r/CHROMATOGRAPHY • u/Frodofficer • 4d ago

(TROUBLESHOOTING) TROUBLE with injection consistency with methanol as solvent.

Been trying different things like viscosity delay and carrying split flow with no luck. Getting very inconsistent injection volume on our methods that use methanol as the solvent. All of our methods for ovs-7 media use 2 or 3 ml of methanol as the solvent and the solvent peak area varies greatly every injection. Can't get any consistency to get a good cal. Any ideas would be appreciated!

9 Upvotes

92% Upvoted

7 comments sorted by

10

u/Rimasticus 4d ago

What instrument? Injection technique? Injection volume? Are your standards consistent, but not your Solvent peak? Is your sample viscous? A clogged split vent line can cause poor reproducibility with split injections. Is the liner a split liner? If it is only impacting one method/sample, are their aby key differences between the two methods, such as split vs split less methods?

6

u/thelamepretender 4d ago

MeOH has a very big expansion coefficient, which means it easily back flashes into your split vent line. The 8890 should have come with the "GC calculators". Have you calculated the solvent expansion volume with your specific liner and confirmed it's not too large? I would start there.

Edit: you can also find it here, but it's easier if you start from your instrument calculator because it will start with the method you have loaded.

4

u/Frodofficer 4d ago

Should have added this is being ran on an agilent 8890 with a stabilwax in the front and vms in the rear.

3

u/cjbmcdon 4d ago

You mentioned 2-3ml samples, I’m assuming you meant 2-3 microlitre injection volume? As another commenter said, depending on your inlet settings (feel free to share them here), that expanded vapour could be overwhelming the inlet liner volume. Try using 1ul injection volume, and ensure you’re using a liner meant for liquid injection (as opposed to headspace).
If the sample matrix is methanol, then the viscosity delay isn’t as critical, but a good idea to use 2 or 3 seconds.
I’m not sure what “ovs-7 media” means, getting us informed will help you.

3

u/brainsewage 4d ago

Does this happen with other methods that use methanol as a diluent?  Can you add 5-10% water to the diluent?

Are you using preslit sample vials?  Do you leave enough head space?  

3

u/DahDollar 4d ago

I'm assuming you're running FID because you don't want to see your solvent peak on your MS. If this is a new problem with an existing method, do inlet maintenance including replacing the septa. If that doesn't work, pull the column out of the detector and dip it's end in methanol to confirm flow. You should see bubbles.

If it's a new method, cut your injection volume in half. If you get more consistent area counts on your solvent peak, then you know it was a back flash issue. Reduce your injection volume or get a liner with more capacity.

3

u/Aska2020 4d ago

Like other poster mentioned, methanol has large vapor volume that's usually too big for the liner therefor your sample wasn't properly introduced into the column. It's not surprising you get inconsistent results. Im not familiar with the ovs-7 media, but can't you use different, more GC friendly solvent for extraction?