Chromatography Forum

Hi everybody,

I'm having problems trying to remove the background noise in my Varian 500MS Ion Trap. even with the LC system unplugged to the ESI chamber so the problem must be in the MS.
I cleaned the Ion source (needle, shield and capillary) as well as the ion optics and the ion trap. (water+sonication; methanol + sonication; drying with argon). However my ion time remains extremely low, around 500 us.

I tried also disconnecting the nebulizer and drying gases, but noise remains, so the gas source cannot be the cause. Background peaks concentrate in a 100-500 m/z window and are rather erratic, no particular peaks seem to be dominant.
I'm getting really frustrated, so if anyone can throw some light on this, i'd really appreciate it.

Thanks in advance to the Chromatography Community!
Cheers!

Something "loose" in/at the detector?

Apparently not. I 've spent weeks since my last post trying to find out where the problem is. At this point i cleaned the ion source really intensively and checked the nitrogen generator by using a N2 bottle from another lab. Signal improved. After a couple of days running with non-in-house N2 noise was reduced.

Then we started to think what the problem could be and hypothesize the following (maybe i'm just paranoid ) Due to space contraints we have both foreline pumps and N2 generator in the same room. When checking the pumps i realized that one of them had a small oil leakage. So maybe the N2 generator is pumping some volatile contaminats from pump oil into the system. The Varian guy looked at me a bit weired but he had no better explanation. The fact is, i replaced the pump, washed a couple of days and everything looked different.

So, although situation got better i still need to undestand why:
The pumping 100% A (water) my spectrum is a dream, completely clean, no noise at all. When gradually increasing B%, noise starts to increase dramatically. [Ok, Organic solvent (Fluka) sucks, will change to another vendor, any suggestions?]. But the weirdest thing happens whenever stopping the flow (no matter what solvent was being pumped). I start getting more noise with no flow. Particular masses such as 136, 217, 227, 371 (i'm at home right now & dont remeber them all) I have no clue where this may be coming from and why I do not detect them when water is on.

This is getting personal
Thanks upfront, guys!
A MS-rookie

Due to my experience with the 217 m/z ion, which is a siloxane (is this correct for you?), the problem may lie in contamination from the septum/liner, injection port, or some variation of the machine itself. I believe these will always pop up when running the machine, but are less apparent when running a sample.