Stable Isotope Geochemistry


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Pratigya Polissar <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Sat, 14 Feb 2004 10:18:42 -0500
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Dear Bernie,
        We have a Delta Plus XP with TC-EA/Conflo III and GC-GCC
inlet systems which we use for D/H analyses of waters and organic
material.  Although you have probably done this, check the m/z 2
backgrounds with He dilution on and off in the Conflo.  Any changes
give you some idea of the H2 background coming from the TCEA.
        Second, I have found that the m/z 2 background varies with
both the tuning and inlet pressure in the source (i.e. He pressure).
The pressure relationship is due (I think) to tailing or scattering
of the m/z 4 ion beam into the m/z 2 cup.  I tested this by varying
the source He pressure from dual-inlet (i.e. no He) to full
continuous flow and doing magnet scans across the m/z 2 and 3 cups.
I found that there is a large sloping background from the m/z 4
region across the m/z 3 and 2 region which develops as the He
pressure increases.  This background is quite stable and we have
successfully run with m/z 2 backgrounds up to 700mV (GCC).  (Our m/z
3 backgrounds are usually right around zero.)  The tuning
relationship comes (I think) from the fact that you are simply trying
to maximize the voltage on the m/z 2 cup.  The m/z 2 current is
derived from both He and H2, therefore tune changes increase either
the He or H2 current are seen as voltage increases.  On the XP, the
autotune program is particularly prone to increasing the m/z 2
background for this reason.  Our solution has been to tune with
relatively large amounts of H2 in the source (m/z 2 total voltages
above 5 V).  This should theoretically maximize the amount of tuning
which occurs for H2 and minimize the He derived m/z 2 increases.

        As far as the H3 factor, 15 does seem high in my experience
(we usually run around 5-7), although as long as it is stable...  We
have crept up to ~12 and this was mostly due to the XP autotune
program again.  The autotune is particularly prone to increase the
extraction voltages which seem to lead to higher H3 factors (no
quantitative data here though).  With CO2, the autotune program
generally gives us horrible linearity (over 0.2/V for both d45/44
and d46/44).  To get acceptable linearity I generally have to
decrease the extraction voltage manually and then retune (autotune)
w/o changing the extraction voltage.

Hope this is helpful,
Pratigya Polissar

   When tuning generally you are trying to maximize
>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

Pratigya Polissar

312 Deike Building
Department of Geosciences
Pennsylvania State University
University Park, PA 16802
(814)865-6720 office
(814)863-8673 lab