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

[Rom]

Forgot... we use 20nA

[John Donovan]

Forgot... we use 20nA

OK, so probably not a problem with the your dead time corrections.

Please check for impurities in your Fe2O3 and use Evaluate.exe to check on agreement with other Fe standards.

[Rom]

Ok, thank you. I am disappearing for several days to check everything.

[Rom]

Greetings!
1. I did not find some impurities in Fe2O3 Taylor with EDS spectrometer (15KeV, 20nA, 10 um, 15-20 min). Energy limit is 15 keV - see photos of screen.
2. I used Evaluate application for evaluate 4 standards with Fe we have in the Taylor block: FeCuS2(28), FeS(16), Fe2O3(33), Fe(32). Sample 7201 - Fe2O3
 synthesized from Fe metal (1200C, CO2 atmosphere, 4 hours) in our lab. See addition picture.

[John Donovan]

I told you Evaluate was easy to use!   

So both your Fe2O3 standards compare low in Fe compared to your Fe metal and also your Fe sulfide.  Maybe they have excess H2O or oxygen?

What are the compositions provided by the commercial vendors?

[Rom]

Yes, the Evaluate is easy and useful application - thank you.

All these standards except 7201-Fe2O3 are in the Taylor block which was supply by commercial vendor.
I thought about O2 or H2O in Fe2O3 Taylor - it is possible but not 2%wt.
I can add to the Evaluate diagram results for extra one Fe2O3 Standard from SPI block which was supply by commercial vendor too.
Also we can see the same composition in synthetic fresh sample 7201. I can't image that this sample obtained in the furnace at the exactly known PO2 contains less then 69.5-70wt% of Fe.

[jon_wade]

I would not rely on your Fe2O3 being 'bang on' stoichiometric - it will almost inevitably contain some mixed valence Fe.

[John Donovan]

I would not rely on your Fe2O3 being 'bang on' stoichiometric - it will almost inevitably contain some mixed valence Fe.

To be clear, it's not my Fe2O3. It's mounted in a Taylor block from Rom's lab at the University of Queensland. Yet someone, somehow manages to call it a "standard" whatever that means!  And advertise it for sale! 

But yes, I agree that makes a lot of sense. Could this be related at all to the inclusion of H2O or OH in hematite?

[jon_wade]

one persons standard is another persons 'where did we find that again?'
 

to be fair, as we all know some things are kinda hard to make/find as microanalytical standards, and something with the potential for mixed valence like this is always going to be suspicious. Trace metals in metals generally is another but by and large, thats the beauty of the matrix correction!

[John Donovan]

one persons standard is another persons 'where did we find that again?'
 

to be fair, as we all know some things are kinda hard to make/find as microanalytical standards, and something with the potential for mixed valence like this is always going to be suspicious. Trace metals in metals generally is another but by and large, thats the beauty of the matrix correction!

True, but the matrix correction can only correct for what it knows about.  That's why specifying unanalyzed elements is so important:

https://probesoftware.com/smf/index.php?topic=92.0

[Rom]

So what could you suggest me to check next?
I recap:
we have 2 commercial Standard blocks with Fe2O3: Taylor block (sorry if it is the fake block, it is out of my responsibility) and SPI block. Also we have "handmade" sample which composition is close to Fe2O3.
3 samples in total (actually we have a pile of samples like last one).

Measuring of the all 3 samples gives us very similar results: Fe is in 1.5-2 %wt. lower than we expect.

What should I check? It is not peaking issue, not dead time, not BG, not APF, not the standard issue (Fe metal), not energy limit... what else?

Thank you very much!

[John Donovan]

Can you do some Mossbauer to determine the Fe+2/Fe+3 ratios?

Try obtaining a synthetic high purity magnetite. That should be close to Fe3O4.

[Rom]

I will try to do something in this direction.

