Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS
9.3.6 Optimization of Sample Volume
The volume of the sample should be selected based on the estimated
distribution constant. The distribution constant can be estimated by
using published values for the analyte or a related compound, with the coating
selected. The distribution constant can also be calculated or determined
experimentally by equilibrating the sample with the fi ber and measuring the
amount of analyte extracted by the coating.
9.3.7 Determination of the Extraction Time
The equilibration time is defi ned as the time after which the
amount of analyte extracted remains constant and corresponds within the
limits of experimental error to the amount extracted after infi nite time. Care
should be taken when determining the equilibration time, since in some cases
a substantial reduction of the slope of the curve might be wrongly taken
as the point at which equilibrium is reached. Determination of the amount
extracted at equilibrium allows calculation of the distribution constants.
When equilibrium times are excessively long, shorter extraction
times can be used. However, in such cases the extraction time and mass
transfer conditions have to be strictly controlled to assure good precision. At
equilibrium, small variations in the extraction time do not affect the amount
of the analyte extracted by the fi ber.
On the other hand, at the steep part of the curve, even small
variations in extraction time may result in signifi cant variations of the
amount extracted. Shorter is the extraction time, larger is the relative error.
Autosamplers can measure the time precisely, and the precision of analyte
determination can be good, even when equilibrium is not reached in the
system. However, this requires that the mass transfer conditions and the
temperature remain constant during all experiments.
9.3.8 Optimization of Extraction Conditions
An increase in extraction temperature increases the extraction
rate but simultaneously decreases the distribution constants. In general,
if the extraction rate is of major concern, the highest temperature that still
provides satisfactory sensitivity should be used.
Adjustment of the pH of the sample can improve the sensitivity
of the method for basic and acidic analytes. This is related to the fact
that unless ion exchange coatings are used, SPME can extract only neutral
(non-ionic) species from water. By properly adjusting the pH, weak acids and
bases can be converted to their neutral forms, so that they can be extracted
by the SPME fi ber.
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