the film of the column is gone at the entrance and the exit of the cryotrap.

When the coating of the column is burned like the picture above, this can be 2 reasons:

- the maximum temperature of the trap is a bit too high for the column. Reduce the temperature with 20°C.

- the LN2 valve is leaking and this effect will appear. 

To solve the problem it is needed to replace the LN2 valve.

2406-4306, LN2 Cryogenic Solenoid Valve