CompactGC manual
12
Figure 3.5: Principle of the TCD
The reference flow is supplied by a Detector Gas Module (DGM), and this flow is also
used for make-up gas according figure 3.5. This results in a better peak shape,
especially for the higher concentrations, and a less critical column connection.
Consequently at the ‘TCD out’ exit both flows are measured together. If the column
flow needs to be checked, the reference flow can be switched off for a short time
(don’t forget to switch off the filament first). The normal reference flow is 1-2 ml/min.
Since the response of the detector is concentration dependant, a higher reference
flow results in lower sensitivity.
Operation conditions
Cell temperature:
10-20 ºC above column temperature
Filament (bridge) temperature: 20-120 ºC above cell temperature, depending on
required sensitivity
Reference flow:
1-2 ml/min
Polarity:
depending on carrier gas
Range:
depending on required sensitivity
FID
Operation principle
In gas analysis, the FID is popular for carbon-hydrogen containing compounds, due its
high sensitivity, good stability, and wide linear response. The response is mass-
dependant, so flow rate does not affect the sensitivity.
In the FID (see figure 3.6), the effluent of the column is mixed with hydrogen, this mixture
is burned as it emerges from a metallic jet. This jet acts as one electrode (polarizing
electrode), while a metallic collar surrounding the flame forms the second electrode. A
potential is applied across the pair of electrodes to accelerate the electrons that are
generated during the combustion process of the organic compounds. The resultant
ionization current is sent to an electrometer impedance amplifier, and converted into a
suitable output signal.
TCD front out
capillary column 1
TCD front
analysis
reference
TCD