Hi All,
We have recently changed our multiplier from a Johnston to a Blazers
and due to this have higher than normal backgrounds as the machine
recovers from being vented. Despite fairly high backgrounds, we
proceeded to run some Fish Canyon sanidine to see where we were at
with respect to reproducibility. We consistently obtained slightly
less radiogenic yields, observed a slight age variation that was
correlated to signal size and MSWD values for 6 analyses of about 2
to 3.
We determined that we could subtract an additional absolute value of
about 4e-18 moles from mass 36 from our measured value and thus
increase our radiogenic yield values and obtain populations with
MSWD's of 1 or less. We refer to this as a phantom 36 correction.
In an attempt to understand why our apparent 36 background was less
during a blank than during an analysis, be began looking at mass 37
and mass 35 with a little more rigor. We found that for an air
analysis or for an unirradiated sanidine we consistently measured
negative 37 and 35 following blank correction. That is, we had less
35 and 37 during analysis than during a blank run. In fact, running
any sample of sanidine, air, biotite or amphibole always returns at
negative 35 signal of about 4e-18 moles.
We believe that it is no coincidence that the negative 35 signal
matches in magnitude the correction that we need to make to mass 36
in order to get quality Fish Canyon sanidine populations. We
hypothesize that introduction of hydrogen from a sample or air split
is reacting with our chlorine backgrounds in the mass spec such that
35 and 37 are reduced during analysis (compared to a blank) and in
turn we create HCl (thus the mass 36 interference) as well as minor
mass 38 (37Cl+H).
Monitoring mass 2 during gas introduction shows that our getters come
to a consistent steady-state value that is non zero (3e-15 moles) no
matter what we put in the instrument. Much of the H is pumped out by
the ion pump after analysis. Also, closing the mass spec getter with
the machine static shows a pretty dramatic hydrogen degassing of the
mass spec that is readily pumped upon opening the getter. Thus, our
getters seem to be working normally.
Our present Mass spec backgrounds are (moles):
40 - 2e-18
39 - 5e-19
38 - 5e-19
37 - 8e-18
36 - 2.5e-18
35 - 2.5e-17
As you can see, 35, 36, and 37 are higher than we'd like, but not
outrageously bad. We believe we have not observed this prior to
venting the machine because our backgrounds were in the E-19 range
and thus any introduction of hydrogen had nothing to react with to
cause our HCl interference.
We would be curious to know if others have seen any similar behavior
to what is described above. If any one has at present the unfortunate
circumstance of relatively high backgrounds, we would appreciate
information on analysis of unirradiated samples (i.e. negative 37) or
any other runs to test for negative 35. Also, anyone willing to take
a look at hydrogen could help us evaluate the quality of our getters.
We think we have exhausted any electronic issue that could cause this
apparent behavior (i.e. decay of signal following measurement of a
large beam, signal non-linearity, etc.). The chemistry of our problem
seems to make sense. Any feedback would be welcomed.
Thanks,
Matt
--
Matt Heizler
NM Bureau of Geology and Mineral Resources
NM Tech
801 Leroy Place
Socorro, NM 87801
Office 505-835-5244
Argon Lab 505-835-5271
Main Office 505-835-5420
FAX 505-835-6333
http://geoinfo.nmt.edu/staff/mheizler/home.html
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