Stable Isotope Geochemistry


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Bernie Genna <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Thu, 12 Feb 2004 13:40:05 -0500
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Hello All,

        We have a Delta Plus XL  connected up to a TCEA that we use with an A200S
liquid autosampler to analyze O18/HD of water samples. Recently, we lost
our filament (after 3 years) and replaced it with a new one from
Thermo-Finnigan. Now it seems, we are having some difficulty obtaining  an
acceptable H3 factor for our hydrogen measurements. Acceptable being less
than 15. I've noticed that our hydrogen backgrounds are 300 and 30 ,
whereas previously they were 200 and 18. Thermo-Finnigan suggests that
acceptable backgrounds are <200 and <20. I can adjust the 3 backgrounds
down by decreasing the electron energy, but short of completely detuning
the source, I cannot find a way to drop the 2 background below 20.
        Questions for the group: Is an H3-factor greater than 15 acceptable if it
does not change by more than .2?  How can I drop the 2 backgrounds to below
20 without detuning the instrument? I fear I may have a contaminated system
somehow, but I see no apparent leak, the CO backgrounds (for running O2 in
waters) are extremely low (all less than 50 mv), and the filament has been
working just fine for about 2 months now (analyzing N2/CO2 samples with our
Carlo Erba  GC).

Any thoughts?

Bernie Genna