Quantitative Analysis of Oxygen (and other light elements)

[Probeman]

This topic is for questions and suggestions regarding quantitative light element analysis in Probe for EPMA, mostly oxygen but we could include nitrogen and boron also if it is desirable.  For carbon quantitative analysis (and carbon contamination issues) see this post instead:

http://probesoftware.com/smf/index.php?topic=48.0

In the meantime here are some documents to start with (attached) that may provide some useful information. The abstract by John Fournelle is a good starting place because almost always Ovonics gets the 2d of their LDE or PC crystals wrong due to the fact that they do not include the refractive index of the Bragg crystal in their calculations. 

This can create confusion when trying to identify interferences from higher order lines unless it is corrected by the instrument operator.

[Probeman]

Another aspect of this, when analyzing oxygen in minerals and oxides, is that one can have the program calculate the "predicted" stoichiometric oxygen and then have the program subtract that from the measured oxygen and see what the excess or deficit oxygen actually is. 

Obviously this gets more difficult when Fe is present due to its multiple oxidation states, but if Fe is the only significant element with multiple oxidation states that might be OK to try to bring the excess/deficit oxygen to zero by modifying the Fe cation and oxygen ratio as seen here:

St  395 Set   1 Magnetite U.C. #3380
TakeOff = 40.0  KiloVolt = 15.0  Beam Current = 30.0  Beam Size =    2
(Magnification (analytical) =   4000),        Beam Mode = Analog  Spot
(Magnification (default) =     3200, Magnification (imaging) =    100)

Port Henry, NY
FeO=30.93% (ISE Carmichael)
Fe2O3=68.85%, FeO=30.92% (as FeO=92.73% + 6.90% O)
(Total FeO=92.73%, by EPMA, JJD)
Number of Data Lines:   3             Number of 'Good' Data Lines:   3
First/Last Date-Time: 09/30/2007 08:07:17 PM to 09/30/2007 08:08:45 PM

Average Total Oxygen:       27.661     Average Total Weight%:  100.054
Average Calculated Oxygen:  20.889     Average Atomic Number:   20.987
Average Excess Oxygen:       6.772     Average Atomic Weight:   33.014
Average ZAF Iteration:        4.00     Average Quant Iterate:     2.00

St  395 Set   1 Magnetite U.C. #3380, Results in Elemental Weight Percents
 
ELEM:       Fe       O       H      Al      Mg      Mn
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    10.00   10.00
BEAM:    29.99   29.99

ELEM:       Fe       O       H      Al      Mg      Mn   SUM 
     4  71.957  27.474    .000    .201    .072    .054  99.757
     5  72.300  27.712    .000    .201    .072    .054 100.339
     6  71.941  27.797    .000    .201    .072    .054 100.065

AVER:   72.066  27.661    .000    .201    .072    .054 100.054
SDEV:     .203    .168    .000    .000    .000    .000    .291
SERR:     .117    .097    .000    .000    .000    .000
%RSD:      .28     .61     .00     .00     .00     .00

PUBL:   72.080  27.803    n.a.    .201    .072    .054 100.210
%VAR:   (-.02)    -.51     ---     .00     .00     .00
DIFF:   (-.01)   -.142     ---    .000    .000    .000
STDS:      395     377       0       0       0       0

STKF:    .6779   .2132   .0000   .0000   .0000   .0000
STCT:   6749.8  2131.2      .0      .0      .0      .0

UNKF:    .6779   .1944   .0000   .0000   .0000   .0000
UNCT:   6749.8  1943.5      .0      .0      .0      .0
UNBG:     84.6    42.6      .0      .0      .0      .0

ZCOR:   1.0630  1.4229   .0000   .0000   .0000   .0000
KRAW:   1.0000   .9119   .0000   .0000   .0000   .0000
PKBG:    80.86   46.59     .00     .00     .00     .00

St  395 Set   1 Magnetite U.C. #3380, Results in Oxide Weight Percents

ELEM:      FeO       O     H2O   Al2O3     MgO     MnO   SUM 
     4  92.572   6.616    .000    .380    .119    .070  99.757
     5  93.014   6.756    .000    .380    .119    .070 100.339
     6  92.552   6.944    .000    .380    .119    .070 100.065

