FID diagram wanted

[Mendicant]

So I have read on the Restek website that "At elevated temperatures, even trace levels of oxygen or water will quickly cause irreversible damage to the stationary phase... To prevent oxygen and moisture from entering the system and damaging the column, the system must be completely leak free. In addition, the carrier gas must be passed through a high-quality oxygen trap before entering the column."

What prevents oxygen and moisture from the room entering through the rear of the column though the FID when the GC is turned off?

I understand how the inlet is sealed between the septum at the injection port and the ferrule around the column. The part I do not understand is how the FID remains sealed. I can see the moisture that is being produced from the flame when I hold a cool watch glass and see condensation. Is there some form of port that allows the exhaust to escape but then remains sealed when the FID is not in use? And if such a port exists, why can I not see it in the diagrams available online?

This is on par with everything I can find by a google image search.


Does anyone possess or know where to find a more technical schematic?

Thank you.

[rb6banjo]

You are correct. It is not sealed. However, it is not good practice to shut off the flow of carrier gas through an installed GC column - even when the flame is off. I keep the flow going all of the time when the column is installed in the instrument. The flow of carrier gas keeps the air from backing up into the column. When possible (on instruments with flow controllers that are electronic), I create standby methods where the carrier gas flow is reduced when not operating BUT there is always flow through them from the inlet when they are installed. I've operated like this for many years and never had a problem with degradation of any of my stationary phases.

[Mendicant]

You are correct. It is not sealed. However, it is not good practice to shut off the flow of carrier gas through an installed GC column - even when the flame is off. I keep the flow going all of the time when the column is installed in the instrument. The flow of carrier gas keeps the air from backing up into the column. When possible (on instruments with flow controllers that are electronic), I create standby methods where the carrier gas flow is reduced when not operating BUT there is always flow through them from the inlet when they are installed. I've operated like this for many years and never had a problem with degradation of any of my stationary phases.

But how does the flow stay on when you power the GC off overnight and over the weekends?

[rb6banjo]

I don't recommend turning them off and on all the time. The only time mine are off is when they will not be used for very long periods of time. In my case, that is never. They go off when we have power outages but those are unplanned and extraordinarily annoying. I don't know of anyone who turns them off and on frequently.

The oxygen is only bad for the columns if you expose them together at elevated temperature. If you must turn your systems off and on, be sure you adequately purge the air out of the columns before running up the temperature.

Please let me reiterate, it's not a good idea to turn these instruments off and on. I fear that you will have a lot more trouble with leaks if you do it.

[Peter Apps]

Except for mass spectrometers (and maybe the new fangled vacuum UV detectors) none of the GC detectors is sealed - usual practice (as the banjo man says) is to leave the carrier gas flowing and the GC switched on except for long shutdowns when you need to remove and cap the column anyway.

Peter

[GOM]

Hi

As long as your column and injector and FID are at room temperature during shut down periods then in general you should not experience any problems when you stop to think about it.

I understand your concerns but you may be overthinking it in an FID instrument with a typical silicone or wax phase. Or I could be underthinking it

To re-iterate some previous comments about temperature, I agree, that is the key.

As Rb6 said "The oxygen is only bad for the columns if you expose them together at elevated temperature. If you must turn your systems off and on, be sure you adequately purge the air out of the columns before running up the temperature."

As a side note - Capillary columns used to be supplied with no end caps - from my experience there was never any degradation of the phase during storage for typical silicone or wax phases.

The main reason for capping on storage is to prevent the ad/absortpion of atmospheric volatiles, where an open column is acting like an SPME

The same applies for a shut down

Upon connecting, or re-starting, any oxygen is flushed out within a very short time.

As Rb6 said - Temperature is the key.

Kind regards

Ralph