NeXL

[Ben Buse]

That's great, I'hadn't checked it was there, I'd just assumed it wasn't, I will give it a go.

[Ben Buse]

Hi Nicholas, I think it's great NeXL screens out unsuitable standards which contain overlaps.

I'm concerned as to whether it should be screening out due to the presence of a minor element

In the emsa file for Sanidine I have 0.76 Ba

Code Select Expand

B4 Sanidine,(Na:1.27),(Al:9.8),(Si:30.27),(K:11.8),(O:46.03),(Ba:0.76)
This results in Sanidine being rejected

Code Select Expand

Info: A material containing Al, Ba, Na, Si, O and K is not a suitable reference for "K K-L3 + 3 others" due to a peak interference.
┌ Warning: A material containing Al, Ba, Na, Si, O and K provides no references for K.
└ @ NeXLSpectrum

Whereas if I delete the Ba concentration from the emsa it is no longer rejected.

Is sanidine with 0.76 Ba a suitable EDS standard.

Thanks

[Ben Buse]

Similarly with Durango Apatite

emsa

Code Select Expand

##D2STDCMP   : B4 Durango Apatite,(P:17.79694),(Ca:38.60773),(Si:0.1589262),(Na:0.170626),(S:0.1481775),(F:3.53),(Cl:0.41),(Ce:0.4268828),(La:0.3410718),(Nd:0.128602),(O:37.432)

Rejected on basis of small amount of Si and S

Code Select Expand

[ Info: A material containing S, Ca, Ce, Nd, O, Cl, Na, F, Si, La and P is not a suitable reference for "P K-L3 + 3 others" due to 2 peak interferences.
┌ Warning: A material containing S, Ca, Ce, Nd, O, Cl, Na, F, Si, La and P provides no references for P.

If remove Si and S it is no longer rejected.

[Nicholas Ritchie]

As you observe, there is a limit below which a trace element no longer matters - at least in practical terms.  However, as implemented, NeXL doesn't even consider the mass fraction when determining whether a material is suitable.  Even 1 part in 10^6 of an element will trigger a rejection.  Clearly, this doesn't always make sense.  But nor is there a clear cut-off between matters/doesn't matter.

As an alternative, I usually suggest to use pure elements or simple stoichiometric compounds as simple references.  If you really want to use a similar standard, fit the unknown with the simple reference, fit the similar standard with the simple reference (and the other elements as necessary) and compute the ratio of the two to get the k-ratio of the unknown relative to the similar standard.  This ratio of k-ratios idea is the key notion behind the k-ratio database.

[Ben Buse]

Yes I agree it's complicated to define an almost limit depending on distance away, x-ray yield and concentration.

I do suspect though it's exacerbated in EPMA as we want to list the trace elements in the standards, which may be at EDS detection limit or negligible contribution compared to the major element being used for the standard.

For EDS I've been trying to use simple oxides Mgo, Al2O3, NaCl, TiO2, but also finding I running out of simple compound standards, and quartz is unstable under the electron beam, at high beam currents, which I can use with small aperture selected on collimator. I'd be interested in simple synthetic standards if anyone is sharing any material

I guess one way would be to read the emsa files exported from PFE remove trace elements and write the files. Recognizing this could cause an error where the interference is a problem

Do you find matrix matching important, or do you find pure metals working for silicates. In terms of your suggestion do I fit the unk and the complex standard as unk, using fitted_spectrum, to give the kratios. Then I take the kratios and divide them by each other.

[Nicholas Ritchie]

Personally, I find pure elements and simple stoichiometric compounds sufficient for almost everything. As JD points out, matrix correction has improved to the point where similar standards are rarely necessary.  He suggests to use secondary standards to validate the measurement rather than similar standards to make the measurement. 

My typical probe currents are on the order of 1 nA so material instability under the beam is not an issue.  However, I understand that for the probe currents necessary for WDS there can be a problem.

Reading and rewriting the composition makes sense to me.  It is a little tedious but it keeps you in control when deciding whether a trace element can be ignored.  I'd be much less concerned about omitting an element for hyperspectral maps where the data spectra are low count. I would be much more careful when ignoring elements on references to be used for point spectra.

I've implemented a mechanism to standardize using similar standards in NeXLSpectrum/standardize.jl  THere is an example here: https://github.com/usnistgov/NeXLSpectrum.jl/blob/master/test/standardize.jl