On Wed, 10 Nov 2004 11:40:56 -0500, Suzanna Richter
<[log in to unmask]> wrote:

>Hi.
>We have a Thermo Finnigan Delta plus and are running cellulose oxygen
samples.
>Over the past couple of months our CO reference gas voltage has decreased
>gradually from 2000 to 300 mV.
>Does anyone know what could have caused this AND how to remedy it?
>
>Thanks in advance,
>Suzanna Richter
>Graduate Student
>Department of Earth & Environmental Science
>University of Pennsylvania

The two primary things that come to mind are source tuning and source
cleanliness. When I would operate our IRMS or any of our mass
spectrometers, I would always do either a manual tune or autotune depending
on the instrument's software. In most cases any change from day to day
would be neglible as long as I was running the same method/sample types and
didn't put a lot of garbage into the instrument. At least I was assumed of
the condition of the instrument before proceding with samples. Also, look
at peak shape and peak centering if the version of software you are running
allows it.
As to source cleanliness - mass spectrometers are dynamic instruments. The
lenses and other source componets get dirty over time and do not focus ions
with the same precision or intensity. The source in a mass spectrometer may
not need to be cleaned for many months or it may need to be cleaned every
couple of weeks (shudder) depending on the type and frequency of use. If
after tuning the source and assuring that the ion beam is properly focused,
you may consider this.

Richard A. Berger
Washington University Medical School
Mass Spectrometry Resource
St. Louis, MO 63110
Voice: 314-362-2602
FAX: 314-362-8188