- Problems With Light Elements
- Area Peak Factors (APFs)
- Experimental Determinations of APFs for Al2O3 and SiO2
The analysis of light elements (fluorine -beryllium) present special problems for the microprobe analyst.Some of these problems are technical in substance, relating tothe measurement procedure, while others are inherent and are dueto physical and chemical effects. The following is just anoverview of these concerns. For a more complete treatment thereader is referred to (Bastin and Heijligers, 1986) and(Goldstein, et.al., 1981).
Physical and Chemical Effects
1. Low fluorescent yields from these k-linescompounded by a relatively poor reflectivity of analyzingcrystals and low sensitivity of detector systems can result inextremely low count rates. Increasing the beam current canproduce unacceptably high deadtime corrections for any metallines present which may also need to be measured. The use oflayered dispersive element (LDE) reflectors can improve thingssomewhat.
2. Errors in the estimation or accuracy of thetake off angle or operating voltage of the microprobe. Whileslight variations in these parameters normally have a littleeffect on a typical analysis, due to the very high absorptioncorrections of low energy x-rays, the effect on the ZAFcorrection can be much larger for the light elements and cansometimes result is large systematic errors.
3. Errors in the mass absorption coefficients(or MACs) for the x-ray absorption cross sections for theseelements are considerable. See Appendix C for a table of MACs forthe light elements from a variety of sources. There isconsiderable disagreement among investigators. Probe for Windowsallows the user to enter MACs from any source for re-calculationpurposes.
4. Chemical bonding effects can results inlarge peak shift and/or shape changes to the analytical x-raylines. In fact the degree of shift and/or shape changes in lightelement K lines and transition metal L lines can sometimes berelated to the degree of oxidation in the compound. But normallythese effects are a nuisance to the analyst. In a few instancesthere has even been documented cases of particular lines beingstrongly enhanced (Ni La in NiAl) or reduced (B Ka in nickel borides) (Pouchouand Pichoir, 1988).
5. Volatilization of certain light elementssuch as sodium and fluorine is often the case in glasses andother amorphous samples. See the use of the Volatile elementcorrection for a powerful software correction to remove thiseffect.
Problems with the Measurement Techniques
1. Higher order interferences from metal linesare everywhere and must be avoided when possible or corrected forwhen not. The elements Ti, Cr, Mn, Zr and Nb are well known inthis respect. Although it would seem appropriate to usepulse-height analysis (PHA) to reduce these interferences it isoften found that narrow PHA settings can introduce more problemsthan they solve. See (Donovan, et.al., 1993). These interferencesare especially severe for minor and trace level measurements.
2. Carbon contamination is a serious problemfor quantitative analysis of light elements, especially ofcourse, carbon but also the other light elements. Contaminationis produced by the polymerization of hydrocarbons at the point ofimpact of the electron beam. Carbon compounds present may includediffusion pump oils, polishing agents, cleaning solvents, amongothers. The use of an air-jet has been shown to dramaticallyreduce carbon contamination. It is suggested that the analystperform measurements to ascertain the actual carbon contaminationrate in their microprobe for each element being analyzed.
3. Trace analysis of light elements isparticularly difficult, especially oxygen and carbon. Keeping ametallic sample surface from oxidizing is not a trivial matter.The use of an ion mill inside the probe to remove the very toplayer of the sample, just prior to the analysis, if available,would be ideal. Note that the increase in temperature ofnon-conducting samples can be significant enough to causeadditional oxidation of the surface. Trace carbon analyses can bedifficult without the use of an air jet to reduce the carboncontamination at the point of impact of the beam.
4. Another problem which has not beenadequately addressed is x-ray production in non-conductivesamples. Effects on x-ray production are seen even when aconductive coating has been applied, possibly due to internalcharge buildup below the conductive layer.
5. The selection of standards is also criticaldue to the already mentioned peak shift and/or shape changes forlight element x-rays. The use of Area Peak Factors (APFs) (Bastinand Heijligers, 1986) can provide the analyst with a practicalmethod for treatment of these effects. Program PROBE supports theuse of APF factors for analysis of light elements.
6. Be aware also, that carbide, boride andnitride standards are often not 100 % dense resulting insignificant errors in the analysis. If single crystals can beobtained, so much the better.
G. F. Bastin and H. J. M. Heijligers,"Quantitative Electron Probe Microanalysis of Carbon inBinary Carbides," Parts I and II, X-Ray Spectr. 15: 135-150,1986
J. I. Goldstein, D. E. Newbury, P. Echlin, D.C. Joy, C. Fiori, E. Lifshin, "Scanning Electron Microscopyand X-Ray Microanalysis", Plenum, New York, 1981
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This menu allows the user to selectively loadempirical APFs (Area Peak Factors) from a supplied ASCII file ordata entry.
