Chemistry Reference
In-Depth Information
2
Extraction and Separation
of Natural Products
Naturally occurring chemical compounds are found in a variety of sources and
invariably are present as mixtures. It is the task of the natural products chemist to
isolate and purify specific components, usually to homogeneity, in order to identify
the chemical structure. A variety of separation techniques is available. Generally,
the first step is the use of a solvent (aqueous, organic, or liquid CO
2
, etc.) to dis-
solve organic material, and the solute is separated from the marc (plant residue) or
the mycelia (fermentation). By concentrating the solvent an ideal outcome would be
crystallization! This is rare and generally other separation steps are required.
2.1 WATER- STEAM DISTILLATION
In its simplest form, water is boiled and the water-steam vapors are passed over and
through the source material, releasing and carrying the (volatile) chemicals away
from the residue. This technique is commonly used to extract volatile plant oils as
shown in Figure 2.1. In many cases, as the steam condenses and cools, the oils float
to the top and are easily removed from the aqueous phase.
2.2 SUPERCRITICAL FLUID EXTRACTION
Supercritical fluid extraction (SFE) is used for separating one component from the
matrix with the application of supercritical fluids as the extracting solvent. Carbon
dioxide (CO
2
) is the most used supercritical fluid. Extraction is usually from a solid
matrix, but can also be from liquids. Extraction conditions for supercritical carbon
dioxide require a temperature around 31°C and a critical pressure of 74 bar.
2.3 SOLVENT PARTITIONING
Upon extraction of the solids and release of desired organics into the extraction sol-
vent, the most common next step is a liquid-liquid extraction, taking advantage of
mixing two (or sometimes three) immiscible solvents, for example, water and ether.
The common rule of thumb is that polar compounds go into polar solvents (e.g.,
amino acids, sugars, and proteins remain in water), whereas the nonpolar com-
pounds generally remain in the organic phase (e.g., steroids, terpenes, waxes, and
carotenoids are normally extracted into a solvent such as ethyl acetate).
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