Air leak problem

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

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I have an odd problem tracking down an air leak in my 5890/5972. During a run, about every 15 minutes I get a brief spike that looks like an air leak. I've tried isolating the MS first by capping the transfer line. No more spikes. After reconnecting with a new ferrule, I checked for leaks with dust-off by scanning ion 51, and everything inside the oven seemed fine, so I did another run, and still had peaks. So, I disassembled the inlet completely and replaced everything from septum, liner, seal & etc. On my next run, the peaks are back.

I did notice something strange. This problem started when I switched to a new SGE column, and looking at it carefully, I see that the polyimide coating on the outside of the column seems to be in poor shave with pitting. Could a column flaw cause this problem?
jkloepping wrote:
I have an odd problem tracking down an air leak in my 5890/5972. During a run, about every 15 minutes I get a brief spike that looks like an air leak.


What are you detecting? Are you referring to a peak in column bleed in full MS scan?

jkloepping wrote:
Could a column flaw cause this problem?


Yes. Do you have an electronic leak detector?
I'm detecting mostly ion 18 but also 28 and 22. Background peaks have counts around 1000 or so (a couple of hours after pumping down). Then I will get peaks with counts of 50k or higher with 18 as the dominant ion.

Yes I have an electronic leak detector, and I know it works. I haven't found any leaks with it.
So you get peak at m/z 18 when you do an injection measuring in full scan from m/z 10.

If your air/water check is OK before/after a run, i wouldn't see this as a big deal. To be honest, I never record from m/z 10 in full scan. Do you have indications that it affects the performance of your instrument?

Maybe some water is stuck somewhere around your inlet or a column connection. If your system is oxygen free, you could do a sort of bakeout overnight: inlet and oven at 250°C for example, with a good flow of carrier gas.
It doesn't affect the analysis, but I like to control all the variables and not knowing the source of this problem is very irritating. I'm going to switch columns back and see if it is a defect in the new column.
I sometimes have a similar problem with a 5975 and a 5973 both running purge and trap. Scanning from 35-350m/z there will be random spikes on the chromatogram that show 40 and 44 masses like there is a momentary air leak. These appear at random retention times maybe one to three times during a 20minute run. I have never found the source. No signs of leaks. The random retention time is what leads me to think that something before the column is outgassing air at random times.

I first thought it was related to the rotation of the valve in the purge and trap, but the moving retention time doesn't correspond to any of the valve switches. After a few days it will disappear, maybe for a few weeks then return again. Sounds like we have similar problems, I hope one of us can figure it out.
The past is there to guide us into the future, not to dwell in.
Running 524.2 I see 44 for the first 4 min ( desorb time), I believe it is from disolved CO2 in the sample. From this system's autotunes there is <2% air and water.
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