5973: random Peaks of mass 40 & 44

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

11 posts Page 1 of 1
SEE UPDATES AT BOTTOM OF THIS POST....

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.
40 is argon and 44 is carbon dioxide
I have had it before on purge and trap systems. It is out gassing or leaks of air into the carrier. It can come from a saturated carrier gas trap or adsorbed into the vespel in the valve. Often it will be timed to the valve switches such as switching from desorb to bake, or bake to standby.

If allowed you can get rid of it in the full scan TIC by starting the scan range at 46m/z (45 will still have some spikes when the 44 shifts just a little and gets picked up as 45).

It may require cleaning or replacement of the rotor in the 6 port valve (or 8 port depending on P&T model).
The past is there to guide us into the future, not to dwell in.
I had the Tekmar 3000 multiport valve resurfaced in 2018. It could be leaking again. But at any rate, the spikes happen all over the The 44 is just a few hundred counts in the baseline. Its only the 40 that peaks. I have not checked to see if there is a corresponding 28 peak. But since I am using SIM to quantitate all the 8260 compounds that might have mass 40 or lower; I can bump the TIC scan up to 42 and get rid of the 40 artifact which will be sufficient for now. Thanks for the suggestion!
James_Ball, I am super pleased to tell you that you are right! I widened the range down to mass 28. I am seeing artifacts of mass 32, 29, and 40 (that order of abundance) now instead of just 40. So I can drop these out of the spectrum by raising my lower mass from 35 to 42 (44 seems negligible).

In addition to a leak from the Tekmar 6-port valve, could this also be caused by leakage in the EPC? Has anyone experienced this problem in that way?
Geoff of Great Lakes Technical Services gave me a great suggestion. Tomorrow I will short circuit the GC and MS carrier lines together to exclude the 6-port valve in the Tekmar 3000. I'll do some runs that way. If that gets rid of the air spikes, I will know its the Tekmar and not my GC inlet system.
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 12/15/21
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.
LALman wrote:
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.


I keep a small tube of Apezion L grease just for this. Clean the groove and the side plate, take the o-ring and wipe it down with kemwipe then put a little grease on your fingers and pull the gasket through them to lightly grease it. Pull it through a kemwipe a few times so only a little of the film remains then reinstall it in the groove.

I have even found a tiny piece of lint on it before that caused a leak.

I would have replied earlier, but have been off a week because my house is gone after the Kentucky tornado. Been a wild week around here.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
LALman wrote:
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.


I keep a small tube of Apezion L grease just for this. Clean the groove and the side plate, take the o-ring and wipe it down with kemwipe then put a little grease on your fingers and pull the gasket through them to lightly grease it. Pull it through a kemwipe a few times so only a little of the film remains then reinstall it in the groove.

I have even found a tiny piece of lint on it before that caused a leak.

I would have replied earlier, but have been off a week because my house is gone after the Kentucky tornado. Been a wild week around here.

I'm very sorry you lost your house! I hope your insurance covers the loss.
Thanks,

Looks like it will and maybe a little more. Honestly the help offered by everyone has been wonderful :)
The past is there to guide us into the future, not to dwell in.
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