AVER:   92.713   6.772    .000    .380    .119    .070 100.054
SDEV:     .261    .165    .000    .000    .000    .000    .291
SERR:     .151    .095    .000    .000    .000    .000
%RSD:      .28    2.43     .00     .00     .00     .00

PUBL:   92.731   6.899    n.a.    .380    .119    .070 100.210
%VAR:   (-.02)   -1.84     ---     .00     .00     .00
DIFF:   (-.02)   -.127     ---    .000    .000    .000
STDS:      395     377       0       0       0       0

Above the Fe is calculated as FeO, so in magnetite (Fe3O4), we see an excess of oxygen. Now by adjusting the Fe cations and oxygens as seen here from the Elements/Cations button in Analyze! we can set them to any value between 0 and 99:



or here as Fe3O4:



Now our output looks like this:

St  395 Set   1 Magnetite U.C. #3380
TakeOff = 40.0  KiloVolt = 15.0  Beam Current = 30.0  Beam Size =    2
(Magnification (analytical) =   4000),        Beam Mode = Analog  Spot
(Magnification (default) =     3200, Magnification (imaging) =    100)

Port Henry, NY
FeO=30.93% (ISE Carmichael)
Fe2O3=68.85%, FeO=30.92% (as FeO=92.73% + 6.90% O)
(Total FeO=92.73%, by EPMA, JJD)
Number of Data Lines:   3             Number of 'Good' Data Lines:   3
First/Last Date-Time: 09/30/2007 08:07:17 PM to 09/30/2007 08:08:45 PM

Average Total Oxygen:       27.661     Average Total Weight%:  100.054
Average Calculated Oxygen:  27.771     Average Atomic Number:   20.987
Average Excess Oxygen:       -.110     Average Atomic Weight:   33.014
Average ZAF Iteration:        4.00     Average Quant Iterate:     2.00

St  395 Set   1 Magnetite U.C. #3380, Results in Elemental Weight Percents
 
ELEM:       Fe       O       H      Al      Mg      Mn
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    10.00   10.00
BEAM:    29.99   29.99

ELEM:       Fe       O       H      Al      Mg      Mn   SUM 
     4  71.957  27.474    .000    .201    .072    .054  99.757
     5  72.300  27.712    .000    .201    .072    .054 100.339
     6  71.941  27.797    .000    .201    .072    .054 100.065

AVER:   72.066  27.661    .000    .201    .072    .054 100.054
SDEV:     .203    .168    .000    .000    .000    .000    .291
SERR:     .117    .097    .000    .000    .000    .000
%RSD:      .28     .61     .00     .00     .00     .00

PUBL:   72.080  27.803    n.a.    .201    .072    .054 100.210
%VAR:   (-.02)    -.51     ---     .00     .00     .00
DIFF:   (-.01)   -.142     ---    .000    .000    .000
STDS:      395     377       0       0       0       0

STKF:    .6779   .2132   .0000   .0000   .0000   .0000
STCT:   6749.8  2131.2      .0      .0      .0      .0

UNKF:    .6779   .1944   .0000   .0000   .0000   .0000
UNCT:   6749.8  1943.5      .0      .0      .0      .0
UNBG:     84.6    42.6      .0      .0      .0      .0

ZCOR:   1.0630  1.4229   .0000   .0000   .0000   .0000
KRAW:   1.0000   .9119   .0000   .0000   .0000   .0000
PKBG:    80.86   46.59     .00     .00     .00     .00

St  395 Set   1 Magnetite U.C. #3380, Results in Oxide Weight Percents

ELEM:    Fe3O4       O     H2O   Al2O3     MgO     MnO   SUM 
     4  99.444   -.255    .000    .380    .119    .070  99.757
     5  99.919   -.148    .000    .380    .119    .070 100.339
     6  99.422    .074    .000    .380    .119    .070 100.065

AVER:   99.595   -.110    .000    .380    .119    .070 100.054
SDEV:     .281    .168    .000    .000    .000    .000    .291
SERR:     .162    .097    .000    .000    .000    .000
%RSD:      .28 -152.92     .00     .00     .00     .00

PUBL:   99.614    .016    n.a.    .380    .119    .070 100.210
%VAR:   (-.02) -805.91     ---     .00     .00     .00
DIFF:   (-.02)   -.125     ---    .000    .000    .000
STDS:      395     377       0       0       0       0


To assist in this process Paul Carpenter sent us this spreadsheet (see attached).

