Page 284 - Inorganic Mass Spectrometry - Fundamentals and Applications

270 Marcus

a flat sample holder and a secondary cathode. The formation of this new conduc-
tive layer on the insulating sample permits the formation of a cathode fall poten-
tial and the acceleration of discharge gas ions to the surface. On the basis of the
of
assumption that sputtering ions will have hundreds electron-volts of kinetic en-
ergy, incoming ions can penetrate distances of 2-10 A [5l]. So long as the metal-
Sam-
lic layer is not too thick, an incoming ion transfers momentum to the sublayer
ple such that analyte atoms are sputtered into the gas phase. If the deposited layer
so only the
is too thick, the incident ions do not penetrate the metallic coating, and
overlayer is sputtered. In this case, the resultant mass spectrum solely comprises
elements making up the secondary cathode material. If the layer is too thin, a po-
tential difference does not develop and sputtering does not occur. plasma also
The
tends to be unstable ,without complete coverage.
On the basis of the preceding discussion, it is not too difficult to imagine the
parameters that are important in optimizing the secondary cathode approach to
nonconductor analysis. Two groups have performed the majority of the evaluations
of this method: Milton and Wutton at VG Elemental [21] and Van Grieken’s group
at the University of
of Antwerp [22-241. It is difficult to treat the works these groups
separately as they mesh to form a very comprehensive picture of the relevant ex-
perimental factors affecting the use of this approach.

The choice of the material from which the secondary cathode (also called the mask
is
or ~~a~~~agm) formed must address a few basic issues. First, the material must
be available in high purity. It must tie kept in mind that the cathode continuously
is
sputtered, and so the constituents of that material become ionized and contribute
to the resultant mass spectrum. Thus, there is a definite concern about the ability
to generate a clean analytical blank. Second, the matrix element should have as
few isotopes as possible, preferably in mass ranges removed from target analytes.
Finally, the sputter rate of the secondary cathode must be sufficiently high to form
the conductive layer under typical discharge conditions, without being so high as
to form too thick an overlayer.
Milton and Hutton C211 evaluated aluminum, copper, silver, indium, lead,
and tantalum as possible secondary cathode materials. The first three candidates
(AI, Cu, and Ag) sputtered at rates too high to allow production of ions character-
istic of the glass sample (i.e., tended produce too thick a metallic layer). Indium
to
and lead are soft materials that lead the overcompression of the insulator-cath-
to
ode-sample sandwich, consistently resulting in electrical short-circuiting between
the anode and cathode. Finally, tantalum does indeed exhibit the desirable charac-
teristics far application as secondary cathode materials. Although not explicitly re-
quired, the fact that Ta is a getter element is likely to provide some added benefits
as well.