Engineering Chemistry S Datta

508 ENGINEERING CHEMISTRY
General method
• The size of the glass plate is 20 × 20, 20 × 10, or 20 × 5 cm.
• 30 g of adsorbent (stationary phase, generally silica gel) is made to a smooth paste with
requisite amount of water or solvent specified (preparation of chromatoplate).
• The slurry is quickly poured to a spreader or applicator and is spread (thickness
0.25 mm).
• The slurry is allowed to set for 3-4 min.
• The plate is transferred to a drier and allowed to dry for 1 hr.
• The plate is transferred to a desiccator over silica gel.
• Spots are given as usual.
• Eluted with mobile phase in a chromatographic chamber.
• The solvent is dried, the plate is developed and the spots are located.
• R f
is measured to identify the components, e.g., R f
for L-lysine is 0.14, DL-alanine is
0.36 and L-leucine is 0.65.
High Performance Liquid Chromatography (hplc)
hplc is a sophisticated technique of liquid chromatography. The mobile phase is a solvent
of very high purity. The stationary phase consists of very small particles of a solid (silica gel)
packed into a long steel tube. The use of fine particles increases the surface area helping to
separate the components in a mixture efficiently. A pump provides very high pressure (1-550
bar, 0.1-55 Mpa, 14.6-8000 p.s.i) to maintain a flow rate of mobile phase at the rate of
0.01-10 ml min –1 . The material (15-25 µl) to be chromatographed is injected through a septum
into the centre of the packing material with the help of a syringe. Various types of detectors
are used.
One application of hplc is to study the fate of drugs administered in the body.
Gas-liquid Chromatography (glc)
glc is a sensitive analytical technique for analysing mixtures of liquids. In a modern gasliquid
apparatus, the stationary phase is a thin film of liquid adsorbed on the inside surface of
a coiled capillary tube of about 30 m long inside an oven. Sample to be analysed is injected into
the hot column with the help of a syringe. The mobile phase is a gas, which carries the vapours
of the sample mixture through column. The components in the mixture separate as they pass
through the column. The components are detected as they come out and the signal detector is
fed to a chart recorder.
The chart recorder shows how long it takes for each component of the mixture to pass
through the column. This time is called retention time (t r
).
Dried carrier
gas in
Replaceable
silicon rubber
septum
Injector port
Recorder
Detector
Exit
Injector oven
Column
Column oven
Fig. 23.2 The main features of gas-liquid chromatography.