Tim,

An unlikely cause but a possibility. Are there any temperature
changes occurring in your lab as the day progresses? Is the air
conditioning system still performing OK? Do the rough runs
correspond to warmer days? We saw some rough stability in our
runs, after checking everything else it was a temperature
fluctuation, the air conditioning was not operating well and we could
put a thermometer near the flight tube and pick temperature
changes (+- 1 oC) corresponding to poor performance

Regards,
Tim Clough


Date sent:              Mon, 22 Oct 2001 18:31:29 -0600
From:                   tim prokopiuk <[log in to unmask]>
Subject:                Delta E Problems
To:                     [log in to unmask]
Send reply to:          Stable Isotope Geochemistry <[log in to unmask]>
Organization:           University of Saskatchewan

> Hello to all...
>
> We've got a FIN MAT dual inlet Delta E in our lab that has been
> running like a top for the past couple of years.  Recently the
> filament blew and was replaced with one from factory, and all our
> troubles began. Sensitivity was up approximately 50%, and I was
> informed by Isomass that this is to be expected: new filaments run
> hotter hence ionize more efficiently than filaments on their last leg.
>
> The problem that occurred was this: running samples first thing in the
> morning generated good traces (traces of raw delta values) and good
> data.  But as the day progressed traces became rougher and
> rougher...to the point that standard deviations were unacceptably
> large.  I noticed that the background values reported for each of the
> three faraday cups in use were larger than usual.  Our electronics
> technician and myself reasoned that it was time to clean the
> source...which was confirmed by Isomass.  After cleaning the source,
> backgrounds were reduced (though not quite to the level they were at
> before) and the Delta once again ran quite well (though not as well as
> before).  After a couple of weeks, the problem has
> resurfaced...rougher and rougher runs are generated as the day
> progresses.
>
> We are running strictly CO2.  I've tried inletting a fresh aliquot of
> standard gas into the standard belows, but this did not improve
> performance, so I've ruled out a leak in my inlet system somewhere.
> I've also confirmed that my sample gases are clean so are not
> responsible for the rough runs.
>
> Any ideas?
>
> thanks
>
> tim prokopiuk
> technician
> geology isotope lab
> university of saskatchewan
> saskatoon, saskatchewan, canada
> [log in to unmask]



xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Dr Tim Clough,
Research Scientist,
Soil, Plant and Ecological Sciences Division,
Soil and Physical Sciences Group,
P O Box 84,
Lincoln University,
Canterbury,
New Zealand.

Phone: (64) (3) 325 2811
Fax:  (64) (3) 325 3607