Bothersome "hump" beginning of VOA analysis (EPA8260 GCMS)

[gabeornelas]

New user here. Have an issue that I was hoping someone would help me with. I am running EPA 8260C VOAs on an Agilent 5975B MSD with an Agilent 6890N GC. Attached is an Archon autosampler with an OI Analytical 4560 Purge and Trap. About 2 weeks ago, I took the mass spec down and cleaned source and changed filaments. Normal run of the mill maintenance. After I pumped down and started running again, I noticed a very annoying and bothersome "hump" at the beginning of my analysis. It comes after the solvent peak (methyl alcohol) and right in the range where my 8260C gases come out (around 1.85 minutes to 2.3 minutes). It is playing havoc with my gases. Does anyone have any ideas as to how to get rid of it or any experiences with this? I have attempted the following things in this order: 1)changed the water management valve on the purge and trap. 2)changed septum and liner on GC injection port 3) Installed a new column. 4) Installed a new trap. None of these things have solved the problem. I have tried a couple of different diagnostic steps with uncertain results. First, I direct injected a standard to see if it was coming from outside the GC or was a GCMS issue. 2) I raised the mass threshold to higher than 35 amu as the library was identifying it as hydroxylamine (MW=31). The direct injection helped, but just a little. Changing the mass threshold also helped, but not enough. Does anybody have any ideas as to what is causing this "hump"? Thanks preemptively.


G. Ornelas

[James_Ball]

Normally for 8260 you start the scan at 35m/z to avoid detection of methanol. In my experience methanol elutes after the first gasses, usually around the time of Bromomethane. Water also elutes near this time and usually gives a rise in the baseline that appears to be most of the masses along your scan range. The only thing I know that elutes before dichlorodifluoromethane or chloromethane is carbon dioxide 44m/z.

If carbon dioxide or argon 40m/z are showing up at the beginning of the run then taper off, it is usually because the ferrule at the Mass Spec interface is a little loose, normally caused when it shrinks after the first few oven cycles. When this happens it will seal as the oven temp rises and become lose when it is cold. Tweaking it tighter will usually solve the problem, but it can be avoided by leaving a few ferrules in the oven in a beaker and allowing them to be pre-shrunk by the cycling oven.

[Bigbear]

As Mr. Ball said, make sure you do not scan below 35 daltons. I notice my water hump is worse right after maint. and gets lower with time.
I desorb for 3 min, and as a result my "water hump" goes out to 5-6 min.