by
tlahren » Fri Oct 14, 2011 2:15 pm
Background really depends on a lot of factors. Do you have specific ions you want to know about? The TIC background can vary quite dramatically depending on type, brand and batch of column you have installed. Also worse if column was not properly conditioned prior to initial use.
Inject a standard mixture of the compounds you are interested in close to the low end of what you are hoping to see (detection limit). If you can see the compounds in this chromatogram I wouldn't worry about where the background is too much.
Also, most background ions are from siloxane bleed from the column and have pretty consistent masses. You will get better sensitivity if you process your samples using EIC (extracted ion chromatogram) or acquire the data in SIM (selected ion monitoring) mode. If this is old news to you just ignore it. If you would like to know more about these let me know. I can send you some links and other info relevant to Chemstation data processing.
Generally I like to see H2O < 1% and N2 less than 2% on a standard auto tune. This usually means there is minimal residual water left in the system and that you have no significant leaks. Most likely if there is a large N2 peak then you have a leak at the transfer line nut (where column leaves the GC and enters the MS). Tighten it maybe a quarter turn and watch the vacuum pressure on the system. If you had a leak this will get better (higher vacuum/lower pressure). Be careful not to over tighten the nut because you could crush the column.
Hope this helps a little. Let me know if you need more info.
Ty