But it means that the main version of our issue is wrong compositions of Fe2O3 Standards.
And all 3+ absolutely independent substances (2 - commercial and 1...100+ samples from our lab) have the same issue - total contaminations are close to 2%wt. Also all binary compositions which close to Cu2O, NiO, ... (the topic starts from this) have the same issue. Everything is possible...

I'll update the topic when collect some new information.

Could you suggest me what I need to see and how I should analyze results of different ZAF calculations?
My question unfortunately drowned the discussion
https://probesoftware.com/smf/index.php?topic=1514.msg11698#msg11698

And extra one question - could you suggest the methodology of obtaining a synthetic high purity magnetite? I want to kill any questions to methodology we used )). Our main way is furnace, hang high purity Iron foil, 1200C, mixture of CO, CO2, 3-5 hours. - the same way we did Fe2O3. The only difference in  furnace atmosphere.

Thank you!

 

[John Donovan]

But it means that the main version of our issue is wrong compositions of Fe2O3 Standards.
And all 3+ absolutely independent substances (2 - commercial and 1...100+ samples from our lab) have the same issue - total contaminations are close to 2%wt. Also all binary compositions which close to Cu2O, NiO, ... (the topic starts from this) have the same issue. Everything is possible...

We're all just trying to help figure out what is going on with your Fe2O3 analyses. But thinking about Jon's suggestion a bit more, I have to wonder if Fe+2/Fe+3 ratio cannot be the issue because Fe2O3 should already be all Fe+3, correct?  If some of the Fe is Fe+2 then that would give us even less oxygen, right?  I am not a mineralogist, so perhaps someone with that expertise could chime in on this question?

But that does still leave the question of OH and H2O as additional contaminants...

Could you suggest me what I need to see and how I should analyze results of different ZAF calculations?
My question unfortunately drowned the discussion
https://probesoftware.com/smf/index.php?topic=1514.msg11698#msg11698

Two points here: first, the results of all the matrix corrections (both ZAF and phi-rho-z) yield low Fe in your Fe2O3 samples relative to your Fe standard, so I don't think matrix corrections are the problem. Second, the historical ZAF corrections, e.g., Philibert, Love-Scott can probably be ignored. The Armstrong and XPP/PAP phi-rho-z corrections are probably the most accurate in general.

What I do find interesting is that in your Evaluate screen capture here:
 

2. I used Evaluate application for evaluate 4 standards with Fe we have in the Taylor block: FeCuS2(28), FeS(16), Fe2O3(33), Fe(32). Sample 7201 - Fe2O3

I note that both your sulfide standards agree within 0.5% absolute with your Fe standard. This suggests to me that you might be doing things mostly correct, but that it's the Fe2O3 standards that are the problem.

And extra one question - could you suggest the methodology of obtaining a synthetic high purity magnetite? I want to kill any questions to methodology we used )). Our main way is furnace, hang high purity Iron foil, 1200C, mixture of CO, CO2, 3-5 hours. - the same way we did Fe2O3. The only difference in  furnace atmosphere.

I am not suggesting that you grow your own Fe3O4. Rather I am suggesting that you try to obtain a synthetic single crystal Fe3O4 from a commercial crystal grower and see how that compares. The document attached to this post has a number of crystal growers listed in the appendix:

https://probesoftware.com/smf/index.php?topic=1415.msg10929;topicseen#msg10929

[Probeman]

I've been running some tests of my own on this question from Rom on his efforts to utilize Fe metal as an Fe primary standard for various Fe oxides where the Fe is present in major concentrations.

As some of you may remember, Rom found that when using Fe metal as a primary standard, he was getting consistently low totals for his Fe oxide standards, but his sulfide standards seem to analyze close to their expected values. I can now report that I am seeing similar effects. 

Why could this be? Well as discussed in the posts above, it could be a question of standard accuracy, e.g., his hematite standards could be contaminated with OH or H2O. But I am seeing similar issues with my own magnetite standard and that has been analyzed for Fe using wet chemistry and for ferric-ferrous ratios using colorimetry by Ian Carmichael many years ago.

