Retention time shifting GCxGC with deans switch

[Peter Apps]

A shift of a whole chromatogram is actually really unusual, and can only be related to something happening at slightly different times at the beginning of the run. The most likely culprit is that column 2 post run pressure is set to zero post run - when the run starts it has to jump to 240 KPa, which it will never be able to do as repeatably as you want. This is putting a variable back-pressure on column 1 just as the injection is being made, which could well shift the whole chromatogram. How repeatable are your peak areas ?, and does peak area correlate with retention shift ?

Peter

Thank you Peter!
Do you think I can change the pressure to 240 kPa at the end of the run (post run)? Will the pressure be set to 0 after the post run? Or are the settings loaded directly from the next run?
I have now figured out the function of how to record the pressures in each run. I will record them from now on and compare them.
I'm working in constant pressure mode, so all pressures need to be stable during the run, is that right?

I cannot see a reason why you should not set the post run pressure to 240. It is certainly worth a try as a part of troubleshooting if nothing else. You should see the pressure staying at 240 all the time. Yes, pressures and (temperatures) need to be stable or smoothly increasing.

Peter

[Peter Apps]

How repeatable are your peak areas ?, and does peak area correlate with retention shift ?

The peak areas are quite repeatable. I insert a picture.

I can't see the image, there is asticky about posting pictures somewhere on the site.

Peter

[emi_r]

How repeatable are your peak areas ?, and does peak area correlate with retention shift ?

The peak areas are quite repeatable. I insert a picture.

I can't see the image, there is asticky about posting pictures somewhere on the site.

Peter

Try to right-click and then open the image in a new window. For me it worked I don't know how to load the pictures here

[dblux_]

I can't see the image, there is asticky about posting pictures somewhere on the site.

Peter

And now ?
https://picload.org/view/ddwgggdw/bild_forum.png.html

[GOM]

Having re-read the posts I agree with the previous posters that a possible cause is a lack of stable/repeatable initial conditions.

Just a note of caution on bake out temperature.

Thanks! I'll bake out the column first. Up to what temperature can I go if the maximum for the column is 325 degrees Celsius.

With both Innowax and HP5 columns in the same oven this will be governed by the the maximum operating temperature of your Innowax column - approximately 260°C? and not that of 325°C for your HP-5 column.

Otherwise, exceeding 260°C may damage your Innowax column.

The maximum operating temperature of your Innowax column would thus also apply to your analysis conditions

[Peter Apps]

There is a definite trend of peaks to be smaller when they elute later (unless they are getting wider as well, which will make them shorter) which is consistent with something to do with gas flows form inlet to column not being repeatable at the start of the run. You should certainly set that post run pressure to 240.

Peter

[emi_r]

With both Innowax and HP5 columns in the same oven this will be governed by the the maximum operating temperature of your Innowax column - approximately 260°C? and not that of 325°C for your HP-5 column.

Otherwise, exceeding 260°C may damage your Innowax column.

The maximum operating temperature of your Innowax column would thus also apply to your analysis conditions

Hallo GOM,
thanks for your hints!
The second column is in the second oven, so we can use other temperatures. I try today with the post run pressure. But I have another question -
I saw the following message in the logbook friday:

"Instrument Setpoint changed at run time: 14.35".

Which stepoints can it be? I have checked and the message has nothing to do with the temperature gradients. Can it be that the device (because it works in contant pressure mode) adjusts the initial flow when it wants? Based on the ambient temperature and the ambient pressure? Can it be that the differences are so relevant to the RT?

[emi_r]

Hello everyone,

I have watched something new. The inlet pressure 297kPa is kept constant by the device, i. the device measures the air pressure and compensates for the differences by lowering or raising the inlet pressure. I have now taken some pressure curves and allegedly the air pressure decreases with each measurement by 1 hPa. Accordingly, the inlet pressure is increased to have the 297kPa. Is it even possible to have such large fluctuations? Do you know where is the pressure sensor and if the room temperature can influence the measurement of the atmospheric pressure? Can this have an impact on the RT?

[Peter Apps]

You are making this more complicated than it needs to be. Set the post run pressure to 240 and see if it solves your problem. If it does you know the cause of the problem, and how to solve it.

Peter

[emi_r]

You are making this more complicated than it needs to be. Set the post run pressure to 240 and see if it solves your problem. If it does you know the cause of the problem, and how to solve it.

Peter

Hello Peter,
I have tried it and and the device has then beeped the whole PostRun time. The indicated flow in column 1 was in the minus range. Can that be the cause of the beeping? Maybe the pressure 240kPa is then too high for the postrun (column 2) when the postrun pressure for column 1 drops from 297 to 157kPa?

[Peter Apps]

OK, set the post run pressure for column 2 at whatever its initial value is at the start of the run. The idea is to have the gasses stabilized for both pressure and flow before the injection.

Peter

[emi_r]

Hello everybody,

thank's for the tipps. It helped me understand the device better

Unfortunately, the change in postrun pressure did not help. But now I have found the cause. After a few days of testing I now know that the RT shifting are dependent on the room temperature. I have a question for you about this:

There is certainly an explanation for why the peaks elute earlier in the constant pressure mode device and elute later in the constant flow mode device when the temperature in the room is turned up?

[Peter Apps]

I have to say that the room temperature affecting retention times is impossible unless something on your GC is at room temperature or very close to it. If it is then the instrument is set up wrong, or has a serious malfunction. What is the room temperature and how much does it vary. what readouts do you get for each of the supposedly hot zones on the GC ?

Peter

[emi_r]

To the time when the problem came back, there were actually variations in the ambient temperature of 24 to 32 degrees celsius. Last few weeks, I installed a thermometer and made sure that the temperature remained at 25 degrees and the fluctuations were very small, no longer relevant.
I thought, so it must be related to the temperature, I have turned up the room temperature at 30 degrees and then measured - the peaks were shifted (eluted earlier). At 25 all were back. I have repeated these tests for several days and it always works. In the same room is another device that works in constant flow mode. During the tests, I also measured there and was also able to observe fluctuations, but in the other direction - when it got warmer, the peaks eluted later.

I made a graphical representation for the GCxGC: RT and Temp. I paste the image:

[Peter Apps]

What about the hot zones on the GC, what are their readouts at the different room temperatures ?

Record flows and pressure as well while you are doing it.

Peter