Quantifying Osmium

[Michael Lance]

I have a customer who needs to measure Osmium but I don’t have an Osmium standard and even if I did have one, there are the safety concerns. Osmium is a transmutation product of W.
 
How can I quantify Os with no standard? Should I just try to collect Net Intensity on my unknown? I also will be measuring Re as it should be present in W after irradiation. I have a Re standard. Could I just use the net intensity of Re for Os since they are adjacent on the periodic table?

Thanks,

Michael

[Probeman]

Hi Michael,
There are two options I can think of.

1. Buy some vacuum re-melted osmium from ESPI. Link is here:

https://probesoftware.com/smf/index.php?topic=1310.msg9389#msg9389

2. Use the "virtual standard" feature in Probe for EPMA as described here:

https://probesoftware.com/smf/index.php?topic=42.msg796#msg796

and here:

https://probesoftware.com/smf/index.php?topic=179.0

For measuring osmium as a virtual standard you will need a rhenium standard and an iridium standard (using a PET crystal). You said you have a rhenium standard and I assume you have an iridium standard so you should be good to go.

[Brian Joy]

Hi Michael,

I feel your pain.  I've posted before on the subject of osmium standards, as I often analyze PGMs, but I don't have a reliable Os standard.  I don't like to use metals as standards except when necessary (like when analyzing steel, for instance) because they tend to develop oxide films.  That being stated, as far as I know, only Os powder is hazardous, as it tends to alter more readily (due to large surface area relative to volume) to OsO₄, which is toxic.

It should be possible to grow synthetic versions of erlichmanite (OsS₂), omeiite (OsAs₂), or osarsite (OsAsS).  While I do grow standard materials in a muffle furnace, sulfides and arsenides are a bit out of my range.  The reason for this is that I don't have much practice using a torch to seal silica glass tubing under vacuum.

Although one might not encounter Os very often, there is a real need for a good standard other than high purity metal.

Brian

[Probeman]

Attached below is a Johnson Matthey "white paper" paper from 1975 discussing oxidation of the platinum group metals in case anyone is interested.

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[Brian Joy]

Attached below is a Johnson Matthey "white paper" paper from 1975 discussing oxidation of the platinum group metals in case anyone is interested.

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Quoting from that paper, "Osmium is much more reactive [than ruthenium]. Even at room temperatures it is covered with a layer of solid OsO₂, which reacts with the air to form the poisonous OsO₄. This has a remarkably high vapour pressure, so that finely divided osmium powder loses weight at a measurable rate."

I know for certain that erlichmanite (OsS₂) has been synthesized, though I haven't yet done a literature search.  I've synthesized RuSb₂ myself successfully, but synthesis of laurite (RuS₂) should be possible as well (under vacuum).

[Brian Joy]

Although I haven’t found any phase diagrams yet, I’ve been able to verify that OsS₂, OsSe₂, OsSb₂, and OsTe₂ have in fact been grown and are thought to be “line compounds,” i.e., they have compositions that are thought to be invariable.  Unfortunately, Sb and Te cause interference problems near the Os Mα peak position, so OsS₂ and OsSe₂ are the only really viable choices among these.  Also, OsP₄ is thought to be a line compound, though it must be grown in a sealed capsule/ampoule.  Os borides and silicides have been grown as well, but their compositions have not been investigated thoroughly.

It’s possible that I could grow OsSe₂ using the technique described below, though I don’t know if I’d have to worry about generation of OsO₄ gas (starting with Os shot/lump, not powder/sponge).  When growing selenides by this technique, it’s important to be mindful of the fact that Se boils at 685°C.  Because Se is relatively volatile, I’ve simply saturated the system in the past (as this can be used ensure production of the desired line compound).

https://probesoftware.com/smf/index.php?topic=1310.0

P.S.  Nominal OsAs₂ has been grown, but it is known to have variable composition that ranges roughly between OsAs_1.8 and OsAs_2.2.

P.P.S.  Stoichiometric OsSe₂ contains 54.64 wt% Os and 45.36 wt% Se.

P.P.P.S.  In my personal experience, I've most commonly encountered wt% (or greater) Os in laurite, nominally RuS₂ and the only(?) commonly occurring Ru-rich PGM.  It forms a solid solution with erlichmanite (OsS₂).  For these reasons alone, a good synthetic erlichmanite standard would be good to have (along with a synthetic laurite standard).  But OsSe₂ might be good enough.

