Dear William (Anderson) and others interested,
> 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).
As others have pointed out already, this is not uncommon and
there is a fix. Here's my story.
The problem of leaking Valco connectors, especially the Valco X-
piece, was one of the first things I remonstrated with Finnigan after
having taken delivery of my Delta S with GC-combustion in 1994.
The problem is less one of which ferrules to use, the culprit is the
Valco X-piece with its high mass (15 g !!!) and, hence, high thermal
inertia. This is clearly demonstrated by the fact that even under
leak-tight conditions polar compounds tend to disappear from the
chromatogram, for the Valco X-piece acts as a large cold spot thus
leading to condensation of polar compounds such as free alcohols,
free MC fatty acids and free amines. This can easily be verified by
running a Grob test (Grob test mixture II) on the system (I regards
this as a must on any GC or GC coupled system). In the presence
of cold spots (active sites in the injector and/or the GC column
have a similar effect) the following compounds either tail, show
reduced peak area or disappear altogether: 2,3 butanediol; 1-
octanol; 2,6-dimethylphenol; 2-ethylhexanoic acid; 2,6-
dimethylaniline; dicyclohexyalamine.
My solution was to replace the Valco X-piece with a custom-made
Gerstel X-piece (for detail see my paper in J. High Res.
Chromatogr. - HRC, vol 18, 28-32, 1995). In essence, this is a
similar solution to using a glass Y-piece.
On the downside, Gerstel products in general and this X-piece in
particular are darned expensive.
On the up-side, by using a cross-piece I maintained the
functionality of the original Finnigan design, i.e. FID split and He-
back flush. Furthermore, the Gerstel X-piece is virtually dead-
volume free as the only dead-volume is the point where the 4
capillary ends meet. Due to their unique design, the Gerstel Graph-
Pack ferrules afford a leak-tight seal and they can be re-used.
To me, this seemed to off-set the initial price disadvantage since
glass splitters are essentially one-way items once a proper seal
has been formed between fused silica and glass.
Once tightened (and re-tightened) after an initial GC oven cycle (40
to 360 C) the seals remain tight over several days and GC columns
can be changed quite easily.
Due to the ease of change and the longevity of GraphPack ferrules,
I even abolished straight glass connectors and replaced them with
Gerstel connectors, e.g. for connecting my retention gap ("pre-
column") to my GC column.
Gerstel have a website in the US: http://www.gerstelus.com and
the above mentioned paper can be found as PDF document on this
site (requires a password from Gerstel Sales). Their European
website is at http://www.gerstel.de/indexe.htm .
Hope this helps.
Cheers,
Wolfram
*****************************************************
Dr. W. Meier-Augenstein, CChem MRSC
Senior Research Fellow
University of Dundee, Dept. of Anatomy & Physiology,
Old Medical School, DUNDEE DD1 4HN, United Kingdom
Tel. (B): +44-(0)1382-34/5124 or /4574
Fax (B): +44-(0)1382-34/5514
e-mail (B): [log in to unmask]
URL: http://www.dundee.ac.uk/anatphys/wma/meieraug.htm
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