It is also unlikely to be a matrix correction or background issue, as discussed above.  Though it might be worth checking the dead time calibration, though at 20 nA that is unlikely to be a significant effect.  So what else could it be?

Now we all know that when analyzing lower Z elements with low energy emission lines where the valence shell is one of the shells involved in the electron transition which produces an x-ray emission, there can be significant peak shape/shift effects from chemical bonding, such that we must usually utilize a primary standard that is at least somewhat similar to our unknown.

For analysis of light elements such as O, N, C or B it is well known that such chemical peak shift/shape effects can be quite large, even when utilizing relatively low resolution modern LDE diffractors.  In such cases, the use of a close matrix matched primary standard is necessary, or one can utilize area peak factors (APFs) to account for these chemical bonding effects:

https://probesoftware.com/smf/index.php?topic=536.0

One can also utilize integrated intensities for the acquisition of WDS intensities which can be quite slow, but can handle these peak shift/shape effects automatically. See the Integrated Intensities options in the Elements/Cations dialog:

https://probesoftware.com/smf/index.php?topic=536.msg2992#msg2992

And even in the case of say Mg, Al and Si Ka, we would not want to utilize a metal primary standard when analyzing those elements in oxides or silicates.  Instead we would utilize an oxide or silicate standard such as MgO, MgAl2O4 or Mg2SiO4 for Mg Ka analysis of oxides and silicates.  An exact matrix match is not necessary for these emission lines, but we can't reliably extrapolate from a pure metal to the oxide/silicate.

However, for higher Z, higher energy emission lines, e.g., Fe Ka, I would have thought that these chemical peak shift/shape effects would be minimal, but perhaps that is not the case.  For example, here is a measurement of Fe Ka using Fe metal as a primary standard, analyzing pyrite as a secondary standard:

St  730 Set   1 Pyrite UC # 21334, Results in Elemental Weight Percents
 
ELEM:       Fe      Mn      Cr      Si       S      Ti
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    40.00   40.00     ---     ---     ---     ---
BEAM:    29.89   29.89     ---     ---     ---     ---

ELEM:       Fe      Mn      Cr      Si       S      Ti   SUM 
   342  46.309   -.007    .000    .000  53.450    .058  99.810
   343  46.229   -.012    .000    .000  53.450    .058  99.725
   344  46.329   -.039    .000    .000  53.450    .058  99.797
   345  46.304    .004    .000    .000  53.450    .058  99.816
   346  46.323    .018    .000    .000  53.450    .058  99.849

AVER:   46.299   -.007    .000    .000  53.450    .058  99.799
SDEV:     .040    .021    .000    .000    .000    .000    .046
SERR:     .018    .010    .000    .000    .000    .000
%RSD:      .09 -288.57     .00     .00     .00     .00

PUBL:   46.550    n.a.    .000    n.a.  53.450    .058 100.058
%VAR:     -.54     ---     .00     ---     .00     .00
DIFF:    -.251     ---    .000     ---    .000    .000
STDS:      526     525     ---     ---     ---     ---

Here we can successfully extrapolate from Fe metal to FeS2 (as Rom found). Another attempt:

St  730 Set   2 Pyrite UC # 21334, Results in Elemental Weight Percents
 
ELEM:       Fe      Mn      Cr      Si       S      Ti
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    40.00   40.00     ---     ---     ---     ---
BEAM:    29.89   29.89     ---     ---     ---     ---

ELEM:       Fe      Mn      Cr      Si       S      Ti   SUM 
   387  46.198   -.032    .000    .000  53.450    .058  99.674
   388  46.255   -.034    .000    .000  53.450    .058  99.729
   389  46.237    .005    .000    .000  53.450    .058  99.750
   390  46.361    .024    .000    .000  53.450    .058  99.893
   391  46.218   -.016    .000    .000  53.450    .058  99.710

