We've used this method in the past, and have had similar difficulties. I
believe that the silica becomes brittle simply because of the high
temperature. It's important to secure the ends of the tubing outside the
oven such that there is no strain or stress put on the capillary.

We also found that tubes broke a lot if we allowed the oven to cool. We
suspect that this is due to the copper expanding and contracting thus
putting pressure on the capillary wall, so we always maintained constant
temperature.



>Dear Isotopers,
>
>is there somebody who has some experience with fused silica ovens? Im
>especially interested in the type of capillary that gives good performance
>and shows sufficient robustness.
>
>Here is what I experienced:
>I used deactivated capillaries from BGB (0.32mm ID) filled with two Cu
>wires (0.1mm). I switched to 500°C to charge with oxygen and did the
>combustion at several temperatures between 700°C and 800°C. That gave
>excellent peak shape and signal intensity -- for one measurement.
>Afterwards the capillary was broken *OUTWARD* the ceramic tube, whereas
>the part from inside the tube could be removed and looked good. The
>problem seems to be that the capillary doesn't like copper: Having
>inserted the wires, the region that came into contact with Cu tends to
>break extremely easy. The material looses most of its flexibility (room
>temperature). I think the ventilation during the cooling phase of the GC
>oven causes some vibration and that might be what ultimately makes the
>capillary break.
>
>Any help is greatly appreciated.
>
>         Best ...
>
>--
>         Uli Flenker

Paul Eby, B.Sc.

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