[Probeman]

Here's a wonderful example of higher order interferences (though not from a LDE or PC xtal Bragg spectrometer), from Paul Carpenter.

[John Donovan]

This post describes how to treat the excess measured oxygen as bonded to OH or H2O:

http://probesoftware.com/smf/index.php?topic=61.msg229#msg229

Suggested by Ed Vicenzi, it allows essentially one to specify hydrogen by stoichiometry to the calculated excess oxygen after accounting for all stoichiometric oxygen from the cation and oxygen formula ratios. The paper attached to the post also describes using the TDI correction and also the analysis of boron in Kernite.

These methods are very useful for hydrated, beam sensitive samples when oxygen must be measured.

[Probeman]

In the meantime here are some documents to start with (attached) that may provide some useful information. The abstract by John Fournelle is a good starting place because almost always Ovonics gets the 2d of their LDE or PC crystals wrong due to the fact that they do not include the refractive index of the Bragg crystal in their calculations. 

This can create confusion when trying to identify interferences from higher order lines unless it is corrected by the instrument operator.

I hate to quote myself but John Fournelle just sent me his complete PPT on this refractive index determination for LDE crystals. See attached below.

Also see this topic on determinations of the refractive index values for different light element crystals:

http://probesoftware.com/smf/index.php?topic=375.0

[Probeman]

I thought that some people might be interested in the so called "sub atmospheric differentially pumped flow detector for dramatically improving light element sensitivity in Ar flow detectors. See attached pdf (must be logged in to see).

When I was at Berkeley I implemented this system on my ancient EPMA using pure propane gas, a micrometer needle valve and an old spare mechanical vacuum pump, and saw a significant improvement in sensitivity especially for N Ka, which was our primary concern at that time.

[Michael Lance]

Hello,

I am getting the following warning when measuring C:
 
WARNING in ZAFSetZAF- the f(x) of C ka is .3917. 
 
What does this mean? My quant is also not what we expected. I used MAN or EDS for all the other elements in this steel sample but I used traditional peak/off-peak measurements for carbon. I am also doing an interference corrections for Cr over C.
 
Thanks

[Probeman]

Hi Michael,
The f(x) term is called "f of chi" and is the inverse of the absorption correction. Your value indicates that there is a significant absorption correction, so that simply means that you might want to look at the MAC value for C ka by the various absorbers in your system, i.e., Fe, Cr and Ni.

I would refer to the "Empirical MACs" dialog in PFE and see how those differ from the default values you are using from the default MAC tables.

But if the carbon is in trace amounts, the background corrections, carbon contamination and spectral interferences will be your main concerns. Here are some other topics to look at:

Trace carbon and contamination:

https://probesoftware.com/smf/index.php?topic=933.0

Also consider the use of a blank standard for obtaining a zero carbon measurement:

https://probesoftware.com/smf/index.php?topic=1005.0

Basically there are two traditional approaches to trace carbon: First the use of multiple trace carbon standards and a calibration curve (e.g., Nippon Steel and Caterpiller). Second the use of an MAN curve type background correction (e.g., Pinard and Donovan). 

Both methods are available in Probe for EPMA in addition to a blank correction method.

Let me know if you would like me to expand on any of these topics.

[Probeman]

If you are attempting to measure trace carbon, you should also doing some carbon contamination tests on your instrument:

https://probesoftware.com/smf/index.php?topic=646.0

https://probesoftware.com/smf/index.php?topic=140.0

This topic is on TDI scanning which is the best method I know of for quant trace carbon:

https://probesoftware.com/smf/index.php?topic=933.0