AVER:   46.254   -.010    .000    .000  53.450    .058  99.751
SDEV:     .064    .025    .000    .000    .000    .000    .084
SERR:     .029    .011    .000    .000    .000    .000
%RSD:      .14 -240.04     .00     .00     .00     .00

PUBL:   46.550    n.a.    .000    n.a.  53.450    .058 100.058
%VAR:     -.64     ---     .00     ---     .00     .00
DIFF:    -.296     ---    .000     ---    .000    .000
STDS:      526     525     ---     ---     ---     ---

Again within 1% relative accuracy.  Now let's analyze the magnetite standard from Carmichael:

St  395 Set   1 Magnetite U.C. #3380, Results in Elemental Weight Percents
 
ELEM:       Fe      Mn      Cr      Si       S      Al      Mg       O
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    40.00   40.00     ---     ---     ---     ---     ---     ---
BEAM:    29.90   29.90     ---     ---     ---     ---     ---     ---

ELEM:       Fe      Mn      Cr      Si       S      Al      Mg       O   SUM 
   322  69.969    .046    .007    .000    .000    .201    .072  27.803  98.098
   323  69.922    .017    .007    .000    .000    .201    .072  27.803  98.021
   324  70.188    .036    .007    .000    .000    .201    .072  27.803  98.307
   325  70.542    .056    .007    .000    .000    .201    .072  27.803  98.681
   326  70.528    .006    .007    .000    .000    .201    .072  27.803  98.616

AVER:   70.230    .032    .007    .000    .000    .201    .072  27.803  98.345
SDEV:     .296    .021    .000    .000    .000    .000    .000    .000    .297
SERR:     .133    .009    .000    .000    .000    .000    .000    .000
%RSD:      .42   64.40     .00     .00     .00     .00     .00     .00

PUBL:   72.080    .054    .007    .000    n.a.    .201    .072  27.803 100.217
%VAR:    -2.57  -40.75     .00     .00     ---     .00     .00     .00
DIFF:   -1.850   -.022    .000    .000     ---    .000    .000    .000
STDS:      526     525     ---     ---     ---     ---     ---     ---

Our relative error has now increased to ~2.5% which is unacceptable. Analyzing other Fe silicate (glass) standards we observe the same low accuracy, here for NIST SRM K-412:

St  160 Set   2 NBS K-412 mineral glass, Results in Elemental Weight Percents
 
ELEM:       Fe      Mn      Cr      Si       S      Mg      Ca      Al       O
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    40.00   40.00     ---     ---     ---     ---     ---     ---     ---
BEAM:    29.89   29.89     ---     ---     ---     ---     ---     ---     ---

ELEM:       Fe      Mn      Cr      Si       S      Mg      Ca      Al       O   SUM 
   352   7.563    .058    .000  21.199    .000  11.657  10.899   4.906  43.597  99.879
   353   7.510    .047    .000  21.199    .000  11.657  10.899   4.906  43.597  99.816
   354   7.498    .029    .000  21.199    .000  11.657  10.899   4.906  43.597  99.785
   355   7.521    .078    .000  21.199    .000  11.657  10.899   4.906  43.597  99.857
   356   7.579    .057    .000  21.199    .000  11.657  10.899   4.906  43.597  99.894

AVER:    7.534    .054    .000  21.199    .000  11.657  10.899   4.906  43.597  99.846
SDEV:     .035    .018    .000    .000    .000    .000    .000    .000    .000    .045
SERR:     .016    .008    .000    .000    .000    .000    .000    .000    .000
%RSD:      .46   33.33     .00     .00     .00     .00     .00     .00     .00

PUBL:    7.742    .077    n.a.  21.199    n.a.  11.657  10.899   4.906  43.597 100.077
%VAR:    -2.69  -30.21     ---     .00     ---     .00     .00     .00     .00
DIFF:    -.208   -.023     ---    .000     ---    .000    .000    .000    .000
STDS:      526     525     ---     ---     ---     ---     ---     ---     ---

