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- Posts: 478
- Joined: Fri Jun 02, 2017 11:10 pm
- Location: Western Colorado
My 5973inert with fastscan electronics is producing mass 40 and 44 artifacts in the baseline in my 8260VOC system. The peaks have a sharp front rise to ~10X the background and decays with a jaggy dropping trailing edge back toward baseline and then straight down from there. These happen in different places in the chromatogram for each instrument run. My TVH(GRO) is typically <25 ppb with an MDL of 100 ppb. I am seeing ~150 ppb of TVH just from these bursts. Scan range is 35-260 amu.
These peaks are happening in the TIC part of the spectrum and the SIM scan part does not seem to be affected. I've changed filaments twice and thoroughly cleaned the source with the second filament change. I also swapped in two different bottles of UHP He. But the problem persists.
Column is a 20m DB-VRX 180um ID x 1.2um coating. Carrier is helium at 1 mL/min. System is running in SIM/SCAN mode. GC is 6890N. Purge gas is UHP LN2. Tekmar 3000 with vocarb 3000 trap (new seals), and Archon autosampler.
I should add that my background has always been mass 40 and 44 AMU maybe 1000-3000 counts.
UPDATE PASTED FROM BELOW on 12/6/21
I took the Tekmar out of the loop and plumbed the EPC line directly to the inlet. I reduced the lower mass to 27 and then did some manual injection runs and indeed I am still getting peaks of 2.5 million counts TIC with N2, O2, Ar and CO2 present in appropriate amounts to be an air leak. There is a constant baseline of N, O, Ar, CO2 at about 4000 counts which must be from the UHP helium supply. The air peaks jump to over a million counts and last for about half a minute then return to baseline. These peaks are random in location and they occur at constant flow constant temp as well as constant flow programmed 40C to 220C like a normal run. No other masses are in the peaks
While monitoring 51 I have tested the sideboard, vent trap, inlet, EPC, EPC outlet and inlet, column inlet, MS inlet, and tune valve. No leaks! But sit and watch the 28 / 32 line and every few minutes N jumps to over a million for 10-30 seconds and then returns to ~7000 counts. Any ideas?
UPDATE posted 12/10/21
I have cooled the source to 100C shut down the turbo, then pulled the column and blocked the MS inlet. MS was then pumped down again and manual spectra were taken. Air leaks are still occurring at least once every 20 minutes. Is this something that would happen if the sideboard seal were leaking?
UPDATE posted 12/15/21
After changing out the fast scan log amp and still getting the same problem...
Decided I did not want to swap the fast scan side board. Instead vented the MS and removed the seal and cleaned out the groove with kimwipes and MeOH then also gently cleaned the vacuum chamber seal and reinstalled it. I used a kimwipe to draw the seal back and forth until all the bumps were spread out evenly (they always stretch when you take them out). Then trimmed the column at both ends and changed out the inlet liner, seal, and gold plate and reinstalled the column. Once pumped down... No more air peaks.
Somehow the side gasket was letting tiny sips of air into the chamber every 5 minutes or so. Problem is solved.