Detector Systems
Thermal conductivity detector (TCD)
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filament. The immediate symptom is a permanent change in detector
sensitivity due to change in filament resistance.
If possible, such offending materials should be avoided. If this is not
possible, the filament may have to be replaced frequently.
Capillary column considerations
The TCD cell filament channel has an internal volume of about 3.5 l.
This small cell volume makes it suitable for use with capillary columns.
In performing capillary analyses with the TCD, the following should be
noted:
Since a portion of the column passes through the TCD heated block
and into the cell itself, zone temperature for the TCD should not be
set greater than the maximum temperature allowed for the column.
A higher zone temperature may cause column bleed.
After setting column flow rate, makeup gas should be set so total flow
rate (column + makeup) is 5 ml/min. Reference gas should be turned
off while making this measurement.
- Measure the column flow rate.
- Open (counterclockwise) the On/Off valve for TCD makeup gas
flow.
- Use a small screwdriver to adjust the variable restrictor at the
center of the On/Off valve as necessary to obtain the desired total
flow rate.
After makeup gas is adjusted, reference gas should be 1.5 times the
total flow from the column plus makeup. Therefore, if column plus
makeup flow is 30 ml/min, then reference flow = 45 ml/min. Also, with
makeup, the column should be pushed all the way up into the detector
and backed off approximately 1 mm. However, when not using
makeup gas (when using Series 530 at a relatively high flow rate),
then the column should be only 1 to 2 mm above the ferrule. If you
want to position the column all the way up for maximum inertness,
then continue to use makeup and set it to 1 to 2 ml/min.