Again about 2.5% relative low for Fe.  I did not have a Mn sulfide standard, but running Mn metal against MnO, I see a similar low accuracy:

St   25 Set   3 MnO synthetic, Results in Elemental Weight Percents
 
ELEM:       Fe      Mn      Cr      Si       S       O
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    40.00   40.00     ---     ---     ---     ---
BEAM:    29.89   29.89     ---     ---     ---     ---

ELEM:       Fe      Mn      Cr      Si       S       O   SUM 
   347    .008  74.428    .000    .000    .000  22.554  96.990
   348    .015  75.138    .000    .000    .000  22.554  97.707
   349    .024  74.848    .000    .000    .000  22.554  97.426
   350    .026  74.670    .000    .000    .000  22.554  97.250
   351   -.002  74.831    .000    .000    .000  22.554  97.384

AVER:     .014  74.783    .000    .000    .000  22.554  97.351
SDEV:     .012    .261    .000    .000    .000    .000    .262
SERR:     .005    .117    .000    .000    .000    .000
%RSD:    81.45     .35     .00     .00     .00     .00

PUBL:     n.a.  77.446    n.a.    n.a.    n.a.  22.554 100.000
%VAR:      ---   -3.44     ---     ---     ---     .00
DIFF:      ---  -2.663     ---     ---     ---    .000
STDS:      526     525     ---     ---     ---     ---

About a 3.5% relative error. And here for Mn2SiO4 synthetic olivine:

St  275 Set   2 Mn2SiO4 (manganese olivine) synthetic, Results in Elemental Weight Percents
 
ELEM:       Fe      Mn      Cr      Si       S       O
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    40.00   40.00     ---     ---     ---     ---
BEAM:    29.89   29.89     ---     ---     ---     ---

ELEM:       Fe      Mn      Cr      Si       S       O   SUM 
   362    .019  52.526    .000  13.907    .000  31.688  98.140
   363    .013  52.363    .000  13.907    .000  31.688  97.971
   364    .008  52.843    .000  13.907    .000  31.688  98.446
   365    .013  52.404    .000  13.907    .000  31.688  98.013
   366    .016  52.316    .000  13.907    .000  31.688  97.927

AVER:     .014  52.490    .000  13.907    .000  31.688  98.099
SDEV:     .004    .212    .000    .000    .000    .000    .209
SERR:     .002    .095    .000    .000    .000    .000
%RSD:    28.58     .40     .00     .00     .00     .00

PUBL:     .000  54.406    .000  13.907    .000  31.688 100.001
%VAR:      .00   -3.52     .00     .00     .00     .00
DIFF:     .000  -1.916    .000    .000    .000    .000
STDS:      526     525     ---     ---     ---     ---

Again about 3.5% relative accuracy.  So not good for major element accuracy!   

Personally I've always utilized pure oxide standards for my primary standards for measurements in oxides and silicates, and so have never observed these effects previously. Has anyone else looked at these peak shift/shape effects for the first transition series elements?  Please share some data with us...

Could these low values be from a subtle chemical peak shape/shift effect?  I am running some detailed wavescans to check... but I suspect it's the most likely explanation.

But again, remember that these subtle primary standard matrix effects are something that may be important for major elements, but probably not for trace elements as discussed here:

https://probesoftware.com/smf/index.php?topic=610.msg11752#msg11752

A 2 or 3% relative error at 100 PPM is going to produce an absolute error of 2 or 3 PPM, generally not something we are concerned with. Even at 1000 PPM that's only an absolute error of 20 or 30 PPM, so still below the detection limit for many measurements.  In other words if you want to measure some first series transition elements at trace levels in an oxide or silicate matrix, and you don't have a suitable pure oxide standard, you are probably OK to use a pure metal standard.