Yokogawa GC8000 Process Gas Chromatograph - Manual (Page 432)

<Appendix A Principle of Gas Chromatograph>
App.A-3
IM 11B08A01-01E
 Thermal Conductivity Detector (TCD)
The TCD utilizes the difference in the thermal conductivity between the measured gas and the
carrier gas and detects the unbalanced voltage produced in a bridge circuit as a measure of
concentration.
Figure 3 shows the fundamental principle of the TCD. As shown, there are two streams, each
having two filaments. One stream passes the carrier gas only and the other, connected to the
column outlet, allows the measured gas to pass during analysis. The fi laments in the two streams
form a bridge circuit such that the filament in one stream is adjacent to the filament in the other
stream. The unbalanced voltage in the bridge is proportional to the concentration of the measured
gas (liquid) component.
The TCD is frequently used to measure the component concentration of the measured gas
(liquid).
F_A03.ai
Comparison
filament
Comparison
filament
Measurment
filament
Measurment
filament
Output
Z1 Z2
Z4 Z3
Carrier gas
Carrier gas
+
Sampling gas
Constant voltage
Figure 3 Fundamental Principle of Thermal Conductivity Detector
 FID
The FID utilizes the phenomenon that carbon molecules in the measured component
(hydrocarbon) are ionized in a hot hydrogen fl ame. That is, it detects the ionization current which
flows between electrodes to which a high voltage is applied. The ionization current is almost
proportional to the carbon number.
The FID is used to measure the component concentration of gases containing low concentrations
of hydrocarbons.
F_A04.ai
Output
+
+
-
+
-
+
+
-
-
Ion collector coil
Hydrogen flame
Jet pipe (nozzle)
Carrier gas
+
Sampling gas
Hydrogen gas for combustion
Figure 4 Fundamental Principle of Flame Ionization Detector
2nd Edition : May 11, 2012-00
App.A