Page 271 - Inorganic Mass Spectrometry - Fundamentals and Applications

The ~m~ssion Ions 257
of

several elements that are limited by the levels of impurities in the filament mate-
rials such as MO in Re filaments. Since MO is more volatile than rhenium, it sub-
Ru. Also, there are
limes preferentially, causing spectral interferences with Zr and
a variety of isotope ratio analyses in which the chloride or nitrate salt is loaded di-
the
rectly onto a single filament, with temperature slowly increased until ion emis-
sion is reached. The mechanisms have never been studied, but it is possible that
the
the element alloys with the filament material before being re-emitted as atomic
cation. Thus these ion emitters may also fall into the category of being trace met-
als embedded into a refractory metal matrix. This provides a research opportunity.

is
Electrodeposition could be a pseudo-S-L type process, although definitive proof
lacking. The element to be ionized, uranium [ 161 or plutonium [ 171, is coelec-
layer
trodeposited with a platinum metal layer, then covered with an additional of
platinum. The U or Pu is believed to be electrodeposited as an oxide, and platinum
is electrodeposited as the metal. Hence there is thought to be a U or PLI oxide buried
in the metal matrix. When this deposit is heated, after a sufficient length of time
atomic cations of U or Pu begin to sublime from the surface without measurable
metal oxide ions. Metal oxide ions should be readily observable if they are pres-
ent in the matrix. Thermodynamic calculations indicate that the hot platinum ma-
U
trix will not reduce the and Pu oxides to the metallic state, and yet the observed
species are atomic ions and not oxide molecular ions.
olten Glass) Ion Emitters

The original work on silica gel ion emitters was first published in 1959 [ 181 and
has typically been referred to as the “silica gel” method. This method has been
widely adapted to the analysis of many elements. The basic method is to deposit
silica gel in an aqueous suspension onto a filament, then to add a solution con-
taining the element to be ionized, then to add either phosphoric acid or boric acid
solutions. The mixture is dried and the filament mounted in the source. The source
is evacuated and the temperature slowly increased to operating temperatures. In
general, it is best to use solutions with the lowest practical acid (HC1, H ~ ~ ~ )
strong
content to prevent dissolving filament material that poisons the emitter. The ele-
by
into
ments that can be analyzed this technique can be classified two main groups:
to
those that are readily reduced the metal and those that are not. It is unlikely that
these two groups have the same ion formation mechanism. Elements in the first
group include Pb, Bi, Ag, Au, Te, Sn, Ru, Pd, and Fe [ 19-25]. The alkaline earth
elements E261 can also be analyzed by using variations of this method; they are
clearly quite difficult to reduce to the metal and therefore constitute the second
group. This analytical technique was originally developed for the isotope ratio
analysis of lead for URb age dating, and this is still the largest single application.