Page 323 - Inorganic Mass Spectrometry - Fundamentals and Applications

al.
et
308 ~alli~a~
is
measurements the ability to resolve small isotopic variations limited, in part, by
the need to correct for mass discrimination and mass fractionation during the course
of
of the analysis, normally by alternating the sample with a standard known com-
position. Natural isotopic fractionations among heavy elements are typically too
small to measure with such techniques. However, Mar6chal et al. (1997), using
MC-ICP-MS, report success with the measurement natural frac~ionations in Cu
of
isotopic compositions using Zn to correct for inst~ment~ mass bias, and vice
versa. Similarly, it appears possible to measure natural Ga isotopic variations in
meteorites by monitoring instrumental mass bias using admixed Ge (Hirata 1997).
Recently, the natural isotopic variations in T1 produced by mass-dependent frac-
tionation have been measured by admixing Pb of known isotopic composition
(Re~~per and Halliday, 1999).
Many of the problems associated with ICP-MS are not relevant to MC-ICP-
MS. Most solution work performed on reasonably pure solutes separated trace
is
of
elements, so artifacts associated with the level of total dissolved solids (Williams
and Gray, 1988) are negligible. Plasma instability problems (Begley and Sharp,
1994) are not so much of a concern with MC-ICP-MS because of the use of static
multiple collection. The effects of plasma instability therefore largely cancel out.
The most recent inst~ments (notably the Micromass instrument) are extremely
stable, especially when used in conjunction with the MCN-6000, Polyatomic ion
interferences (Date et al., 1987; Gray and Williams, 1987; Marshall and Franks,
1990) are only of concern below about mass 80 and most applications of MC-ICP-
MS are used for studying higher masses. The contributions of hydrides and oxides
are negligible because a dry plasma is used, either in laser ablation mode or with
a desolvating nebulizer. Use of adjustable high resolution, as on all the latest in-
struments, makes it possible to see which interferences may be present. Use of the
hexapole collision cell, as on the Micromass instrument, provides a means to elim-
inate many of them altogether.
Problems with internal standar~zation and calibration of signal intensities
for concentration measurements with ICP-MS (Walsh, 1992) are irrelevant to MC-
ICP-MS because most applications are concerned with isotopic compositions and
Yi
isotope dilution measurements (Lee and Halliday, 1995b; et al., 1995). Internal
standar~zation is only of significance in the application of laser ablation tech-
niques to the measurement of paren~daughter ratios by MC-ICP-MS. Work has
started in this area (Halliday et al., 1998a), and though the results of the first ex-
periments are encouraging, they mandate a great deal of further effort.

The potential range of application of MC-ICP-MS is enormous. Reviews of some
of the geochemical applications can be found in Halliday et al. (1998b). Here we