baseline problems

[afischer]

I am having a drifting baseline problem that I can't seem to solve on an HP 5890 series II. The baseline seems to go up as the oven temperature increases. The perplexing thing is that it responds differently when hexane is injected versus a blank run (no injection). With no injection the baseline still increases, but only varies between 4.4 and 9.6 pa. I assume this acceptable drift? When I inject the hexane the baseline varies between 4.4 and 129pa. I have done all of the following.

- replaced septum, liner and gold plated seal
- replaced FID jet
- installed and conditioned a new column
- replaced oxygen and moisture traps
- checked for leaks and installed new air canister

The GC had been sitting for several months before I started attempting to revive it. Is there anything else that I am missing that could be the cause of this problem? Thanks.

[CE Instruments]

You seem to have checked the obvious on the GC and a few others things as well.
If you get no baseline shift when you do not inject anything the source of the rise must come from the injection.

Change the syringe, try another source of Hexane or another solvent. If using an autosampler change the wash solvent and the waste vial/cap.

The only way it could be the injector is if it is washing contamination into the system. You do not state the injection volume nor type but we assume ssl not PTV ? Injection volume should be 1ul or less regardless of injection technique. You have already changed the liners etc. so with 1ul injection this should have eliminated any possible problems from the injector.
Regards
Richard

[chromatographer1]

I would guess from your post that your GC is working normally.

The baseline signal increase with the increase in temperature is normal column bleed. You did not state what column you have installed.

The high bleed with hexane injection can be from the hexane or it could be from a contaminated injection liner from which the hexane dissolves and deposits 'greasy stuff'? onto the column.

I would suggest you change your injection liner and septum. These may have who knows what on them. If you still get a large rise in baseline, rinse your gas lines into the GC with acetone methanol. Disconnect the EPC or gauges etc of course first. Rinse the tubing all the way into the injection port itself.

You are dealing with unknowns here. Clean up the Gc and it shall work well for you.

Good luck.

Rod

[afischer]

I am using a Omegawax 320 capillary column and an ssl injection. The method (FAME - Fatty Acids) calls for a 1.5-2ul injections. I have been injecting 1.5ul. I have tried a different bottle of hexane, but that produced the same result.

[chromatographer1]

Sounds like the lines from the flow controllers to the injection port should be cleaned.

[Hafiz Allah Mehr]

Hi, Try constant pressure or constant flow modes if awailable on your GC.

[theavenger]

"The GC had been sitting for several months before I started attempting to revive it. "


I would raise the temp on the injection port and detector to about 10 degrees below your column maximum temperature and let "bake" overnight. (Be sure you have carrier gas) Just sounds like a contaminated injection port to me. Might as well get the detector too. Simply replacing parts doesn't always cure all contamination. Even new parts need to be heated above their normal operating temp for a few hours to clean them up. This will most likely cure your problem; its worked for me for, well, ....many years. If not, sonicate all of the injector and FID parts in a chlorinated solvent for an hour or so, rinse with hexane, then methanol, Let air dry, reassemble everything and repeat the bake of the two zones overnight.


Glenn