Bill,

that's something that causes constant trouble in our lab as well. The
whole region (GC-column -> oven, GC-column  -> backflush and the backflush
itself) generally tends to become leaky.

Here comes what I experienced: The standard VALCO X-piece for splitting
the GC effluent usually becomes leaky every 15 measurements or so.
CHROMPACK has an Y-shaped press fit splitter (CP4798). Its made from glass
and is usually absolutely leak tight even when heavy thermal stress is
present.  The problem is that you have to change the splitter when some of
the capillaries have to be removed. I don't know whether a similar X-pece
is available, but when you can do without the FID thats the best solution
I experienced so far. Additionally you can treat the ends of the
capillaries carefully with a little bit of polyimide which gives more
resistance to vibration.

VALCO has special ferrules for multiple reuse. There is an additional
liner in the ferrule itself (Its part FSR.4-5 in the european catalogue).
These parts also are more resistant towards temperature changes and are
very leak tight. At least they are not worse than the standard ferrules.
Probably you will have to use other than standard fittings at the
backflush then, but its certainly worth a try.

I also made good experience with the use of blind ferrules in which I
drilled a hole slightly smaller than standard. This also seems to give
better performance under thermal stress, especially at the backflush
valve.

So far I never heard of somebody in GC-IRMS who was *not* concerned with
these problems. I think its all far from optimum. Probably someone else
has to contribute something more elaborate ... ?

I hope this helps a bit.

Good luck ...

        Uli Flenker
        Institute of Biochemistry
        German Sports University Cologne
        Carl-Diem-Weg 6

        50933 Cologne / Germany

        Phone 0049/0221/4982-493


On Wed, 28 Jun 2000, william anderson wrote:

> Hello All,
>
> A quick question.  We keep on noticing that on our GC the Valco tee
> fittings for the He back-flush (for the column), do not stay tight.
> I realize that overtime fittings will become lose do to vibration
> (our GC sits on top of the IRMS), but I am observing this occurring
> after a standard ramping cycle of the ovens (80 to 300 C, and then
> back down).
>
> Is there a fix?  Every morning I come into the lab, and the fittings
> are a little lose after running a few samples the night before (and
> backgrounds are high).
>
> Thanks,
>
> Bill
>
>
>
>
>
>
> --
> William Anderson
> Assistant Professor
> Geology Department and Southeast Environmental Research Center
> Florida International University
>
> Mailing address:
> Geology Dept., PC 344
> FIU
> 11200 S.W. 8th Street
> Miami, FL  33199
> USA
>
> office: 305 348 2693, sec. 348 3572
> SERC stable isotope lab:  305 348 3044
> fax: 305  348 3877
> email: [log in to unmask]
>