Paul,
Lots of different places, it doesn't make a difference. However,
generally at 35.8.
Matt
>Hi Matt,
>
>Where are you measuring your baselines?
>
>Paul
>
>>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
--
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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