John and Oregon,
Thanks for the information on the blank correction method. Seems like it will be very helpful in many of the upcoming tasks for this lab and I can indeed measure 0 in standard provided.
Some background on this project. I am the interim lab manager at the University of Iowa with about a year of experience working in the lab under my belt and a month as manager. I am working on developing a procedure for measuring the amount of Cu, Zn, Ni, Co, Fe, Mn, V, Al, and Cr that have been included (doped)in a synthetic uranium crystal lattice. Crystals are grown in a organic acid so I have applied a glow discharge and gold coat to stop sample degassing (this was a huge issue). The concentrations of these elements in the crystal is expected to be between .5-2 wt%. The uranium standard that I was provided is reproducing excellently but is rather heterogeneous and void of all the aforementioned doped elements. I expect this to be reflected upon the doped unknowns so that is not an issue.
I have been able to measure 0 for Cu in my standard and I got an intensity of -3.7 cps and I am not sure what value to apply for Cstd and Clevel.
Also, is this method appropriate for values this high.
Cheers and Thanks,
Sam
Hi Sam,
Welcome to the field! You've come to the right place for questions and answers on EPMA...
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Interesting problem. You say the uranium was grown in a synthetic acid so the material contains some organics that are (were) out gassing? I would be concerned about beam damage. You might want to perform some time dependent intensity (TDI) measurements to see if the x-ray intensities are stable over time.
The blank correction could be performed by hand, but it is intended to be included in the matrix iteration for best accuracy (it is built into the Probe for EPMA quantification code). But performing the blank correction off-line in Excel is better than not performing any correction at all.
Basically the "blank level" is the known concentration in your blank standard and the "blank value" is what you actually measure in your unknown. The difference between these can be applied to your unknown concentration results, in your case the standard would be your pure uranium crystal and the unknown would be your doped uranium crystal lattice materials.
In PFE the concentration difference is converted to an intensity and applied during the matrix iteration, since technically the matrix is being adjusted, so the matrix corrections should be re-calculated. In practice the changes to the matrix composition are trace to minor levels in magnitude, so they might be ignored depending on the physics details and performed in Excel using just concentrations.
Basically the blank correction is intended for correction of continuum artifacts such as the holes in some PET crystals under the Ti Ka peak or the curved background under Al Ka in quartz. However, it can be applied as an accuracy correction even for major elements, though you want to be careful that it is being applied appropriately.
Attached below is a white paper I produced some time ago on using the blank correction for determining water in glasses by measuring the oxygen content directly.
See also here for more details on the blank correction:
http://probesoftware.com/smf/index.php?topic=204.msg3467#msg3467But I would suspect that, due to the delicate nature of the organic matrix in your materials, you may have bigger problems than just measuring the background. That is, the TDI correction might be much more important. You should check out the method described here and perform some of your own tests to check the stability of your materials:
http://probesoftware.com/smf/index.php?topic=11.0For example, see the last few posts in this topic on measuring the absorbed current during the analysis to check for beam damage over time.
The other method we use in PFE for beam sensitive trace analysis is the "on and off-peak alternating" backgrounds described here:
http://probesoftware.com/smf/index.php?topic=29.msg387#msg387This allows the intensity measurements to be distributed over time between the on and off-peak positions...
Feel free to call me any time if you want to chat off-line.
john
541-346-4632