Thermo Scientific FOCUS GC, Instruction Manual

Page: 300 / 356

Chapter 15
Maintenance and Troubleshooting
TCD Troubleshooting
300 Instruction Manual
TCD Troubleshooting
Sensitivity
Sensitivity is related to the detector temperature (increasing temperature reduce
sensitivity) and to the flow rate of the carrier, reference and make-up gas. A gain
in sensitivity can be obtained increasing the gap between the temperature of the
block and that of the filaments or increasing the filaments voltage.
Sensitivity is strictly related to the state of the filaments and to their operating
conditions. A significant reduction of the detector sensitivity may be caused by
the contamination of the filaments due to degradation of high molecular weight
compounds inside the cell or to contaminated gases.
Low temperatures of the detector block may cause high boiling compounds
condensate on the filaments reducing sensitivity.
Baseline drift
A small baseline drift normally occurs during a temperature program and does not
indicate a trouble. This effect is due to the decrease of the carrier gas flow rate as
temperature increases.
A baseline that suddenly goes out of scale and a rapid growth of the signal could
indicate that filaments are likely to be burnt and have to be replaced.
Negative peaks
Negative peaks are normally generated by the sample components that have a
thermal conductivity higher than carrier gas. For instance, using nitrogen or argon
as carrier gas, negative peaks are obtained for helium, hydrogen or methane.
To revert the polarity of the detector, refer to Detector Signal Parameters in
Chapter 9.