Introduction to Thermal Conductivity Detector (TCD)
The Thermal Conductivity Detector (TCD), also known as a katharometer or hot-wire detector, is a commonly used, early-developed, and widely applied detector in gas chromatography. Its working principle is based on the fact that different gases have different thermal conductivities.
Solutions to the Fault of Inability to Zero the Baseline of TCD in Gas Chromatographs
- Attenuation Gear Test
When the baseline shows an offset from the zero point, adjust the attenuation gear from low to high and observe whether the baseline offset gradually decreases.
- Function Check of Zero Adjustment Knobs
Turn the coarse, medium, and fine adjustment knobs respectively, and check if the baseline responds.
- Dual-Channel Flow Rate Check
On the basis of leak testing of the gas circuit, use a soap film flowmeter to measure the flow rates of the two gas channels separately and check if the difference between them is too large.
- Check for Resistance Errors Between Hot Wires
Measure the resistance values at the terminal sockets of the hot wires at all levels in the thermal conductivity cell.
Generally, the resistance difference between each group of hot wires should not exceed 0.2~0.5 Ω. If it exceeds this range, follow the method in Step 6 to address it.
- Hot Wire Touching the Cell Wall or Contamination
- Whether the hot wire touches the cell wall can be confirmed by measuring the insulation resistance between the hot wire and the cell body.
- Severe contamination of the hot wire can be eliminated or partially eliminated by cleaning the thermal conductivity cell body.
- Asymmetric Hot Wires or Wrong Lead Connections
- If the measured resistance error between hot wire groups is between 0.5~3 Ω, it can be solved by connecting a resistor in parallel across the resistor in the TCD zero-adjustment circuit.
- Another solution for asymmetric hot wires is to pair the hot wire with the largest resistance with the one with the smallest resistance into a group, and pair the two hot wires with medium resistance into another group. This pairing method often better resolves the TCD baseline zero-adjustment issue.
- Treatment for Excessively Large Dual-Channel Flow Rate Difference or Gas Circuit Leakage
If the flow rate difference between the two channels is too large, it can be resolved by adjusting the gas circuit control valve. However, there should be no leakage in the two gas channels during this process.
- Open Circuit in the Zero-Adjustment Circuit
Check and repair the zero-adjustment circuit to eliminate the open circuit fault.
- Recorder Open Circuit or No Response
Inspect the recorder and its connecting circuits to fix the open circuit or address the issue of no response.