too high column bleed

[gandreou]

Hello everybody!

The problem that I am facing with my gc/ms analysis is that the masses from the column (eg 207) are too much higher that the masses from the alkane standard that I can not recognize each peak, and last ones are hidden behind the column bleed.

I conditioned the column (db5ms) twice for 3hours at 325 degrees but the problem still appears. is there anything else that I can do?

Georgia

[Peter Apps]

Hi Georgia

This is very likely to be caused by oxygen and/or water in your carrier gas. First you need to install an oxygen and moisture scrubber on your helium suppply to the GC, then leak check the whole system very carefully and fix all the leaks.

Until you have oxygen and water-free carrier gas "conditioning" the column will only make the bleed worse.

Regards Peter

[gandreou]

hello Peter

There are big oxygen, water and hydrocarbon scrubbers installed to the helium supply for some time now. Yet I can see a small water peak just before the solvent peak. I can not change the scrubbers at the moment. Is there anything else to do?


thanks for your help

Georgia

[Earl]

Does your software allow you to subtract your background? Have you tried doing a selective ion monitoring run to eliminate background so you only see the ions you want?

If you have a second column to try, that will tell you if the problem is a degraded column. Also, double check and make sure you are not going above the maximum temperature for that phase (350C). Also, if nothing else helps and you are willing to throw money at the problem, you could get an XTI-5 column from Restek which is stable up to 360C.

Good luck

[Peter Apps]

Hi Georgia

Scrubbers that have been installed "for some time" might be exhausted, depending on how long "some time" is and how much gas has gone through them.

Air and water can get into your carrier gas where ever there is a leak downstream of the scrubbers.

If you can see a water peak you are almost certainly injecting water with your samples, which will cause the dramatic column bleed that you observe and destroy the column quite quickly. You need to dry your samples (I presume that they are in a voaltile organic solvent and not in water !) using sodium sulphate or something similar. Exact details will depend what the samples are.

Good luck

Peter