Page 175 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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

466 Gas Chromatography

molecules, flowing through the ionization chamber, are
bombarded by the radioactive rays from the source of ra-
3
diation (usually a foil containing 63 Ni or H) incorpo-
rated into the detector body. In a rather complicated pro-
cess, radicals, positive ions, and low-energy electrons are
created. Application of electric potential between the elec-
trodes permits the easily collected electrons to be contin-
uously monitored as the so-called standing current (typ-
ically, around 10 −9 A). This steady current provides a
baseline value for the measurement of substances with
a strong affinity to such low-energy electrons. When an
electron-capturing solute enters the detector, it decreases
the population of electrons by an electron attachment pro-
cess. A decrease of standing current thus occurs during
the passage of a solute band, resulting in a negative chro-
matographic peak.
FIGURE 10 The flame ionization detector.
The decrease of standing current due to the electron-
capture process is proportional to the solute concentration
in a process reminiscent and formally similar to Beer’slaw
increasing the current in accordance with the solute con-
of optical absorption, except that thermal-energy electrons
centration. With most flame-ionization detectors, this cur-
rather than photons are involved:
rent increase is linear with the solute concentration up to
six orders of magnitude. E = E 0 exp(−Kxc), (12)
The flame-ionization detector is a carbon counter; each
where E is the number of electrons reaching the anode
carbon atom in the solute molecule that is capable of hy-
per second, E 0 is the initial number of electrons, K is
drogenation is believed to contribute to the signal (com-
the electron-capture coefficient (a function of molecular
pounds with C—C and C—H bonds), while the presence
parameters), x is the detector geometrical constant, and c
of nitrogen, oxygen, sulfur, and halogen atoms tends to
is the solute concentration.
reduce the response. The detector is most sensitive for hy-
The electron-capture detector is a selective measure-
drocarbons. Practically, no response is obtained for inor-
ment device since only certain compounds exhibit appre-
ganic gases, carbon monoxide, carbon dioxide, and water.
ciable affinities toward the low-energy electrons. Among
Because of its high sensitivity (the minimum detectable the structures exhibiting strong electron affinities are var-
amounts are of the order of 10 −12 g/s), linearity, and ease
ious halogenated compounds, nitrated aromatics, highly
of operation, this detector is most popular, in spite of the
conjugatedsystems,andmetalchelates.Thedetectorisex-
somewhat incomplete understanding of the physical (ion- −12 −15
tremely sensitive (amounts between 10 and 10 g can
ization) processes involved.
be detected) to various pesticides, herbicides, dioxins, fre-
ons, and other substances of great environmental concern.
3. The Electron-Capture Detector To achieve this extremely high sensitivity for normally
noncapturing types of molecules (e.g., hormones and drug
This detector is a device based on certain gas-phase ion-
metabolites), various electron-capturing moieties can be
ization phenomena within the ionization chamber. Its
introduced via chemical derivatization (a controlled sam-
schematic diagram is given in Fig. 11. The carrier gas
ple alteration).
4. Other Detection Techniques
Several additional detectors were developed for GC. A
major aim of such measurement devices is selectivity to-
gether with high sensitivity. Selective detectors should be
blind to compounds in a mixture that do not possess cer-
tain unique structural features (i.e., chromophores or het-
eroatoms). In practice, some detectors qualify for such
FIGURE 11 Electron capture detector. [From Fenimore, D. C., selectivity; in other cases, certain substances merely en-
Loy, P. R., and Zlatkis, A. (1971). Anal. Chem. 39, 1972.] hance the detector response.