Too high oxygen with SnO2

[Rom]

Hi everyone,
I'm using a JXA-8200 Probe, with both JEOL and PfEPMA software. I'm really excited with Probe software due to the wide range of possibilities that this software allows me to perform. But I faced with some issues of direct oxygen measurements in oxide standards. Currently I'm using the LDE1 crystal (FCS WDS, 15kV, 20nA, probe diameter is 10-50 microns).

I have the problem of incorrect oxygen measurements in simple oxides (MgO, SnO2, etc). In particular, oxygen concentration is 2-3 wt.% higher in MgO, than standard concentration, while 15wt.% higher in SnO2.

I believe the lower oxygen concentration in SiO2 can be explained by the build-up of negative charge in a very nonconductive sample, but how can we explain the increasing of oxygen concentration in simple oxides?

The discrepancy between measured and expected oxygen concentration in known standards can be reduced, but not completely, if we reduce the acceleration voltage to 2kV and measure the oxygen concentration only, and metal concentrations will be set as constants. In the table below you can see the normalised (cps/(1nA*1 sec)) oxygen counts, measured on Fe2O3, MgO, Al2O3, SiO2, SnO2 corresponding measured oxygen concentrations (wt.%) and relative deviations from the expected stoichiometric concentrations.

                                             Fe2O3      MgO     Al2O3     SiO2       SnO2
wt.% O in standards (stoichiometric)         30.06      39.70   47.08     53.26      21.24
cps/(1nA*1 sec)                         2kV    4.7        6.1     6.6       6.6        4.1
                                        6kV   31.4       39.7    43.4      44.1       16.2
                                       15kv   54.2       61.9    64.9      60.5       13.9
                        
(cps/(1nA*1 sec)) / wt.% O              2kV    0.16       0.15    0.14      0.12       0.19
                                        6kV    1.04       1.00    0.92      0.83       0.76
                                       15kv    1.80       1.56    1.38      1.14       0.65
                         
wt.% O (standard is Fe2O3)              2kV   30.63      45.50   49.01     49.56      26.04
                                        6kV   29.63      42.96   48.10     50.71      28.83
                                       15kv   29.59      40.69   47.27     48.34      35.13
                        
delta wt.% O, rel.%                     2kV    1.91      14.61    4.11     -6.94      22.65
                                        6kV   -1.42       8.23    2.17     -4.79      35.75
                                       15kv   -1.54       2.50    0.42     -9.23      65.43
                                                                                         ?   charge   
Fe2O3 was used as a standard for oxygen.
You can see the abnormal behaviour of oxygen intensity on SnO2, as well as huge discrepancies of oxygen determination at any acceleration voltage. The discrepancy for MgO is not so critical, but still questionable. The change of the matrix correction procedure, as well as change the oxygen standard did not help. Oxygen concentration in SnO2 can be measured correctly, if SnO2 is selected as a standard, but in this case the deviation of oxygen measurements for other oxides will be high.

My goal is to develop and experimental set-up such that the oxygen level of a multi-oxide system can be measured accurately. Any advice would be most welcome.

Thank you!

[Probeman]

Hi Rom,
Welcome to the world of low energy analysis!   

Two things you need to incorporate into your quant calculations. Fortunately Probe for EPMA makes this easy.

The first is the use of empirical mass absorption coefficients (MACs). See this topic for details:

https://probesoftware.com/smf/index.php?topic=1340.0

The second is area peak factors (APFs). This topic will introduce you to the APF concepts:

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

A good demonstration of using these parameters is found in a white paper here:

https://epmalab.uoregon.edu/reports/Withers%20hydrous%20glass.pdf

Once you have looked these topics over please let us know if you have further questions. 

[Rom]

Hi Probeman,
Thank you so much for quick reply and useful links.
I'm studying them now. And I have several short questions now:
1) Where can I find XMAC program? Is it application in CalcZAF software?
2) Why do we see the higher oxygen concentration in the samples with high MAC? I suppose, if MAC is high the Ka of oxygen will be heavily adsorbed and resulted oxygen will be lower. Am I right?

[John Donovan]

1) Where can I find XMAC program? Is it application in CalcZAF software?

The XMAC program is shipped with the STRATAGem thin film processing software.

However I don't think you need it as the empirical MAC table in Probe for EPMA already contains all the empirically measured MACs for oxygen emitted in all the matrix elements that you have discussed.

2) Why do we see the higher oxygen concentration in the samples with high MAC? I suppose, if MAC is high the Ka of oxygen will be heavily adsorbed and resulted oxygen will be lower. Am I right?

The higher the MAC, the higher the *absorption* correction (not adsorbed), and therefore the higher the reported concentration.  You should spend a little time to study the physics of matrix corrections.

