Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

9 SOLID PHASE MICRO-EXTRACTION AND HEADSPACE TRAPPING EXTRACTION
9.3.9 Determination of the Linear Dynamic Range of the
Method
Modifi cation of the extraction conditions affects both the sensitivity
and the equilibration time. It is advisable, therefore, to check the previously
determined extraction time before proceeding to the determination
of the linear dynamic range. This step is required if substantial changes in
sensitivity occur during the optimization process.
SPME coating includes polymeric liquids, such as PDMS, which
by defi nition have a broad linear range. For solid sorbents, such as Carbowax/DVB
or PDMS/DVB, the linear range is narrower because of the limited
number of sorption sites on the surface, but it still can span over several orders
of magnitude for typical analytes in pure matrices. In some rare cases
when the analyte has extremely high affi nity to the surface, saturation can
occur at low analyte concentrations. In such cases, the linear range can be
expanded by shortening the extraction time.
9.3.10 Selection of the Calibration Method
Standard calibration procedures such as external calibration
can be used with SPME. The fi ber blank should fi rst be checked to ensure
that neither the fi ber nor the instrument causes interference with the determination.
The fi ber should be conditioned prior to the fi rst use by desorption
in a GC injector or in a specially designed conditioning device. This process
ensures that the fi ber coating itself does not introduce interference. Fiber
conditioning may have to be repeated after analysis of samples containing
large amounts of high molecular weight compounds, since such compounds
may require longer desorption times than the analytes of interest.
A special calibration procedure, such as isotopic dilution or
standard addition, should be used for more complex samples. In these
methods, it is assumed that the target analytes behave similarly to spikes
during the extraction. This is usually a valid assumption when analyzing
homogeneous samples.
9.3.11 Precision of the Method
The most important factors affecting precision in SPME are:
• Agitation conditions
• Sampling time (if non-equilibrium conditions are used)
• Temperature
• Condition of the fi ber coating (cracks, adsorption of high molecular
weight species)
• Geometry of the fi ber (thickness and length of the coating)
• Sample matrix components (salt, organic material, humidity,
etc.)
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