Laure-Amelie wrote:

> I work on a GC-C-IRMS delta + (MAT 252). For one 2 weeks, I have had a
> problem. The amplitude of the CO2 reference peaks decreased during the week
> (from 2.5V to 1V in mass 45). So I should adjust the potentials many times
> in the week. Now it happens 3 times in the day.
>
> The CO2 bottle and the pression are OK. Does any one know the reason of this
> kind of probleme. Should I only wash the ion source ?
>

When was the last time the filament was replaced on your
instrument?

I assume there is no problem with CO2 inlet as such (CO2/He
mixing module and capillary inlet into the source). Have you
checked the N2 signal (air background)? Does this decrease in
size too over time?

If the answers are and yes to the former and to the latter,
respectively, chances are your filament is on the blink leading to
reduced ionization and, hence decreasing the signal intensity.

Changing the resistance of your 45 amplifier as a first intervention
measure is not such a good idea (as a last resort perhaps). I trust
you have tried to re-focus the beams using the pen recorder
provided by Finnigan, which is plugged directly into the head
amplifiers by-passing the A/D boards?

Should you decide to change the filament have a look at the source
for discolouration on the focal plates etc. just in case.

Hope this helps.


Best regards,

                          Wolfram



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

University of Dundee, Dept. of Anatomy & Physiology,
Old Medical School, DUNDEE  DD1 4HN, United Kingdom

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