Interference on Mn-L Line?

[Dominik]

Hello,

we are trying to setup a measurement program at low voltages (10 kV/5 kV) to analyse small mineral grains. So we use L-lines for elements such as Ti, Mn, Cr or Fe. There is one problem with Mn: in an ol standard with Mn below the d.l. (for L-lines), Mn appears with almost 4 wt%. I cannot find a sensible interference to explain this. The Mg SKA4 and 5 lines are close, but imho not close enough. When I measure a mineral with higher Mn-concentration (20 or 40 wt%), I obtain perfect Mn results. The count rates for Mn are really low – maybe that is a problem?
I attach a couple of screenshots for illustration.

 Thanks,  Dominik

[Probeman]

we are trying to setup a measurement program at low voltages (10 kV/5 kV) to analyse small mineral grains. So we use L-lines for elements such as Ti, Mn, Cr or Fe. There is one problem with Mn: in an ol standard with Mn below the d.l. (for L-lines), Mn appears with almost 4 wt%. I cannot find a sensible interference to explain this. The Mg SKA4 and 5 lines are close, but imho not close enough. When I measure a mineral with higher Mn-concentration (20 or 40 wt%), I obtain perfect Mn results. The count rates for Mn are really low – maybe that is a problem?

Using the Standard application (included with the free CalcZAF software) I selected an olivine standard (USNM 2566), edited the Mn line from Ka to La, changed the crystal from LiF to TAP, and selected the Misc Options | Interferences menu dialog and calculated the possible interferences:



The reported composition for this standard is:

St  474 Olivine USNM 2566 (Fo 83) Springwater
TakeOff = 40.0  KiloVolt = 15.0  Density =  3.300  Type = olivine

Analysis (wet chemistry) by Gene Jarosewich
Oxide and Elemental Composition

Average Total Oxygen:       41.817     Average Total Weight%:   99.470
Average Calculated Oxygen:  41.820     Average Atomic Number:   12.535
Average Excess Oxygen:       -.003     Average Atomic Weight:   21.723

ELEM:     SiO2     FeO     MgO     MnO   Cr2O3       O
XRAY:      ka      ka      ka      la      ka      ka
OXWT:   38.951  16.620  43.582    .300    .020   -.003
ELWT:   18.207  12.919  26.281    .232    .014  41.817
ATWT:   28.086  55.847  24.305  54.938  51.996  16.000
KFAC:    .1310   .1099   .1758   .0007   .0001   .2366
ZCOR:   1.3901  1.1758  1.4952  3.1287  1.1083  1.7678
AT% :   14.157   5.052  23.615    .092    .006  57.078
24 O:    5.953   2.124   9.929    .039    .002  24.000

The results from the (nominal) interference calculations are:

On Peak Interferences for : St  474 Olivine USNM 2566 (Fo 83) Springwater

For Si ka    TAP at  7.14182 angstroms, at an assumed concentration of 18.207 wt.%
  Interference by Fe SKB      IV    at  6.98090 ( 75.9239) ( -1.7502) =       .1%
  Interference by Fe SKB``    IV    at  6.99440 ( 76.0708) ( -1.6034) =       .2%
  Interference by Fe KB3      IV    at  7.02780 ( 76.4340) ( -1.2401) =      1.2%
  Interference by Fe KB1      IV    at  7.02780 ( 76.4340) ( -1.2401) =      2.1%
  Interference by Fe SKB`     IV    at  7.03850 ( 76.5504) ( -1.1237) =       .2%
  Interference by Fe SKBN     IV    at  7.13340 ( 77.5825) ( -.09161) =       .2%
  On Peak Position   -------------  at  7.14182 ( 77.6741)

For Fe ka    LiF at  1.93746 angstroms, at an assumed concentration of 12.919 wt.%
  On Peak Position   -------------  at  1.93746 ( 134.724)

