thank you all a lot for the comments and advices. I really don't like this but the
leak disappeared the next day. Trying to trace a leak, I checked and tightened
every nut in the GCC Interface and replaced one ferrule with a broken capillary
inside, but there was no immediate effect. Spraying the argon around every
connection did not affect the mass 40 intensity in the mass spec. I decided to
postpone the solution of a problem and leave the system heated overnight.
Next day it was only 0.5 mV (mass 40, cup3) short-term spike when swithing
the BF valve.
So I am happy with my system so far..
Tim Sobolevsky, PhD
Moscow Anti-Doping Center
On Wed, 18 Jun 2008 16:07:03 -0400, Tim Sobolevsky
<[log in to unmask]> wrote:
>as a newbie to the field of GCC-IRMS, I may not understand some basic
>but I have found a leak in my system that I cannot explain nor cope with it.
>The system is Trace GC / Combustion Interface III / Delta V Advantage,
>intended for analysis of steroids.
>When switching from straight mode to the backfkush, if open split remains
>I see a short but intense leak. Monitoring mass 40 at cup3, the intensity may
>reach 1.5V for a moment of several seconds (mass 28 at cup3: up to 10V).
>The intensity depends on how long the backflush valve was closed (the signal
>maybe 300 mv Ar or something if I close it for 10-20 sec), and the time lag
>between backflush opening and seeing the leak in mass spec is ca. 5-7 sec.
>I disassembled the valve and replaced the vespel seat and PTFE seal..
>Argon near every suspicious place.. closed the restriction capillary at SGE
>valve with septum.. connected the valve to the argon tank instead of
>pressurized air - all this did not change a bit.
>Additionally, I 've tried to exclude the GC part by making helium to flow as
>follows: sge valve -> reduction furnace -> nafion -> open split. All in vain
>Sorry if I gave to few technical details, I promise to improve myself..
>Tim Sobolevsky, PhD
>Moscow Anti-Doping Center