Page 272 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

9 CAS CHROMATOCRAPHV

at 600-800 OC while a plasma is sustained in the region of the bead by hydrogen
and air support gases. A reaction cycle is so produced in which the rubidium
is vaporised, ionised and finally recaptured by the bead. During this process an
electron flow to the positive collector electrode occurs and this background
current is enhanced when nitrogen or phosphorus compounds are eluted, due
it is thought to the production of radicals in the flame or plasma which accelerate
the rate of rubidium re~ycling.~' The selectivity of this detector can facilitate
otherwise difficult analyses, e.g. the determination of pesticides such as Malathion
and Parathion.
Another example is the flame photometric detector (FPD) which offers
simultaneous sensitivity and specificity for the determination of compounds
containing sulphur and phosphorus. The operating principle of the FPD is that
combustion of samples containing phosphorus or sulphur in a hydrogen-rich
flame results in the formation of luminescent species that emit light characteristic
of the heteroatom introduced into the flame. Selection of an appropriate filter
(394 or 526nm bandpass) allows selectivity for sulphur or phosphorus,
respectively. It is advantageous to use nitrogen as the carrier gas and mix it
with oxygen at the column exit; hydrogen is introduced at the burner
to initiate comb~stion.~~
The principles and applications of element-selective detectors have been
re~iewed.~~
The element specificity of atomic absorption spectrometry has also been used
in conjunction with gas chromatography to separate and determine organo-
metallic compounds of similar chemical composition, e.g. alkyl leads in petroleum;
here lead is determined by AAS for each compound as it passes from the gas
~hromatograph.~~
Finally, a high degree of specific molecular identification can be achieved by
the interfacing of the gas chromatograph with various spectroscopic instruments.
Although thequantitative information obtained from achromatogram is usually
good, the certainty of identification based only on the retention parameter may
be suspect. In the case of spectroscopic techniques, however, the reverse situation
applies since these provide excellent qualitative information, enabling a pure
substance to be identified, but less satisfactory quantitative information is
often obtained from their signals. The combination of chromatographic and
spectroscopic techniques thus provides more information about a sample than
may be obtained from either-instrument independently. The chief combined
techniques are gas chromatography interfaced with mass spectrometry (GC-MS),
Fourier transform infrared spectrometry (GC-FTIR), and optical emission
spectroscopy (GC-OES).76

9.3 PROGRAMMED TEMPERATURE GAS CHROMATOGRAPHY
Gas chromatograms are usually obtained with the column kept at a constant
temperature. Two important disadvantages result from this isothermal mode
of operation.
1. Early peaks are sharp and closely spaced (i.e. resolution is relatively poor in
this region of the chromatogram), whereas late peaks tend to be low, broad
and widely spaced (i.e. resolution is excessive).
2. Compounds of high boiling point are often undetected, particularly in the