Standards for Sulfur

[knshugart]

At the EPMA TC, there was a discussion of good versus bad standards for Sulfur. And I forgot to write down the advice I heard, whoops!

What are your favorite standards for Sulfur? Which ones are extremely beam sensitive and should be avoided?

And while you're thinking about beam sensitive standards: are there any others for other elements that those new in the field frequently chose without thinking carefully enough? Trying to avoid these pitfalls before I jump right into them.

Thanks!

[Probeman]

At the EPMA TC, there was a discussion of good versus bad standards for Sulfur. And I forgot to write down the advice I heard, whoops!

What are your favorite standards for Sulfur? Which ones are extremely beam sensitive and should be avoided?

And while you're thinking about beam sensitive standards: are there any others for other elements that those new in the field frequently chose without thinking carefully enough? Trying to avoid these pitfalls before I jump right into them.

Thanks!

Hi Kathleen (?),
It kind of depends on whether these are sulfides or sulfates. There is a pretty significant peak shift between the two due to the chemical bond- which by the way can be used to get an idea on the chemical state of sulfur.  This sulfur peak shift measurement is often used by igneous petrologists to get oxygen fugacity information in  basaltic glasses.  Jeez, I almost sound like a geologist!   

So for sulfides I would use a natural single crystal pyrite.  Analyze it for traces but it will be stoichiometric.

For sulfates synthetic BaSO4 (Barite) can be obtained and is more stable than CaSO4 (Anhydrite).

As for other difficult elements/standards, I would say that a good fluorine standard can be hard to find, but I've used a synthetic BaF2 single crystal electronic material with success.

Also the other halides and alkalis can be problematic.  See here for more discussion:

http://probesoftware.com/smf/index.php?topic=301.msg2921#msg2921

[Ben Buse]

Hi,

For sulphur in basaltic glass VG2 and VG99 are good secondary standards - one has 1000 ppm and one has 100 ppm. The one with a 1000 ppm also allows you to peak-up on the sulphur which will be closer than pyrite or basalt if you can't peak up on the actual sample.

Ben

[Probeman]

Hi,

For sulphur in basaltic glass VG2 and VG99 are good secondary standards - one has 1000 ppm and one has 100 ppm. The one with a 1000 ppm also allows you to peak-up on the sulphur which will be closer than pyrite or basalt if you can't peak up on the actual sample.

Ben

Yes, VG2 makes a good secondary standard. It's actually about 1350 PPM sulfur.

What we do is peak up on both pyrite and say, anhydrite, then calculate the spectrometer position 1/3 down from pyrite towards anhydrite and that's usually very close to where you will find the sulfur peak in basaltic glasses.

Here's some references:

http://probesoftware.com/smf/index.php?topic=343.msg1803#msg1803

And interestingly, the sulfur peak position changes but not it's shape, so you can pretty much use any sulfur standard so long as you are at the peak position for your standard, and also at the peak position for your unknown as shown here:

http://probesoftware.com/smf/index.php?topic=127.msg502#msg502

[Probeman]

A grad student here wants to perform some scans on different sulfur containing compounds, one of them being elemental sulfur.

A quick look at Wikipedia shows 1 Pa vapor pressure at 375 K (~100 C), that might be OK though a little high for my comfort. But its vapor pressure would be 10 Pa at 400 K, so obviously that would not be good.

But using the CalcZAF temperature rise dialog, and assuming a thermal conductivity of .2 W/(MC) for sulfur, we would see only about an 18 C increase in temperature at 15 keV and 50 nA and 1 um beam, so maybe that would be OK? Only 1.8 C with a 10 um beam.

Has anybody tried hitting elemental sulfur with the beam in an EPMA?   Does any one have data for sulfur vapor pressure at say 50 C?
john

[sem-geologist]

sorry for bringing old post,
I had hit sulphur once with SEM with quite low beam current (as far I remember it was about 1nA), and after the few seconds (2-3 s) no EDS of sulphur was any more getting acquired, as after turning beam scanning back there was no sulphur at that position anymore. It had octoedral form and was in some volcanic rock speciment from Italy. Unless You have cryo-stage you should not hit raw sulphur with e-beam.