A good introductory guide is found here:

https://epmalab.uoregon.edu/pdfs/Corrections3%20(Chap%209).pdf

[John Donovan]

Rom: I wanted to double check that all the oxygen absorbers you mentioned in your original post above are contained in the EMPMAC.DAT empirical mass absorption coefficient file in Probe for EPMA and it looks like they are all there:

  "o"     "ka"    "li"       1600         "Bastin  (1992)"
  "o"     "ka"    "b"       8550         "Bastin  (1992)"
  "o"     "ka"    "o"       1200         "Bastin  (1992)"
  "o"     "ka"    "f"       1850         "Love et al. (1974)"
  "o"     "ka"    "ne"       2750         "Love et al. (1974)"
  "o"     "ka"    "na"       3630         "Love et al. (1974)"
  "o"     "ka"    "mg"       5170         "Bastin  (1992)"
  "o"     "ka"    "mg"       5918         "Donovan  (2011) MgO"
  "o"     "ka"    "al"       6720         "Bastin  (1992)"
  "o"     "ka"    "si"       8790         "Bastin  (1992)"
  "o"     "ka"    "p"       9820         "Love et al. (1974)"
  "o"     "ka"    "s"       12400        "Love et al. (1974)"
  "o"     "ka"    "cl"       14300        "Love et al. (1974)"
  "o"     "ka"    "ar"       16100        "Love et al. (1974)"
  "o"     "ka"    "k"       20500        "Love et al. (1974)"
  "o"     "ka"    "ca"       24600        "Love et al. (1974)"
  "o"     "ka"    "sc"       26800        "Love et al. (1974)"
  "o"     "ka"    "ti"       19900        "Bastin  (1992)"
  "o"     "ka"    "ti"       21046        "Donovan  (2011) TiO2"
  "o"     "ka"    "v"        35463        "Donovan  (2011) V2O3"
  "o"     "ka"    "v"        40509        "Donovan  (2011) V2O5"
  "o"     "ka"    "cr"       2900         "Bastin  (1992)"
  "o"     "ka"    "mn"       3470         "Bastin  (1992)"
  "o"     "ka"    "fe"       4000         "Bastin  (1992)"
  "o"     "ka"    "co"       4500         "Bastin  (1992)"
  "o"     "ka"    "ni"       5120         "Bastin  (1992)"
  "o"     "ka"    "cu"       5920         "Bastin  (1992)"
  "o"     "ka"    "zn"       6350         "Bastin  (1992)"
  "o"     "ka"    "ga"       7090         "Bastin  (1992)"
  "o"     "ka"    "y"       15100        "Bastin  (1992)"
  "o"     "ka"    "zr"       16200        "Bastin  (1992)"
  "o"     "ka"    "nb"       17100        "Bastin  (1992)"
  "o"     "ka"    "mo"       18000        "Bastin  (1992)"
  "o"     "ka"    "ru"       19700        "Bastin  (1992)"
  "o"     "ka"    "sn"       15050        "Bastin  (1992)"
  "o"     "ka"    "ba"       4560         "Bastin  (1992)"
  "o"     "ka"    "la"       3600         "Bastin  (1992)"
  "o"     "ka"    "ta"       10600        "Bastin  (1992)"
  "o"     "ka"    "w "       11300        "Bastin  (1992)"
  "o"     "ka"    "pb"       11000        "Bastin  (1992)"
  "o"     "ka"    "bi"       12100        "Bastin  (1992)"
  "o"     "ka"    "u"        8973         "Donovan (2011)"

[Rom]

Hi John,
Thank you so much for quick reply and useful links.

[Rom]

Am I right that:
1. Overstatement of oxygen in SnO2 is the cause of secondary fluorescence.
2. MAC procedure in Probe for EPMA realises correction of theoretical A and F  coefficients in ZAF. At this way MAC bases on experimental depends of intensities from accelerating voltage (X Mac program results). 

[Probeman]

Am I right that:
1. Overstatement of oxygen in SnO2 is the cause of secondary fluorescence.

