Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

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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. 188
EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS 11.5 Stepwise Operations in Process-scale HPLC Various aspects should be taken into consideration for operating process-scale HPLC and stepwise scale-up, as follows. • Develop a robust analytical method and scale up the method for process scale using the same stationary phase. • The main objective of method development is a simple, wellautomated and robust separation process able to run 24 h per day. • Optimize sample loading, fl ow rate and column pressure. • Select the best solvent for both sample preparation and elution. • The injection solvent should be optimized because sharp peaks and high loadability are important goals. • In process-scale separations, control the fi rst two runs manually and observe the process. If no technical problems occur, subsequent runs can be performed automatically. • Collect fractions and re-analyze them for purity using the analytical method. • Peak purity at three points (i.e. up slope, apex and down slope) should be confi rmed. 11.6 Problems Encountered in Preparative Scale-up 11.6.1 Purity of Crude Extract A typical problem encountered in process scale-up is that the plant material or enriched fraction used during method development had been produced in analytical scale and differs in solubility and impurity from the material that is being processed in pilot scale. The process-scale plant material can be either of a different quality or show larger amounts of the same impurities, and, in some instances, even new impurities can arise. If an impurity profi le shows larger amounts of the same impurities or new impurities, the chromatographer must retest the separation method at analytical scale before starting the process-scale separation. If during a process scale up, a compound shows higher purity, the solubility in the weak solvent chosen during optimization may not be good enough. In this instance, productivity can be lower than expected because the amount separated in each run will be less. Because scale-up is linear, the chromatographic run takes the same time in preparative scale as in analytical scale, and the substance is not stressed longer in the separation equipment. 189
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CONTRIBUTORS Chapter 6 Aqueous Alco
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