Extraction Technologies for Medicinal and Aromatic ... - Capacity4Dev

EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS
14.4.3 TLC versus HPTLC Layers
High performance TLC (HPTLC) plates use thin layers of adsorbent
(100 μm instead of 200-250 μm) and smaller particles (5-6 μm versus
10-12 μm) of more homogeneous size (4-8 μm versus 5-20 μm). Moreover,
they give better resolution (5- to 10-fold more) over shorter runs (3-6 cm versus
8-15 cm), reduce separation time (3-20 min versus 20-200 min), accommodate
more samples per plate (more than double), use smaller sample
volumes (0.1-0.5 μl versus 1-5 μl) with improved detection limits (100-500
pg), and signifi cantly improve the precision, accuracy and sensitivity. HPTLC
plates are substantially more expensive (4- to 6-times more) than normal
plates but are an effi cient alternative when high sensitivity, accuracy and
precision are required in situations demanding high performance. More improvements
in adsorbent layers include use of spherical particles of narrow
size distribution (reducing resolution time and size of spots while improving
the detection limit) and ultrathin layers (10 μm) that improve the resolution
and sensitivity and drastically reduce the development time.
14.4.4 Selection of the Mobile Phase
Infi nite combinations and a wide choice of solvents are available
for TLC developments. Unlike HPLC, where choice is limited, TLC provides
no or few restrictions. A mobile phase with 1-3 components is preferred
over a multicomponent mobile phase. The polarity of the compounds
of interest is the key to selection of a mobile phase. Personal experience applied
to existing knowledge and a trial and error method is used to select the
composition of the mobile phase. The mobile phase is freshly prepared for
each run and the constituting solvents are mixed outside before transferring
to the developing chambers. It is advised to allow the developing chamber to
saturate unless otherwise specifi ed. Saturation of the chamber is quickened
by lining half or more of the total area of the inside walls with fi lter paper
and pouring the mobile phase over it. Closing the chamber and allowing it
to stand at room temperature saturates the chambers. It is possible to use
another solvent alongside the mobile phase for chamber saturation in twin
troughs, e.g. ammonia placed in one trough and mobile phase in another.
The TLC results are sensitive to temperature and humidity variations.
All operations during which the plate is exposed to the air should be
carried out at a relative humidity of 50%-60% under controlled temperature
of 20°-30° C.
14.4.5 Application of Sample
Three typical options of delivering the sample solution onto the
plate are manual, semi-automatic and automatic application. Manual application
is done using a capillary, which can have a specific volume of 1, 2 or 5 μl
for quantitative purposes. The solution is applied by the technique of touch
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