Hi Rom,
I don't see how.  A single phase can only exhibit self fluorescence.  If I enter SnO2 into CalcZAF I get these correction factors which show no fluorescence:

ELEMENT  ABSCOR  FLUCOR  ZEDCOR  ZAFCOR STP-POW BKS-COR   F(x)u      Ec   Eo/Ec    MACs
   Sn la   .9761  1.0000  1.0743  1.0486  1.1630   .9237   .9007  3.9290  3.8178 371.925
   O  ka  9.6969  1.0000   .7524  7.2961   .6325  1.1897   .0723   .5317 28.2114 18441.6

 ELEMENT   K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL                                       
   Sn la  .00000  .75114  78.764   -----  33.333   1.000   15.00                                       
   O  ka  .00000  .02911  21.236   -----  66.667   2.000   15.00                                       
   TOTAL:                100.000   ----- 100.000   3.000

In fact the absorption correction using the default MAC is quite large and I suspect the "overstatement" you are seeing in the oxygen concentration is due to not using an experimentally measured MAC. So let's see what happens if I select the empirical MAC for oxygen absorbed by Sn. Now we obtain these results:

ELEMENT  ABSCOR  FLUCOR  ZEDCOR  ZAFCOR STP-POW BKS-COR   F(x)u      Ec   Eo/Ec    MACs
   Sn la   .9761  1.0000  1.0743  1.0486  1.1630   .9237   .9007  3.9290  3.8178 371.925
   O  ka  6.0034  1.0000   .7524  4.5171   .6325  1.1897   .1167   .5317 28.2114 12108.8

 ELEMENT   K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL                                       
   Sn la  .00000  .75114  78.764   -----  33.333   1.000   15.00                                       
   O  ka  .00000  .04701  21.236   -----  66.667   2.000   15.00                                       
   TOTAL:                100.000   ----- 100.000   3.000

Note how the absorption correction is lower than before. I suspect you will obtain a better oxygen analysis using this empirical MAC for oxygen absorbed by Sn.

2. MAC procedure in Probe for EPMA realises correction of theoretical A and F  coefficients in ZAF. At this way MAC bases on experimental depends of intensities from accelerating voltage (X Mac program results).

I am not clear on what you are trying to say here.

[Rom]

Hi Probeman,
Thank you so much for informative and quick reply.
1. Could you explain "A single phase can only exhibit self fluorescence"?
 
If I understand correctly, the problem of oxygen concentration overestimation on Sn is due to very high A coefficient in ZAF by default. The empirical MAC coefficient is lower and results will be better.

I carefully looked through  the table of empirical MAC coefficients. It is interesting how to measure MAC of "O "in "O" or "O" in other gases (At, Cl, etc.)

2. Sorry for my English. I meant that MAC correct theoretical A and F coefficients in ZAF correction. And MAC are experimental results obtained at measurement of intensities depending on acceleration voltage (in XMAC software). But we are discussing this question in the question above (#1).

[Probeman]

Hi Probeman,
Thank you so much for informative and quick reply.
1. Could you explain "A single phase can only exhibit self fluorescence"?

Hi Rom,
No worries.

Simply that a single phase can only fluoresce the elements within that phase. And for the phase SnO2 there is no significant self fluorescence for oxygen or Sn as absorbers as shown in the CalcZAF results.
 

If I understand correctly, the problem of oxygen concentration overestimation on Sn is due to very high A coefficient in ZAF by default. The empirical MAC coefficient is lower and results will be better.

Exactly correct.

I carefully looked through  the table of empirical MAC coefficients. It is interesting how to measure MAC of "O "in "O" or "O" in other gases (At, Cl, etc.)

Yes. Empirical MAC measurements of emitters in gases is not performed in the electron microprobe. See Henke for methods to measure these fundamental parameters.

2. Sorry for my English. I meant that MAC correct theoretical A and F coefficients in ZAF correction. And MAC are experimental results obtained at measurement of intensities depending on acceleration voltage (in XMAC software). But we are discussing this question in the question above (#1).

OK, but what is your question?

[Rom]

Thank you a lot. The question #2 is disappeared because I start understanding what MAC in Probe software is.

[Probeman]

Thank you a lot. The question #2 is disappeared because I start understanding what MAC in Probe software is.

Not a problem.

Here is a good topic on assessing accuracy using CalcZAF:

https://probesoftware.com/smf/index.php?topic=508.0

It gets into the issue of mass absorption coefficients (MACs).

[Rom]

Hello, everything good to everyone.
I studied the conceptions of MAC and APF corrections and returned to the problem of incorrect measuring oxygen in simple oxides.
the problem is incorrect measuring oxygen in oxides:
MgO +2 - +4 wt% (depends of MAC coefficient was chosen - we have 2 of them)
SiO2 -3 wt % (MAC using)
SnO +4 wt% (MAC using)
etc.

the standard is Fe2O3 (MAC using), 15kV, 20nA, beam 10mkm, crystal LDE1, 20sec peak position, 10+10sec BG.

If we'll not use MAC correction, the results change but not become correct.

WS's spelled out neatly after PHA setting and performing peak position.
I cannot find any peak overlays on peak/BG positions or peak shiftings which can correct with APF.

So, could you recommend me next step or direction which help me  fight the problem. Lets start from MgO, I think it is the easiest because its APF with Fe2O3 is 1.
Thank you!