HI Herb-

If you tune on CO2, then do a magnet scan to see the other masses, mass 18 can
run from <500 mV to more than a few volts when running the GCC in combustion
mode (yeah, a  bit redundant).  When looking at DH, the GC/TC setup, then it
should always stay low.

A few things:
Switching valves sometimes give a little air pulse, sometimes there's a small
leak in the connection between GCC O2 or backflush He regulator and the
switching valve.  You'll can see a bit of moisture if the humidity is high.

Open splits are pretty good, but not perfect.  The lines going in the 
outer tube
can sometimes create a path for a small air leak.  He line in open 
split has to
near the bottom, not on it. On the sample side, make sure the line to the mass
spec always goes into the coupler holding the sample input line.  It can get
stuck on the lip or coil up, creating problems.

SGE valves on mass spec???  Don't turn on that valve heater (or only 
use it for
very short periods of time).  The teflon seal can "melt".  Also, if you open
and close it a lot, it will start to leak--and since the leak is under 
the cap,
using Ar to check it for leaks doesn't work well.

The nafion tubing can only handle so much sample load and then you have 
to give
it a rest an let it dry out.  How far can you push it depends on your
particular setup.  Do a bunch of runs, with intervening standard runs, to see
how far you can push it before the 13C values start to skew a bit.  You can
also periodically stop it a check the water levels. And even do linearity
checks with the standard gas at the different water levels.  The nafion tubing
can also get coated with gunk on the inside, so it won't work as well as it
should.  You may have to clean it every year or two--or when it gets bad.

You can start out too dry when running the GCC.  Breaking out the heat gun and
overcooking the valves to get it below 200 mV to start is not a good idea.

You can get a bad tank of He.  Even with a drying tube on the He line, a bad
tank can wipe that out in no time.

good luck...

take care

gerry



Quoting Herbert Tobias <[log in to unmask]>:

> Hello All,
>
> I would like to ask if there is someone out there that runs a MAT 253 with
> a continuous flow device (such as GC-C)  that can give me a number on the
> "routine" water background signal (in the middle of the three CO2 cups).
> Please, I would like people with actual measured numbers to respond.   We
> all know what these things should be in a perfect world.
>
> Thanks much,
>
> -Herb
>