A powerful feature in Probe for Windows allowsthe user to select an Area-Peak-Factor (APF) correction for usein correcting x-ray intensities for peak shift and shape changesbetween the standard and the unknown. This can occur especiallywith Kalines of the light elements such as oxygen, nitrogen, carbon andboron. With these elements, differences in the bond energiesbetween say TiC and SiC will produce significantly different peakshapes resulting in systematic errors in the analysis of thesematerials when only the peak x-ray intensities are used.
For example, when analyzing oxygen and usingMgO or Fe2O3 as an oxygen standard, analyses of SiO2 will produceerrors up to about 4.5%. This effect is independent of the matrixcorrection and can be corrected for only by the use of theappropriate APFs.
The APF concept was proposed by Bastin andHeijligers (Bastin and Heijligers, 1986) to provide a simplecorrection scheme for this effect. It basically requiresmeasuring the ratio of the peak intensity to the integratedintensity between a primary standard to a number of secondarystandards on the light element spectrometer used. The wavelengthscan feature in Probe for Windows can be used to acquire the peakshape profiles. After correcting for background and removal ofextraneous peaks from interfering lines, the APF can becalculated. The calculation of the APF factor is shown below :
is the integrated intensity of the secondary standard
is the peak intensity of the primary standard
is the peak intensity of the secondary standard
is the integrated intensity of the primary standard
The peak shape and shift of light element peakscan be easily demonstrated using MgO and SiO2 standard samples.First select a suitable layered dispersive element (LDE)analyzing crystal for oxygen Ka analysis. Tune the spectrometer using the MgO standardand acquire a standard sample for MgO. Assign MgO as thecalibration standard for oxygen. Then move to the SiO2 standardand acquire another quantitative standard sample on the SiO2sample. Note that the analysis of SiO2 using MgO as a calibrationstandard will result in a low total for the SiO2, somewherearound 96-97 %. This is not an error in the mass absorptioncoefficient (MAC), and no attempt should be made to correct thisproblem by entering an arbitrary MAC value.
Instead, the proper APF correction will need tobe applied as shown below. Although the values below are relativeto Fe2O3, you will note that the APF for MgO is 1.000, whichmeans that these correction factors apply equally well relativeto MgO. In the Oxygen Area Peak Factor table below you will notethat oxygen in the presence of SiO2 needs to be corrected by afactor of 1.0444 when MgO or Fe2O3 is used a a calibrationstandard. After entering this APF correction factor for theoxygen channel and re-analyzing, note that the total is now closeto 100 %.
It is very important to note that the APFvalues selected or entered are always measured relative to somestandard sample. For example, if measuring C Ka and using Fe3C asthe primary standard for carbon, then any C Ka APF values usedmust be those measured relative to Fe3C. For the same reason, ifusing APF values for a particular (light) element and one decidesto re-assign the standard for that element, the APF values forthat element must also be changed to reflect the standardre-assignment. See the section above for information onre-calculating the APF values relative to another standard.
For the above reason it is usually mostefficient to simply always use the same standard for each lightelement analyzed. Typically (in order to utilize the APF valuesin the supplied EMPAPF.DAT file) these will be :
- Oxygen : MgO or Fe2O3
- Nitrogen : AlN
- Carbon : Fe3C
- Boron : B metal
The APF correction in Probe for Windows willallow the user to enter empirical APF values in each run. Theuser may enter one or more APF factors for each emitting elementalthough they are generally applied to soft x-ray lines. Notehowever that even S Ka exhibits peak shift and shape changes when comparingsulfide and sulfate peaks. The APF for each absorber will besummed according to it's weight fraction in the composition andapplied to the emitting element counts during the ZAF orPhi-Rho-Z iteration.
If it is desired to correct a complete unknownfor peak shape changes, simply perform a wavelength scan on thestandard being used for the light element emitter (oxygen,carbon, etc.) and also on the unknown. Calculate the APF usingthe expression above and from the Analytical | Empirical APFsmenu, enter the same APF for all absorbers in the unknown. Theprogram will then sum the APFs which will be the same as thesingle measured APF. Note that this APF may only apply to asingle composition. One can perform additional wavelength scanson other samples to confirm this however.
The APF correction values are defined in theEMPAPF.DAT file in the XRAYDATA sub directory (usuallyC:\PROBEWIN\XRAYDATA). The file contains some 50 or 60 valuesthat may or may not be applicable, depending on the analyzingcrystals and standards available. The user may edit the fileusing any ASCII text editor such as NotePad or NoteBook to inserttheir own measurements. Be careful to avoid adding any<tab> characters when editing the file (use space or commadelimited values only). Note that the elements may be in anyorder although they have been sorted to facilitate editing.
