Page 270 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

9 CAS CHROMATOCRAPHV

signal is fed to a recorder thus producing the chromatogram. The differential
technique used is thus based on the measurement of the difference in thermal
conductivity between the carrier gas and the carrier gaslsample mixture.
The TCD is generally used for the detection of permanent gases, light
hydrocarbons and compounds which respond poorly to the flame-ionisation
detector (FID). Used in conjunction with a Porapak column it is useful for the
determination of water (see Section 9.5) and it has also been employed in gas
chromatographic studies of metal chelates, e.g. for the quantitative determination
of mixtures of beryllium, aluminium, gallium and indium trifluoroacetylacetonates
(see Section 9.2(2)). For many general applications, however, it has been replaced
by the FID, which is more sensitive (up to 1000-fold), has a greater linear
response range, and provides a more reliable signal for quantitative analysis.
Ionisation detectors. An important characteristic of the common carrier gases
is that they behave as perfect insulators at normal temperatures and pressures.
The increased conductivity due to the presence of a few charged molecules in
the effluent from the column thus provides the high sensitivity which is a feature
of the ionisation based detectors. Ionisation detectors in current use include the
flame ionisation detector (FID), thermionic ionisation detector (TID), photo-
ionisation detector (PID) and electron capture detector (ECD) each, of course,
employing a different method to generate an ion current. The two most widely
used ionisation detectors are, however, the FID and ECD and these are described
below.
Flame ionisation detector. The basis of this detector is that the effluent from
the column is mixed with hydrogen and burned in air to produce a flame which
has sufficient energy to ionise solute molecules having low ionisation potentials.
The ions produced are collected at electrodes and the resulting ion current
measured; the burner jet is the negative electrode whilst the anode is usually a
wire or grid extending into the tip of the flame. This is shown diagrammatically
in Fig. 9.2(b).
The combustion of mixtures of hydrogen and air produces very few ions so
that with only the carrier gas and hydrogen burning an essentially constant
signal is obtained. When, however, carbon-containing compounds are present
ionisation occurs and there is a large increase in the electrical conductivity of
the flame. Because the sample is destroyed in the flame a stream-splitting device
is employed when further examination of the eluate is necessary; this device is
inserted between the column and detector and allows the bulk of the sample
to by-pass the detector.
The FID has wide applicability, being a very nearly universal detector for
gas chromatography of organic compounds, and this, coupled with its high
sensitivity, stability, fast response and wide linear response range ( - IO7), has
made it the most popular detector in current use.70

Electron capture detector. Most ionisation detectors are based on measurement
of the increase in current (above that due to the background ionisation of the
carrier gas) which occurs when a more readily ionised molecule appears in the
gas Stream. The electron capture detector differs from other ionisation detectors
in that it exploits the recombination phenomenon, being based on electron
capture by compounds having an affinity for free electrons; the detector thus
measures a decrease rather than an increase in current.