Page 141 - Inorganic Mass Spectrometry - Fundamentals and Applications

I~d~ctively ~o~pled Plasma Mass Spectrometry 131

for the analysis of geological samples. A few selected areas of geology in which
ICP-MS has been used are described here.

ant~~nide and Actinide

of
Insight into melting and fractionation during the formation rocks can be gained
uring the relative concentrations of rare earth elements. Therefore, ICP-
been used since its introduction for the measurement of rare earth
elements in geological materials [282]. Recently, basalt from Mauna
investigated by ICP-MS using this approach [288].
A wide variety of sample types, sample preparations, and processes have
been used. Poydered rock samples were fused with KZ zCO,, followed by
precipitation of the potassium using perchloric acid, separation with methanol-
perchloric acid, evaporation to a residue, and dissolution of the residue in dilute
nitric acid. Detection limits in the solid were in microgram per gram (pglg) to
the
nanogram per gram (ng/g) levels in the solid without preconcentration.
Rare earth elements have been measured in fluid inclusions using flow
injection [289]. Rare earth oxides cause spectral overlaps, so ion c~omatography
or other separation techniques are often used. High-resolution mass spectrometers
have also been used. By reducing the amount of water entering the ICP using a
be
desolvation and membrane, molecular oxide ion signals can reduced by a factor
of 100 [290]. E~anced isotopes for a light rare earth element and a heavy rare
earth element were used for mass bias correction, internal standardization, and
isotope dilution so that all of the rare earth elements could be determined in one
measurement [290].
Naturally occurring zircon crystals are very stable and include structures
from a succession of geological events. Therefore, rare earth measurements in
into
zircons have also been great interest [291]. Zircons are often difficult to put
of
solution completely so laser ablation is often used. Laser ablation is also partic-
ularly advantageous in probing small zircons embedded in other materials and in
measuring different structures within a zircon crystal.

Precious Metals and
Trace element measurements by ICP-MS have been used for geological explora-
tion [292]. The use of ICP-MS for measurement of precious metals in rocks has
been recently reviewed [293]. Typically sample preparation includes extraction
and concentration of precious metals and separations from potentially inte~ering
elements [293]. For example, HfO+ of masses 194, 195, and 196 overlaps with
Pt+ isotopes. Limits on dissolved solid concentrations to about 0.2% also limit
the detection of small quantities of precious metals in the solid.
Sample preparation is especially key to the measurement of precious metals.