stages of development.
11.4.4 Washing Steps
The accumulation of impurities on the column can decrease
the resolution of the subsequent separation, and late-eluted impurities can
spoil the collected fractions of the subsequent separation. Therefore, washing
steps are often implemented between chromatographic runs. Solvent
gradients and recycling steps are sometimes necessary to increase the
resolution for diffi cult separation problems. Sometimes temperature programming
is used to remove strongly held impurities.
11.4.5 Recycling
Sometimes gradients and recycling steps are required for better
preparative separation of complex mixture of compounds.
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