Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS
traction process; there is typically a large interface between sample matrix
and headspace; and the diffusion coeffi cients in the gas phase are typically
higher by four orders of magnitude than in liquids. The concentration
of semivolatile compounds in the gaseous phase at room temperature is
small, and headspace extraction rates for these compounds are substantially
lower. They can be improved by using effi cient agitation or by increasing
the extraction temperature.
In the third mode (SPME extraction with membrane protection),
the fi ber is separated from the sample with a selective membrane, which
lets the analytes through while blocking the interferences. The main purpose
for the use of the membrane barrier is to protect the fi ber against adverse
effects caused by high molecular weight compounds when dirty samples
are analyzed. While headspace trapping serves the same purpose, membrane
protection enables the analysis of less volatile compounds. Use of
thin membranes and an increase in extraction temperature result in shorter
extraction times.
9.3 Calibration, Optimization, Precision and
Suitability of SPME
9.3.1 Selection of Fiber Coating
The chemical nature of the analyte of interest determines the
type of coating used. A simple general rule, “like dissolves like”, applies
very well for liquid coatings. Selection of the coating is based primarily on
the polarity and volatility of the analyte. Poly(dimethylsiloxane) (PDMS) is the
most useful coating and should be considered fi rst. It is rugged and able to
withstand high injector temperatures, up to about 300° C. PDMS is a nonpolar
liquid, thus it extracts non-polar analytes very well with a wide linear
dynamic range. However, it can also be applied successfully to more polar
compounds, particularly after optimizing extraction conditions.
Both the coating thickness and the distribution constant determine
the sensitivity of the method and the extraction time. Thick coatings
offer increased sensitivity, but require much longer equilibration times. As a
general rule, to speed up the sampling process, the thinnest coating offering
the sensitivity required should be used.
9.3.2 Selection of the Extraction Mode
Extraction mode selection is based on the sample matrix composition,
analyte volatility, and its affi nity to the matrix. For dirty samples, the
headspace or fi ber-protection mode should be selected. For clean matrices,
both direct and headspace trapping can be used. The latter is applicable for
analytes of medium to high volatility. Headspace trapping is always preferen-
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