MSD leak or something else?

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

7 posts Page 1 of 1
Hello All,

I recently took down a 5890 GC 5972 MSD for a cleaning. I put it back together and pumped down overnight, when i came in the next day I ran the Air Water Check and the water was ok (~3%) but the 28 m/z was at 53% of the 69 m/z. I'm in a pretty arid climate so i figured it must be a leak. I closed of the MSD with a no-hole ferrule and ran the check again, same thing. I grabbed some Dust Off and monitoring mass 51 sprayed around the MSD nut and the edges of the top board with no detection. I opened it back up and swapped the filaments again and gave it a once over, nothing stood out, but it did not change anything. If it's not a leak what is it?

Thanks in advance
Do you also have oxygen (m/z 32) in a roughly 1:4 ratio with nitrogen (28)? This will give you more confidence that it's in fact an air leak.

Aside from that, you followed a good practice. For the dust off sprayer test, did you -only- monitor the mass 51? Because if i perform such a test, i look at the peak of 43. It's safer to just scan between 10-100 and see if there's any signal increasing when you spray.
Some of the Dustoff type sprays will have a mass in the 80 range depending on which chemical is used.

Also spray around where the transfer line interface seals against the side of the analyzer and where the diffusion pump attaches, you can get leaks in those seals also.
The past is there to guide us into the future, not to dwell in.
It's definitely an air leak.
oxygen (m/z 32) in a roughly 1:4
was there, it escaped me at the time that it was oxygen :shock: . The Dust Off I'm using has 1,1-difluoroethane hence the monitoring of mass 51. I checked every seal, no luck. I replaced the o-ring on the base plate and the gasket for the transfer line then replaced the ferrule. It did absolutely nothing. At this point I'm at a loss. The only thing left is the diff pump.
Also with the interface heater off, place the tip of the Dustoff spray tube beside the interface nut so that it is spraying inside the insulation that surrounds the interface. There are welds there for the inner tube and the metal bellows that can have leaks also, and can be difficult to find.

If the heights of the 28 peak drops when the calibration valve is closed, you could also have a leak in the seal on the PFTBA vial.
The past is there to guide us into the future, not to dwell in.
I figured it out. The MSD nut had a flaw in the threading causing a small leak. I used a steel brush on the transfer line and replaced the nut. Problem solved. Thanks for all the advice.
Another tip to help with the ferrules on the transfer line interface is to keep a few spares in the oven in a 40ml beaker. The heat cycling of the oven will pre-shrink the ferrules and you won't have to retighten them after a few oven cycles, which is a cause of many leaks on a newly installed column.
The past is there to guide us into the future, not to dwell in.
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