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Hi everyone,

I've sorted through the archives and see a lot of troubleshooting of rising mass 
30 during EA-IRMS but I am having some issues with mass 30 spiking up at 
the end of a GC-IRMS nitrogen run and then slowly decreasing.  

The issue started with a routine reoxidizing of the oxidation reactor, after 
which there was the normal excess of 30 which was removed by solvent 
injections.  But once mass 30 was gone, I still was unable to get nice mass 28 
peaks with my standard.  They were all just blips.  We have a quadrupole 
mass spec in parallel with our IRMS so I know that the compounds were 
coming out of the GC column just fine.  So we replaced the reduction reactor.  
No real improvement, peaks barely discernible.  There was a lot of water in 
the system so we thought maybe it was building up in the capillary and 
blocking the compounds (is this possible?) so we ran in carbon mode to make 
sure there was no blockage (carbon was fine).  So over the weekend I made 
sure He flow was high to purge the system of water.

Then we had a power outage (thunderstorm + building UPS failed) and I came 
in the next day and the turbos were off.  So of course it took a few days to 
get the water out of the system.  Now, I am seeing mass 28 peaks in the 
beginning of the run, but then I get a rise in mass 30 near the end of the run, 
and don't see any of the mass 28 peaks I should see.  Running a blank of just 
my solvent didn't yield a mass 30 peak.

Does this make any sense to anyone?  Not sure what to do next...

Jen Stern
NASA GSFC