Bryan:

A major beam oscillation of 1E-10 A is not a very large oscillation, not
when sample analyses are typically performed at beam currents from 1E-9 to
1.5E-8 A.  The valves for CO2 and N2 in your reference injector box could
be at fault, as could the valve for the He carrier gas (a small change in
the He pressure causes a marked change in the beam currents of the injected
CO2 or N2).  You may also want to check your regulators on your CO2, N2,
and He compressed gas cylinders to make sure your supply is not pulsing
significantly.

Still, given this minor oscillation in beam currents,  I still don't see
why you are having problems with poor precision.  When one does linearity
tests, one changes the injected CO2 or N2 pressure by as much as 25 psi,
resulting in beam currents that vary from 5E-10 A to 2E-8 A.  If your
machine is linear, then your precision with standards should be in the
vicinity of stated specs, ±0.3 per mil, even if you have beam currents that
vary an order of magnitude.  So, assuming your machine passed a linearity
test, just how bad are the precisions?

Steve


>Dear list subscribers:
>
>        Help!  Our Fisons Optima IRMS has a problem that we have been
>unable to fix.  Has anyone seen this before?  A brief synopsis:
>
>        We noticed poor precision of organic carbon and nitrogen standards
>in continuous-flow mode (using a Carlo Erba elemental analyzer), so we ran
>a 24-hour time scan on CO2 from the EA's reference gas injector.  All beams
>and ratios OSCILLATE synchronously with a period of ~30-60 minutes; the
>amplitude of the major beam oscillations is ~1E-10 A (with a trap
>current=200 uA tuning).  The story is similar for injected N2.  This
>problem showed up suddenly, within a 6-day span of unuse.  We have since
>found the same oscillations (with smaller amplitude) in a time scan of CO2
>in the dual inlet.
>
>        We have tested everything we can think of in order to track this
>problem down, without success.  If anyone has suggestions or has seen this
>problem before, I'd love to hear about it.
>
>Thanks much!
>
>Bryan
>
>______________________________________
>Bryan Bemis
>Department of Geology
>University of California
>Davis, CA 95616
>
>Lab (916) 752-3311
>Fax (916) 752-0951
>[log in to unmask]

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