Indetermination Fe, Cu,... in simple stoichiometric oxide

[Rom]

Hi dear colleagues,
Our issue is indetermination on 0.5-1wt.% metals (Fe, Cu,...) in simple stoichiometric oxides if we use standards pure metals.
We didn't find peak shifts when come from metals to oxides.
K-line, Lif.
To simplify our task we use specify (not measured) oxygen concentration.
What direction should we look? These are strong lines and use APF or MAC corrections looks unusual.
Thank you.

[Probeman]

Our issue is indetermination on 0.5-1wt.% metals (Fe, Cu,...) in simple stoichiometric oxides if we use standards pure metals.
We didn't find peak shifts when come from metals to oxides.
K-line, Lif.
To simplify our task we use specify (not measured) oxygen concentration.
What direction should we look? These are strong lines and use APF or MAC corrections looks unusual.

1. Please explain "indetermination".

2. One should not observe peak shifts with high energy emission lines (e.g., Fe Ka, Ni Ka, etc.). Peak shifts due to chemical states are usually only seen when the transition shell is also the bonding shell (e.g., C Ka, N Ka, Si Ka etc.).

3. Either pure metal (if unoxidized) or oxide primary standards should be fine as the most important correction is the background correction at low concentrations:

4. Please explain "What direction should we look? These are strong lines and use APF or MAC corrections looks unusual."

5. Here are some additional readings for you:

https://probesoftware.com/smf/index.php?topic=1404.0

https://probesoftware.com/smf/index.php?topic=1378.0

[Rom]

1.Indetermination: the measured concentration of metal in oxide is lower in 0.5-1 wt% its stoichiometric (real) concentration.
2. Yes
3. Yes
4. I just kindly asked where should I looking for my mistake (because 2,3 - yes).
5. Thank you

[Probeman]

1.Indetermination: the measured concentration of metal in oxide is lower in 0.5-1 wt% its stoichiometric (real) concentration.

Did you mean to say: "the measured concentration of metal in oxide is lower than 0.5-1 wt% which is its stoichiometric (real) concentration."?

[Rom]

Yes. You are right.

[Probeman]

For minor elements of first row transition elements I wouldn't be too worried about MACs (certainly not APFs), but did you perform careful wavescans to check the background positions?  That's going to have the biggest effect on accuracy for minor elements.

Also are the totals good?  Did you include oxygen from ferric iron in the calculation?

https://probesoftware.com/smf/index.php?topic=92.msg8593#msg8593

What is the oxide matrix?  What are the minor elements in question?

[Rom]

I didn't understand about minor elements. I told about metal measure in pure oxides, which are very close to stoichiometric compound (Fe2O3, Cu2O etc.)

Yes, I performed careful wavescans to check the peak shifts and background positions.

Totals are 98.5-99.5. I used both: measured or specify (not measured) oxygen concentration.

My issue is absolutely the same question of your topic! Thank you!
But my results didn't change if I use "calculate excess oxygen from Ferrous/Ferric ratio" checkbox (attached file). In excel file I see zeroes in FeO, Fe2O3, excess oxygen etc. columns.



 

[Rom]

There are 3 questions:
1. What I should check If I want to use "calculate excess oxygen from Ferrous/Ferric ratio" - how can I start this options;
2. How I can start this options for system Cu-O or Sn-O etc., without Fe (it is necessary if we research  alloys after quenchingfrom liquid  for instance).
3. What is the fundamental base of  "the measured concentration of metal in oxide is lower than 0.5-1 wt% which is its stoichiometric (real) concentration"? Everything is clear if we discuss Oxygen concentration but why this correction affects on metal concentration?
Thank you a lot.

