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Greetings again...
To answer some of the questions that my query generated:
We are indeed getting poor precision on different nitrogen standards
(some in house, some IAEA), so it is not an issue of a bad standard
(i.e. some sort of contamination).
Water background is at about 190 mV.
Performing N2 standard on on offs gives me a mass 30 background value
of a consistent 100 mV...not rising during the run. During the
reference pulse itself it does rise, but then falls when the N2
capillary is removed. But it has always done this...as far as I know
this is normal as we've observed this behaviour from the time of
installation of the EA. My understanding is that part of the peak is
15N15N and part is also 14N16O from the cracking of N2 in the source
combining with the small amount of oxygen that is present in the mass
spec. This latter species causes the mass 30 peak to rise and fall
sluggishly.
Could it be a moisture issue? All standards are stored in a
dessicator though, and the Mg(ClO4)2 is stored with two sealing caps,
so should be leak tight. I've tried a fresh water trap with no
improvement.
Nitrogen peak shapes look good...flat topped with steep shoulders.
I don't think a mass calibration would help at all as we do a peak
center prior to each sample analysis, so we should be centered on
mass 29.
The nitrogen peaks are not tailing either...they look the same both
before when we were getting great data and now. I can't see it being
related to the timing of the arrival of the oxygen pulse (i.e.
sampling delay). I did change the delay, but one of my tests was to
run at the same delay as back when my data was good...no improvement.
I've received notification that the Ar leak spec through a peripheral
device at full temperature is not 70 mV but 30 mV. I will hunt for
leaks in the EA with a leak detector, though I must say that we've
been running at this 70 mV level for a long time, and our great data
was generated at this same level. Blanks come out negligible, so I
can't see a leak being the issue.
But given all that I have tried I must pursue all possibilities.
Thanks for those who answered...I will email the list when I find the
solution.
As for my TC/EA troubles, it appears that our hot zone has indeed
migrated downwards slightly. Repacking with the distance between the
top of the ceramic and the top of the crucible being 28 cm instead of
27 cm has brought our AgPO4 samples back to normal. Thank you to all
who responded on that issue.
Tim Prokopiuk
-------------------------------------------------
Tim Prokopiuk
B. Sc. Geology/Technician
Saskatchewan Isotope Laboratory
Room 241
Department of Geological Sciences
University of Saskatchewan
114 Science Place
Saskatoon, Saskatchewan, Canada
S7N 5E2
Phone: (306) 966-5712
Fax: (306) 966-8593
Email: [log in to unmask]
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