impurities in W and Mo

I'm reading an old paper from Physical Review in which the author heats
a filament of W or Mo in a primitive mass spectrometer (just an evacuated
toroidal tube in a magnetic field) to detect ions evaporated from the filament.
He notes the field strengths and temperatures at which there are peaks in the
current and computes the mass of the ions. That seems pretty innocent. Based
on the masses, he identifies some of the ions as being sodium, some as
potassium (two isotopes), some as aluminum (rejecting J.J.Thomson's claim
in earlier experiments of this type that the ions of mass about 27 or 28
were ions of carbon monoxide). At higher temperatures, after "aging" the
filament, apparently W only gives off W ions and Mo only gives off Mo ions.
He thinks that the reason the W (resp. Mo) ions are emitted is due to
recrystallization of the metal in the filament.

That was a long time ago and maybe people know more about these processes
now. So, I'd like to know that. Also, he claims that it was not possible
by ordinary spectroscopy to detect the impurities. The detection of impurities
is a chemical question, so I'd like to know how hard that would be to do now
by chemical or other methods. For all I know, the filament is not the source
of the sodium, potassium and aluminum.

Here is another issue related to filaments, but not directly to this paper.
It was apparently difficult to know what is going on at the surface of
thoriated tungsten filaments. When the filament is heated, the distribution
of the thorium on the surface might change and also some might evaporate.
There could be patches of pure tungsten and thoriated patches. 70 years
later, we probably have very good ways of knowing what the precise situation
is on the surface of a thoriated filament after it has been heated. What are
they? Could any of them be used without having to destroy a perfectly
good vacuum tube to examine the filament?

This seems like a good excuse to start reading about surface chemistry,
a subject I've completely neglected so far. Suggestions on what to read
to get started are welcome, especially if they are likely to shed some
light on the kinds of questions I have been asking about filaments.
--
Ignorantly,
Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx>
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.
.