Agilent Thermal Conductivity Detector Settings

Answer

This Information Applies To: Agilent 6850, 6890, 7820, 7890, 8850, 8860, 8890, Intuvo 9000 GC Systems with a Thermal Conductivity Detector (TCD)

Issue

The TCD temperature, column (carrier), makeup, and reference flow rates need to be optimized to ensure proper operation and sensitive detection. This article guides the user on setting the correct temperature and gas flow rates on the TCD.

Steps to Follow

Detector temperature 

• Set the detector temperature to 30 to 50 ^°C greater than the highest oven ramp temperature of your method. The detector filament will not turn on when the detector temperature is less than 150 ^°C.
  
  

  Caution: The TCD temperature must not exceed the manufacturer recommended maximum column temperature limit as exceeding this limit will damage your column.

Gas flows

There are three gases flowing into the TCD (Figure 1): 

• Analytical column gas flow
• Makeup gas flow. It combines with the analytical column flow to optimize the total flow into the detector
• Reference gas flow. It switches at a frequency of 5 Hz, alternating this flow past the filament and non-filament sides of the detector.

Figure 1. TCD gas flows.
Left: Column effluent is forced away from the filament. TCD measures reference gas.
Right: Column effluent is forced toward the filament. TCD measures column flow sample peaks (if present).
1. Column flow 2. Make up gas flow 3. Reference gas switched towards the detector filament 4. Reference gas switched away from the detector filament.

1. The analytical column, makeup, and reference gases must all be the same type of gas (typically helium or nitrogen). 

2. Analytical column (carrier) gas flow. The typical column flow rate depends on the column diameter (Table 1):
   

    

   Warning: Do not use hydrogen as the carrier gas in packed columns, and where possible, you can use it as the carrier gas in capillary columns. Hydrogen is flammable and presents an explosion hazard when mixed with air in an enclosed space. The use of hydrogen as the makeup gas is not supported by the gas chromatograph hardware or software.

   
   

   Column type	Column size	Carrier flow rate, mL/min
   		Hydrogen	Helium	Nitrogen
   Packed	1/8-inch	Do not use	30	20
   	1/4-inch	Do not use	60	40
   Capillary	0.05 mm id	0.5	0.4	NA
   	0.10 mm id	1.0	0.8	NA
   	0.20 mm id	2.0	1.6	0.25
   	0.25 mm id	2.5	2.0	0.5
   	0.32 mm id	3.2	2.6	0.75
   	0.53 mm id	5.3	4.2	1.5

   Table 1. Recommended column flow rate

3. Makeup gas flow: 
   • Packed column flow 2 to 3 mL/min
   • Capillary column flow 5 to 15 mL/min.
4. Reference gas flow is calculated as a factor of the total makeup flow plus the analytical column flow (Figure 2). The reference gas flow should be two to three times higher than the column flow plus the makeup gas flow to effectively switch the column/makeup flow between the two sides of the detector. To calculate the reference gas flow:
   1. Add the column gas flow and the make up gas flow in mL/min e.g. total column gas + makeup gas is 20 mL/min.
   2. Using the graph from Figure 2, read the ratio value that corresponds to the total column + makeup gas. Any ratio within ±0.25 of what is in the figure is suitable, e.g., the ratio is 2.1

5. Multiply the total column + make up gas by ratio value to get the reference flow rate,  e.g., 2.1*20 mL/min = 42 mL/min

   Tips: The instrument itself maintains a minimal continuous reference flow of about 5 mL/min whenever the detector is enabled. Thus, any entered value less than about 5 mL/min has no effect on the actual flow rate.

   • See Table 2 for other worked examples.

     
     
     Figure 2. Calculate the reference gas flow
     1. Total column + makeup gas flow mL/min, 2. Ratio of reference gas flow for the total column + makeup gas flow
     
     

     Tips: *Makeup gas is recommended with all capillary columns. It allows the column to be inserted all the way into the detector and withdrawn 1 mm. If makeup is not used, the column must be no more than 3 mm above the ferrule. Minimum makeup flow is 1 mL/min

     
     

     Column*	Column flow mL/min	Makeup flow mL/min	Combined mL/min	Reference flow ratio (Figure 2)	Reference flow mL/min	Total detector flow mL/min
     Small bore capillary column: 25 m × 0.2 mm	0.75	5	5.75	3 * combined flow	3 × 5.75 = 17.25	5.75 + 17.25 = 23
     Small bore capillary column: 25 m × 0.32 mm capillary	10	2	12	2.25 * combined flow	2.25 × 12 = 27	12 + 27 = 39
     Large bore capillary column: 10 m × 0.53 mm capillary	15	2	17	2.1 * combined flow	2.1 × 17 = 35.7	17 + 35.7 = 52.7
     Packed column: 1/8-inch stainless steel	30	0	30	1.75 * combined flow	1.75 × 30 = 52.5	30 + 52.5 = 82.5

     Table 2. Worked example reference gas flow calculations

Learn how to effectively operate and troubleshoot your Thermal Conductivity Detector (TCD): GC-0GEN-1013s - GC Detectors Theory and Operation GC-7890-2281r - Agilent 7890 Series GC Thermal Conductivity Detector (TCD) Troubleshooting, e-learning courses available from Agilent Education