Page 315 - Inorganic Mass Spectrometry - Fundamentals and Applications

source are sampled via very inefficient assembly of skimmer and sample cones,
a
is
as in ICP-MS. This “interface” region where most inst~mentation development
is clearly needed in the coming years. However, despite the transmission advan-
tage of TIMS it is difficult to obtain high-precision concentration and/or isotopic
data for certain elements using such a method simply because they are difficult to
ionize. High-precision isotopic composition and concentration isotope dilution
(ID) measurements of high-mass elements that are difficult to ionize other meth-
by
ods are relatively s~aightforward using MC-ICE”-MS (Halliday et al., 1995, 1997).
is
One of the elements of great interest to isotope geochemists hafnium (Hf)
because of the decay of 176Lu to 176Hf (Patchett, 1983). Of course, Hf has a high
first ionization potential (6.65 eV), rendering it a notoriously difficult element to
measure at high precision by TIMS. Therefore, the measurement of Hf isotopic
compositions has received a great deal of attention from MC-ICP-MS users. The
standard method of sample admission utilizes a peristaltic pump or free uptake (the
Venturi effect in the nebulizer and capillary action) deliver dissolved samples to
to
as
a pneumatic nebulizer. However, desolvating higher-efficiency nebulizers, such
the Mistral, marketed by Elemental, or the MCN6000, marketed by Cetac Tech-
VG
nologies, yield significant (factor of >lo) increases in sensitivity and extremely
good reproducibili~ for small samples (<l00 ng of Hf or W) (Walder et al., 1993b;
Lee et al., 1997). At the time of writing, the sensitivity of all the inst~ments de-
scribed when using the MCN6000 is typically in the range of 0.2% to 0.3% for Hf
the
of
(total ions detected per atom used), with very good plasma stability. For all
new instruments discussed, the sensitivity is being worked on as a matter of pri-
ority and improvements are expected in the coming years.
The precision and reproducibility of MC-ICP-MS data are comparable to
those acquired with modern multiple-collector TIMS a similar sized ion beam.
for
to
The values obtained by Walder et al. (1993a,c) are, within uncertainty, identical
recommended values and the accuracy of the method is not in question, provided
there are no interferences (such as s6Kr on 86Sr). This is illustrated for Hf in Fig.
8.7. The isotopic composition of Hf standards is reasonably well established and
represents an extremely good starting point in terns of establishing the power of
MC-ICP-MS for high-precision isotopic meas~rements of elements that are diffi-
cult to ionize. Figure 8.7 shows isotopic data for the JMC-475 standard used by
most workers, It yields mean 176Hf/177Hf 0.282161 * 0.000013 (20) at the Uni-
of
a
versity of Michigan and 0.282163 0,000012 (20) at Lyons (Blichert-Toft and Al-
it
barkde, 1997), in excellent agreement with the value 0.282154 it 0.000014 now
of
quoted as the best mean value for TIMS measurements (Nowell, pers. comm.,
1996).
Hi~h-precision isotopic ratio measurements with MC-ICP-MS have now
been made on a large range of elements with high first ionization potential, in-
cluding Cu and Zn (~lichert-Toft et al., 1996; Markchal et al., 1997); Ga and Ge
(Hirata, 1997); MO (Lee and Halliday, 199%); Ru, Pd, and Ag (Reh~amper and