Potassium and Chlorine Standards

[Jacob]

This might be a more appropriate topic for the general EDX or microprobe forums, but I thought I'd ask here given that I'm using DTSA-II.

I'm looking at some reaction products that include both potassium and chlorine.  My first thought for a standard was potassium chloride, so I grew some large crystals and went for it.  The software throws an error message ("Potassium Chloride is not suitable as a reference for this ROI") when I try to use the standard bundles created.  Is this due to insufficient separation of the peaks?  Fluorescence concerns?

-Jacob

[Nicholas Ritchie]

The problem is the proximity of the K and Cl peaks.  As with WDS, measuring the background is critical to measuring the peak intensity with utmost accuracy.  In DTSA-II, measuring the background on a standard requires about one FWHM on each side of the peak.  (Note at this isn't true about unknowns because a high quality peak shape reference allows us to deconvolve even the nastiest interferences.)  While the K and Cl peaks are separated, they are not sufficiently separated.  You will need "peak shape references" to fit these peaks - table salt (NaCl) while not a good standard makes an excellent Cl reference.  For K, either KBr or KAlSi3O8 can be used as a reference.

Remember, a reference does not need to meet some of the requirements of a standard - it needn't be absolutely stable under the beam and it needn't be prepared with the same care (surface finish, topography) and you don't need to know the exact composition (just which elements are present).  Thus materials like grains of table salt can be used as a reference for Na or Cl while they are less well suited to be used as a standard.

[Jacob]

Thanks Nicholas!

That worked great.  I wound up using some freshly cleaved pieces of NaCl and KBr from some absolutely ancient optical single crystals originally used for FTIR.

-Jacob

[rieders.n]

I am having a related issue.  DTSA-II is forcing me to specify a reference spectra for Carbon even though I have not identified it in the unknown spectra, nor have I selected it for quantitation.  The same is true for Oxygen? 

[Nicholas Ritchie]

Reference requirements have much more to do with the standards than the unknown.  If there is an interference in the standard, you may need to provide a reference for an element not present in the unknown.  This is probably why you are being asked for a C reference even though C does not exist in the unknown.