[crystalgrower]

If you are looking for a trace S content, please look at tugtupite.  It has an "intermediate" sulfur state:  there are S4 clusters in a oxide silicate matrix.  Very beam stable and very little difference in composition between collection sites.  Also a good source for Cl as well as S.

I bought some on ebay which was fine for probe work.  It never comes in large single crystals, the best will be a mass of intergrown fine crystals about 0.1mm.  It helps to precast these into a drop of epoxy before trying to mount. 

The critical test for tugtupite is, does it get darker red when exposed to bright light (a sunny window is good).  A "probe expert" told me that there was a "white tugtupite" but John Rucklidge confirmed all published info, that only the tenebrescent pink (changing to red) stuff is tugtupite. 

For high level sulfide try ZnS which is 99.999+% in electronics grade and not expensive.  Very beam stable.

Good anhydrite is hard to find, it often has Fe2O3 as a fine intergrown phase in natural cores. 

I provided a rock with S crystals to a student once, to calibrate SEM electron backscattering as S matches zircon.  I asked about the outcome--got two words "didn't work". 

[sem-geologist]

The ZnS (sphalerite) is ideal standard for normal PET, as ZnS is very beam stable AND it does not oxidize compared to pyrite, pyrhotite or chalkopyryte. Now I don't know about JEOL, but If You have Cameca with large crystals, then the weird spectral artefacts, particularly Zn Ka 3rd order back-side reflection from XTAL will make it not so good standard (someone says that back-side reflection/diffraction is completely impossible and that it is caused by XTAL segmentation due to some manufacturing weaknesses, but then why all LPET's (at least 4 which we currently have) show those artificial peaks at exactly same positions?). Such artificial 3rd order (back-side) reflected peak is exactly at S Ka position. This is one of those cases (and I know plenty) which would try explaining discrepancy with matrix model and MAC shortcomings, while it is pure technical measurement problem.

[Probeman]

ZnS is an excellent standard for elemental sulfur measurements. I also utilize a ultra pure pyrite FeS2 which is also excellent (we have ICP-MS on the traces so we can assume stoichiometry on the Fe and S). 

Obviously due to the peak shift of sulfur with oxidation, an sulfate standard would be better for sulfate measurements.  In fact, in typical basaltic glasses which often have a mixture of sulfide and sulfate at the 1000 PPM level or so, a good "rule of thumb" is to peak on the pyrite/sphalerite standard, then detune the spectrometer by 1/3 towards the sulfur peak on anhydrite. On a Cameca instrument that is about a 10 points lower sin theta (*10^5) spectrometer position. The perform your basaltic glass measurements at that spectrometer position.

Remember, for high accuracy trace elements, the standard choice is relatively unimportant compared to the accuracy of the background determination.  In fact, looking at the detection limit expression from Scott and Love 1983, we can see that the higher the standard intensity (normalized to the pure element concentration), which is in the denominator of the expression, the lower the calculated detection limit for the unknown.  In other words, the higher the standard intensity per concentration (which is highest in the pure element due to less absorption of the primary emitted X-ray), the better the trace element statistics.

For ultra high accuracy trace element measurements the best methods utilize the appropriate background measurement type (off-peak vs. MAN vs. multi-point bgd) and a matrix matched blank standard, which is run as an unknown sample. That is, a standard with a similar matrix to the unknown in question, which contains a *zero* concentration of the trace element.

As Mike Jercinovic says "If you can't measure something, trying measuring nothing, because if you can't measure nothing, you can't measure anything".

A nice summary of considerations of background measurement choices is this topic here:

https://probesoftware.com/smf/index.php?topic=1378.0

See also:

https://iopscience.iop.org/article/10.1088/1757-899X/32/1/012012/meta

and

https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/abs/multipoint-background-analysis-gaining-precision-and-accuracy-in-microprobe-trace-element-analysis/55B6A8EC500550EEDCFBF5BE50D81555

An extended discussion of these trace element considerations with Mike Jercinovic can be found here:

https://probesoftware.com/smf/index.php?topic=928.0