The format of the EMPAPF.DAT file is shownbelow :
"b" "ka" "c"1.02 "B4C/B/STE"
"b" "ka" "n" 1.2"BN/B/STE"
"b" "ka" "al"1.12 "AlB2/B/STE"
"b" "ka" "al"1.01 "AlB12/B/STE"
"b" "ka" "si" 1"SiB3/B/STE"
"b" "ka" "si".92 "SiB6/B/STE"
"b" "ka" "ti".75 "TiB/B/STE"
"b" "ka" "ti".88 "TiB2/B/STE"
"b" "ka" "v" 1"VB2/B/STE"
"b" "ka" "cr" .9"CrB/B/STE"
"b" "ka" "cr"1.1 "CrB2/B/STE"
"b" "ka" "fe"1.1 "FeB/B/STE"
"b" "ka" "fe"1.25 "Fe2B/B/STE"
"b" "ka" "co"1.2 "CoB/B/STE"
"b" "ka" "co"1.02 "Co2B/B/STE"
"b" "ka" "ni"1.2 "NiB/B/STE"
"b" "ka" "ni"1.06 "Ni2B/B/STE"
"b" "ka" "ni".98 "Ni3B/B/STE"
"b" "ka" "zr" .8"ZrB2/B/STE"
"b" "ka" "nb" .8"NbB/B/STE"
"b" "ka" "nb" .9"NbB2/B/STE"
"b" "ka" "mo".94 "MoB/B/STE"
"b" "ka" "la" .9"LaB6/B/STE"
"b" "ka" "ta".88 "TaB/B/STE"
"b" "ka" "ta"1.1 "TaB2/B/STE"
"b" "ka" "w" .98"WB/B/STE"
"b" "ka" "u"1.04 "UB4/B/STE"
"c" "ka" "b"1.01 "B4C/Fe3C/WSi/59.8"
"c" "ka" "si".933 "SiC/Fe3C/WSi/59.8"
"c" "ka" "ti".868 "TiC/Fe3C/WSi/59.8"
"c" "ka" "v".873 "V2C/Fe3C/WSi/59.8"
"c" "ka" "v".873 "VC/Fe3C/WSi/59.8"
"c" "ka" "cr" .8"Cr7C3/Fe3C/STE"
"c" "ka" "cr".83 "Cr3C2/Fe3C/STE"
"c" "ka" "cr" .8"Cr23C6/Fe3C/STE"
"c" "ka" "zr".88 "ZrC/Fe3C/WSi/59.8"
"c" "ka" "nb".79 "NbC/Fe3C/STE"
"c" "ka" "mo".82 "Mo2C/Fe3C/STE"
"c" "ka" "hf".83 "HfC/Fe3C/STE"
"c" "ka" "ta".96 "TaC/Fe3C/STE"
"c" "ka" "w" .97"WC/Fe3C/STE"
"c" "ka" "w"1.02 "W2C/Fe3C/STE"
"n" "ka" "si"1.103 "Si3N4/AlN/WSi/59.8"
"n" "ka" "ti".997 "TiN/AlN/WSi/59.8"
"n" "ka" "v"1.0226 "VN/AlN/WSi/59.8"
"n" "ka" "cr"1.018 "Cr2N/AlN/WSi/59.8"
"n" "ka" "fe"1.012 "Fe2N/AlN/WSi/59.8"
"n" "ka" "zr".9952 "ZrN/AlN/WSi/59.8"
"n" "ka" "hf"1.002 "HfN/AlN/WSi/59.8"
"o" "ka" "b"1.0628 "B6O/Fe2O3/WSi/59.8"
"o" "ka" "mg" 1"MgO/Fe2O3/WSi/59.8"
"o" "ka" "al"1.0213 "Al2O3/Fe2O3/WSi/59.8"
"o" "ka" "si"1.0444 "SiO2/Fe2O3/WSi/59.8"
"o" "ka" "ti".9796 "TiO2/Fe2O3/WSi/59.8"
"o" "ka" "cr".993 "Cr2O3/Fe2O3/WSi/59.8"
"o" "ka" "mn"1.0121 "MnO/Fe2O3/WSi/59.8"
"o" "ka" "fe".9962 "Fe3O4/Fe2O3/WSi/59.8"
"o" "ka" "co"1.0133 "CoO/Fe2O3/WSi/59.8"
"o" "ka" "ni"1.0153 "NiO/Fe2O3/WSi/59.8"
"o" "ka" "cu".9946 "Cu2O/Fe2O3/WSi/59.8"
"o" "ka" "cu".9943 "CuO/Fe2O3/WSi/59.8"
"o" "ka" "zn".9837 "ZnO/Fe2O3/WSi/59.8"
"o" "ka" "ga" 1"Ga2O3/Fe2O3/WSi/59.8"
"o" "ka" "zr".9823 "Y3Fe5O12/Fe2O3/WSi/59.8"
The first column (in the example is"b" for Boron) is the atomic symbol of the emittingelement. The second column ("ka" in the example equalsKa) isthe x-ray line of the emitter. The third column is the atomicsymbol of the absorber element. The fourth column is the actualarea peak factor (APF) that has been experimentally measured. Thelast parameter is a string that contains the conditions underwhich the APF was measured. For example, the comment"B4C/B/STE" indicates that the peak and integratedintensities for the APF were measured on a B4C (boron carbide)sample, relative to a B (elemental Boron) primary standard, usinga Pb Stearate analyzer. The reason that this information isneeded is because the APF is a relative measurement (to astandard) and the value is dependent on the resolution (crystaltype) used. If a LDE (layered dispersive element) analyzer isused, the 2d of the analyzer should be indicated also as seen inthe last two lines of the example.
