Rugged baselines

Chromatography Forum: GC Archives: Rugged baselines
Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Tuesday, August 21, 2001 - 02:40 pm:

Greetings, everyone:

The GC in our lab is HP6890 chemstation with FID, we use Restek Stabilwax column (15mx0.53mmx1.0Ám) to analyze fatty acids in biological samples such as bile, lymph, serum. It worked well previously and was not used for about 3 weeks. However, now it shows rugged baseline at the beginning of a new session. We thought it was column bleeding and changed a new Stabilwax column. It still has the rugged baseline. Somebody says the inlet or the detector may be contaminated . Also possibly gas trap has some problem.
Could you please tell me the possible reasons for the problem? And where can I find information or reference related to similar problems?
Thank you very much.

David


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Russ on Wednesday, August 22, 2001 - 09:03 am:

By rugged baseline, I am assuming you mean a rising baseline and/or a lot of unusual peaks present. This could be caused by a problem with your inlet, detector, or gasses. It could be caused by a bad or incorrect septum. You could also have a leak in your system which is allowing air onto the column and what you are seeing is from degradation of the column. If the problem only occurs at higher temperatures, I would tend to eliminate the detector, as I would expect most problems with it to show up at any temperature. If you have some fused silica tubing without any bonded phase or coating (often used for guard columns, retention gaps, transfer lines), you could replace your column with a section of this and see if you have the same problem. If you don't, it would tend to indicate the unusual baseline is from the column; though something in your system could be causing this column problem. Hope this helps. Good luck.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Wednesday, August 22, 2001 - 03:52 pm:

Russ:

I think you are right since the baseline is bumping up and down at higher temperature (init T 200, problem starts at T 240) but straight at the beginning for 4 min. I will exclude the possibility of a detector problem.
I changed septum but no improving was observed. I checked a textbook saying "normally septum bleeding causes ghost peaks, while column bleeding causes irregular baseline". So I guess septum is not the point.
I just came up an idea that maybe I need to increase column carrier gas head pressure since at high temperature pressure drops. I don't know if this assumption is right or not.
I will check other possibilties and tell you the result.

Thank you very much for the help.

David


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, August 23, 2001 - 06:15 am:

If your head pressure drops at higher temperature, you probably have a leak. If you have any graphite-Vespel ferrules in the system try tightening the fittings. These ferrules tend to shrink a little with thermal cycling, so a system may be leak tight initially, then leak after use.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Russ on Thursday, August 23, 2001 - 09:34 am:

Does your 6890 have the electronic pressure control? We had problems using short (15 m) 0.53 mm ID columns on ours. The actual pressure would drop below what the instrument thought it should be so it would shut the carrier off. If your carrier was off while the temperature was elevated, you may have ruined the column.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, August 23, 2001 - 10:05 am:

Great help tips from you guys.
This morning I tried to increase column 5.0 ml/min (the optimum value is 4.2 on the manual). And I maintained the temperature at 220 instead of 240. The baseline is quite straight now. The problem is that the later peaks of FAMEs are wider than before.
I guess Russ is right. We use EPC for 6890 and I noticed that the rugged baseline happens mostly during temperature elevation instead of isothermatic period. I guess this problem is related to EPC.
I will try more and tell you guys.
Thanks again.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Friday, August 24, 2001 - 01:18 am:

Recently I found a plausible exlanation for cycling baseline from column bleed. In one case that drove us mad it started at a period of 30-40 secs then speeded up as the temperature program progressed. It was suggested that part of the column was not properly shielded from the cool-down air stream, so column bleed condensed on short sections of dozens of coils, to emerge in the next run as "waves". We counted them and sure enough the number was about equal to the number of coils. After re-aligning the column to stop it sagging, the waves were reduced although they were never eliminated. Better oven/fan design should stop this but I am not naming any names.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Friday, August 24, 2001 - 07:56 am:

I missed the fact that you were using a short megabore column before. The EPC on a 6890 will not control pressure well below 5 psi. Anything lower than that will cause flow variations, including dropping down close to 0 headpressure, which if you are running a low split ratio can introduce air into the column.

If you don't need the sample capacity of a 0.53 mm column, I would suggest going to a 0.32 mm column with a 0.5 phase thickness. A 15 m column of this configuration will work better with the EPC, and the run time and resolution should be similar.


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