For Mg ka    TAP at  9.91164 angstroms, at an assumed concentration of 26.281 wt.%
  Interference by Fe KA1      V     at  9.68180 ( 105.299) ( -2.4997) =      1.3%
  Interference by Fe KA2      V     at  9.70130 ( 105.511) ( -2.2877) =       .9%
  On Peak Position   -------------  at  9.91164 ( 107.799)

For Mn la    TAP at  19.4925 angstroms, at an assumed concentration of 0.2323353 wt.%
  Interference by Mg SKB`     II    at  19.3330 ( 210.265) ( -1.7347) =       .8%
  Interference by Mg SKBN     II    at  19.3450 ( 210.395) ( -1.6042) =      1.3%
  Interference by Mg SKA9     II    at  19.3950 ( 210.939) ( -1.0604) =      6.4%
  Interference by Mg SKA6     II    at  19.4560 ( 211.603) ( -.39694) =     19.0%
  Interference by Mg SKA7     II    at  19.4850 ( 211.918) ( -.08153) =     22.5%
  On Peak Position   -------------  at  19.4925 ( 212.000)
  Interference by Mg SKA5     II    at  19.5080 ( 212.168) ( .168610) =     21.9%
  Interference by Mg SKA8     II    at  19.5370 ( 212.484) ( .484024) =     17.4%
  Interference by Mg SKA4     II    at  19.6160 ( 213.343) ( 1.34322) =     24.1%
  Interference by Mg SKA3     II    at  19.6490 ( 213.702) ( 1.70213) =      7.1%
  Interference by Mg SKA`     II    at  19.6950 ( 214.202) ( 2.20242) =       .3%

For Cr ka    LiF at  2.29113 angstroms, at an assumed concentration of 0.014 wt.%
  On Peak Position   -------------  at  2.29113 ( 159.317)

For  O ka  WSi60 at  23.9756 angstroms, at an assumed concentration of 41.817 wt.%
  On Peak Position   -------------  at  23.9756 ( 110.052)

Note the interference of Mn La by the Mg Ka (II), second order line.  That's probably your problem.  You'll want to apply the quantitative interference correction in Probe for EPMA using an interference standard that contains Mg, but no Mn, say MgO:

https://probesoftware.com/smf/index.php?topic=69.0

Alternatively you could utilize the "low overvoltage" method for reducing the size of the analytical volume. For example analyzing the Mn Ka line just above the edge energy of the Mn K edge, which is 6.539 keV, so say using a beam energy of 7 or 8 keV.

This also has the benefit of not only reducing the interaction volume, but one still retains Ka quantitative accuracy for the matrix correction, not to mention avoiding peak shape and shift issues. See here for more tips on improving spatial resolution:

https://probesoftware.com/smf/index.php?topic=1354.0

Note that you can quickly model interaction volume sizes using this dialog in CalcZAF:

https://probesoftware.com/smf/index.php?topic=86.msg309#msg309

though for rigorous modeling of the interaction volume you'll want to use a good Monte Carlo software such as Casino or Penepma.

[Dominik]

Thanks – I also thought about the MgKa(ii) and used a window in the PHA. Maybe this is insufficient and some X-rays still penetrate?
I am a bit hesitant regarding an interference correction, given the very low counts. We are trying at 10 kV and 5 kV. But, right, it might be worth trying what counts we get with the 'low overvoltage' method.

[Probeman]

Thanks – I also thought about the MgKa(ii) and used a window in the PHA. Maybe this is insufficient and some X-rays still penetrate?
I am a bit hesitant regarding an interference correction, given the very low counts. We are trying at 10 kV and 5 kV. But, right, it might be worth trying what counts we get with the 'low overvoltage' method.

PHA window filtering is not completely effective, especially at low energies.

Note that the iterated quantitative interference correction in Probe for EPMA  is completely accurate even for 1000% percent overlaps on trace elements.  See the original paper for an example of trace Co interference correction by Fe Kb in an alloy:

https://epmalab.uoregon.edu/publ/Improved%20Interference%20(Micro.%20Anal,%201993).pdf