Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

9 SOLID PHASE MICRO-EXTRACTION AND HEADSPACE TRAPPING EXTRACTION
9.4.6 Need for Headspace
When we go to a rose-fi eld full of bloomed roses, we detect a
pleasant smell in the atmosphere and expect the same fragrance when we
use the bottled perfume or 100% genuine essential oil extracted from the
same roses. However, this is not true. The fragrance we detect in the fi eld
is completely different from the bottled perfume or essential oil, for the following
reasons:
a) When a fl ower or herb is processed to obtain the essential
oils, the low volatile compounds cannot always be recovered
and often evaporate. These low volatiles are important for
aroma.
b) During the processing of an herb, many chemical reactions
take place, such as saponifi cation, trans-esterifi cation, polymerization
and condensation. These reactions actually
change the character of the oil, so that its aroma no longer
resembles that of the actual plant and the product is different
in composition. Many stereoterpenes, which are highly
volatile in nature, cannot be extracted and remain in the
herbs. These stereoterpenes do not contribute directly to
the odor but, in combination with other ingredients, impart a
synergic effect to the overall odor quality.
9.5 Types of Headspace Trapping
Headspace trapping can be static or dynamic, which is generally
called the purge-and-trap method. In static headspace trapping, gas extraction
is carried out in a single step or in a limited number of steps. On the
other hand, the purge-and-trap technique consists of two or three separate
steps, the fi rst of which is continuous gas extraction.
9.5.1 Static Headspace Trapping
This is a single-step gas extraction procedure (Figure 3). By
thermosetting the sample for a certain time at a preselected temperature,
equilibrium is reached between the sample phase and the gas phase of the
sample vial. Subsequently, a single aliquot of the headspace is introduced
into the carrier gas fl ow, which then carries it to the column where the volatile
compounds are separated in the usual way.
The equilibrium of the two phases in the sample vial is characterized
by a partition coeffi cient (Ki) representing the ratio of the analyte’s
concentration in the sample phase and in the gas phase.
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