FMF - Friends of Minerals Forum, discussion and message board

[tantalumgirl]

This is my first post, I hope I'm in the right forum. This morning I attempted to analyze some molybdenite samples. Instead of seeing peaks for Mo and S as expected, I got a single peak at ~4.65eV. Let me say up front that my SEM is about 10 years old, and I would not describe myself as a "power user". I examinined 7 ore samples marked MoS2, and all gave the same result. Any ideas? Thanks in advance, Bonnie

[Pete Modreski]

Bonnie,
I believe the answer is, that as you have found, the Mo L-alpha and S K-alpha peaks in the energy spectrum basically overlap or just about coincide, and you really cannot discriminate between them in the spectrum you get on a standard SEM. To confirm the presence of Mo you have to go look at the much higher-energy part of the spectrum, to look for the Mo K-alpha line (which may not appear very strong, as the energy to produce it may approach that of the excitation energy of the electron beam you are using). I think I've said this correctly. Does this help?
Pete

[Cesar M. Salvan]

In fact, Mo and S lines overlap. The problem is that Mo L-alpha and sulfur K shell emission does not appear at 4.65 eV. I recommend that "tantalumgirl" share an image of the spectrum and inform about the model and method used for record it.

[tantalumgirl]

Unfortunately, I did not save my work yesterday, I will have to run it again to show you the spectrum. I just filled the dewar with LN, so while everything is cooling down, I'll give you some background information.

A colleague left a box of unmarked and vaguely marked samples in his office when he departed the company. That is a big no-no around here. I was given the task of identifying them for proper disposal. One of the projects he was working on was Mo doping, and many of the samples are marked MoS2, and identified by the mine that they come from (Toquepala, Cuajone, Butte Mountain, etc). The samples range from as found grey and yellow rock samples, through all stages of refinement. The final product looks and feels like graphite and is very messy.

There were other samples in the box as well, and I was able to identify Zr, ZrO2, and spherical SnO2 (which was very cool to look at with the microscope ;o), so I assume my machine is working.

The machine is about 10 years old. It is an RJ Lee PSEM, the company is now called Aspex. The EDX from Noran is a replacement, so it is somewhat newer than the SEM.

The samples were attached to the stub with carbon tape and were uncoated. I used an accelerating voltage of 20kV, spot size 30, and working distance of 23.5mm.

According to the software on my machine, Mo peaks are at 2.3, 17.5, and 19.6eV. Sulfur peaks are at 2.33 and 2.47eV.

What I got across the board (except for the "dirtiest" sample) was a single peak at 4.65eV. I looked it up, and Cs or La does not make sense.

Could I be doing something wrong? I am not accustomed to examining ore samples, my work typically involves refined refractory metals.

Or am I doing it right and the samples are mislabled?

Thanks again, Bonnie

[Pete Richards]

I'm somewhat of a beginner with SEM work, but I believe that with an accelerating voltage of 20kV you won't get those peaks above 15eV. I don't have a clue why you're not getting the sulfur peak at 2+eV, however, or what the peak you do get represents. Have you checked the working distance? I don't know how much variation in geometry there is, but I've been working at 12 or 13 mm (JEOL), and had trouble getting much of anything one day when the working distance was set at ~20 and I did not notice it. Good luck!

[Cesar M. Salvan]

Bonnie, without an image of spectrum is quite difficult to guide you, so my guess is based on a good spectrum, with low noise and a great and narrow peak.

In first instance, I think in a calibration trouble. First of all test the EDS with a known sample (I usually use a copper grid also used for calibrations)
Then, follow the calibration procedures of your instruments.
In think about it because a single peak at 4.6 eV is nonsense for me.

Other possibility is a failure in detector. If your detector suffered succesive heatings due to lack of liquid N2 or large storage at ambient temperature, the semiconductor detector could be damaged.

[tantalumgirl]

Ok, great information, I will turn down the accelerating voltage and try again. The recommendation for working distance on our machine is ~19-20mm. But since we don't have an automatic Z axis, I know that using 4 1/2 shim plates will put me in the ballpark. Adjusting it is laborious, as we have an enormous forensics chamber that takes a long time to vent, but it gives me something else to try.

Thanks Pete ;o)

[tantalumgirl]

Thanks Cesar, I will run it again and post the results. You may be right about it being a bad detector, it has been replaced before. That's the problem with letting many people use the machine, but that's not in my control.

[Cesar M. Salvan]

Ok, let us know your results.

I think that your acceleration voltage and working distance is OK. Other problem after your description could be the mechanical adjustments in the chamber...
So, I would check the mechanical adjustments, test EDS with pure metal samples (and assure you can see all the relevant peaks. For example, with copper, you must see 3 peaks at 0.9, 8 and 8.9 keV) to cover the range of energies and follow the manual guidance on calibrations.

As I said, a lone peak at 4.6 is quite nonsense. The only possibility is barium, but you should have other peaks (sulphur...). Also, with samples, take care on the geometry and the possibility of interferences and angles. The beginners usually obtain odd spectra due to the geometry of sample, orientation, cavities...

[tantalumgirl]

After many hours of work yesterday and today, it appears that my detector is not working properly. I put in some known samples, and the results are completely off.

Thank you to everyone that took the time to respond to me. At least my problem helped me to find this most interesting forum .

Cheers, Bonnie ;o)