Dear List members,

> Kim Suan Gan wrote:
>
>  To check the yield
> of CO from water, I compare both the TCD and mass spec outputs of water
> standards with beet sucrose standard. CO yield from water standards are
> usually 98 - 99.5 % of what's observed from beet sucrose. These results were
> obtained with no modification to the method described in Farquhar (1997)
> paper (at 1080 oC) other than removing the little hiccups from the previous
> efforts.
>
> The problem with our system is still memory effect which I'm looking into
> as well.  I've examined your paper with Barbara Kornexl (1999) and I wonder
> if the trick of eradicating memory effect lies with high temperature
> (1400oC), use of glassy carbon grit and tube or a combination of all. If I
> read correctly, your choice of high temperature is to permit analyses of
> inorganic samples such nitrates, sulphates and phosphates. I wonder if high
> temperature is also crucial in minimizing memory through efficient
> pyrolysis. The answer is probably obvious and I would appreciate your
> comments based on your experience in developing the technique with Barbara.
> I hope my answer to your query helps.

Kim's message (and many more from other list members) highlites the tremendous
interest in EA-pyrolysis for measuring 18O (and deuterium). There have obviously
been many publications (Farquar, Werner, Kornexl, Kelly, etc) that have shown
how useful this technique can be to give us isotopic data that was previously
difficult to obtain (especially organic H and O). However, data for water was
not previously that difficult to obtain. Equilibration methodology still very
much being the standard way of analysing oxygen-18. I guess that from the
interest shown in using in EA-pyrolysis for measuring water isotopes that many
users perceive it to be a faster and easier way of getting data.

If this is the case, there is an alternative continuous flow 'high temperatue
conversion' method for water that is emerging. This being the one reported by
Begley and Scrimgeour in Analytical Chemistry in 1997. Rather than using a
'large volume' EA system, they used a modified GC-IRMS system. In comparison to
the EA method it gives; on-line distillation and a reduction in the amount of
sample required.The sample memory (which clearly concerns most people), owing to
the low dead volume of the system and tiny amount of water (5 nL) reaching the
reaction centre, is limited to one injection. This means that with the ability
to use a standard GC auto-injector and the rapid analysis time (2 minutes per
injection) you can afford to do multiple injections of each sample and disregard
the 'memory injection'.

Best regards,

Steve


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