Trace GC Ultra column K factor low

[Arne]

Hi All,

I have a question about column evaluations on a Trace GC Ultra from Thermo. Our column (30m, 0.25mm id, Restek #13623) passes the leak check but its K factor is much lower than expected (0.8 instead of the expected 1.7). I also understand that a meaningful leak check might depend on a column evaluation performed with a truly leak-tight system.

I checked all connections with an electronic leak detector and found no leaks.

Does anyone know what the underlying problem could be? Is this a problem at all?

Thank you so much!

Arne

[CE Instruments]

It's about right for a 0.32mm id column
Low K factor indicates a lower column back pressure.
How new is the septa ?

If the GC thinks youir column is wider than it is then it will set too low a pressure when calculating the flow so constant flow modes will give slower than expected chromatography. Have you injected a solvent to see if the time is later than expected ?

[trozen]

1. do you have vacuum compensation function turned off? this would decrease the K-factor as well
2. are you sure the column is not broken somewhere in the middle? any bubbles going out at the end of the column merged in a solvent?

[Arne]

Hi,

thank you so much for the replies!

CE Instruments:

I already noticed that it would be about right for a 0.32mm column.
I was also aware that a lower K factor indicates a lower backpressure.
I don't know what expected retention time should be since I recently took over the instrument after the previous operators left my place of work. RTs seem reasonably in agreement with what I see in published work. I will inject a bit of diethyl ether to determine the dead time and calculate whether that's appropriate - great idea.

trozen:

1. I have the vacuum compensation tuned on when the column is connected to vacuum and turned off when it isn't conencted to the mass selective detector. The obtained K factors are very similar.

2. I am sure that the column is not broken somewhere in the middle. I see bubbles come out in solvent at a rate that responds to changes in flow rate settings.



I am not sure what to do next...

Best,

Arne

[Arne]

One of the silver brazes on the injector had developed a leak. What an unlikely source of a leak! We had it re-brazed and the system is fine. Thanks for the input.

[Peter Apps]

One of the silver brazes on the injector had developed a leak. What an unlikely source of a leak! We had it re-brazed and the system is fine. Thanks for the input.

Thanks for the feedback. One of the supposed advantages of back-pressure regulation of column head pressure is that the pressure is robust to leaks. It does not look as if that was the case here.

Peter