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