Try checking the flow of counterflow helium in the Nafion drier, and
perhaps turn it up to see if that helps. It should be 10x the column flow.
You can also try gentle heating of components of your system while
monitoring mass 18 - this can give you a hint where the water is.
Conversely, you can put a cold trap in various parts of the system to
isolate the source. Perhaps a bad He tank?
At 04:46 PM 07/07/2010, you wrote:
>We are getting high water background levels on our GC/C/IRMS and
>have had difficulty reducing them. The mass 18 signal on the IRMS
>was at saturation (14000 mV), and we have had limited success
>lowering it to about 7500 mV by allowing the system to flush over a
>number of days. This is this case in backflush-mode with the open
>split in (take the split out and it only drops to 3500mV). We see
>little difference when we turn off the backflush so it does not seem
>to be coming from the GC-combustion side. We have replaced the
>nafion trap in the GC/CIII interface and this only reduced the
>background slightly but not significantly (to the present 7500mV we
>see presently). We have been running liquid samples (amino acids in
>ethyl acetate). However, we have not injected anything in more than
>two weeks, and have seen high water levels throughout this time.
>The background levels for argon, nitrogen and carbon dioxide are
>normal for a leak-free system in backflush and non-backflush-mode,
>suggesting that we do not have a leak. The He gas is dry, since the
>water problem disappears when the IRMS is switched over to our other
>This is the first time we have done liquid samples of this nature on
>this system and are wondering whether this is a "normal" state of
>affairs for such samples or has anyone else had this problem
>before? Do you have any suggestions where our water is coming
>from, or how to get rid of it?