Chromatography Forum

Hello everyone!

I have a very strange problem with the GC-MS I'm using. If I measure several times the same sample (benzyl alcohol in water extracted to ethyl acetate) the intensity of the peak decreases with every consecutive run, while the baseline is also decreases. I tried heating the column overnight, but it didn't help. However, after some time of stand-by (overnight or couple of days), the peak of the analyte returns to be almost the same, but decreases again if I measure it repeatedly. The filaments, liner and septum are all new. I'm confused. Could it be the ion source or some kind of contamination?

Thanks a lot if you could help me with this.

Since water will dissolve somewhat into the ethyl acetate, it could be the water is getting into the vacuum pump oil and the source and staying there a day or two. If you continue to inject the samples, does the sensitivity drop finally reach a low stable state or continue dropping? If it finally stabilizes, then you have reached saturation of the water in the system and it will run ok from that point on until you stop samples for a few days. Similar things happen with a new system when using purge and trap until you reach equilibrium with the water in the system.

If you use some anhydrous sodium sulfate to dry the extracts before injection it could tell you if it is water causing the problem or something else. If you remove the water from the ethyl acetate and the problem goes away then you know it was the water, if not then you will have to look at something else causing the sensitivity drop.

Since water will dissolve somewhat into the ethyl acetate, it could be the water is getting into the vacuum pump oil and the source and staying there a day or two. If you continue to inject the samples, does the sensitivity drop finally reach a low stable state or continue dropping? If it finally stabilizes, then you have reached saturation of the water in the system and it will run ok from that point on until you stop samples for a few days. Similar things happen with a new system when using purge and trap until you reach equilibrium with the water in the system.

If you use some anhydrous sodium sulfate to dry the extracts before injection it could tell you if it is water causing the problem or something else. If you remove the water from the ethyl acetate and the problem goes away then you know it was the water, if not then you will have to look at something else causing the sensitivity drop.

Thank you James!
I'll try what you suggested. As long as I tried I couldn't reach any stability, instead the peak of the analyte almost disappeared.
The strange thing that I had been working with the same setup for several months and it was fine, the problem appeared suddenly.

hi Igor,

Have you found out the solution of your problem?