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Chapter III CHN Analysis, FT IR …..
In this method the sample weighing 2 -3 mg is placed in a tin capsule. After
folding the capsule the sample is placed in the auto sampler. The tin capsule
enclosing the sample falls into the ignition chamber where excess oxygen is
introduced before. At about 990 °C the material is mineralized. Formation of
carbon monoxide is probable at this temperature even under these conditions of
excess oxygen. The complete oxidation is reached at a tungsten trioxide catalyst
which is passed by the gaseous reaction products. The resulting mixture should
thus consist of CO2, H2O and NO2. The product gas mixture flows through a
silica tube packed with copper granules. In the zone held at about 500 °C
remaining oxygen is bound and nitric/nitrous oxides are reduced. The leaving
gas stream includes the analytically important species CO2, H2O and NO2.
Eventually, included SO2 or hydro-halogenides are absorbed at appropriate
traps. High purity helium is used as carrier gas. Finally, the gas mixture is
brought to a defined pressure/volume state and is passed to gas
chromatographic system. Separation of the species is done by so called zone
chromatography. In this technique a staircase type signal is registrated. Step
height is proportional to the substance amount in the mixture. The schematic
diagram of CHN element analyzer is shown in figure (3.1).
Figure: 3.1 Schematic diagram of CHN elemental analyzer.
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