Repeatability issue GC-TOFMS

[chromy2709]

Hi all,

I am having 7890 GC-TOFMS. I am currently having peak repeatability issue where my peak are not having same intensity. Some are higher some are lower. My retention time shifted slightly but not much.I believe this has got something to do with GC related issue. Anyone knows what could likely cause this problem ?

[Don_Hilton]

There are a number of reasons that you can be getting this kind of variability. Wandering retention times indicate a change in gas flow, temperature, or stationary phase. Given that it is wandering back and forth, we can rule out stationary phase. And temperatures are typically pretty stable on modern instruments - so that is not likely.

On gas flow. Be sure the pressure being delivered to the back of the instrument is at least 20 lb higher than the greatest demand in the inlet. And be sure there are no leaks. It is possible to have a bad flow controller.

The other clue you have here is varying peak size. Three possibilities - you have variable amounts of material getting into the instrument, variable detection of the compound or some of the compund is getting back out of the instrument after injection.

Let's assume that we are looking at a single problem. A leak in the system that would allow material to escape could give both symptoms described.

So, when is the last time you changed the septum? A corred septum is a common leak in the GC. If you are not sure, change the septum and see how things look. And check for leaks around the inlet anyhow. The ferrule holding the column will loosen after a number of heat/cool cycles. be sure it is snug. A leak sniffer is a good investment.

If this does not solve the problem, a bit more information can help narrow things down.