Figure 4.19
Thermal conductivity detector (TCD).balance signal in the bridge circuit. The imbalance is registered as a deflection from the baseline.
Sensitivity, which is determined by the difference in thermal conductivity between the reference and
sample streams, is highest when hydrogen or helium is used as the carrier gas. The thermal conductivity
detector is robust and reliable but has only moderate overall sensitivity which varies widely from
compound to compound. Response has a limited linear range and is sensitive to changes in temperature
and flow rate so that it is not particularly suitable for quantitative work.
Ionization Detectors
Ionization detectors depend on the principle that the electrical conductivity of a gas is directly
proportional to the concentration of charged particles within it. Effluent gas from the column passes
between two electrodes across which a dc potential is applied. An ionizing source partially ionizes the
carrier gas allowing a steady current to flow between the electrodes and through a resistor where a
corresponding voltage drop is amplified and fed to a recorder. When a sample component is eluted from
the column, it is also ionized in the electrode gap thereby increasing the conductivity and producing a
response in the recorder circuit. Ionization detectors are very sensitive, respond rapidly, usually linearly,
and are mostly stable to variations in temperature and flow rate.
Flame Ionization Detector (FID)
A schematic view of a flame ionization detector, which is one of the most widely used, is shown in
Figure 4.20. Effluent gas from the column is mixed with hydrogen and air and burned at a small metal
jet. The jet forms the negative electrode of an electrolytic cell, the positive or collector electrode being a
loop of wire or short tube placed just above the flame. The potential difference applied across the
electrodes is about 200 V. The FID responds to virtually all organic compounds except formic acid, air
and other inorganic gases. Its response to water is very low. It has a very high sensitivity and the widest
linear range (10^7 ) of any detector in common use (Table 4.11).