Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS
precise composition of mobile phase and flow rate can only be assured by accurate
pumps. Gases dissolved in the mobile phase are a source of flow-rate
inaccuracies and errors in detector response.
Retention time variations are often discussed to know the tolerable
limits. The retention time is affected by flow rate, column temperature, mobile
phase composition and integration. An error in flow rate leads to changes in
the retention time to the same extent. Small variations in column temperature
have more significant effects on retention time. Ideally, a column is thermostatted
to ±0.2° C. However, a high precision of 0.1% in retention time requires the
column to be thermostatted to ±0.04° C. Changes in mobile phase composition
leave a stronger impact on the retention time. It is estimated that in a typical
isocratic elution, a variation of ±1% in mobile phase composition occurs, which
introduces an error of 0.4%-0.7% in retention times. The observed variations in
composition of the mobile phase are more in the gradient elution. Recording
devices also introduce variations in the retention time through faulty recording,
but the effect is much smaller (in the range of 0.1% to 0.04%).
Peak area is affected by all the factors that affect retention
time. Additionally, the recorder response in marking the start and end of the
peak is crucial; this has been seen to be the main source of error in recording
peak areas.
Several more factors, like injection volume, connecting tubing,
end fi ttings and detector volume, also have bearings on the fi nal results.
Large injection volume and quantity of analyte result in broadening of the
peak. Preparing the sample in the mobile phase produces the best result
and should be taken as the fi rst choice.
14.5.7 HPLC in Quality Control of Plant Products
HPLC is the most popular technique among all the analytical
techniques used today. It is therefore understandable that most happenings
are taking place in the modernization of this technique. As discussed in the
section on TLC, HPLC can be used for similar purposes. There are two applications
of HPLC: one to generate the profi le, for which TLC is preferred,
and one to estimate the quantity of markers, where HPLC is preferred. The
initial steps of sample preparation are similar to those for TLC with the exception
that the samples for HPLC are fi ltered through a fi lter of 0.45 μm
or less. Furthermore, it is assured that the test sample does not contain
substances which are permanently retained on the HPLC column, which
means in most cases purifi cation procedures are applied to extracts before
injecting them onto the column.
After the sample has been prepared, it is injected onto the
column and the response is recorded preferably using a variable wavelength
ultraviolet detector. As the nature of all the compounds of the extract cannot
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