Liner activation !? PTV large volume injection

[ortelli]

Dear all,
i develop a method with large volume injection on PTV injector with GC-MSMS for PCB and dioxins analyses. I inject standard solution in hexane and it work well but only one time. When I used a new liner, the peak are nice (peakwith ~0.2 min.). For the second injection, I obtain poor peak resolution with peakwith ~0.5min! By changing the liner, I have again a good peak shape for the first injection. The liner used are either multibaffle liner or packed with glass wool.

What could be the reason of this degradation?
I think about liner activation. Every suggestion and comment will be welcome.
Best regards
Didier

[Don_Hilton]

Are you holding the inlet at a constant temperature or programming it? And what temperature(s) are you using? Are you using commercially made standards? And, is there anything eles in the mix? Do you see all analytes in the standards come off the column?

[i.horsting]

Maybe your solvent vent time is to short.
Than you're overloading the column with solvent.
then yes, the first run is kinda oke, but if you do more runs, you will have problems. What kind of PTV are you using?

The glass wool can posse a problem. because it can break when it's put in the liner, en than you have active places.

the peak area's remain the same?

How much are you injecting?
what are your parameters of the PTV and GC?

[ortelli]

The standard of PCB and dioxins are commercially made standard and just diluted with hexane to 1ppb. There is nothing else in the mix. All compounds are present either in first and second injection.

The PTV injector use a temperature programm as follow:

Speed control injection of 20µl at 0.2µl/sec at 50°C with VentFlow at 30ml/min, Vent Pressure 2 psi and vent end time 0min. It means that as soon the injection finish, the split valve is closed and the temperature of injector is raised to 300°C at 720°C/min. The injector stay at 300°C during 10min. A injector purge is then performed at 80ml/min for 3min.

The problem also occured when using multibaffle liner without glasswool.

The peak area remain the same for first and second injection. There is just a peak broadening (~3times larger) and a little shift of retention time (27min. instead of 26.85min.)

System Agilent 7890 with PTV
GC in constant flow 1ml/min (helium) with standard temperature gradient
column 60mx0.25mm, 0.25µm

Thanks for your help. Best regards

[bhuvfe]

Just few thoughts.

Have you tried a liner from a different supplier?

I suppose you are in MRM mode. Maybe is worth to run in scan mode 1st injection and 2nd injection?

What happens to a 3rd injection of your standards? Broader peaks or the same?

Could it be that the final time of your PTV is not long enough and that you have some high molecular weight material that affects the next injection?


Regards,
bhuvfe

[ortelli]

We didn't try liner from other supplier. Do you have recommandation?

3rd injections and all following are the same than the second. The system is stable with poor performance!

It's a good idea to make a full scan acquisition. I will try.

However, if I get back good performance with a new liner, don't you think it is a problem of liner degradation?

What can stay in a liner after 10min at 300 ° C following by a purge of 80ml/min at 3ml/min?

Is it possible that after one injection, the liner is no longer deactivated?

[bhuvfe]

Restek would be one supplier but, to be honest, I think that either you have a bad batch of standards with some contaminant or an air leak in the inlet that activates your liner after only 1 PTV run. The latter seems more likely. When you run in full scan you will find out.

Regarding "What can stay in the liner..." question.
There are plenty of semi-volatiles around that can create chemical noise and active sites.

[i.horsting]

I think you're injection speed is too slow.
I know that you are doing 20 ul in multibaffled liner but if your injection takes too long. 10 sec is a bit to long, expecially with 30 ml/min vent flow.
You have created a sort of desorbtion method, because when you start the GC, almost all of the solvent will be gone. and this relates into a peak broadening because of less refocusing effect (very important).

Also please check your ramp rate, because 12°C/sec is way too much for a multibaffled liner. I think 5°C/sec would be sufficient to get no backflash. (which can later result in many ghost peaks and overall carry over).

good luck getting the parameters.

[AICMM]

ortelli,

PCB's and dioxins are not really subject to degradation in the same manner as some pesticides. So I personally would not attribute the poor peak shape to degradation. Instead, I would follow along the track of earlier posts and look at solvent effects. A number of things I would look at. First, I would hold the ramp until about a minute (or so) after the injection is done to eliminate the balance of solvent. Two, I would look at the initial column temperature at the end of the inlet ramp. Third, I would go split very quickly after the inlet has reached its final temp, something on the order of 20-30 seconds at the most. Finally, I would play a bit with the inlet temperature while venting solvent.

Best regards.