7890 GC Retention Time Shifts

[GB6477]

I am running a 7890 GC with a cool-on-column. On my last run I ran 4 standards. The first 2 ran perfectly with no shift. Each compound in my standard has identical retention times. Then my third standard ran. The retention times lined up perfectly till about 10 minutes into the run then they started to shift. They shifted so bad that they shifted into the next compund. The 4th standard also ran about the same as the 3rd standard but with not quite as bad a shift. What can I all look at as a problem. The column is new, just installed last Wed. I have reinstalled it a couple times and don't think it is leaking. Any ideas would be great.

[KM-USA]

Septum loose or leaking (if retention times are getting longer?) ?

[Peter Apps]

First thing; post enough detail for us to troubleshoot without having to do it by guesswork. What are you analysing, ? what are the retention times and how much do they shift ?, what are all the analytical conditions.

Peter

[GB6477]

Spent about an hour today talking with agilent on the phone. We think we have narrowed it down to ferrules. We are using the Vespel Graphite when we should be using the graphite ferrules. We are doing oil characterization looking for C10 through C60. It is a cool-on-column and the run is about 33 minutes long. The temp ramps from 60 degrees C to 430 degrees C at about 12 degrees a min. I don't have the right size ferrules but I am trying some a size to big now to see how the run comes out. The agilent tech says the vespel ferrules melt after 350 degrees C and we are probably creating a worse leak with each injection. We will see.

[Peter Apps]

Is the cool on-column injector back-pressure regulated ? If it is the inlet pressure and therefore carrier flow should be robust to leaks - if there is a leak at the inlet you just get less gas out through the pressure regulator. On the other hand, if you have mass flow control a leak will give you longer retentions.

The simplest test is to check for leaks with a leak seeker.

The other possibility is poor oven temperature control.

Peter

[GB6477]

You will have to excuse my ignorance on this subject a little because all of my experince is with the split/splitless GC and not the cool-on-column setup. I am going to stay on the safe side and say that the back injector is not back pressure regulated. If it is I guess I am not sure how to check that.

The problem I was having was shifting the peaks to the right but only on injections 3 and 4. Injection 3 started to shift to the right at about 21 minutes and injection 4 started to shift at about 11 minutes. Injections 1 and 2 ran identical with their retention times.

I just ran 3 more standards and documented the oven temps at 5 min increments and they were pretty much the same for all three injections. The problem of the retention time shift has seem to be fixed. Hopefully it was just the ferrules that were causing me the leaks. Unfortunately we don't have the leak checker so I was not able to do that test.

I will put a full run on starting now and see how it comes out in the morning.

Thank you.