Yes, there is a possibility of leak. Did you remove the vent line copper tubing to clean? If yes, check both ends brass nut using Snoop to ensure no leakage after reassembling back. If you happen to remove the Split Vent Trap assembly to replace the filter cartridge, check that both O-rings on the cartridge are still there and ensure that there's no extra O-ring (from previous filter) left inside the Trap holder.
Of course, the common leak points would be the septum, O-ring of liner, the tightness of the Septum nut. Change a new column ferrule if in doubt.
Hope these would help.
Kelvin has some good pointers, although I would avoid the use of snoop as it can raise the background on your instrument with certain detectors if it makes it into your gas lines. A mix of 50/50 methanol and water will make small bubbles and is preferred over snoop. I would also check the split vent trap if that was changed as a trap oring may not be in place. I have also seen where an old oring sticks in the trap and you end up with two in there when the cartridge is replaced.
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Thanks very much for the responses Kelvin and James- much appreciated. Yes, I removed the vent line copper tubing for cleaning. It turns out that the brass nut was not screwed on properly - my supervisor had to bend the copper tubing so that the brass nut could be screwed back on properly. you both have provided useful information for future reference!