[John Donovan]

It appears no one has responded to your last post here, but I will begin again with a quote from your post yesterday in the other topic:

Actually my target is simple. I try to understand why the measured content of Fe in Fe2O3 standards (Taylor and SPI) is 2wt% lower than it should be.
...
Fe standard - Fe metal in the same block with Fe2O3 (carbon coating is the same).
O standard - Fe2O3 or MgO at the same block.
Doesn't matter: oxygen measured or calculated, use or not Fe+2/+3 correction, width and shape BG (from detail WSs analysis).
FeKa is a strong line, so APFs for Fe wont affect.
MAC, APF for O change O. Of course Fe changes a bit as well but not so successfully as it needs.
The only point I thought was a wrong peaking. But also not, everything is good.

Certainly, my affords with searching the solution give me new knowledge but...

You and your colleagues many times recommended to look at results of calculations with different corrections. I did. But what should I see is not clear. For instance here are results for calculation with default MAC table (LINEMU Henke...) and  FFAST table. Empirical MAC and APF values are not use in the calculations.

Let's start by assuming you are measuring Fe Ka using Fe metal as a primary standard and Fe2O3 as a secondary standard and calculating oxygen by stoichiometry (2:3). Then your only concern should be the measurement of Fe Ka.

The following should not be concerns:

1. Peaking issues should not be an issue (little to no peak shifts for Fe Ka)

2. MAC issues should not be a concern as MACs are very small for Fe Ka in oxygen as shown here:
MAC value for Fe ka in O =      22.55  (LINEMU   Henke (LBL, 1985) < 10KeV / CITZMU > 10KeV)
MAC value for Fe ka in O =      22.20  (CITZMU   Heinrich (1966) and Henke and Ebisu (1974))
MAC value for Fe ka in O =      22.25  (MCMASTER McMaster (LLL, 1969) (modified by Rivers))
MAC value for Fe ka in O =      22.26  (MAC30    Heinrich (Fit to Goldstein tables, 1987))
MAC value for Fe ka in O =      22.25  (MACJTA   Armstrong (FRAME equations, 1992))
MAC value for Fe ka in O =      20.85  (FFAST    Chantler (NIST v 2.1, 2005))
MAC value for Fe ka in O =      22.00  (USERMAC  User Defined MAC Table)

3. Carbon coating differences should not be an issue as long as the over voltage is reasonable (15 keV or higher).

4. Background corrections should not be an issue using the LiF Bragg crystal.  However analyzing Fe Ka on a PET Bragg crystal could be very problematic for the background will be very curved at such a low sin theta.

5. Surface polish should not be an issue for an energetic line such as Fe Ka, but always worth making sure the samples are well polished.

However, these issues could be problematic:
1. Your Fe metal standard could have surface oxidation, but if so this would tend to raise the concentration of Fe in your Fe2O3 secondary standard (the primary standard intensity is in the denominator of the k-ratio).

2. I note that you have only analyzed for Fe cations. Have you checked that your Fe2O3 standard from SPI/Taylor is actually 99.99% pure?  Is it natural or synthetic?  Common natural impurities are Si, Ti, Al, Mn, H2O. This alone could explain your observations.

Again, as I have said before, we need to identify, obtain and distribute at least two high purity synthetic minerals for each of the (at least to begin with, common) geological elements, on a global basis, so that we can actually begin to rigorously compare our results with each other. This problem is described here for those that have not seen it:

https://probesoftware.com/smf/index.php?topic=1415.0

For example, Will Nachlas has documented that a so called 99.99% Rh metal standard in one of his commercial mounts actually has ~4 wt% Fe in it!   

Quantitative analysis of oxygen as a major element is a whole separate endeavor, and here is a link to one of those discussions:

https://probesoftware.com/smf/index.php?topic=197.0

But I suggest that we first figure out what is going on when you try to analyze Fe2O3 using Fe metal and calculating oxygen by stoichiometry.  I would start by analyzing for the trace and minor elements, but maybe start by examining the Fe2O3 with EDS using a long count time to obtain sufficient sensitivity.

[Rom]

Hi John,
The results below are with measured oxygen (Standard is MgO).
Yes, of course I utilizing the Fe Ka emission line.

What topic should I look?
Thank you!

[John Donovan]

Hi John,
The results below are with measured oxygen (Standard is MgO).
Yes, of course I utilizing the Fe Ka emission line.

