Dear all,
I have just tried to analyze galena on a flat polished sample (until 1/4 micrometers abrasive size).
I said "tried" because I have been a bit surprised by the results. I used to have fun analyzing pyrite and finding the perfect stoechiometry, but this time nothing in common. Not only do I never find PbS, but there is also a peak corresponding to Al whereas the galena is optically fine. Is there a problem related to the fact that Pb and S peaks are almost superimposed (sum peak?) Why is Al analyzed? I have really paied a great care to sample preparation, I am working in the optimized geometry (WD and so on) for analysis, DT around 15, but I don't analyze galena correctly. Any feedback would be greatly appreiated. THanks in advance.
I am not a specialist of EDX/EDS , I just discovered the technique a few years ago and used it for the determination of the sulfur/metal ratio in a polymer powder. I found that using powders may be a simple approach to get a correct (1-2% uncertainty) element ratio. The first result were publishes in Anal. Chem. 2019. Did you try to use synthetic or pure PbS as a test ? I am just curious...
Of course it is possible to find Al if you polish your specimen with alumina, especially if your specimen is somewhat porous or soft. As was said by @ M. Farooq Wahab you may want to check for Al on unpolished (fractured) surface.
It is better to use certified standards if you want better results.
And I did not understand about not finding PbS. Did you analyse M-line for Pb? (accelerating voltage of 20 kV is needed)
Dear all, many thanks for replies and sorry for the delay. I have performed standardless analysis, and I am surprised because as explained above I did not use any further calibration for pyrite some time ago, without any problem. Yes, I had finished the diamond polishing using very fine alumina (0,05 micrometers). Although it seems invisible, it looks like alumina grains are plucked in galena. I will polish again my sample, this time I will end polishing at the 0.25 micormeters step. Let's see...
First: carefully wipe the polished surface with pure ethanol, after cleaning it with ultrasonication. That should remove any relics of colloidal alumina, though some particles might remain locked in plucked cleavage pits (but these can be avoided for analysis).
Second: check the specimen surface with SE and BSE, perhaps also at a high angle. Any Al-phase on a PbS-substrate will have very low contrast, so a sideways view will perhaps reveal more. The SE signal is most sensitive to topography - if anything is sitting on the galena surface, it will show (though maybe not sharp due to resolution limits).
What settings are you using to operate the SEM instrument? These are important to identify the possible source of the Al-signal artefact. State kV, nA, Torr (mPa), EDS live/dead time, WD in mm, etc. Please specify whether or not the specimen was coated to reduce charging. As you don't mention that, you may have operated your SEM in low-vac mode, which greatly increases the chance for stray signal, easily from an area up to 60-100mu around the point of incidence.
Note that a heavy sample material (galena) will backscatter probe-beam electrons with little loss of energy, and relatively many at a high angle.
While I support Mr. Farooq Wahab that if the EDS detects Al there must be a source for it, it seems unlikely to me that the source is within the specimen.
Instead, I suggest that the Al-signal probably represents a stray signal from aluminium metal present in the chamber. Open the chamber door (unscrew the bolts if JEOL) and check to identify all aluminium parts. If I do so for my Leo VP1450, I see aluminium metal in the stage movement and specimen holder, in the collar of the SE's charge collector, in the CL detector, and in the BSE detector right under the pole piece (custom device to go with the CL).
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