Page 153 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005F220 June 15, 2001 20:44

Elemental Analysis, Organic Compounds 395

FIGURE 1 Perkin-Elmer 240C Elemental Analyzer connected to a microcomputer. [Courtesy of Perkin-Elmer
Corporation.]

CHN analyzer, an apparatus that determines all three el- CO 2 ,H 2 O, and N 2 is performed by three pairs of thermal
ements simultaneously on the basis of the chemical reac- conductivity cells connected in series. A trap between the
tions discussed above. Several models of this apparatus are first pair of cells absorbs H 2 O from the gas mixture before
available commercially. An example is shown in Fig. 1, it enters the second cell so that the signal is proportional
in which the CHN analyzer is connected to a microcom- to the amount of H 2 O removed; another trap between the
puter. With this equipment, after the accurately weighed second pair of cells removes CO 2 so that the signal is pro-
sample has been placed in the combustion tube and the portional to the CO 2 removed; and the last pair determines
sample weight entered on the keyboard, the analysis is N 2 by comparing the remaining sample gas plus helium
started and all operations of combustion: measurement of with pure helium. The instrument is calibrated with a pure
CO 2 , and H 2 O and N 2 ; calculations; and printout of results known organic nitrogen compound such as acetanilide be-
are carried out automatically. Figure 2 shows a schematic fore samples are run.
diagram of the combustion train. The boat that contains Adaptation of the CHN analyzer for total automatic
the organic sample is put in the ladle and introduced into operation when a large number of samples have to be ana-
the combustion tube by opening the sample fitting plug. lyzed within a short period involves changing the combus-
By means of the magnet, the boat is pushed to the high tion train from horizontal to the vertical arrangement. The
heat coil area. The automated process now commences. head of the combustion tube is connected to a turntable
The organic compound is combusted in the pyrolysis tube device known as the autosampler. Figure 3 depicts the
in an oxygen atmosphere under static conditions. The car- schematic flow diagram of the PE 2400 CHN Analyzer
bon dioxide, water vapor, and nitrogen oxides produced equipped with an autosampler which holds 60 samples in
are then swept by a stream of helium into the reduction the carousel sample tray. Figure 4 shows a complete pic-
tube, where nitrogen oxides are converted to nitrogen gas ture of this instrument connected to the microprocessor
and the excess oxygen is removed. The gaseous mixture and microbalance. Instead of being placed in an open mi-
is conducted into the analyzer, where the measurement of croboat, each sample is weighed in a tin or an aluminum