Extraction Technologies For Medicinal And Aromatic Plants - Unido

9 SOLID PHASE MICRO-EXTRACTION AND HEADSPACE TRAPPING EXTRACTION
Figure 2: Modes of SPME operation: direct extraction (a), headspace trapping (b) and
membrane-protected SPME (c)
In direct extraction, the coated fi ber is inserted into the sample
and the analytes are transported directly from the sample matrix to the
extracting phase. To facilitate rapid extraction, some agitation is required
to transport the analytes from the bulk of the sample to the vicinity of the
fi ber. For gaseous samples, natural fl ow of air (e.g. convection) is usually
suffi cient to facilitate rapid equilibrium for volatile analytes.
In headspace mode, the analytes are extracted from the gas
phase equilibrated with the sample. The primary reason for this modifi cation
is to protect the fi ber from the adverse effects caused by non-volatile,
high molecular weight substances present in the sample matrix (e.g. human
acids or proteins). Here, the amount of an analyte extracted by the fi ber
coating does not depend on the location of the fi ber, in the liquid or gas
phase; therefore, the sensitivity of headspace trapping is the same as that
of direct sampling as long as the volumes of the two phases are the same
in both sampling modes. When no headspace is used in direct extraction, a
signifi cant sensitivity difference between direct and headspace trapping can
occur only for very volatile analytes. However, the choice of sampling mode
has a signifi cant impact on the extraction kinetics. When the fi ber is in the
headspace, the analytes are removed from the headspace fi rst, followed by
indirect extraction from the matrix.
In general, the equilibration times for volatile compounds are
shorter for headspace SPME than for direct extraction under similar agitation
conditions, because of the following reasons: a substantial portion of
the analytes is present in the headspace prior to the beginning of the ex-
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