The first and third columns (emitting atomicsymbol and absorber atomic symbol) must be valid element symbols.The second column (x-ray line) must be "ka","la" or "ma". All element and x-ray symbolsmust be enclosed in double quotes. The fourth column (APF value)must be a real number greater than 0.0. The comment string mustbe enclosed in double quotes.
These values can then be loaded from theAnalytical | Empirical APFs menu item in Probe for Windows. Notethat the comments listed after each APF value can be used toindicate the experimental conditions under which the APF wasmeasured. For consistency sake, the formula of the compoundmeasured is listed first, then the primary reference standardused and finally the analyzing crystal type (and 2d spacing ifdesired) used. Edit the EMPAPF.DAT file in the XRAYDATA subdirectory to add additional APFs from your own measurements.
The following is a short listing of sometypical APFs taken from Bastin and Heijligers (1986). Because thefactors are spectrometer and crystal dependent these should beused as a guide only.
Oxygen Area Peak Factors (APF)
Relative to Fe2O3 or MgOusing W/Si LDE (2d = 59.8)
Area-Peak Factor (APF)
Carbon Area-Peak Factors
Relative to Fe3C using W/SiLDE (2d = 59.8):
Nitrogen Area-Peak Factors(Video) Dark & Light Element Analysis w/ Knight Mare & Knight Light
Relative to AlN using W/SiLDE (2d = 59.8):
Area Peak Factors and Standard Assignments
Another consideration in assigning standards,concerns the use of Area Peak Factors (APF) for correction oflight element peak shift and shape changes. When analysis oflight elements is to be performed (typically oxygen, nitrogen,carbon and boron but possibly heavier elements such as silicon,aluminum and sulfur), you need to plan the use of APF correctionsfor the most accurate results. The actual APF values selectedwill depend the standard assigned for calibration of the lightelement.
For example, when analyzing for C Ka and using Fe3C asyour carbon standard, your APF value for C Ka in a Cr7C3 matrix might be0.80. However, if instead, you re-assigned your carbon standardto TiC, you must use APF values relative to TiC rather than Fe3C.If they are available in the Empirical APF database (EMPAPF.DAT),just change the APF values for C Ka. Otherwise you would needto calculate the Fe3C APF values relative to TiC. Some typicalFe3C APF values for C Ka, recalculated relative to TiC and Cr7C3, are shown inthe table below :
|APF Relative To :||Fe3C (APF/1.000)||TiC (APF/0.868)||Cr7C3 (APF/0.80)|
G. F. Bastin and H. J. M. Heijligers,"Quantitative Electron Probe Microanalysis of Carbon inBinary Carbides," Parts I and II, X-Ray Spectr. 15: 135-150,1986
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In an attempt to verify Bastin's published oxygen Ka APF values for Al2O3 and SiO2 using thesame model instrument (SX51) and analyzing crystal (W/Si 60 A2d), I ran a series of wavescans on MgO (standard for oxygen),Al2O3 and SiO2. Four wavescans were acquired on each standard at15 keV, 40 nA, 10 um beam and 200 points with 15 second counttime each point using a 1 um stage step every 60 sec.
Using the equation P/I(std) * I/P(unk) to obtain the APF, thefollowing calculations were performed (P/I is the peak intensitydivided by the integrated intensity and I/P is the integratedintensity divided by the peak intensity) :
|MgO||Al2O3||Al2O3 APF||SiO2||SiO2 APF|
|(I/P)||1050.8||1081||(1.0213 Bastin)||1124.5||(1.0444 Bastin)|
As you can see, my average APF for Al2O3 relative to MgO is1.0285, while Bastin got 1.0213. For SiO2 I got 1.070 whileBastin got 1.0444. This measurements illustrate the importance ofcalibrating APFs on the actual instument that they will be usedon.
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