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you have mentioned two things that have caused me trouble with probe analysis before.
>> I wonder if this particular sample is not adequately grounded. What
>> does the beam current and the sample current look like during the
>> analysis, and is the sample current similar to that on andesite
>I keep coming back to this explanation myself, actually. The glass is
>surrounded by a metal capsule in a brass mount holder, and we've
>coated it twice now, so it really shouldn't be a problem. Checking
>the sample current is a good idea, though -- I'll definitely have him
>try that. I guess the next step would be getting out the carbon or
I have had specimens that look OK and don't appear to charge but gave low results.
When I used my EDS/EDX detector and checked where the backgound x-rays were going to zero counts at the high energy end (the Duane-Hunt limit), I found that I had about 2kV of charge build up that was decelerating the beam. The standards did not have this charging problem. So I was effectively measuring standards at one voltage and doing analysis at another. On my machine it's easy to do a qualitative ED analysis while the probe is doing a WDS analysis. Coating grounds the surface of the specimen but does not always remove sub-surface charge build up.
>> I guess the other question is what oxygen mass absorption
>> coefficient did you use? . . .
>It should be the standard value in JEOL's data -- I'll check that too.
This has also caused me trouble before. My old 8800 JEOL (15 years) uses Henke (1966) MACs for longer wavelenghts ( > 1.2nm) and Heinrich (1966) MACs for shorter wavelenght x-rays. My newer 8200 EPMA (4 years) claims to use Henke (1966, 1982) for the longer wavelenghts and the same Heinrich (1966) MACs for shorter wavelenght x-rays. When I looked up the actual value in my new probe for O Ka in Si I was given 8790.0, the same value John gave before as Bastin 1992. I don't know the age of your probe, or what MACs it uses, but changing MACs has affected my results (different sample) by about the same amount you are talking about. You may be able to edit you MAC table and recalculate to see if it makes a difference for you.