Page 276 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

9 CAS CHROMATOCRAPHV

based on those adopted for the Car10 Erba Elemental Analyser; further details
may of course be obtained by referring to the manufacturer's manual.
Determination of C, H and N. The weighed samples (usually about 1 mg), held
in a clean, dry, tin container, are dropped at pre-set intervals of time into a
vertical quartz tube maintained at 1030 OC and through which flows a constant
stream of helium gas. When the samples are introduced the helium stream is
temporarily enriched with pure oxygen and flash combustion occurs. The
mixture of gases so obtained is passed over Cr,O, to obtain quantitative
combustion, and then over copper at 650 OC to remove excess oxygen and reduce
oxides of nitrogen to N,. Finally the gas mixture passes through a chromato-
graphie column (2 m long) of Porapak QS heated to approximately 100 OC.
The individual components (N,, CO,, H,O) are separated and eluted to a
thermal conductivity detector, the detector signal being fed to a potentiometric
recorder in parallel with an integrator and digital print-out. The instrument is
calibrated by combustion of standard compounds, such as cyclohexanone-2,4-
dinitrophenylhydrazone.
It is appropriate to mention here the determination of Total Organic Carbon
(TOC) which is important in water analysis and water quality monitoring,
giving an indication of the overall amount of organic pollutants. The water is
first acidified and purged to remove carbon dioxide from any carbonate or
hydrogencarbonate present. After this treatment, a small measured volume of
the water is injected into a gas stream which then passes through a heated
packed tube where the organic material is oxidised to carbon dioxide. The latter
is determined either by infrared absorption or is converted to methane for
determination by gas chromatography using a flame ionisation detector [see
Section 9.21.
Determination of oxygen. The sample is weighed into a silver container which
has been solvent-washed, dried at 400 OC and kept in a closed container to
avoid oxidation. It is dropped into a reactor heated at 1060 OC, quantitative
conversion of oxygen to carbon monoxide being achieved by a layer of
nickel-coated carbon (see Note). The pyrolysis gases then flow into the
chromatographic column (1 m long) of molecular sieves (5 x cm) heated
at 100 OC; the CO is separated from N,, CH,, and H,, and is measured by a
thermal conductivity detector.
Note. The addition of a chlorohydrocarbon vapour to the carrier gas is found to
enhance the decomposition of the oxygen-containing compounds.
Determination of sulphur. The initial procedure for flash combustion of the
sample is essentially that described for the determination of C, H and N.
Quantitative conversion of sulphur to sulphur dioxide is then achieved by
passing the combustion gases over tungsten(V1) oxide, WO,, and excess oxygen
is removed by passing the gases through a heated reduction tube containing
copper. Finally the gas mixture passes through a Porapak chromatographic
column heated at 80 OC in which SO, is separated from other combustion gases
and measured by a thermal conductivity detector.
9.6 DETERMINATION OF ALUMINIUM BY GAS CHROMATOGRAPHIC ANALYSIS OF ITS
TRIS(ACETYLACETONAT0) COMPLEX
The purpose of this experiment is to illustrate the application of gas chromato-