Page 54 - Inorganic Mass Spectrometry - Fundamentals and Applications

44 ~arshi~k

type of operating mode is being employed; however, the primary consideration is
the formation of ions for subsequent mass analysis. The variations employed often
result from personal preference combined with the desire to perform a certain type
of experiment or analysis or from availability of equipment.

re~aratio~ Schemes
Unlike solution based methods in which the sample is dissolved in an aqueous
medium that usually includes some small percentage of mineral acid, in CDMS
the sample is not a homogeneous liquid, but a solid, and can therefore take a num-
ber of forms (e.g., machined hollow cathodes, compacted disks, and dried solution
residue). In the next section, cathodelanode geometry is discussed; in the section
presented here, an equally important but often overlooked issue, the physical form
of the sample, is discussed.
The simplest sample preparation involves machining the material into the
desired shape immediately prior to analysis. Even with this approach, however,
the sample is usually etched in a dilute mineral acid or electropolished to remove
surface impurities, rinsed with an organic solvent remove oil residue, and dried
to
in an oven. Often, however, the sample is not large enough to be machined into
a cathode, or it is in the form of a powder. In this latter case, the sample is usually
pressed into the desired shape in a die. The mechanics of this process varies
widely, but there are usually a grinding step, a homogenization step, and a
pressing step. In our laboratory we press pin-shaped cathodes by drilling a 2-mm
hole in a polyethylene slug 2.54 cm in diameter by 5.08 cm in length. The powder
is poured into the void made by the drill bit, and 11 metric tons of pressure is
applied to the slug. This compresses the slug enough to compact the powder into a
rod -1.5 mm in diameter. When the pressure is released, the sample falls out of the
slug. The pin is then trimmed to the desired length prior to analysis. This technique
works equally well for conducting and nonconducting samples. We have found
that powders of 325 mesh hold together better than those that are more coarse.
A
slight variation of this approach is to mix a nonconducting sample with a conduct-
ing binder prior to pressing the mixture. This allows the material to be run with a
conventional direct current (dc) discharge. We have found empirically that sam-
ples with greater than 10 weight percent nonconducting material perform poorly
in the discharge. A similar approach to compacting samples has been applied to
dried solution residues [63]. In a series of articles, Barshick et al. [63-651 demon-
strated how as little as 100 pL of a l-ppm solution can be analyzed as a dried
to
residue homogenized with silver powder. Unique this approach was the homog-
enization of the residue with a binder. This provided a stable ion signal for more
than 1 hour, compared with a transient signal lasting less than 15 minutes when the
solution is dried on the surface of a target cathode [66,67]. If an additional step