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Hi Jeff,

Ar leak check definitely the way to go.  If that does not help, check 
the flow out the bleed out lines.  Those can vary by setup, but are 
typically by the He feed for the backflush and the O2 input.  They 
should have a very low flow, 0.5 mL/min or less with 2 mL/min He flow 
from GC.  No flow could mean a leak upstream or at valve.  High flow 
would mean clog downstream--or leak across valve.

Not sure if this is what is happening in your case, but a couple of hard 
to find problems are:

1. Shaft seal leaks on valve.  You can't always find that using Ar since 
the leak is buried.

2. Capillary pulled out a little bit so it's not all the way thru the 
ferrule.  Instead of tightening around the capillary tube, you end up 
sealing on edge--eventually pushing the capillary out. Re-tightening 
seems to help, but then the leak opens up again. You can end up pinching 
off the flow if you tighten it enough.

When I was playing on GCC, I ended up putting in a union on the line to 
the open split just so I could check flow to the open split.  Depending 
on your exact set up, you may have something similar in place already.  
Just remember to set it up so the mass spec won't be sucking in air when 
play with the plumbing--and open it back up for the Ar leak testing...of 
course if you are picking up a leak via Ar testing with the mass spec 
closed off, you might have other problems ;)

good luck,

gerry



On 10/20/2016 1:02 PM, Dyckmans, Jens wrote:
> Hi Jeff,
>
> I suspect you have a leak between BF valve and open split. This way you basically have no He flow into the open split but air diffusing in. Opening the BF does hardly flush the capillary leading to the open split because the leak is too large. Can you put m/z 40 on the first cup to get a readable value and Ar spray the system downstream of the BF?
>
> Best,
> Jens
>
> Von: Stable Isotope Geochemistry [mailto:[log in to unmask]] Im Auftrag von Jeff Salacup
> Gesendet: Donnerstag, 20. Oktober 2016 19:31
> An: [log in to unmask]
> Betreff: [ISOGEOCHEM] GC-TC-irMS leak
>
> Hi all,
>
> Me again. We've got a large leak somewhere. With the GC split in and the backflush closed we get a slowly increasing Ar signal starting around 400 sec and terminating a minute or so later at a full 50 V. Opening the BF and pulling the GC split out results in a slowly decaying Ar signal that can take an hour to really leave the system.
>
> I initially troubleshot it down to the BF. A flow check at the BF vent suggested it wasn't closing and removing it from the flow path got rid of the problem. So I ordered a new one and installed it. It opens and closes but the problem remains.
>
> I've removed the column from the flow path and the problem remains.
>
> The reactor is new... twice now, just to double check.
>
> The inlet liner ferrule is new.
>
> What am I missing?
>
> Thanks,
>
>