Page 174 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GQT Final
Encyclopedia of Physical Science and Technology EN006F-275 June 29, 2001 21:12

Gas Chromatography 465

mechanism has been utilized. Synthesized optically active While most GC determinations are performed with so-
−9
polymers are highly effective in resolving various racemic lute quantities between 10 −6 and 10 , certain selective
mixtures. Certain metal chelates, used as additives to the detectors can reach down to the 10 −15 -g levels, represent-
common stationary phases, can retain selected solutes ing some of the most sensitive measurement techniques
through the formation of reversible complexes. Finally, available to the chemist. Some GC detection principles
highly organized liquids (such as various liquid crystals) are based on the measurement of certain transport prop-
tend to retain more strongly the molecules of elongated erties of the solutes (e.g., thermal conductivity or optical
rather than bulky structures. properties), while other detectors are transducers, mea-
Through advances in synthetic chemistry and polymer suring ultimately some product of a solute molecule (e.g.,
research, new GC stationary phases will become available. gas-phase ionization products). The latter detectors are
Additional column selectivities can also be achieved by destructive to the solutes.
mixing the existing stationary phases with each other, in
suitable proportions.
1. The Thermal-Conductivity Detector
This detector, occasionally referred to as the hot-wire de-
IV. DETECTORS AND ANCILLARY tector or katharometer, operates on the basis of measuring
TECHNIQUES the difference in thermal conductivity of pure carrier gas
and the carrier gas plus a solute. Typically, the column ef-
The detector has an extremely important role in the overall fluent is passed through a thermostatted cavity (measuring
processofGCanalysis.Thecurrentpopularityandsuccess cell) that contains a resistor element heated by passage of
of GC as an analytical method is attributable in great part a constant current. Various changes in the thermal conduc-
to the early development of highly sensitive and reliable tivity of the surrounding gas causes the element tempera-
means of detection. In sensing the vapor concentration ture (and, consequently, its electric resistance) to decrease
at the column outlet, the detector provides information on or increase. Pure carrier gas is passed, under the same
thedistributionofindividualpeakswithinachromatogram conditions, through a reference cell of identical design.
(which compound?) as well as their relative amounts (how The resistor elements of both cells are parts of a Wheat-
much?). The area measured under a chromatographic peak stone bridge circuit that records any imbalance caused by
is generally related to the quality of the compound. the passage of individual solutes.
Many detection principles in GC were investigated over In thermal-conductivity measurements, it is advisable
the years, but only a few pass the criteria of reliability to choose a carrier gas that differs maximally from the
needed for precise analytical measurements. Detectors organic solutes (e.g., hydrogen or helium). The detector
can broadly be classified as universal or selective. Uni- is a truly universal and simple device, but its sensitivity
versal detectors measure all (or nearly all) components is marginal; at best, submicrogram amounts are detected.
of a mixture, although their response to the same quan- The thermal-conductivity detector is most typically em-
tities of different compounds is seldom similar. Selective ployed for the analysis of permanent gases and light hy-
detectors respond only to mixture components that pos- drocarbons.
sess a unique structural feature in their molecules. For
example, a typical gasoline sample contains a number of
2. The Flame-Ionization Detector
organic components which, after being separated by an
appropriate chromatographic column, are all detected by This detector is the workhorse of GC. It operates on the
a universal detector. However, if a lead-selective detector basis of decomposing the solute-neutral molecules in a
is used instead, only a few peaks are recorded, those due flame into charged species and electrically measuring the
to the lead-containing additives in gasoline, while the re- resultant changes of conductivity. A cross-sectional view
maining mixture constituents are ignored. The so-called of a flame-ionization detector is shown in Fig. 10. A small
ancillary techniques go a step further as highly selective flame is sustained at the jet tip by a steady stream of pure
detectors, because they actually characterize the individ- hydrogen, while the necessary air (oxidant) is supplied
ual GC peaks qualitatively. through the diffuser. At the detector base, the column
effluent is continuously introduced, mixed with hydro-
gen, and passed into the flame. Conductivity changes be-
A. DETECTORS
tween the electrodes are monitored, electronically ampli-
The most important analytical properties of a GC de- fied, and recorded. A conventional carrier gas contributes
tector are sensitivity, linearity over an extensive concen- little to the flame conductivity; however, when organic so-
tration range, long-term stability, and ease of operation. lute molecules enter the flame, they are rapidly ionized,