P.P.P.P.S.  I'm not tremendously worried about generation of OsO₄ gas when using the above technique because 1) the amount of Os is very small and its surface area relative to volume will be small (shot/lump, not powder/sponge), 2) the Os will be fully immersed in Se liquid at 221°C, and 3) all reactants will be encapsulated by B₂O₃ liquid at about 450°C.  Thus, significant interaction with the atmosphere will end within a few hours of turning on the furnace; the reactants will probably need to sit in the furnace for at least a week at ~600°C to ensure complete reaction of the metal.  Also, OsO₄ is supposed to have a distinctive garlic-like odor.  I'm happy for input from anyone on the safety issues regarding working with osmium.

[sem-geologist]

What are you going to do with OsSb2 and OsTe2; If You want throw it out I would happy buy these, as I checked those interferences do not look bad at all even for LPET (and for PET  that is good and easy).

[Brian Joy]

What are you going to do with OsSb2 and OsTe2; If You want throw it out I would happy buy these, as I checked those interferences do not look bad at all even for LPET (and for PET  that is good and easy).

I haven't grown these yet.  I've only done a quick literature search to see which Os compounds have been grown by others.  If I do grow an osmium compound, it will probably be OsSe₂.  But I'm swamped with work...

[sem-geologist]

Oh ok, I think for standard it is also important to be e-beam-resilient (no composition changes due to electron bombardment). I saw many vendor standards with Se and Sb which are quite beam sensitive. On the other I can't tell so about tellurium based standards. i.e. I find synthetic PbTe2 much better standard than pyromorphite or crocoite for Pb.

Consider Te as option too, I am attaching the wds scan pictures with metal Os and metal Te overlay (green is Os, teal is Te). In first figure Os is selected and background for it is shown, in second Te is selected and background is shown (at same positions). The truth is it could cause small issues if someone would use such standard without aware where to set bkgd positions, but for anyone aware of it that cause no problem. Also Te overlap is second order, maybe time will come with better PHA, and possibility to cut out 2nd order peaks (and ghm... background). OsTe2 also looks as better standard for EDS than OsS2 or OsSe2 where S and Se lines would be a bit too close to Os M lines (would cause complaint in DTSA-II). Oh, and I see it even does not cause a problem for TAP too, where it is possible to resolve it.

[Brian Joy]

Oh ok, I think for standard it is also important to be e-beam-resilient (no composition changes due to electron bombardment). I saw many vendor standards with Se and Sb which are quite beam sensitive. On the other I can't tell so about tellurium based standards. i.e. I find synthetic PbTe2 much better standard than pyromorphite or crocoite for Pb.

I've found that elemental Se is somewhat beam sensitive.  I've grown PbSe and Ag₂Se, and both of these hold up fine under the beam (noting that Ag is often difficult in this respect).  Te Lα(2) is too close to the Os Mα peak position for my liking, especially on TAP.  Generally, I find that second order reflections are difficult to suppress using pulse amplitude discrimination.

It's likely to be a while before I get to this, anyway.  I need to think about it some more.

[Probeman]

This is a fascinating discussion on these compounds I have to say.

Anyway, I was curious, so today I went into the lab and pulled out my vacuum re-melted osmium standard.  Even after 4 years in the desiccator it still looked smooth and shiny under the reflected light microscope:



Maybe it's because it's coated with 200 angstroms of carbon it's protected from oxidation in the desiccator?  I also looked at my ruthenium, rhenium and boron metal standards in there and they also looked smooth and shiny!  But they also are carbon coated.

Back to the opening post in this topic, I don't know for sure, but I suspect Michael wants to analyze trace osmium, and as we all know, for trace elements, accuracy is not so much dependent on the standard, but rather with the background determination.

So whether he decides to use the vacuum re-melted osmium from ESPI, or a virtual standard based on rhenium (or tungsten) and iridium, I think he'll be OK.

But if anyone does come up with a stoichiometric osmium compound, I would be very interested in making some measurements on it...

[Michael Lance]

Thank you for the great discussion! Since we need to measure Os around 0.5 wt%, I will just use my Re and Ir standards as John suggests.

-Michael