Stable Isotope Geochemistry


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Joe Lambert <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Fri, 9 Jan 2009 10:50:40 -0500
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Hi all,

    I am writing to see if anyone else has experienced similar behavior with
their machine.  In a nut shell, our CO2 beam from REF1 (27 PSI) on our
GasBench has dropped from ~5800 mV (mass 44) to below 2000 mV over the span
of about 2 months.  We have a Delta Plus that is about 7 years old and our
Gasbench is less than 2 years old.  We're typically running very clean
sample gases (CO2 and H2, never S2) and probably have less sample throughput
than most labs. The bulk of the work lately has been carbonate powders where
our sample heater block is at 50 oC and we are doing manual acid injection.

    Problems first appeared the end of November, 2008 when both reference
and sample peaks during a 25-hour GasBench run dropped dramatically (but
gradually) from sample 1 to 96. We stopped measuring samples and started to
diagnose the problem. What we discovered was that if we did a peak center
followed by a scan of the 3 beam intensities through time, was that the beam
intensities dropped (see linked PDF below).  The rate of "beam decay" varied
but could be ~200 mV per hour.  Our 5800 mV beam was down to 5400 mV after
just a couple hours; however, the next morning the beam would be back up to
5800 mV (pump time helped?). We introduced CO2 from a separate source
(Conflo III) and from the dual inlet system and saw similar results
(PDF).  The Conflo and GasBench share a common CO2 ref tank and He
carrier tank. Never did we see the CO2 beam increase during a scan through
time. We've tried using a different He tank but the results are the same and
the CO2 tank is ~1-year old and does not appear to be at the end of it's life.

    We have put in a new filament/cathode and tried multiple tunings for the
source settings, none of which has had an effect.  Our water background
looks reasonable (PDF). A mass spectrum (background) scan with
the SGE/needle valve closed resembles a scan we did before the problems
started.  Several months ago (>6) we did have some minor problems with the
sample needle possibly sucking in some acid but I'm not sure if this could
be a problem that is just now showing up in the source.  It should also be
noted that it has been about 2 months since any real samples have been
measured so I would think the system should have purged itself by now...but
maybe acid is stickier than we realize.

     We also experienced times where the beam would decay through time (one
day to the next), but in most cases we could adjust source setting to
reacquire the full beam.  However, over a long time period our maximum
possible beam has gone from ~5800 mV at the end of November, to ~3000 mV
before Christmas break, to <2000 mV today.  I do not know if we have two
problems superimposed on each other or if one problem is causing the other.

    Possible causes of the problem in my mind could be 1) contaminated CO2
or He gases/capillaries, 2) contaminated source, or 3) electronics issues. 
Since the behavior is similar between GasBench, Conflo, and dual inlet gas
sources I would somewhat lean toward a contaminated source; however, the
recovery time with pumping suggests something in the gases.  As mentioned,
our machine is 7 years old but we have never cleaned the source because it
looks almost as clean as day 1 and we've never experienced any "dirty
source" type problems (until now?).  Would a dirty source result in such a
quick decay in the beam?  I had noticed a few comments on ISOGEOCHEM
regarding similar problems caused by electrical issues, but I am not sure if
they symptoms were the same as ours.  Our machine has experienced many harsh
uncontrolled shutdowns because of no back up power supply so wear and tear
on the electronics is not out of the question.

   Sorry for the long message but I wanted to cover as many things as
possible to hopefully rule some things out.  I'm sure I forgot to clarify
some things though.  

Thanks in advance,

Joe Lambert
Alabama Stable Isotope Laboratory