Detector Systems

Thermal conductivity detector (TCD)

As Figure 7•9shows, however, the lower the detector zone temperature, the greater is the temperature difference between the filament versus the surrounding detector body temperature. Thus, for maximum sensitivity, the detector zone should be operated at the lowest temperature possible (limited by highest boiling components condensing inside the detector). Also, a second advantage is one of increased filament lifetime.

Gas flow rates

After the detector and instrument are allowed time for thermal stabilization (about 1/2•hourafter the oven and zones achieve desired setpoint values), two gas flow rates, column and reference gas, must be set independently.

After setting optimal carrier gas flow rate through the column, Figure 7•10may be used to determine the corresponding optimal reference flow rate.

Figure 7-10

60

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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50

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Relative

 

 

 

 

 

 

Column Flow Rate:

 

 

 

 

 

Response

 

 

 

 

 

 

30 ml/min

 

 

 

 

 

 

 

 

 

45

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Column Flow Rate:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

20 ml/min

 

 

 

 

 

 

 

 

 

40

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Column Flow Rate:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

60 ml/min

 

 

 

 

 

 

 

 

 

35

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

20

40

60

80

100

Reference Gas Flow Rate (ml/min)

TCD Response versus Reference Flow Rate

147