Stable Isotope Geochemistry


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Gerard Olack <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Wed, 11 Feb 2004 20:00:12 -0500
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Hi German--

A few things to try, but last, clogged sample capillary to mass spec,
might be the problem.  The order is rather random, sorry about that.
1.  Have you done a magnet scan from 8 to 50 M/z (don't have magnet
settings off hand) with reference down to see everything coming into the
different cups?
2. How's the pressure in the source?  If you had to cut the glass
capillary going from the conflo into the mass spec, you can be bleeding
in too much gas, hence a very large signal.  Apparently the ConfloIII's
let in more gas than the confloII's to give better sensitivity, and
higher values are ok since the detectors go to 50V.  Though this is
tuning dependent, amongst other things, my  ref CO2 values range from
597 (20 psi He, 5 psi CO2) to 20,000 (8 psi He, 35 psi CO2).
3. You've already checked the capillary tube positions, so you don't
have to worry about the ref tube hanging up right next the to mass spec
inlet tube and the He tube isn't jammed against the bottom of the split.
  If you're very careful, you can manually move the capillary tubes
slightly in the open split--use something smooth to gently lift them up,
e.g. a quarter inch wrench, and let them back down.
4. Also, is the signal coming in from the EA open split alright?  If the
tube going from the sample open split to the mass spec gets clogged,
you're no longer drawing in that He (or sample if one is coming
through), which means that He isn't there to dilute the reference gas
coming into the mass spec via it's capillary.

good luck


German Mora wrote:

>We have a problem with our Conflo-III system, which is attached to a Delta
>Plus XL. When we turn on any of the reference gases (CO2 or N2), we get a
>large signal (>14 volts).  My first thought was that the pressure was too
>high. We decreased the pressure using the gauge in the Conflo unit, but
>there was no luck. We checked the pistons and the distances of the
>capillaries inside the open split glass cell when they are on and off, and
>all of them are ok.  I also thought that we were not getting any or low flow
>of helium. We measured the flow of the helium stream entering the open split
>glass cell and it is what is expected (~2 ml/min).  We even disconnected the
>EA to see if somehow the problem was with the EA, but we still see the same
>problem.  What makes this problem so particular is that when we turn on the
>CO2 reference gas we only get a large signal on mass 44. Masses 28, 40, or
>18 only show a slight increase. When we turn on the N2 reference gas we only
>get a large signal (>14 volts) on mass 28, not on any other mass.  I ran out
>of ideas, so I would  really appreciate any help.