What topic should I look?
Thank you!

Doesn't matter: oxygen measured or calculated, use or not Fe+2/+3 correction, width and shape BG (from detail WSs analysis).

You said (quoted above), that you saw the same low totals when calculating oxygen by stoichiometry.  So let's deal with the quantification of Fe first and once that is working we can look at the measurement of oxygen, which is much more complicated.

Please share with us an example of measuring Fe in Fe2O3 using Fe metal as your primary standard and calculating oxygen by stoichiometry.

[Rom]

Please, see attached picture.
It should be noted that Fe met of the Standard  includes (estimate) 0.6-0.7%wt of oxygen. But it doesn't matter on the current step.
Default MAC table (LINEMU Henke...), default ZAF correction

[John Donovan]

OK, it is clear that your Fe metal standard and your Fe2O3 standard do not agree, so there is definitely a problem. You didn't mention the conditions (keV, nA, etc.), so that would be worth knowing also...

By the way, the oxygen values you show are the same for every data point (all three) so they are not being calculated by stoichiometry, but rather being loaded from the standard database as a fixed concentration (note the "Published" value for oxygen). But no matter because the real problem is the measured Fe concentration in your Fe2O3 standard.

2. I note that you have only analyzed for Fe cations. Have you checked that your Fe2O3 standard from SPI/Taylor is actually 99.99% pure?  Is it natural or synthetic?  Common natural impurities are Si, Ti, Al, Mn, H2O. This alone could explain your observations.

But I suggest we start by trying to figure out what is going on by analyzing your Fe2O3 secondary standard using Fe metal as a primary standard and calculating oxygen by stoichiometry. I would begin by analyzing your Fe2O3 standard using EDS with a long count time time and check for trace/minor impurities. You won't be able to check for H2O, but it's a start.

I would start with examining your Fe2O3 standard and check it for impurities. It seems unlikely to have that much "undeclared" contamination, but it is possible.

Also there is an excellent application that comes with Probe for EPMA that can be used to check how well your standards agree with each other called Evaluate.exe.  It was developed with John Fournelle and I think Dan Kremser back in the day, but is still very useful.

Basically you can select any PFE probe database file with standards that has been acquired and you can plot them up one element at a time, to see how well the standards all agree with each other.  Here is a topic that describes the application:

https://probesoftware.com/smf/index.php?topic=340.msg10757#msg10757

Worth checking out.

[Rom]

Quite many directions. So lets move step by step.
1. 15 keV, 10 um diameter (we used from 0 to 100 um, in general all the same), 25 sec on peak, 8+8 sec BG, WS for Fe and Fe2O3 on the picture.
2. You a right (sorry), the oxygen has taken from the standard composition. On the picture lower - the oxygen, calculated from stoichiometry.
3.

I would start with examining your Fe2O3 standard and check it for impurities. It seems unlikely to have that much "undeclared" contamination, but it is possible.

Yes. What I did.
-I measured two completely different Standards of Fe2O3 - in Taylor block and in SPI block. Both give close results. Both are natural.
-I checked WSs "from edge to edge" which I collected on Fe2O3 Taylor. Nothing found. But I'll check again Si, Ti, Al, Mn - thank you for show the direction of searching.
-May be I need to check energy limit with EDS?
-Ok, I'll scan one of my Fe2O3 standards using EDS with a long count time time and check for trace/minor impurities.
4. I didn't use Evaluate.exe. Hope the application is not too difficult for me.

[John Donovan]

1. 15 keV, 10 um diameter (we used from 0 to 100 um, in general all the same), 25 sec on peak, 8+8 sec BG, WS for Fe and Fe2O3 on the picture.

You should never use more than a 20 um beam diameter to avoid Bragg defocus effects.

But the most important condition parameter to my mind is the beam current.  One possible concern is that you are using a very high beam current and if your dead time correction is out of calibration that could explain some of what you are seeing.

4. I didn't use Evaluate.exe. Hope the application is not too difficult for me.

The Evaluate application is very easy to use.