Page 55 - Inorganic Mass Spectrometry - Fundamentals and Applications

Glow Discharge Mass Spectro~et~ 45

(low-temperature ashing) is applied to the solution prior to homogenization,
metals can be analyzed in organic solutions (e.g., oil) [65].
Several disadvantages to using compacted samples have been cited [68],
including surface contamination of the sample from the die, adsorption of water
to
vapor on the individual powder grains, need homogenize the material carefully
to ensure a representative sample, and need for a high-purity binder. These prob-
lems, although often easily overcome, have prompted individuals to search for
alternative methods of sample preparation. One method is to mix the sample with
gallium [69]. At slightly elevated temperatures (~30°C) gallium is a liquid. Once
it
homogenized with the powder, can be poured into an appropriately shaped mold
and cooled. The cooled metal is removed from the mold and analyzed in a
cryogenically cooled discharge cell as if it were a machined solid.
in
A second approach that has been used extensively our laboratory is to roll
a high-purity indium pin (99.999%) in the powder we wish to analyze [70]. Be-
cause indium is soft, a fraction of the powder is impregnated into the metal. The
the
indium serves solely as a host, supporting the discharge processes (i.e., sample
is atomized along with the indium). This approach has proved especially useful
to
when we have small amounts of powder analyze (c 1 mg) or when the material
is difficult to get into solution. Using this approach, we have observed ion signals
lasting up to 1 hour from discharge initiation and precision comparable to that in
other sample preparation methods. Battagliarin et al. have reported a variation of
this approach [68] that uses a special pressurized vessel of their own design. The
powdered sample is placed in this vessel. h indium rod is inserted into the vessel
so that it is brought into contact with the powder. The die is heated for 2 hours at
190T at atmospheric pressure until the chamber has reached a uniform tempera-
ture; the oven is then pressurized to 70 bar with IN,. The high pressure forces the
now-liquid indium into a mold. On cooling, the solid sample can be removed from
the die and trimmed to the appropriate length for analysis.
Hess et al. have developed a novel sample preparation scheme based on
electrochemical deposition [71]. Although this approach was not designed for
routine sample analysis, it has been advocated for preparing standard materials
to aid in quantitative analysis. The procedure is as follows: silver pins (1.5 mm in
diameter X 20.0 mm in length) are first pressed from pure silver powder. These
pins are then polished to a high luster and inserted into an electrochemical cell
consisting of a working electrode, an auxiliary electrode, and a salt bridge. The
IBM EC/225 voltametric analyzer with a
necessary potentials are supplied by an
variable potential of - 5 to + 5 V. The metal ions used for deposition are obtained
by pipetting the appropriate amount of a 1OOO-pg/mL standard solution into a
reaction vessel. After removal from the electrochemical cell, the samples are
washed with deionized water (while the voltage is still applied) and dried over-
night at 110°C. Once they are dried, the cathodes are weighed, crushed, and