Dear Ian,

Having had my Delta PlusXL delivered just recently, I can't provide
any valuable input (see below) but I would be dearly interested in
your progress with this problem.

> We are currently experiencing problems with our Finnigan Delta PlusXL.
> Since Monday the 45/44 ratio for the CO2 reference gas has changed from
> 1.157812 to 1.169340 and consequently all measured values are incorrect.
> The 45/44 ratio of an injected C19 n-alkane standard has remained
> constant over the same period of time. We have checked the reference gas
> inlet assembly and everything is working as it should.
>
> The 45/44 value measued on our DeltaS, for the same cylinder of gas has
> remained constant at 1.1500.

I would seem you have covered all the bases. I take it you are
absolutely sure there is no mass discrimination happening along
the flow path of the reference inlet on the XLPlus? No idle hands
having tinkered with the capillaries, unions or the pressure
regulators (incl. bleed lines)?

Raw data isotope ratios are by no means constant, hence the need
for a reference but one would have thought that any shift should
affect all measured ratios more or less the same (at least that's the
premise we're working with). Since your C19 remains constant
(within reason) and only your CO2 gas changed the finger of
suspicion would point to the CO2 line (from cylinder to reference
inlet assembly). Yet, you checked it and it seems fine.

One SWAG in could offer (SWAG = scientific wild ass guess) is a
change in software settings for the reference peak. The other one
would be a break-in of moisture into your CO2 line resulting in
protonation of the CO2 (44) leading to HCO2 (45).

Why not ask Willie Brand what he makes of it? I think there's
hardly anything in IRMS he hasn't seen yet.


Best regards,
                       Wolfram





Dr. W. Meier-Augenstein, CChem MRSC
Senior Research Fellow

University of Dundee, Dept. of Anatomy & Physiology,
OMS, Small's Wynd, DUNDEE  DD1 4HN, United Kingdom

Tel.: +44-(0)1382-34/5124, /4574, /4968
Fax:  +44-(0)1382-34/5514

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URL: http://www.dundee.ac.uk/anatphys/wma/meieraug.htm


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